KRAUSE’S ESSENTIAL HUMAN HISTOLOGY FOR MEDICAL STUDENTS

Third Edition

William J. Krause, Ph.D. Professor of Anatomy Department of Pathology and Anatomical Sciences University of Missouri School of Medicine Columbia, Missouri

1 Table of Contents

1. The Cell 5 7. Special Connective Tissue: Protoplasm 5 Hemopoietic Tissue 79 Cytoplasmic Organelles 6 Embryonic and Fetal Hemopoiesis 79 Cytoskeleton 18 Bone Marrow 80 Cytoplasmic Inclusions 20 Development of Erythrocytes 83 Nucleus 21 Development of Granular Summary 23 Leukocytes 84 Development of Platelets 86 2. Mitosis 29 Development of Monocytes 87 Summary 31 Marrow Lymphocytes 87 Interrelationships of 3. Epithelium 32 Hemopoietic Cells 88 Sheet/Barrier Epithelium 32 Summary 88 Classification of Epithelia 32 Cell Attachments 34 8. Muscle 89 Junctional Complexes 34 Classification 89 Specializations of Epithelial Skeletal Muscle 89 Membranes 37 Cardiac Muscle 97 Glandular Epithelium 39 Smooth Muscle 99 Histogenesis 41 Histogenesis 101 Summary 42 Summary 102

4. General Connective Tissue 43 9. Nervous Tissue 105 Organization 43 Neurons 105 Classification 43 Neuroglia 113 Subtypes of Loose Connective Spinal Cord 115 Tissue 49 Cerebellar and Cerebral Dense Connective Tissue 50 Hemispheres 115 Special Connective Tissue 50 Meninges 117 Histogenesis 51 Autonomic Nervous System 117 Summary 51 Histogenesis 118 Summary 118 5. Special Connective Tissue: Cartilage, Bone, and Joints 52 10. Cardiovascular and Lymph Vascular Cartilage 52 Systems 120 Bone 53 Heart 120 Joints 56 Blood Vessels 123 Histogenesis 59 Arteries 124 Repair of Cartilage and Bone 63 Capillaries 126 Summary 64 Veins 128 Special Circulations 130 6. Special Connective Tissue: Blood 67 Lymph Vascular System 130 Components of Blood 67 Organogenesis 132 Erythrocytes 68 Summary 132 Platelets 70 Leukocytes 71 Blood in Children 77 Summary 77

2 11. Lymphatic Tissues and Organs 134 17. Male Reproductive System 224 Tonsils 137 Testes 224 Lymph Nodes 138 Seminiferous Tubules 225 Spleen 140 Excretory Ducts 229 Thymus 142 Accessory Sex Glands 231 Organogenesis 145 External Genitalia 233 Summary 145 Organogenesis 235 Summary 236 12. Skin 149 Structure 149 18. Female Reproductive System 239 Epidermis 149 Ovaries 239 Dermis 153 Ovarian Follicles 239 Appendages of the Skin 153 Oviduct 243 Organogenesis 157 Uterus 244 Summary 158 Vagina 248 External Genitalia 249 13. Respiratory System 160 Pregnancy 249 Nose 160 Breasts (Mammary Glands) 253 Pharynx and Larynx 162 Organogenesis 255 Trachea and Extrapulmonary Summary 256 Bronchi 162 Intrapulmonary Air Passages 163 19. Endocrine Glands 258 Respiratory Tissue 164 Pineal Gland 258 Pleura 166 Parathyroid Glands 259 Control of Respiratory Rate 167 Thyroid Gland 260 Organogenesis 167 Adrenal Gland 262 Summary 168 Paraganglia 264 Hypophysis 265 14. Digestive System 170 Organogenesis 270 General Structure 170 Summary 271 Oral Cavity 170 Salivary Glands 172 20. The Eye 273 Major Salivary Glands 173 General Structure 273 Teeth 174 Corneoscleral Coat 273 Pharynx 176 Uveal Layer 275 Tubular Digestive Tract 176 Retina 277 Gastrointestinal Mucus 191 Lens 281 Liver 192 Vitreous Body 282 Pancreas 197 Accessory Structures 282 Organogenesis 199 Organogenesis 283 Summary 203 Summary 285

15. Urinary System 206 21. The Ear 287 Kidneys 206 External Ear 287 Extrarenal Passages 217 Middle Ear 287 Organogenesis 218 Inner Ear 288 Summary 219 Organogenesis 293 Summary 294 16. Meiosis 222 Summary 223 Index 296

3 Preface the associated text can be found quickly from the bold type in the appropriate segment.

A segment entitled either Histogenesis or The format of Essential Human Histology departs Organogenesis provides an introduction into the considerably from that of the usual presentations development of each tissue and/or organ and on human tissue/organ biology. This presentation provides another means of reinforcement that was not designed as a formal textbook but as a tool contributes to an overall understanding of the tissue solely for students and is designed for rapid or organ being considered. student learning as well as rapid review in Summaries briefly outline the preparation for USMLE examinations. The structural/functional relationships and serve to narrative used was developed primarily around the draw the information together and to provide an second edition of “Essentials of Human Histology” additional review of the topic. and incorporates a series of lectures presented at During preparation of Essential Human Histology, the University of Missouri, School of Medicine. three major considerations were kept in mind: (1) Essential Human Histology focuses the beginning most curricula place considerable time constraints student’s attention on the most important aspects on the student; (2) function and structure are of this discipline which are presented as a series of inextricably related; and (3) the learning process learning units. In general, the text follows the essentially is a matter of repetition and traditional and logical sequence of cells to tissues to reinforcement. The narrative strives to present the organs, but within this sequence, the discussion on vast amount of information available on this topic, mitosis is presented immediately after the cell and in a concise and logical manner, without sacrificing discussion on meiosis just prior to a consideration the detail that is necessary for a basic understanding of the reproductive systems. of human tissue and organ biology.

Use of the Text William J. Krause, PhD, To understand human structural biology, it is Professor of Human Anatomy essential to learn a specialized vocabulary and to Department of Pathology and Anatomical Sciences assimilate a large body of facts. Learning, as distinct School of Medicine from memorization, depends to a great degree on University of Missouri repetition and reinforcement and is made easier if Columbia, Missouri the material to be learned can be presented in discrete, manageable segments. The format of August, 2005 Essential Human Histology is designed specifically to meet these requirements and, if used properly, will E-MAIL: [email protected] enable the student to master this knowledge quickly and efficiently. WEB: www.missouri.edu/~pathwjk The subject matter is broken down into small learning units, each of which is introduced by a vocabulary appropriate to that unit. The vocabulary introduces the main features of the subject to be discussed and provides the basic vocabulary for that unit. As each segment is read, note the vocabulary words (identified by bold print) in the text and how they contribute to the discussion. After completing the narrative segment, return to the vocabulary words, using them as prompts to recall the details of the material just read. The vocabulary serves as a summary of the topic and provides a means for rapid review. If a vocabulary word fails to prompt a response, it and

4 constituents of protoplasm are calcium, potassium, 1 The Cell sodium, and magnesium present as carbonates, chlorides, phosphates, and sulfates; small quantities Multicellular organisms are made up of two distinct of iron, copper, and iodine; and trace elements such structural components: cells and those products of as cobalt, manganese, zinc, and other metals. The cells that form the intercellular (extracellular) inorganic materials have many functions, including substances. Cells are the fundamental units of living maintenance of intracellular and extracellular material and show a variety of functional osmotic pressures, transmission of nerve impulses, specializations that are essential for the survival of contraction of muscle, adhesiveness of cells, the individual. Each cell is a distinct entity; each activation of enzymes, transport of oxygen, and contains all the machinery necessary for maintenance of the rigidity of tissues such as bone. independent existence and is separated from its Water makes up about 75% of protoplasm. Part surroundings by an individual cell membrane. It is of the water is free and available as a solvent for estimated that the human body consists of between various metabolic processes, and part is bound to 80 and 100 trillion cells. protein.

Protoplasm Properties of Protoplasm

VOCABULARY: protein, carbohydrate, lipid, VOCABULARY: irritability, conductivity, nucleic acid, inorganic material, water contractility, absorption, metabolism, secretion, excretion, growth, reproduction Protoplasm, the living substance of a cell, consists of protein, carbohydrate, lipid, nucleic acids, and Protoplasm is characterized by several physiologic inorganic materials dispersed in water to form a properties that distinguish it from inanimate complex, semifluid gel. The consistency of material. All living cells show these properties, but protoplasm differs in different cells and may change in some cells a particular property may be from a viscous to a more fluid state. emphasized. Protein, alone or combined with lipid or Irritability is a fundamental property of all living carbohydrate, forms the major structural cells and refers to the ability to respond to a component of the cell and the intercellular stimulus. substances. Enzymes and many hormones are Conductivity refers to the ability of a cell to proteins. The major carbohydrates are glucose, transmit a stimulus from the point of origin to which is the chief source of energy in human cells, another point on the cell surface or to other cells. and glycogen, the storage form of glucose. Conductivity also is a property of all cells but, like Complexes of carbohydrates and proteins form the irritability, is most highly developed in nerve tissue. main constituents of intercellular substances that Contractility is the ability of a cell to change bind cells together. Other carbohydrate-protein shape in response to a stimulus and generally is complexes form some hormones and antibodies. indicated by a shortening of the cell in some Lipids serve as an energy source; they also have direction. This property is most prominent in important structural functions and are major muscle. components of the membrane systems of cells. Absorption involves transfer of materials across Nucleic acids are divided into two classes. the cell membrane into the interior of the cell, Deoxyribonucleic acid (DNA) represents the genetic where it might be used in some manner. All cells material and is found primarily but not exclusively show the ability to absorb material, some very in the nucleus. Ribonucleic acid (RNA) is present in selectively. the cytoplasm and nucleus and carries information Metabolism refers to the ability of a cell to break from the nucleus to the cytoplasm. It also serves as down absorbed material to produce energy. a template for synthesis of proteins by the cell. Secretion is the process by which cells elaborate Inorganic materials are as much an integral and release materials for use elsewhere. Excretion, component of protoplasm as proteins, on the other hand, is the elimination from cells of carbohydrates, and lipids; without them physiologic metabolic waste products. processes are impossible. Among the inorganic

5 Growth of an organism can occur by increasing the Cytoplasmic Organelles amount of cytoplasm in existing cells or by increasing the number of cells. There are limits to Organelles are specialized units of the cell that the size a cell can attain without sacrificing the perform specific functions and constitute part of efficiency with which nutrients and oxygen reach the living substance of the cell. Organelles include the interior of the cell. Beyond a maximum size, structures such as the plasmalemma, granular and increase in the amount of protoplasm occurs smooth forms of endoplasmic reticulum, through reproduction (division) of cells. ribosomes, Golgi complexes, mitochondria, lysosomes, peroxisomes, and centrioles. Many Organization of Protoplasm organelles are limited by membranes that are similar in structure to the boundary (plasmalemma) of the VOCABULARY: nucleus, nuclear envelope, cell itself. These membranes, including the cell cytoplasm, plasmalemma (cell membrane), membrane, are metabolically active sheets that are organelle, inclusion, cytoskeleton, cytoplasmic essential to the life of the cell. In electron matrix (cytosol), karyolymph micrographs, the membranes exhibit a trilaminar structure consisting of inner and outer dense lines Although cells differ in size, shape, and function, separated by a light zone. Because the trilaminar the protoplasm of each cell consists of two major structure is representative of all biologic components: nucleus and cytoplasm. The nucleus membranes of a cell, it has been called a unit contains the hereditary or genetic material and is membrane. The thickness of the unit membrane completely surrounded by cytoplasm, from which it varies from organelle to organelle and generally is is separated by a nuclear envelope. The greatest where it forms the plasmalemma. While the cytoplasm is limited by a plasmalemma (cell membranes of different organelles appear to have membrane), which separates the cell from the only minor morphologic variations, they vary external environment. Within the cytoplasm are considerably in chemical composition, enzymatic several structures representing organelles and properties, and functions. The membranous inclusions. Organelles are highly organized, living organelles act to compartmentalize the cell interior structural units of the cytoplasm that perform into discrete functional units so the biochemical specific functions in the cell. Inclusions generally events associated with one organelle do not represent inert cell products or metabolites that interfere with those occurring in adjacent often are only temporary components. Most, but organelles. not all, of the organelles are membranous structures whose size and concentration vary with the type Plasmalemma (Cell Membrane) and activity of the cell. The different organelles tend to be localized in discrete areas of the VOCABULARY: unit membrane, phospholipid cytoplasm so that they and their associated bilayer, integral membrane protein, metabolic processes remain separate from other transmembrane protein, peripheral membrane components of the cell. Cell shape is maintained by protein, glycocalyx, ion channel-linked receptor a three-dimensional cytoskeleton that provides proteins, G-protein-linked receptors, enzyme- structural support for the cell and serves in cell linked receptors, aquaporins, endocytosis, motility and intracellular transport. The phagocytosis, pinocytosis, micropinocytosis, cytoskeleton consists of various microfilaments and fluid-phase micropinocytosis, transcytosis, microtubules. adsorptive (clathrin-mediated) The organelles and inclusions are suspended in an micropinocytosis, coated pit, coated vesicle, amorphous medium called the cytoplasmic matrix clathrin, endosome, exocytosis, regulated (cytosol). Nuclear material also is suspended in a exocytosis, constitutive exocytosis structureless ground substance called the karyolymph. Other than their differences in Each cell is enclosed by a plasmalemma that is 8 to location, karyolymph and cytosol appear to be 10 nm thick and has the typical trilaminar structure equivalent. of the unit membrane. It appears as two dense lines, each 3 nm wide, separated by a 2 to 4 nm clear zone.

6 The plasmalemma is composed of proteins, lipids, glycosphingolipids. Important glycolipids of the glycolipids, and carbohydrates. Lipid forms about plasmalemma are gangliosides and 50% of the mass of the plasmalemma and consists galactocerebrosides, major components of nerve of three major classes of lipid: phospholipids, cell membranes and myelin, respectively. glycolipids, and cholesterol. Although membrane lipids form the foundation of Of these phospholipids are the most abundant. the bilayered structure of the plasmalemma, Four major types of phospholipid have been membrane proteins are primarily responsible for its identified in the plasmalemma. These are specialized functions. By weight, proteins constitute sphingomyelin, phosphatidylserine, about 50% of the plasmalemma. The membrane phosphatidylcholine, and proteins are able to move laterally in the lipid phosphatidylethanolamine. Each phospholipid bilayer over the surface of the cell if they are not molecule has a polar head region that is charged bound to filamentous proteins in the underlying and an uncharged, nonpolar tail region that consists cytoplasm. Membrane proteins function to of two chains of fatty acids. The polar heads are transport molecules into or out of cells (membrane hydrophilic and so lie on both the inner and outer pump proteins, ion-channel proteins, carrier surfaces of the plasmalemma. The nonpolar tail proteins), act as receptors for chemical signals regions are hydrophobic and line up in the center of between cells (hormone receptors) and generate the plasmalemma. The result is two parallel layers messenger molecules that diffuse into the of phospholipid molecules lying tail to tail and cytoplasm, attach elements of the cytoskeleton to forming a phospholipid bilayer in which the the plasmalemma, attach cells to the extracellular hydrocarbon (fatty acid) chains are directed inward matrix (cell adhesion molecules), or may even and the polar groups are directed outward (Fig. 1- possess specific enzymatic activity when stimulated. 1). Phosphatidylcholine and sphingomyelin are There are two basic types of membrane proteins: usually found in the outer layer of the plasmalemma integral and peripheral proteins. Various subtypes whereas phosphatidylethanolamine and of integral and peripheral membrane proteins exist. phosphatidylserine are located in the inner layer adjacent to the cytosol. The plasmalemma behaves as a fluid due to the lateral movement of adjacent phospholipid molecules. Therefore, breaks and tears seal spontaneously due to the polar nature of the phospholipids. Cholesterol molecules within the plasmalemma tend to limit the lateral fluid movement of the phospholipids and contribute to the mechanical stability of the plasmalemma. Because of the polar lipid composition of the plasmalemma, it is highly permeable to substances such as oxygen and nitrogen, and small, uncharged molecules. It is impermeable to charged ions such as potassium, sodium, chloride, calcium, larger uncharged molecules, and all charged large Fig. 1-1. Fluid mosaic model of the plasmalemma. Lipids molecules. Entry of these substances into the cell are arranged in a bilayer. Integral proteins are embedded in the bilayer and often span it, forming a channel. Peripheral depends on integral proteins of the plasmalemma. proteins do not span the bilayer. Glycolipids - lipids with attached sugar residues - are a minor component (about 10%) of the lipids Integral membrane proteins are firmly comprising the plasmalemma. Sugar residues of this embedded in the lipid bilayer and cannot be class of lipid usually extend from the external removed. Some integral proteins are surface of the plasmalemma and contribute to the transmembrane proteins that span the entire negative charge of the cell surface. Glycolipids are width of the plasmalemma and protrude from both important in cell-to-cell and cell-to-interstitial surfaces. This type of integral protein has three matrix interactions. They also may play a role in parts: a region to the cell exterior, a region passing immune reactions. Human glycolipids are derived through the lipid bilayer, and a region to the interior primarily from ceramide and are called of the cell.

7 The amino acids of this type of integral protein do proteins. The infinite variety of molecular form a α-helix, and the protein is referred to as a configurations of the subunits of the large tripartite single-pass integral membrane protein. Not all polysaccharides that extend from the plasmalemma transmembrane proteins are of this type. forms the basis for cell recognition. Some integral Transmembrane proteins that make multiple passes membrane proteins consist of a polypeptide chain through the plasmalemma also occur, and most linked to a carbohydrate (sialic acid) and function as transporters and ion channels identified thus far are adhesion molecules; these are called cell-adhesion multipass transmembrane proteins. Specific molecules or CAMs. Several CAMs have been transmembrane proteins occur in areas of the identified: one for neurons, one for hepatocytes, plasmalemma specialized for attachment to other one for muscle, one for the adhesion of glial cells to cells or the extracellular matrix. Here they pass neurons, and several others. through the lipid bilayer and link cells together or Thus, the plasmalemma is a selectively permeable anchor the cell to the extracellular matrix. Not all membrane in which ions and small water-soluble integral proteins are transmembrane proteins. molecules (amino acids, glucose) must be pumped Peripheral membrane proteins are defined as through protein-lined channels that traverse the those proteins which can be removed from the plasmalemma to gain access to the cell interior. plasmalemma without disrupting the lipid bilayer. The most common ion channel-linked receptor Peripheral membrane proteins are generally proteins are voltage-gated ion channels that require a attached to the surface of the plasmalemma - transmembrane potential to open, mechanically-gated usually the inner surface - and contribute to its ion channels that sense movement in the stability. Peripheral membrane proteins can attach plasmalemma that stimulate them to open, and to the surface of the plasmalemma by ionic neurotransmitter-gated ion channels. Neurotransmitter- interactions with an integral protein, another gated ion channels are receptors that bind peripheral membrane protein, or by interaction with neurotransmitters and mediate ion movement. the polar head groups of the phospholipids. These include the glycine receptor, the N-methyl- Examples of peripheral membrane proteins are D-aspartate receptor, nicotinic acetylcholine spectrin and ankyrin, which are found on the receptor, the 5-hydroxytrptamine serotonin cytoplasmic surface of the erythrocyte receptor, and the γ-aminobutyric acid receptor. plasmalemma. Both function to anchor elements of The channel proteins undergo an allosteric change the cytoskeleton to the cytoplasmic surface of the that opens the channel when stimulated. Thus, the plasmalemma. Similar peripheral membrane movement of solutes across the plasmalemma proteins are present in most other cells. Peripheral depends on the activity of specific transmembrane membrane proteins also function to keep the transport proteins. Movement of a single solute molecules of the plasmalemma from separating and (molecule) by transmembrane transport proteins is the cell membrane from tearing apart. referred to as a uniport mechanism. The movement of The membrane proteins include proteoglycans. two or more solutes across the plasmalemma in the The protein core of this molecule spans the lipid same direction involves a symport or cotransport bilayer, and the portion of the long molecule mechanism. Coupled transport involving the bearing the carbohydrate side chains projects from movement of two or more solutes, but in opposite the exterior surface of the plasmalemma. The sugar directions across a cell membrane, is referred to as a residues of the carbohydrate portion of these countertransport or antiport mechanism. molecules, as well as glycoproteins and glycolipids, G-protein-linked receptors are proteins that form the fuzzy coat observed by electron span the plasmalemma and are linked to G proteins microscopy that is referred to as the glycocalyx. (trimeric GTP-binding proteins). Important G- Such a coat is present on all cells, and the ionized protein-linked receptors are the dopamine receptor, carboxyl and sulfate groups of the polysaccharide the glucagon receptor, the α- and β-adrenergic units give the external surface of the cell a strong receptors, and the muscarinic acetylcholine negative charge. The glycocalyx also plays an receptor. These receptors activate a chain of cellular important role in determining the immunologic events either through a calcium ion pathway or properties of the cell and its relationships and through a cyclic adenosine monophosphate (c-AMP) interactions with other cells. Carbohydrates offer pathway. far greater structural diversity for recognition than

8 The latter pathway increases or decreases c-AMP opposite side of the cell, fuse with the levels by stimulation or inhibition of adenylate plasmalemma and discharge their contents. In cyclase. The calcium ion pathway activates certain cells, such as endothelial cells, this process is phospholipase C, the enzyme which splits termed transcytosis and is an important phosphatidylinositol biphosphate into inositol mechanism for moving material across a cellular triphosphate and diacetylglycerol. barrier to the extracellular spaces (Fig. 1-2). Inositol triphosphate promotes the release of Specific proteins known as caveolins coat the external calcium ions from the endoplasmic reticulum surface of the caveolae and makes the transport resulting in the activation of calcium across the endothelial cell cytoplasm to the ion/calmodulin-dependent protein kinase (Cam- plasmalemma of the opposite side of the cell kinase). Diacylglycerol activates protein kinase C. possible without interference. Caveolae also can Enzyme-linked receptors are transmembrane form from the plasmalemma of a variety of cell proteins linked to an enzyme. When the appropriate types and some are involved in the selective signal binds to the receptor, a chain of cellular transport of specific molecules. Three different events is put in motion which may eventually affect caveolin proteins have been identified and are gene transcription within the nucleus. Some currently under study. Other proteins known as important enzyme-linked receptors include the coatomer proteins coat a variety of other cytoplasmic fibroblast growth factor (FGF) receptor, the insulin transport vesicles and provide a mechanism for the receptor, and the epidermal growth factor (EGF) movement of materials from one organelle to receptor. another including transport to the plasmalemma. Aquaporins are channel-forming integral membrane proteins that function as selective pathways to facilitate water transport across the plasmalemma of several cell types in different organs. Ten different types of aquaporins have been identified in mammalian tissues to date. The plasmalemma also plays an active role in bringing macromolecular materials into the cell Fig. 1-2. Diagrammatic representation of fluid-phase micropinocytosis. (endocytosis), as well as discharging materials from the cell (exocytosis). Adsorptive (clathrin-mediated) Phagocytosis is a form of endocytosis in which micropinocytosis is the selective uptake of specific particulate matter is taken into a cell. During the macromolecules at certain receptor-binding sites in attachment phase, particles bind to receptors on the the plasmalemma. This form of micropinocytosis is plasmalemma, while during the ingestion phase, the important in the internalization of nutrients, growth cytoplasm forms pseudopods that flow around the factors, antigens, recycling receptors, and particles, engulf them, and take them into the pathogens. Short, bristle-like projections may be interior of the cell in membrane-bound vacuoles. In present on the cytoplasmic surface at these sites and a similar way, fluid may be incorporated into the form coated pits from which coated vesicles cell in small cytoplasmic vesicles by a process called arise. The cytoplasmic surfaces of the vesicles are pinocytosis. Phagocytosis and pinocytosis can be coated with clathrin, a protein that appears as seen by light microscopy. radiating spikes that give a fuzzy appearance to the Some materials may enter the cell in even smaller vesicles. Clathrin may prevent fusion of coated vesicles formed by minute invaginations of the vesicles with membranous organelles (Fig.1-3). plasmalemma. This process, called micropinocytosis, is visible only with the electron microscope. Nonselective fluid-phase and selective adsorptive types of micropinocytosis have been recognized. In most cells, fluid-phase micropinocytosis occurs with the formation of small vesicles or caveolae from the plasmalemma. Fig. 1-3. Adsorptive (receptor-mediated) The caveolae, which contain fluid and substances micropinocytosis via coated vesicles. dispersed in the fluid, traverse the cytoplasm to the

9 Following the formation of the coated vesicle, the proteins, and proteins destined for use by clathrin coat is lost and the vesicles fuse with mitochondria, peroxisomes, and the nucleus. preexisting vacuoles to form structures called Individual free ribosomes are not active; only when endosomes. they are attached to mRNA to form polyribosomes As the molecules being transported are released do they become active in protein synthesis. from these early endosomes into the cytosol, Similarly, ribosomes on the endoplasmic reticulum portions of the endosome membrane containing must be associated with mRNA before they engage unoccupied receptors bud off as small vesicles, in the synthesis of protein. return to the plasmalemma, and fuse with it. Thus, Messenger RNA is formed in the nucleus on a some receptors are recycled. Late endosomes may template of uncoiled deoxyribonucleic acid (DNA). fuse with lysosomes and their contents are broken It contains a message that is encoded as successive down. sets of three nucleotides called codons that specify Materials such as secretory granules are released the sequence in which amino acids are to be from the cell by exocytosis, a process in which incorporated into newly forming protein. During SNARE proteins in the limiting membranes of the synthesis, mRNA enters the cytoplasm from the granules recognize and then fuse with the nucleus and attaches to ribosomes that move along plasmalemma before discharging their contents. In the mRNA, translating the code and assembling this way, a breach in the limiting plasmalemma is amino acids in the proper order. On reaching the avoided. The excess membrane incorporated into end of the mRNA, ribosomes detach and the plasmalemma during exocytosis is removed by simultaneously release the newly synthesized endocytosis of small vesicles. Two types of protein molecule. Amino acids are brought to the exocytosis are known to occur: regulated and ribosomes for incorporation into the protein by constitutive. transfer RNA (tRNA), another form of Regulated exocytosis is stimulus-dependent and ribonucleoprotein. There is a specific tRNA for occurs in cells specialized for the release of a large each of the amino acids. Ribosomal and transfer volume of secretory material as in pancreatic acinar RNA are formed by transcription of specific cells. Constitutive exocytosis occurs in cells in segments of the DNA molecule. Thus, unlike which the secretory product is not concentrated in a mRNA, formation of ribosomal RNA is directed by number of large granules but is released a specific region of a chromosome. continuously in small vesicles. Endoplasmic Reticulum Ribosomes VOCABULARY: tubule, cisternae, granular VOCABULARY: ribonucleoprotein, free endoplasmic reticulum (GER), ribosome, ribosome, polyribosome (polysome), smooth endoplasmic reticulum (SER) messenger RNA (mRNA), protein synthesis, codon, transfer RNA (tRNA) The cytoplasm of almost all cells contains a continuous, irregular network of membrane-bound Ribosomes are small, uniformly sized particles of channels called the endoplasmic reticulum. Typically, ribonucleoprotein 12 to 15 nm in diameter and this organelle appears as anastomosing tubules, but composed of large and small subunits. They may be the membranes also form parallel, flattened saccules attached to the membranes of the endoplasmic called cisternae. Small transport vesicles not reticulum or be present as free ribosomes attached to tubules or cisternae may be present also suspended in the cytosol, unassociated with and are considered to be part of the endoplasmic membranes. Free ribosomes often occur in clusters reticulum. Smooth and granular forms of called polyribosomes (polysomes); in which endoplasmic reticulum can be distinguished. individual ribosomes are united by a thread of Granular endoplasmic reticulum (GER) ribonucleic acid (RNA) called messenger RNA usually consists of an array of flattened cisternae (mRNA). Free ribosomes are sites of protein bounded by a membrane. The outer surface is synthesis, the protein formed being used by the studded with numerous ribosomes (Fig. 1-4). cell itself rather than secreted. Free ribosomes synthesize cytosolic proteins, peripheral membrane

10 The granular endoplasmic reticulum is the site of protein synthesis for secretory proteins, lysosomal enzymes, and plasmalemmal proteins (receptors). The proteins are synthesized by ribosomes on the external surface of the GER, and then enter the lumen of the endoplasmic reticulum, where they are isolated from the surrounding cytoplasm. Proteins destined for secretion, organelles, and some membranes have an NH2-terminal signal sequence (a leader sequence) that is recognized by a Fig. 1-4. Diagrammatic representation of granular signal-recognition particle in the cytosol as it is endoplasmic reticulum. being translated from mRNA. The signal- recognition particle (SRP) binds to both the signal Smooth endoplasmic reticulum (SER) lacks peptide and the ribosome, an event that stops the ribosomes and consists primarily of a system of translation process. The SRP together with the interconnecting tubules without cisternae (Fig. 1-5). ribosome and the newly translated NH2 terminus of In most cells, one form of endoplasmic reticulum the forming polypeptide become bound to an usually predominates. Protein-secreting cells such as integral membrane-docking protein called a signal- pancreatic acinar cells or plasma cells are recognition receptor (ribosome receptor protein) in the characterized by an abundance of GER, whereas membrane of the endoplasmic reticulum. After the smooth type predominates in cells that secrete docking translation resumes, and the signal peptide steroid hormones. In still other cells, such as liver together with the forming polypeptide enters a cells, both types of endoplasmic reticulum are transmembrane channel (pore) to gain access to the present in nearly equal amounts and may be cisternal lumen of the granular endoplasmic continuous. Granular endoplasmic reticulum is reticulum. Once inside the lumen of the known to be involved in the synthesis of protein, endoplasmic reticulum the signal sequence of the but the precise role of SER varies in different cells. polypeptide is clipped off by a signal peptidase It functions in the synthesis of steroid hormones in located along the inner membrane surface of the certain endocrine cells, in the synthesis of granular endoplasmic reticulum. Following membrane phospholipids, cholesterol, ceramide translation of the message and synthesis of the and glycogen, in the detoxification of drugs using secretory polypeptide, the ribosome is released cytochrome P450 , and in the elongation of fatty from the endoplasmic reticulum and the acids. The smooth endoplasmic reticulum also has a transmembrane channel is obliterated by lateral role in the release and recapture of calcium ions movement of phospholipid molecules in the during contraction and relaxation of striated membrane of the endoplasmic reticulum. Once muscle. inside the cisternal lumen these polypeptides and forming proteins remain isolated from the remainder of the cytosol. Here the nascent, unfolded proteins undergo folding into a new three- dimensional configuration. Most proteins synthesized by the granular endoplasmic reticulum also become glycosylated after entry into the cisternal lumen the process of which is completed in the Golgi complex.

Fig. 1-5. Diagrammatic representation of smooth endoplasmic reticulum.

11 Golgi Complex maturing face. Secretory materials within the vacuoles become more concentrated, and the VOCABULARY: negative image, saccule, condensing vacuoles eventually mature into forming (cis)-face, maturing (trans)-face, secretory granules. transport vesicle, condensing vacuole, secretory It is now known that this simplified version of granule, trans-Golgi network Golgi function is much more complex. The GER and SER synthesize a large number of proteins and The Golgi complex (apparatus) does not stain in lipids, respectively. Some for export as secretory ordinary histologic preparations, nor is it visible in products and others destined to become living cells; it does sometime appear as a negative incorporated into the structural components of the image - a nonstaining area of the cytoplasm usually cell itself. The Golgi complex functions in the post- close to the nucleus. The size and appearance of the translational modification, packaging, and sorting of the Golgi complex vary with the type and activity of the proteins and lipids synthesized by the endoplasmic cell and may be small and compact or large and reticulum. It is also involved in membrane netlike. Some cells contain multiple Golgi recycling. Essential enzymes involved in complexes. glycosylation and other functions are found on the In electron micrographs, the Golgi complex is luminal side of the endoplasmic reticulum and seen to consist of several flattened saccules Golgi membrane cisternae. As a result, the Golgi (cisternae), each limited by a smooth membrane. complex can be subdivided into functional The saccules are disc-shaped, slightly curved, and compartments depending on the enzymes present often appear to be compressed near the center and within its cisternae. The cis-compartment of the dilated at the edges. The saccules are arranged in forming face receives transport vesicles that have stacks and are separated by spaces 20 to 30 nm budded off from the transitional elements of the wide. Because of the curvature of the saccules, the endoplasmic reticulum. This compartment is highly Golgi complex has convex and concave faces. The fenestrated and appears as a network of convex face usually is directed toward the nucleus anastomosing tubules and vesicles. Following and is called the forming or cis-face; the concave modification of the proteins and lipids received maturing or trans-face is oriented toward the cell from the endoplasmic reticulum, vesicles bud off membrane. The forming face is associated with from cisternae of this compartment and fuse with numerous small transport vesicles, and at this face cisternae of a medial compartment. After the the outer saccule is perforated by many small transported lipids and proteins are acted on by openings. The saccules at the maturing face tend to enzymes in this compartment, vesicles form once be more dilated than those at the convex face. again and transport these molecules to the cisternae Secretory products are concentrated in the Golgi of the trans-compartment at the maturing face of complex, whose size varies with the activity of the the Golgi complex. Some terminal cisternae of this cell. In protein-secreting cells, peptides first region are highly fenestrated and form a network of accumulate within the lumen of the GER and then anastomosing tubules and vesicles. This specialized are transported to the Golgi complex in small region of the Golgi complex is called the trans- transport vesicles, which are formed by budding Golgi network. It is in the trans-Golgi network off from ribosome-free areas of the GER adjacent that proteins, glycoproteins, and lipids are sorted to the Golgi complex. The coatomer-coated into different transport vesicles. The specific transport vesicles carry small quantities of protein chemical groups added to the proteins in the to the Golgi complex, where they coalesce with and endoplasmic reticulum and later modified in the contribute membrane to the developing outer various compartments of the Golgi complex saccule at the forming face. Proteins accumulate designate where specific proteins will go within the within the cisternae of the Golgi membranes and cell. Thus, sorting takes place at the maturing face are modified as they pass through the Golgi within the trans-Golgi network, where certain complex. At the maturing face, the saccules expand proteins are designated to be packaged into larger and bud off to form limiting membranes that secretory granules, some are enclosed in small, enclose the protein in structures called condensing smooth-surfaced vesicles, and others are placed in vacuoles. Addition of new membrane to the clathrin-coated vesicles. forming face balances loss of membrane from the

12 The larger, membrane-limited secretory granules are Lysosomes usually destined for regulated exocytosis (stimulated secretion). Those products in small, smooth- VOCABULARY: acid hydrolase, autolysis, surfaced vesicles are involved primarily in primary lysosome, secondary lysosome, constitutive exocytosis as well as transport of phagocytosis, heterophagy, phagosome, membrane to other organelles. Many cell membrane heterophagic vacuole, residual body, proteins (receptor proteins) packaged into autophagy, cytolysosome, multivesicular body nonclathrin-coated vesicles are inserted into the plasmalemma via this mechanism. The small coated Lysosomes are small, membrane-bound, dense vesicles that transport enzymes (acid hydrolases) bodies measuring 0.2 to 0.55 µm in diameter. More fuse specifically with endosomes to form than 50 enzymes have been identified in lysosomes. developing (primary) lysosomes. Since they are active at an acid pH, lysosomal During release of secretory granules, the enzymes often are referred to as acid hydrolases. membranes of the secretory granules fuse only with The limiting membrane of lysosomes protects the the plasmalemma and become incorporated into the remainder of the cell from the effects of the cell membrane. The membranes bounding secretory contained enzymes which, if released into the granules have specific proteins known as vSNAREs cytosol, would digest or lyse the cell. Such an that recognize and bind specifically with distinct occurrence is called autolysis and is presumed to plasmalemmal proteins known as tSNAREs and occur normally during such diverse events as not with the membranes of other organelles. Both regression of the mesonephros during kidney of these proteins are necessary for the phospholipid development and regression of mammary tissue bilayer of both membranes to join together thereby after cessation of lactation. Increased lysosomal incorporating the granule membrane into the activity also occurs during the regression of some plasmalemma and releasing the contents contained tumors. within the granule to the exterior of the cell. Thus, The appearance of lysosomes varies according to the membranous packaging provided by the Golgi the state of activity in which they are observed and complex provides the vacuoles/vesicles at the their association with cell structures or material maturing face with special properties. The limiting brought into the cell. Because lysosomes exhibit membranes of the secretory granules have the such variable morphologic features, a histochemical capacity to recognize and fuse only with the apical test identifying at least two acid hydrolases must be plasmalemma to discharge their contents during done for positive identification. regulated exocytosis. Other Golgi vesicles recognize Primary lysosomes are those which have been the basolateral plasmalemma and fuse only with this newly released at the Golgi complex and have not region during constitutive exocytosis. Yet other engaged in digestive activities. The enzymes of the vesicles fuse with other specific organelles or with primary lysosomes are synthesized in the GER and endosomes and are not discharged from the cell as transported to the Golgi complex. Within the Golgi is the case during the formation of primary cisternae, mannose components of enzymes lysosomes. destined for lysosomes are phosphorylated, and as There is a continuous movement of membrane they move through the Golgi complex, they are through the cell, from endoplasmic reticulum to bound to a transmembrane glycoprotein (mannose- transport vesicles, to Golgi complex, to secretory 6-phosphate receptor) on the luminal side of granules, and then to the plasmalemma. membranes forming the trans-Golgi network. Internalization of plasmalemma for use within the These receptor-enzyme complexes are then cell occurs during phagocytosis, pinocytosis, and segregated to regions of the trans-Golgi network micropinocytosis. Proteins associated with the that form small vesicles coated with the protein membrane bounding these vesicles, regardless of clathrin. Coated vesicles containing receptor-bound type, are thought to aid in guiding the membranous acid hydrolases bud from the trans-Golgi network vesicles to their destination and then recognize and soon after formation lose their clathrin coat. complementary proteins within the membranes of The small transport vesicles, filled with inactive acid the target organelles. hydrolases, then fuse with a late endosome derived from the plasmalemma which contains H+-ATPase.

13 This enzyme produces an acidic environment (pH 5.0) that activates the acid hydrolases of the forming primary lysosome. Primary lysosomes usually remain within the cell and are not secreted. However, some cells (neutrophils, osteoclasts) do release primary lysosomes into the extracellular environment. Secondary lysosomes are vacuolar structures that represent sites of past or current lysosomal activity and include heterophagic vacuoles, residual bodies, and cytolysosomes. The relationships of these structures are best understood from a description of the processes involved in phagocytosis (Fig. 1- 6). Fig. 1-6. Schematic representation of heterophagy and autophagy. Some cells, such as macrophages and some granular leukocytes of the blood, have a special capacity to engulf extracellular materials and destroy Autophagy refers to the lysosomal breakdown of them. The process by which substances are taken cytoplasmic organelles in normal, viable cells. The into the cell from the external environment and lysosomal system is involved in the destruction of broken down by lysosomal activity is called excess or damaged organelles and in the remodeling heterophagy. The process involves invagination of of the cytoplasm. During the process, a portion of the cell membrane and containment of the material the cytoplasm containing excess or damaged in a membrane-bound vacuole. Thus, the organelles becomes surrounded by a membrane to extracellular material taken into the cell is form an autophagic vacuole. The membrane is thought sequestered in a vacuole called a phagosome and to be derived from SER. The vacuole fuses with a remains isolated from the cytoplasm. As the primary lysosome to form another type of phagosome moves through the cytoplasm of the secondary lysosome called a cytolysosome. The cell, it encounters a primary lysosome. The fate of the materials within the autophagic vacuoles membranes of the two structures fuse and the (which may be the cell's own mitochondria, enzymes of the lysosome are discharged into the ribosomes, endoplasmic reticulum, and so forth) is phagosome. The combined primary lysosome and the same as that of heterophagic vacuoles and again phagosome is now called a heterophagic vacuole, results in the formation of residual bodies. In many a type of secondary lysosome. The material within cells, indigestible substances within autophagic the heterophagic vacuole is digested by the vacuoles form a brownish material called lipofuscin lysosomal enzymes, and any useful materials are pigment, the amount of which increases with age. transferred into the cytosol for use by the cell. In addition to these activities, lysosomes form an Nondegradable materials such as some dye intracellular digestive system with the capacity of taking particles, asbestos fibers, silica, or carbon may in and breaking down most molecules produced in remain within the vacuole, now called a residual excess by cells. The absence of a specific lysosomal body. A residual body is another form of secondary enzyme results in the accumulation of its normal lysosome that is thought by some to be eliminated substrate within the lysosome. Large accumulations from the cell by exocytosis. However, in many cells of substrate within lysosomes seriously affect the residual bodies accumulate and persist for long lysosome function and are the basis of numerous periods of time. lysosomal storage disorders such as Tay-Sachs and Gaucher's diseases (Table 1-1).

14

Table 1-1. Lysosomal storage diseases.

Disease Missing enzyme Accumulating substance

Sphingolipidoses Gaucher’s disease β-Glucosidase Glucocerebroside Fabry’s disease α-Galactosidase Ceramide trihexoside Krabbe’s disease β-Galactosidase Galactocerebroside Farber’s disease Acid ceramidase Ceramide, gangliosides Niemann-Pick disease Sphingomyelinase Sphingomyelin Tay-Sachs disease N-Acetyl-β-glucosaminidase A Ganglioside GM2

Glycoproteinoses Fucosidosis α-Fucosidase Fucoglycoproteins Sialidosis Sialidase Sialyl oligosaccharides Mannosidosis α-Mannosidase Mannose-containing oligosaccharides Mucolipidoses I cell disease Severe deficiency of several Variety of undegraded hydrolases substances Mucolipidosis Ganglioside sialidase Gangliosides

Mucopolysaccharidoses Sanfilippo disease Heparan-N-sulfatase Heparan sulfate Morquio’s syndrome N-acetyl-galactosamine-6-sulfatase, Keratan sulfate galactose-6-sulfatase Maroteaux-Lamy syndrome Arylsulfatase Dermatan sulfate Sly syndrome β-Glucuronidase Heparin sulfate Dermatan sulfate Other lysosomal storage diseases Pompe’s disease α-1,4-Glucosidase Glycogen Wolman’s disease Acid lipase Cholesterol ester, triglycerides Cystinosis Defective transport system in Cysteine disulfide lysosomal membrane

Another form of lysosome is the multivesicular Peroxisomes contain amino acid oxidase and body, a membrane-bound vacuolar structure, 0.5 to hydroxy acid oxidase that generate a considerable 0.8 µm in diameter that contains several small, clear amount of hydrogen peroxide (H2O2) which in vesicles. Its origin, function, and exact relationship excess is lethal to cells. Peroxisomes also contain an to other lysosomes is obscure. abundance of the enzyme catalase which can make up as much as 40% of the total peroxisomal Peroxisomes enzyme. The excess hydrogen peroxide produced by this organelle is converted to oxygen and water VOCABULARY: nucleoid, hydrogen peroxide, by catalase and other peroxidases. Peroxisomes are catalase essential in the oxidation of several substrates, particularly very long chain fatty acids by fatty acid β- Peroxisomes, or microbodies, comprise another oxidation enzymes. Peroxisomal oxidases also class of the membrane-bound organelles. Usually function to neutralize free radicles, normal byproducts larger than lysosomes, their internal structure varies of cellular metabolism, which if allowed to and can be crystalline or dense. The crystalline accumulate are detrimental to the health of the cell. structures are called nucleoids. Peroxisomes lack Formation of peroxisomes does not appear to acid hydrolases but do contain as many as 40 other involve the Golgi complex. enzymes.

15 Peroxisomes are thought to form by the growth many enzymatic reactions. The inner mitochondrial and fission of existing peroxisomes and their membrane contains enzymes of the electron contents (proteins, phospholipids, membrane lipids) transport chain (NADH dehydrogenase, succinate imported from the cytosol. Peroxisomes are dehydrogenase, ubiquinone-cytochrome c abundant in metabolically active cells such as oxidoreductase, cytochrome oxidase), ATP hepatocytes (liver cells) and proximal tubular cells synthase (confined to elementary particles), and of the kidney. ATP-ADP translocator. The ATP-ADP translocator moves ADP into the matrix and ATP Mitochondria out of the mitochondrial matrix. The narrow space between inner and outer VOCABULARY: outer mitochondrial membranes, the membrane space, is continuous membrane, porins, inner mitochondrial with the small intracristal space within each crista. membrane, cristae, elementary particles, The membrane space is rich in hydrogen ions. membrane space, intracristal space, intercristal The inner mitochondrial membrane surrounds the space, mitochondrial matrix, tricarboxylic acid larger intercristal space that contains a slightly cycle (TAC), DNA, ribonucleoprotein, matrix more electron-dense material called the granule, production of ATP mitochondrial matrix (Fig. 1-7). Enzymes of the tricarboxylic acid cycle (TAC), fatty acid β- Mitochondria are membranous organelles that play oxidation enzymes, amino acid oxidation enzymes, a vital role in the production of energy required by carbamoylphosphate synthetase I, ornithine cells. They are visible in living cells examined by transcarbamoylase, and pyruvate dehydrogenase phase contrast microscopy and can change shape as reside within the mitochondrial matrix. Substrates they slowly move about in the cytosol. metabolized in the matrix produce acetyl CoA Mitochondria can be stained in appropriately fixed which is oxidized by the tricarboxylic acid cycle to tissues, where they appear as rods or thin filaments. carbon dioxide. The energy from this oxidation is They usually are not visible in routine tissue captured by nicotinamide adenine dinucleotide sections because of the lipid solvents used during (NADH) and flavin adenine dinucleotide (FADH2). tissue preparation. As these nucleotides are oxidized they produce Ultrastructurally, mitochondria show a variety of hydrogen ions and electrons. The electrons are shapes and sizes, but all are enclosed by two transferred along the electron transport chain and membranes, each of which has the typical trilaminar the hydrogen ions move into the membrane space. substructure. However, the inner and outer Hydrogen ions then flow from the membrane space mitochondrial membranes differ markedly in their through transmembrane hydrogen ion pores and chemical composition and physiologic properties. into the matrix. It is within the matrix that ATP The outer mitochondrial membrane is a synthase catalyzes the reaction converting continuous, smooth structure that completely adenosine diphosphatase (ADP) to adenosine envelops the organelle. It contains specialized triphosphatase (ATP). transmembrane transport proteins called porins that allow permeability to certain metabolic substrates. An inner mitochondrial membrane runs parallel to the outer membrane but is thrown into numerous folds, the cristae, that extend into the interior of the mitochondrion. Cristae greatly increase the surface area of the inner mitochondrial membrane and may be either shelf-like or tubular in shape; the tubular form is seen most often in cells involved in steroid synthesis. The inner mitochondrial membrane is studded with club- shaped structures called elementary particles. Each consists of a globular head and a narrow stalk. The inner mitochondrial membrane is the site of Fig. 1-7. Structure of a mitochondrion.

16 Mitochondria are unique among organelles because Centrioles they contain their own complement of DNA and are capable of self-replication. Mitochondrial DNA VOCABULARY: centrosome, diplosome, differs from nuclear DNA in its lower molecular microtubule, triplet, procentriole, centriolar weight and is unusual in that it consists of branched satellite, basal body, ciliogenesis, procentriole filaments of variable thickness arranged in a circular organizer manner. Mitochondrial DNA represents less than 1% of the total DNA in a cell. The human Under the light microscope, centrioles appear as mitochondrial genome contains 16,569 nucleotides. minute rods or granules usually located near the These nucleotides can encode 13 of the protein nucleus in a specialized region of the cytoplasm subunits involved in electron transport and called the centrosome. In some cells centrioles are oxidative phosphorylation. Therefore, mitochondria located between the nucleus and the free surface of must import the majority of their proteins from the the cell at some distance from the nucleus. Two cytosol. The mitochondrial matrix also contains centrioles usually are present in the nondividing cell small particles of ribonucleoprotein that are 12 and together form the diplosome. As seen in nm in diameter and are similar in structure and electron micrographs, the two centrioles that make function to cytoplasmic ribosomes. Messenger up the diplosome lie perpendicular to each other. RNA and transfer RNA also have been identified in The wall of each centriole consists of nine subunits, mitochondria. each of which is made up of three fused Scattered throughout the mitochondrial matrix are microtubules; the subunits are referred to as the more conspicuous matrix granules, which triplets. The nine sets of triplets are so arranged in measure 30 to 50 nm in diameter. These granules the centriolar wall that they resemble a pinwheel contain calcium and magnesium ions and are when seen in cross section. The microtubules thought to regulate the internal ionic composition within each triplet are called the A, B, and C of the mitochondrion. microtubules, the innermost being the A Thus, mitochondria function in oxidative microtubule, the central tubule being the B, and the phosphorylation (which results in the production most peripheral tubule being the C microtubule. of ATP driven by electron transfer to oxygen), fatty The clear center of the centriole contains a thin acid oxidation, amino acid oxidation, and in the filament that passes in a helix immediately adjacent production of acetyl coenzyme A (CoA). Adenosine to the inner surface of the centriolar wall. triphosphate (ATP) is a primary source of energy Multinucleated cells contain several centrioles. for cell activities. Centrioles are self-replicating organelles that duplicate just before cell division. A new centriole, Annulate Lamellae called a procentriole, forms at right angles to each of the parent centrioles. Initially, the wall of the The annulate lamellae are membranous organelles procentriole consists of a ring of amorphous consisting of parallel cisternae arranged in stacks. material with no microtubules. As the procentriole At regular intervals along their lengths, the cisternae elongates by addition of material at its distal end, show numerous small pores that appear to be microtubules appear in the wall, and the new closed by thin, electron-dense diaphragms. Because structure assumes the configuration of the parent they contain pores, they exhibit a morphologic centriole. Immediately after duplication, each parent similarity to the nuclear envelope. The cisternae are centriole together with a newly formed daughter spaced uniformly throughout the stack, and the centriole migrates to the opposite poles of the cell, pores in successive cisternae may be aligned. where they function in the development of the Annulate lamellae often have a perinuclear location mitotic spindle. Small, dense bodies, the centriolar and might be continuous with elements of GER. satellites, are associated with the centrioles and They have been seen in germ cells, various somatic initiate the development and polymerization of cells, and in tumor cells but are relatively microtubules during formation of the mitotic uncommon. The functional significance of the spindle. annulate lamellae is unknown.

17 In some cells, centrioles migrate close to the distributed to form an extensive, interwoven net- surface, where they form basal bodies from which work. Actin-binding proteins such as filamin link arise cilia or flagella; these also are microtubule- actin filaments together under certain conditions to containing structures. During the process of form a stiff supportive framework for the ciliogenesis, dense spherical bodies (procentriole plasmalemma. Actin also may be linked to organizers) appear in the cytoplasm, and transmembrane proteins in specialized regions of numerous procentrioles form around each. Multiple the plasmalemma at junctional complexes, newly formed centrioles migrate to sites contributing further to the cytoskeleton of immediately beneath the plasmalemma and become individual cells. A number of actin-binding proteins oriented perpendicular to it. The two innermost exist and interact with either G-actin monomers or microtubules (A and B) of each centriolar triplet F-actin filaments and influence the distribution and begin a rapid polymerization of tubulin and serve as function of actin in the cytoplasm (Table 1-2). templates for formation of the microtubules of the Under different conditions actin filaments axoneme. Elongation of the cilium is usually participate in cell locomotion, ruffling of the cell complete within 1 hour. membrane, invagination of the plasmalemma during endocytosis, and formation of the Cytoskeleton contraction ring of dividing cells. In this case, a thin layer of actin filaments located along the The cytoskeleton gives structural support to the cytoplasmic surface of the cell membrane may cytoplasm and consists of microfilaments, interact with myosin. Myosin is an actin-activated intermediate filaments, microtubules, and a ATPase protein that exists in several monomeric microtrabecular lattice. Interaction between the forms. cytoskeleton and the plasmalemma is essential for Myosins are a diverse protein family consisting of cell movement, intracellular transport, endocytosis, 18 different subtypes, of which myosin II of muscle focal mobility of the plasmalemma, maintenance of is the best characterized. Myosin V also is present cell shape, stabilization of cell junctions, and spatial in most cells and interacts with actin filaments to orientation of enzymes and other molecules in the generate movement of intracellular vesicles and cytosol. organelles.

Microfilaments

VOCABULARY: actin, myosin II, actin- binding proteins, myosin V

Cytoplasmic filaments are responsible for contractility, a property shown in some degree by almost all cells. Filaments are best developed in muscle cells, where the proteins actin and myosin II form two different types of filaments whose interactions are responsible for the contractile properties of muscle cells. Microfilaments have a diameter of 7 nm and usually are located immediately beneath the plasmalemma. Actin shows the same dimensions as the microfilaments and has been identified chemically in microfilaments. Actin is an important component of the cytoskeleton and occurs in two molecular forms: G-actin, which consists of globular monomers of actin, and F-actin, the polymerized form that consists of two identical strands helically coiled around one another. Actin filaments may be organized into parallel bundles or randomly

18

Table 1-2. Actin-binding proteins.

Protein Action

Profilin Binds to G-actin monomers preventing polymerization and thus providing a pool of monomer for future use

Capping protein Binds to ends of actin filaments limiting a further increase in length

Fimbrin Binds parallel actin filaments into bundles

Filamin Cross-links actin filaments into a three-dimensional network

Gelosin Fragments actin filaments into shorter segments

Vinculin Mediates binding of actin to plasmalemma

α-Actinin Mediates binding of actin to plasmalemma

Spectrins I & II Links actin protofilaments to plasmalemma and cytoplasmic vesicles

Tropomyosin Stabilizes actin filaments

Myosin V Interacts with actin filaments in locomotion of cells and movement of vesicles and organelles

Myosin II Actin filaments slide between myosin filaments during contraction of muscle and other contractile cells

Intermediate Filaments provide for the mechanical integration of the contractile proteins actin and myosin; VOCABULARY: keratin, vimentin, desmin, neurofilaments are found only in nerve cell bodies neurofilament, glial filaments and their processes and often occur in bundles. Glial filaments are confined to the supporting The thicker intermediate filaments measure 9 to 12 (glial) cells of the central nervous system and nm in diameter and form a more diverse population consist of glial fibrillary acidic protein. Glial filaments of filaments; they may be present as individual and neurofilaments provide internal support for strands or loose bundles. Intermediate filaments their respective cell types. Knowledge of this often are attached to the internal surface of the restricted distribution is often useful for the plasmalemma at cell junctions and contribute to the assessment of malignant tumors to determine their cytoskeleton of the cell, providing support. site of origin. Microfilaments and intermediate filaments may occur in the same cell. Microtubules Although indistinguishable structurally, five types of intermediate filaments have been identified by VOCABULARY: tubulin, microtubule- immunocytochemical means. Most cells contain associated proteins (MAPs) only a single type, but occasionally a cell type may contain two. Keratin filaments are present in Microtubules are straight or slightly curved, epithelial cells and form bundles that end in cell nonbranching tubules measuring 21 to 25 nm in attachments. Such an arrangement aids in linking diameter and several micrometers in length. Their individual cells into structural units. They are most walls, about 6 nm thick, consist of 13 parallel abundant in the stratified squamous epithelia, protein filaments, each 4 to 5 nm in diameter, especially in the skin. Vimentin filaments occur in arranged in a helix. The central zone is electron- fibroblasts and other cells derived from lucent, so the structure appears to be hollow. mesenchyme; desmin filaments are found in Microtubules are composed primarily of the protein skeletal, cardiac, and smooth muscle cells and tubulin, which occurs in alpha and beta forms.

19 The number of microtubules in the cytoplasm Cytoplasmic Inclusions varies from one cell type to another and at different times within the same cell. Microtubules can rapidly Inclusions are nonliving elements found in the form or disappear depending on cell activity. cytoplasm and include such diverse materials as Microtubules are in dynamic equilibrium with a pigment granules, glycogen, lipid droplets, and large reserve of soluble tubulin in the cytoplasm. crystals. They are not essential to the life or Once formed, microtubules elongate by adding functioning of the cell and represent metabolic subunits at one end and usually depolymerize at the products, storage materials, or foreign substances distal end, recycling subunits to the cytoplasm. taken into the cell from the environment. Small quantities of high-molecular-weight proteins called microtubule-associated proteins (MAPs) Pigment Granules also have been isolated and are thought to correspond to the slender, lateral filamentous VOCABULARY: melanin, melanosome, projections of the microtubules. Four different hemosiderin, lipofuscin types of MAPs have been identified: kinesin, dynein, dynamin, and axonemal dynein. Kinesin links Naturally occurring pigments in human cells include transport vesicles to microtubules and through melanin, hemosiderin, and lipofuscin, each of which cyclic interaction moves the vesicle toward the has its own inherent color. Melanin is contained forming end of the microtubule. Dynein also links within melanosomes, which are membrane-bound transport vesicles to microtubules, but those granules formed and found in melanocytes and destined to move toward the end of the sometimes secondarily deposited in certain other microtubule where depolymerization occurs, cells. They are especially prominently at the skin's providing for two-way traffic within a cell. Dynamin dermal-epidermal junction, contributing to the links neighboring microtubules into bundles, and color of the skin. The pigment epithelium of the axonemal dynein is responsible for the sliding action retina and iris and certain cells of the brain also of microtubules within cilia and flagella, resulting in contain melanin. their beating action. Hemosiderin is a golden brown pigment derived Microtubules usually are scarce in resting cells but from breakdown of hemoglobin present in red are present in large numbers in dividing cells, where blood cells. Phagocytic cells of the liver, bone they make up the mitotic spindle. After the cell has marrow, and spleen normally contain this type of divided, most of the microtubules disappear. In pigment. interphase cells, microtubules form prominent Lipofuscin is found in many cells throughout the components of centrioles, cilia, and flagella and, in body, particularly in older persons. Sometimes some cells, contribute to the cytoskeleton. Within called the "wear and tear" pigment, lipofuscin is platelets, microtubules form stiffening elements that light brown in color, increases with age, and help maintain their shape. In nerve cells, represents an end product of lysosomal activity. microtubules are important for transport of Lipofuscin pigment provides an indication of free material from one region of the cytoplasm to radical damage and consists of phospholipids another. A transient increase in the number of complexed with proteins. Neurons, hepatocytes, microtubules is seen in nondividing cells during and skeletal muscle cells normally contain this type changes in shape associated with cell movement of pigment. and during differentiation. It also has been suggested that a three-dimensional Glycogen molecular framework (a microtrabecular lattice) exists in the cytoplasmic matrix, that links the cell VOCABULARY: glucose polymer, beta organelles and cytoskeleton into a coordinated particle, alpha particle functional unit during cell movement and other activities. Glycogen is a large glucose polymer and is the storage form of glucose. The glucose units are linked by α1, 4 glycosidic bonds.

20 Glycogen synthesis is catalyzed by the enzyme cardiac muscle cells, and liver cells, may possess glycogen synthase and glycogen break down to glucose two nuclei. Giant cells, such as the osteoclasts of is catalyzed by the enzyme glycogen phosphorylase. bone, megakaryocytes of marrow, and skeletal Glycogen cannot be seen in usual tissue muscle cells, may have several nuclei. preparations unless selectively stained. Hepatocytes The shape of the nucleus varies and may be (liver cells) and skeletal muscle cells contain the spherical, ovoid, or elongated corresponding to the largest glycogen stores of cells in the body. The cell shape, or it might be lobulated as in the total amount of glycogen stored in skeletal muscle granular leukocytes of the blood. is greater than that of the liver although the liver The nucleus contains all the information necessary has the highest content of glycogen per gram of to initiate and control the differentiation, tissue. Ultrastructurally, glycogen appears either as maturation, and metabolic activities of each cell. beta particles or glycosomes, which are irregular, The nondividing nucleus is enclosed in a nuclear small, dense particles 15 to 45 nm in diameter, or as envelope and contains the chromatin material and alpha particles, which measure 90 to 95 nm in one or two nucleoli. These are suspended in a diameter and represent several smaller beta particles nuclear ground substance called the karyolymph or of glycogen clumped to form rosettes. nuclear matrix.

Lipid Chromatin and Chromosomes

VOCABULARY: fat cell, lipid droplet VOCABULARY: karyosome, heterochromatin, histone, euchromatin, diploid, haploid, Fat cells are the chief storage sites for lipid, but homologous chromosome, sex chromosome, many other cell types store lipid droplets of autosome, karyotype, metacentric, acrocentric, various sizes. Lipid synthesized by a cell submetacentric accumulates in cytoplasmic droplets that lack a limiting membrane. Intracellular lipid serves as a Genetic information is stored in the molecules of source of energy and as a supply of short-chain DNA that make up the chromosomes. Each carbon molecules for synthesis by the cell. During chromosome contains a single long molecule of preparation of routine tissue sections, lipid usually DNA that consists of two linear polymers of is extracted, and sites of lipid storage appear as clear nucleotide subunits each made up of a phosphate vacuoles. In electron micrographs, preserved lipid group, a pentose sugar (deoxyribose), and four droplets appear as homogeneous spheres of organic bases (adenosine, cytosine, guanine, different densities. thymidine). The bases project toward and bind to complementary bases of the adjacent polymer. Crystals Together the two polynucleotide chains intertwine and form the antiparallel double helix of the DNA Crystalline inclusions are normal constituents in molecule. Genetic information of the DNA several cell types and may be free in the cytoplasm molecules is encoded in the sequence of the bases. or contained within secretory granules, A gene refers to that unit of heredity that involves a mitochondria, endoplasmic reticulum, the Golgi sequence of bases necessary for the synthesis of a complex, or even the nucleus. Many of the protein or a nucleic acid. It is estimated that the observed intracellular crystals are believed to human genome consists of between 20,000 and 25,000 represent a storage form of protein. genes. In non-dividing nuclei, chromosomes are largely uncoiled and dispersed, but some regions of Nucleus the chromosomes remain condensed, stain deeply, and are visible by the light microscope as The nucleus is an essential organelle present in all chromatin. Nuclear DNA is associated with a complete “true” cells. The only cytoplasmic structures variety of proteins that form chromatin. These in which nuclei are absent are mature erythrocytes proteins can be divided into two general categories: and blood platelets; these should not be regarded as histones and nonhistone proteins. true cells. Generally, each cell has a single nucleus, but some, such as the parietal cells of the stomach,

21 The latter include structural proteins, regulatory division that they assume the appearance of proteins, and the enzymes needed for nuclear discrete, solid, rodlike structures. Analysis and study function (DNA and RNA polymerases). Histones of chromosomes can be carried out most are the most abundant proteins in the nucleus and conveniently in dividing cells that have been form the inner core of a DNA-protein complex arrested in metaphase. Alkaloids such as the Vinca called a nucleosome. The nucleosome is the basic unit drugs and colchicine interfere with spindle formation of the chromatin fiber. The nucleosome has a string and permit intracellular accumulation of metaphase of beads appearance and represents the basic chromosomes for study. structural unit of chromatin. Each fiber consists of The number of chromosomes typically is constant a double helix of DNA wrapped about a core of for each species but varies considerably between histones. Individual masses of chromatin are called species. In humans, the chromosome number is 46. The karyosomes, and although not entirely constant, figure given is for the diploid number in somatic the chromatin masses do tend to be characteristic in cells. Germ cells (ova and sperm) contain half this size, pattern, and quantity for any given cell type. number and are said to be haploid. The Collectively, the karyosomes form the chromosomes present in somatic cells represent the heterochromatin of the nucleus and represent inheritance of two sets of chromosomes, one from coiled portions of the chromosomes (Fig. 1-8). each male and female parent. In the male and Heterochromatin is believed to be complexed to female sets, chromosomes that are similar are called histones and to be non-active. Histones are simple homologous chromosomes. In many diploid proteins that contain a high proportion of basic higher animals, a pair of sex chromosomes is amino acids. The dispersed region of the chromatin specialized for and participates in the determination stains lightly and forms euchromatin, which is of sex; all other chromosomes are called active in controlling the metabolic processes of the autosomes. In humans there are 44 autosomes and cell. The distinction between heterochromatin and a pair of sex chromosomes that are homologous euchromatin disappears during cell division when (XX) in the female and heterologous (XY) in the all the chromatin condenses and becomes male. metabolically inert. Homologous chromosomes can be recognized at metaphase and are arranged in groups representing the karyotype of a species. Individual chromosomes often can be identified by the length of their arms and the location of the centromere. If the centromere is in the middle of the chromosome and the arms (telomeres) are of equal length, the chromosome is said to be metacentric. If the centromere is close to one end, the chromosome is acrocentric, and if the centromere is between the midpoint and the end, the chromosome is submetacentric.

Nuclear Envelope

VOCABULARY: unit membrane, perinuclear space, nuclear pore, nuclear pore complex, lamins, nuclear lamina

Fig. 1-8. Diagrammatic representation of nuclear structures. The nuclear envelope consists of two concentric unit membranes, each 7.5 nm thick, separated by Chromosomes are permanent entities of the cell a perinuclear space 40 to 70 nm wide. The inner and are present at every stage of the cell cycle, but membrane is smooth, whereas the outer membrane their appearance depends on the physiologic state often contains numerous ribosomes on its of the cell. At interphase the chromosomes form cytoplasmic surface and is continuous with the delicate, tortuous threads, and it is only during cell surrounding endoplasmic reticulum.

22 At irregular intervals around the nucleus, the inner is synthesized. Since these sites (nucleolus- and outer membranes of the envelope become organizing regions) are located on five different continuous with one another to form small chromosomes, any one cell may contain several octagonal openings called nuclear pores. The nucleoli. Usually only one or two large nucleoli are pores measure about 10 nm in diameter and are found, since the nucleolus-organizing regions tend closed by a nuclear pore complex that consists of to associate and the RNA produced at these regions two rings, one of which faces the cytoplasm. Eight aggregates into larger masses. radial spokes extend inward from the rings toward a As seen in electron micrographs, nucleoli lie free central granule. The number and distribution of in the nucleus, not limited by a membrane. They nuclear pores depend on the type of cell and its show two regions, each associated with a particular activity. form of ribonucleoprotein. The dominant region, The nuclear envelope aids in organization of the the pars granulosa (nucleolonema), consists of a chromatin and controls the two-way traffic of ions network of dense granules of RNA 13 to 15 nm in and molecules moving between the nucleus and diameter. The second region, the pars fibrosa, cytoplasm. Molecules less than 10 nm in diameter tends to be centrally placed and consists of dense pass through the nuclear pore complex by passive masses of filaments 5 nm in diameter. diffusion, whereas large molecules (entering newly Deoxyribonucleoprotein also is associated with synthesized proteins, exiting ribonucleoproteins) the nucleolus and is present in filaments of require an energy-dependent transport mechanism. chromatin that surround or extend into the It is thought that a signal sequence of amino acids nucleolus. This chromatin forms the nucleolus- directs them to the nuclear envelope, and after associated chromatin; its DNA constitutes the binding of a signal sequence to a receptor in the template for synthesis of rRNA. In humans, rRNA nuclear pore complex, the nuclear pore opens much genes represent clusters of DNA segments located like an iris diaphragm of a camera to permit passage near the tips of chromosomes 13, 14, 15, 21, and of the larger molecules. 22. RNA polymerase I catalyzes the formation of A thin meshwork of filaments called lamins is ribosomal RNA. Elsewhere within the nucleus made up of three polypeptides and lies along the RNA polymerase II catalyzes the formation of inner surface of the nuclear membrane to form the messenger RNA (mRNA) and RNA polymerase III nuclear lamina. The lamins are structurally similar catalyzes the formation of transfer RNA (tRNA). to intermediate filaments and are classified as types Ribosomal gene transcription occurs in the fibrillar A, B, and C according to their location and region of the nucleolus. Following transcription, the chemical properties. Type B lamins lie nearer the rRNAs are processed and then assembled in the outer surface of the nuclear lamina and binds to pars granulosa. With the addition of the small specific integral (receptor) proteins of the inner ribosomal subunit, ribosomes are then transported nuclear membrane. Types A and C lamins lie along to the cytoplasm through nuclear pores. the inner surface of the nuclear lamina and link the Nucleoli are found only in interphase nuclei and membrane-bound lamin B to chromatin. The three are especially prominent in cells that are actively lamins are thought to function in the formation and synthesizing proteins. They are dispersed during cell maintenance of the nuclear envelope of interphase division but reform at the nucleolus-organizing cells. They also may aid in maintaining the shape of regions during reconstruction of the daughter the nucleus. nuclei after cell division.

Nucleolus Summary

VOCABULARY: ribosomal RNA (r-RNA), Cells are the fundamental units of structure of all nucleolus-organizing region, pars granulosa tissues and organs and perform all the activities (nucleolonema), pars fibrosa, nucleolus- necessary for the survival, growth, and associated chromatin reproduction of an individual. They carry out energy transformations and biosynthetic activities A nucleolus appears as a dense, well-defined body, and are able to replicate themselves. 1 to 3 µm in diameter, contained within a nucleus. Nucleoli are sites where ribosomal RNA (rRNA)

23 The functional activities of a cell are carried out by important role in muscle contraction, being specialized structures, the organelles, many of responsible for the release and recapture of calcium which consist of or are bounded by biologic ions. The GER is involved in the synthesis of membranes essential to the organization of the cell. proteins and provides a membrane for the transport In addition to forming the interface between the vesicles that carry newly formed protein to the cell and its external environment, they also are Golgi complex, where it is modified, sorted and important in transportation of materials, packaged into secretory granules. Golgi membranes organization of different energy transfer systems, are able to synthesize carbohydrate, complex it to transmission of stimuli, provision of selectively protein, and form glycoproteins. The Golgi permeable barriers, and separating various complex also acts in the synthesis of lipoproteins, intracellular compartments. being responsible for complexing lipid to proteins All materials that pass into or out of a cell must produced by the endoplasmic reticulum. Thus, the cross the plasmalemma; this structure is primary functions of the Golgi complex are post- instrumental in selecting what enters and leaves the translational modification of synthesized product, cell. Large molecules are taken in by pinocytosis, packaging of the product, and sorting of the particulate matter can be incorporated into the cell product enabling specific products to reach varied by phagocytosis, and small molecules enter the cell but very specific destinations. by diffusion. The rate at which material diffuses Synthesis of protein occurs on ribosomes, and into the cell depends on whether the material is protein formed by ribosomes attached to the GER soluble in lipid or water. Lipid-soluble materials usually, but not always, is secreted by the cell, readily dissolve in the lipid matrix of the whereas that formed on free polysomes is used by plasmalemma and pass through the cell membrane the cell itself. relatively unhindered. The transmembrane proteins Mitochondria perform a number of functions, of the plasmalemma act as sites for passage of chiefly the production of energy for the cell. water-soluble substances and serve as transport Pyruvate, produced by the degradation of glucose, proteins for materials such as glucose, ions and enters the mitochondrion, where, along with amino other molecules. acids and fatty acids, it is processed by the enzymes Although simple in microscopic appearance, the of the TAC cycle located in the mitochondrial plasmalemma is very complex and variable in its matrix. Transfer of electrons and hydrogen atoms molecular organization and function. Significant to oxygen is mediated by enzymes of the electron physiologic and biochemical differences may occur transport system, which are located on or within the at specific regions of the plasmalemma according to inner mitochondrial membrane that forms the the function of the cell. The plasmalemma contains cristae. Energy released by the oxidations is special receptor sites that can react with agents such incorporated into the high-energy bonds of ATP, as hormones or neurotransmitters to transfer which then diffuses into the cytoplasm, where it is information to the cell and elicit specific responses. available for use by the cell. Mitochondria also play The structure and composition of the plasmalemma a role in the synthesis of steroids. are important in determining the immunologic Lysosomes mainly function in intracellular properties of the cell and its relationships and digestion, which occurs in membrane-enclosed interactions with other cells. The plasmalemma may digestion vacuoles that isolate the lysosomal contain enzyme systems that act as ion pumps or enzymes from the remainder of the cytoplasm. initiate and control cellular activities by generating Materials may be taken into the cell during secondary messenger molecules. Membrane phagocytosis, or portions of cytoplasm containing specializations also are involved in cell-to-cell excess or damaged organelles may be sequestered attachments and communication. and digested by lysosomal activity. Usable end The endoplasmic reticulum is associated with products diffuse into the cytoplasm, and the many synthetic activities of the cell. Smooth undigested residue remains within the vacuoles to endoplasmic reticulum in liver cells is believed to form residual bodies. Lysosomes have other serve in lipid and cholesterol metabolism, glycogen functions in normal cells. synthesis and storage, and detoxification of drugs. In endocrine cells, the SER has been implicated in the synthesis of steroid hormones. It plays an

24 They have been implicated in the degradation of This structure controls the growth, differentiation, glycogen, in the removal from cells of excess maturation, and metabolic activities of the cell. The substances such as unsecreted products, and in the nucleus also is equipped to duplicate and pass on release of thyroid hormone by splitting off the DNA to its daughter cells. The nucleolus, which is globulin to which the hormone is conjugated during the site of production of rRNA, plays a central role its synthesis. Rupture of lysosomes in some cells in the control of protein synthesis by the cell. All may initiate mitosis. the nuclei in an individual contain identical Peroxisomes are similar in morphology to genomes, but some portions are actively expressed lysosomes but contain a number of different in some cells and repressed in others. The enzymes involved in neutralizing free radicles and differential activity of genes is due to differential in the metabolism (oxidation) of very long chain activation, not to selective removal of genes. fatty acids. In doing so they generate a considerable It must be kept in mind that several genetic amount of hydrogen peroxide. High levels of defects are recognized which can directly affect the catalase prevent the accumulation of lethal levels of function of organelles or elements of the hydrogen peroxide by breaking it down to oxygen cytoskeleton. Genetic defects in the production of and water. specific acid hydrolases or in transport proteins of Microtubules, intermediate filaments, and the lysosomal membrane results in lysosomal microfilaments form the cytoskeleton of cells and storage diseases. Diseases, such as perform a number of functions. Generally, adrenoleukodystrophy, are due to defects in intermediate filaments act as supporting elements peroxisomal enzymes of peroxisomes and present within cells but also aid in linking individual cells as metabolic disturbances. Mitochondrial cytopathy into structural units as in epithelium. Microtubules syndromes also occur in which abnormal contribute to the structure of cilia, flagella, mitochondrial DNA can disrupt mitochondrial and and centrioles, provide supporting structures cell function. Mitochondrial diseases may present at (stiffening rods) in some cells, have been implicated any age from early childhood to well into adult life. in intracellular transport of materials, and are Mitochondrial cytopathy is characterized by essential for cell division and motility. metabolic disturbances, muscle weakness or Microfilaments (primarily actins) together with degenerative disease of the central nervous system. myosin function in cell movement, focal Genetic disease involving elements of the movements of cell membranes, endocytosis, cytoskeleton such as microtubules in cilia/flagella movement of cytoplasmic vesicles and organelles, (Kartagener’s syndrome) and defects in proteins and are responsible for the contraction ring during such as dystrophin associated with the mitosis. They also have a role in stabilization of the plasmalemma of muscle cells (Duchenne’s muscular plasmalemma. dystrophy) also are well established. All the information needed to initiate and regulate the activities of a cell is encoded in the nucleus.

25

Table 1-3. A summary of key structural components within a cell and their functions.

Membranous organelles Morphologic features Functions

Cell membrane (plasmalemma) Electron-lucent layer (3.5-4.0 nm) Selectively permeable membrane; cell- separating two electron-dense layers to-cell communication, receptors for (2.5-3.0 nm) hormones, cell recognition, ion pumps and channels; generates messenger molecules

Granular endoplasmic reticulum Network of membrane-bound tubules Site of protein synthesis to be and cisternae; outer surface is covered discharged from cell, lysosomal with ribosomes proteins

Smooth endoplasmic reticulum Network of branching, anastomosing, Varies with cell type; implicated in smooth, membrane-bound tubules synthesis of steroid hormones, cholesterol, triglycerides, glycogen, lipoproteins; detoxification of lipid- soluble drugs; acquisition and release of calcium ions

Golgi complex Stacked, parallel array of several Site of post-translational modification, flattened, membrane-bound saccules sorting, and packaging of cell products; (cisternae); convex surface is the synthetic activity for complex forming face; concave surface is the proteoglycans, lipoproteins, maturing face glycoproteins, lysosomal enzymes

Annulate lamellae Parallel stacks of membrane enclosing Unknown cisternal lumina 30-50 nm in diameter; become continuous at regular intervals forming fenestrations (pores) in cisternae; pores often lie in register with one another

Mitochondria Size variable; usually round, oval, or Provide energy in form of phosphate rod-shaped; consist of two membranes; bonds (ATP) for cell activities; an outer smooth membrane enveloping involved is synthesis of steroid a thinner, highly folded inner membrane hormones in specific cell types in the form of lamellar or tubular cristae that project into the interior (matrix); matrix granules prominent

Lysosomes Demonstration of 2 acid hydrolases needed for conformation

Primary Round, membrane-bound dense bodies Yet to be activated or begin digestive 0.25-0.55 µM in diameter; interior activity variable in density

Secondary Membrane-bound bodies, ovoid or Activated hydrolytic enzymes that are irregular in outline, size variable. Usually currently or have been involved in contain material of some type digestive activity (crystalloids, debris, organelle remnants, lipofuscin pigment)

Multivesicular body Numerous small vesicles limited by a Unknown single, smooth membrane

Peroxisomes Spherical, membrane-bound bodies 0.2- Oxidation of very long-chain fatty 0.4 µM in diameter; matrix may contain acids, neutralize free radicals and dense crystalline structures (nucleoids) toxins

26 Non-membranous organelles

Ribosomes Dense granules (15-25 nm) comprised Protein synthesis of a large and small subunit; Polyribosomes are several ribosomes united by a strand of mRNA

Centrioles Cylindrical structures (0.2 X 0.5 µM) Essential for formation of with an electron-dense wall containing microtubules of mitotic spindle, cilia, nine triplet microtubules surrounding an and flagella electron-lucent center; occur in pairs oriented at right angles

Components of cytoskeleton

Microtubules Long, slender tubules 25 nm in Maintain cell shape and participate in diameter, wall 5-7 nm in width shape changes; forms axonemes of surrounding an electron-lucent lumen cilia and flagella; intracellular transport; membrane stabilization

Microfilaments Actin – 6 nm microfilament Cell movement; control focal movement of plasmalemma as in endocytosis, contribute to stability

Intermediate filaments 8-11 nm in diameter, subdivided Stabilize cell shape and attachments, immunocytochemically into five classes: maintain position of ion channels and 1. Keratin filaments – occur in receptors in cell membrane; link epithelia individual cells into structural units 2. Desmin filaments – occur in three types of muscle 3. Vimentin filaments – occur in fibroblasts and mesenchymal cell derivatives 4. Neurofilaments – occur in nerve cells 5. Glial filaments – occur in glial cells

Cytoplasmic inclusions

Glycogen 1. Beta particles (20-30 nm) occur Storage form of glucose singly and are irregular in outline 2. Alpha particles are variously sized aggregates of the beta form

Lipid Spheroidal droplets of varying size and Primarily a storage form of short density; lack a limiting membrane carbon chains (triglycerides)

Pigment granules

Melanin Dark brown; complexed in ellipsoidal Inert melanosomes Hemosiderin Gold-brown; ultrastructurally appears as Degraded product of hemoglobin; collections of 9-nm particles inert

Lipofuscin Coarse, irregularly shaped, brown-gold End product of lysosomal activity; granules inert

27 Crystalline structures Vary considerably in size and structure; Most thought to be a storage form of may occur in cytoplasmic matrix, protein organelles, or nucleus

Secretory granules Membrane-bound; vary in size, density, Materials synthesized to be released by and internal consistency cell

Nuclear constituents

Nuclear envelope Consist of inner and outer membranes Specialized segment of endoplasmic that become continuous around nuclear reticulum that bounds nucleus; nuclear pores; outer membrane studded with pores permit communication between ribosomes, inner membrane smooth; cytoplasm and nucleoplasm are separated by a perinuclear cistern; a nuclear pore complex is associated with each pore

Nuclear lamina Thin network of interwoven filaments Stabilized inner nuclear membrane; attachment site for components of chromatin

Heterochromatin Dense staining; condensed chromatin Inactive; part of genome not being expressed

Euchromatin Light-staining; dispersed chromatin Active; part of genome being expressed

Nucleolus Conspicuous round body in nucleus; Synthesis of ribosomes nucleolonema consists of dense granules in a matrix of filaments; amorphous component may be present

28 catalyze the DNA replication. Shortly thereafter, 2 Mitosis the chromosomes begin to coil, shorten, and become visible within the nucleus, and the cell Nearly all multicellular organisms grow by enters the prophase of mitosis. increasing the number of cells. The zygote, which is formed at conception, divides repeatedly and gives rise to all the cells of the body. Every true cell in the resulting individual contains a nucleus, and each nucleus possesses identical genetic information. In the adult, most cells have a finite life span and must be replaced continuously. Proliferation of somatic cells is the result of mitosis, which can be defined as the production of two daughter cells with exactly the same number of chromosomes and the same DNA content as the original parent cell. Mitosis usually lasts from 30 to 60 minutes and involves division of the nucleus (karyokinesis) and the cytoplasm (cytokinesis). Both events usually take place during mitosis, but karyokinesis may occur without division of the cytoplasm, resulting in formation of multinucleated cells, such as the megakaryocytes of bone marrow. Although mitosis is a continuous process, for descriptive purposes it is convenient to divide mitosis into prophase, metaphase, anaphase, and telophase. The time between successive mitotic divisions constitutes interphase and is the period when the cell performs its usual functions, contributes to the total economy of the body, and makes preparations for the next division.

Interphase

VOCABULARY: replication of DNA, telomeres, centromere

Replication of DNA occurs during interphase, Fig. 2-1. Semiconservative replication of DNA. Each before the cell visibly enters into mitosis. The replicated DNA molecule contains one of the original DNA strands (parental strands shown as thin black lines) double helix of the chromosome unwinds, and each and one newly made strand (daughter strand shown as strand acts as a template for the development of a thick black lines). complementary strand of DNA that contains an Each new molecule of DNA consists of two linear polymers of nucleotide subunits made up of a phosphate exact copy of the sequence of molecules in the group, a pentose sugar (deoxyribose), and four organic original DNA. A new double helix is formed that bases (adenosine (A), cytosine (C), guanine (G), thymidine (T)). The bases project toward and bind to complementary contains a new strand and a parent strand of DNA bases of the adjacent polymer. Together the two (Fig. 2-1). Replication begins at the ends of the polynucleotide chains intertwine and form the antiparallel chromosomes, the telomeres, and progresses double helix of the DNA molecule. toward the center, where a small area of the chromosome, the centromere, remains unduplicated. The telomeres contain nucleotide sequences that allow for the replication of the DNA molecule along its full length controlled by the enzyme telomerase. DNA polymerase α and δ

29 Prophase The final act of metaphase is duplication of DNA at the centromere, after which the centromeres VOCABULARY: chromatid, centriole split. The two chromatids of each chromosome replication separate and begin to migrate toward the centrioles at the opposite poles of the cell. Duplication of the In prophase, the cell assumes a more spherical centromeres and migration of chromatids occur shape and appears more refractile, while in the simultaneously in all chromosomes of a given cell. nucleus, chromosomes become visible and appear as threadlike structures. At prophase each Anaphase chromosome consists of two coiled subunits called chromatids that are closely associated along their VOCABULARY: daughter chromosome lengths. Chromatids are the functional units of chromosomes, and each contains a double strand of The initial separation of chromatids marks the DNA. As prophase progresses, the chromatids beginning of anaphase. As the chromatids move continue to coil, thicken, and shorten, reaching toward the opposite poles, the centromeres travel in about one-twenty-fifth of their length by the end of advance of the telomeres (arms) of the prophase. The chromosomes then appear to chromosomes that trail behind. Movement of approach the nuclear envelope. chromatids, which now are called daughter As these events occur, nucleoli become smaller chromosomes, is an active, dynamic process, but and finally disappear, and the nuclear envelope the mechanisms by which the movement is effected breaks down. When this occurs, the center of the are not certain. cell becomes more fluid, and the chromosomes move more freely, making their way to the equator Telophase of the cell. Simultaneous with the nuclear events, the centrioles replicate, and the resulting pairs VOCABULARY: reforming nuclear envelope, migrate to the opposite poles of the cell. nucleolus-organizing region, karyokinesis

Metaphase As the daughter chromosomes reach their respective poles, discontinuous portions of VOCABULARY: mitotic spindle, equatorial endoplasmic reticulum form around each group of plate, continuous fiber, centromere, chromosomes and begin to reform the nuclear chromosomal fiber, kinetochore, splitting of envelope. This event initiates telophase. When the centromere nuclear envelope has been reformed completely, the chromosomes uncoil and become indistinct, and The disappearance of the nuclear envelope marks the two nuclei reassume the interphase the end of prophase and the beginning of configuration. The normal complement of nucleoli metaphase, which is characterized by formation of also reappears at this time. Their development is the mitotic spindle and the alignment of associated with specific nucleolus-organizing chromosomes along the equator to form the regions present on certain chromosomes. With this equatorial plate. The mitotic spindle is a event karyokinesis is complete. somewhat diffuse body formed mainly of microtubules; those which pass from pole to pole of Cytokinesis the spindle are called continuous fibers. Other microtubules extend from the poles of the spindle VOCABULARY: interzonal fiber, midbody to attach to the centromere of each chromosome and form the chromosomal fibers. Protein During telophase the mitotic spindle begins to complexes known as kinetochores assemble at the disappear. The fibers between the two forming centromere and bind to the microtubules of the nuclei appear stretched and often are called mitotic spindle. The centromere is a specialized interzonal fibers. Midway between the two nuclei, region of unduplicated DNA and protein that holds in the region formerly occupied by the equatorial the chromatids of each chromosome together and plate, a constriction of the plasmalemma forms a forms an attachment site for the chromosomal fibers. furrow that extends around the equator of the cell.

30 The constriction extends deeper into the cytoplasm, Duplication of the entire DNA complement is separating the nuclei containing daughter essential for cell division. Depriving cells of chromosomes until they are joined only by a thin thymidine blocks DNA synthesis and prevents cells cytoplasmic bridge, called the midbody that from entering mitosis. Provision of thymidine to contains interzonal fibers. Eventually the daughter such cells results in a wave of DNA synthesis cells pull away from each other by ameboid followed by a wave of mitosis. Again, however; movement, thus completing the separation of the DNA duplication may be completed long before cells and ending cytokinesis. Cell organelles are division occurs, and the existence and maintenance evenly distributed between the two daughter cells. of polyploid cells argue against DNA synthesis Immediately after division, the daughter cells enter being the trigger for mitosis. a phase of active RNA and protein synthesis, The intense coiling and contracting of resulting in an increase in the volume of the nucleus chromosomes at metaphase results in small, and cytoplasm. The endoplasmic reticulum and the compact units that can be transported more easily Golgi complex are restored to their original to the poles of the cells. The destination of the concentrations, mitochondria reproduce by fission, chromosomes, their orderly arrangement at and the centrioles replicate in the daughter cells just metaphase, and the plane of cell cleavage are before the next division. determined by the mitotic spindle. It would seem reasonable that one of the preparations for mitosis Apoptosis must be synthesis of protein specifically for the formation of the spindle. Certain amino acid Apoptosis or programmed cell death does not invoke analogues result in synthesis of faulty spindle an inflammatory response. It is characterized by proteins, and cells cultured in the presence of these chromatin clumping into a distinct crescent pattern analogues fail to divide. When removed to normal along the inner margin of the nuclear envelope. media, the cells enter mitosis but only after a delay, Eventually the chromatin (the nucleus) condenses during which the faulty spindle protein is replaced into a dense mass of nuclear material. The by newly synthesized normal protein. condensed chromatin is eventually cleaved by a Centrioles are responsible for polarization of the specific endonuclease into DNA fragments as the mitotic spindle and play a role in assembling the cell is broken down and destroyed. Apoptosis spindle, but how this is achieved is unknown. normally occurs during embryogenesis and during Centrioles complete their duplication before mitosis the normal growth and establishment of tissues and begins, and cell division can be prevented by organs. It also may be induced by cytotoxic T suppressing the replication of centrioles. lymphocytes (or other immune cells) or may be the Mitosis must be an energy-consuming process, result of viral cell death. but cell division is not a period of intense respiratory activity. Restriction of energy sources Summary and inhibition of respiration and oxidative phosphorylation do not stop mitosis once it has

begun. Thus, it can be assumed that the energy Mitosis produces daughter cells that have a genetic needs are met during preparation for cell division. content identical to that of the parent cell. What Cell division often occurs in waves, with patches determines if and when a cell divides is not known, of cells or whole tissues undergoing synchronous but there are certain requirements that must be met division, implying some kind of cellular or tissue before a cell can enter mitosis. There is some control. Substances that promote or initiate mitosis relationship between cell mass and cell division. In have been isolated from epidermis and salivary general, each daughter cell achieves the mass of the glands in several species and other substances that parent cell before it can divide. However, the appear to suppress mitosis (chalones) have been relationship between cell growth and mitosis is not identified in a number of different tissues. a causal one, and the two events can be separated in Thus, the cell population and the rate of cell time. An example of this is seen in cleavage of ova, division may be governed by the reciprocal action where growth of the cell occurs long before of agents that control the rate of entry into mitosis division takes place. either by stimulation or suppression of mitotic activity.

31 overlying epithelium. The basal lamina and the 3 Epithelium interface of the underlying connective tissue (reticular lamina) together make up the epithelial Epithelium is one of the four basic tissues, the others basement membrane that is visible with the light being connective, muscle, and nerve tissue. A basic microscope. Additional support for the basement tissue is a collection of cells of similar type that, membrane is provided by anchoring fibers of type together with their associated extracellular VII collagen which extend from the basal lamina to substances, are specialized to perform a common form loops around collagen fibers (type I) in the function or functions. Each basic tissue is present, subjacent connective tissue prior to terminating in in variable amounts, in all organs that collectively anchoring plaques. A glycoprotein known as filamin make up the individual. links the basal lamina to elastic fibers within the The various epithelia consist of closely aggregated cells underlying connective tissue. with only minimal amounts of intervening intercellular With rare exceptions, epithelium is avascular and substances. depends on diffusion of substances across the basement membrane for its nutrition. However, Sheet/Barrier Epithelium epithelium is innervated. Epithelial cells also can be specialized for secretion of various substances and VOCABULARY: sheet (layer), basal lamina, form the cellular components of glands. Generally, lamina lucida, lamina densa, basement epithelial cells have a high capacity for regeneration. membrane, avascular, gland Classification of Epithelia The cells of this type of epithelium are organized into sheets or layers that form barriers which VOCABULARY: simple epithelium, stratified cover the external surface of the body and line the epithelium, squamous, cuboidal, columnar digestive, respiratory, cardiovascular, and urogenital systems as well as the major body cavities. Thus, The lining or covering forms of epithelia may substances that enter or leave the body must pass consist of a single layer of cells, forming a simple through an epithelial barrier. Cells within an epithelial epithelium, or multiple layers of cells may be sheet are bound firmly together and resist forces present to form a stratified epithelium. The that tend to separate them. The space between the epithelial cells themselves can be divided into three cell membranes of adjacent epithelial cells is narrow categories according to their geometric shapes: (about 20 nm) and contains a small amount of squamous (thin, platelike cells), cuboidal (cells in proteoglycan that is rich in cations, chiefly calcium. which the height and width are approximately Glycoproteins (integral transmembrane equal), and columnar (cells in which the height is glycoproteins) associated with the plasmalemma act greater than the width). Cells intermediate in height as specialized adhesion molecules that aid in between cuboidal and columnar do occur and holding adjacent epithelial cell membranes in close frequently are referred to as low columnar. apposition. Epithelial cells rest on a basal lamina that Simple Epithelia separates the epithelium from underlying connective tissue. The basal lamina consists of an VOCABULARY: simple squamous, simple electron-lucid lamina lucida immediately adjacent cuboidal, simple columnar, pseudostratified to the epithelium and a denser layer, the lamina columnar, apical surface, basal surface, lateral densa, next to the underlying connective tissue. surface, polarity, mesothelium, endothelium The basal lamina, primarily the product of the epithelial cells, consists of the glycoprotein, laminin, Epithelium consisting of a single layer of cells can limited mainly to the lamina lucida; and a proteoglycan be classified as simple squamous, simple rich in heparan sulfate, fibronectin, and type IV collagen cuboidal, or simple columnar, depending on the limited mainly to the lamina densa. It is reinforced shape of the constituent cells. A fourth type of on the connective tissue side by a layer of reticular simple epithelium, pseudostratified columnar, is fibers embedded in proteoglycan. The basal lamina composed of more than one type of cell whose provides support and a surface for the attachment of the nuclei are at different levels, falsely suggesting that

32 the epithelium is made up of two or more layers. layers of cells are transformed and filled with a While all the cells in this type of epithelium are in protein called keratin. Keratin is an especially tough contact with the basement membrane, not all protein. Cells comprising this layer are resistant to extend to the surface. mechanical injury and are relatively impervious to The surfaces of cells within an epithelial sheet bacterial invasion or water loss. When this show different orientations. The apical surface is transformation occurs this type of stratified free, the opposite or basal surface is directed epithelium is referred to as keratinized stratified toward the underlying basement membrane, and squamous epithelium. The keratinized layer of the lateral surfaces face adjacent epithelial cells. cells at the surface is constantly being shed and Differences in the orientation of the plasmalemma replaced. In other regions the stratified squamous (apical and basolateral) are associated with type of epithelium is found lining a moist differences in cell function and morphology. environment and recognizable, intact cells rather Organelles, including the nucleus, frequently take than flat, keratin-filled cells are sloughed from the up preferred locations in the cell, in which case the surface. This type of epithelium is called a wet or cell is said to show polarity. An epithelial cell may nonkeratinized stratified squamous epithelium. be polarized apically or basally with respect to the Transitional and germinal epithelia are not distribution of its organelles, depending on the classified according to the cells of the surface layer. functional status of the cell. Likewise the apical Transitional epithelium (uroepithelium) is plasmalemma contains receptors, ion channels and restricted in distribution to the lining of the urinary transporter proteins that are functionally quite passages and extends from the minor calyces of the different from those located in the basolateral kidneys to the urethra. It is a thick stratified plasmalemma. epithelium consisting of large rounded cells the The term mesothelium is the special name given appearance of which varies considerably, depending to the simple squamous epithelia that lines the on the degree of distension to which it is subjected. pleural, pericardial, and peritoneal cavities. A The superficial cells of a non-distended organ specific name, endothelium, is given to the simple (urinary bladder for example) appear rounded or squamous epithelium that lines the cardiovascular dome-shaped and often show a free convex border and lymph vascular systems. that bulges into the lumen. In the distended organ, the superficial cells may vary in shape from Stratified Epithelia squamous to cuboidal. Germinal epithelium is restricted in distribution to the seminiferous tubules VOCABULARY: stratified squamous, stratified of the testes. It is a complex, stratified epithelium cuboidal, stratified columnar, keratinized that consists of supporting cells and spermatogenic stratified squamous, nonkeratinized stratified cells. This type of stratified epithelium will be squamous, transitional epithelium, germinal presented in the male reproductive system. epithelium A classification of epithelia and the locations where each type can be found is presented in Table Stratified epithelia are classified according to the 3-1. shape of the superficial (surface) cells, regardless of the shapes of the cells in the deeper layers. Except for those in the basal layer, cells of stratified epithelia are not in contact with the basement membrane, and only the most superficial cells have a free surface. In the thicker stratified types, the deeper cells often are irregular in shape. The types of stratified epithelia are stratified squamous, stratified cuboidal, and stratified columnar. Stratified squamous is the most abundant of the stratified epithelia with stratified cuboidal and columnar having a limited distribution. In regions where the surface is dry or subject to mechanical abrasion, as in the epidermis, the outer

33 Table 3-1 Locations of epithelia. Type of Epithelium Location Specialization

Simple squamous Endothelium, mesothelium, parietal layer of Bowman’s capsule, thin segment of loop of Henle, rete testis, pulmonary alveoli Simple cuboidal Thyroid, choroid plexus, ducts of many glands, inner surface of the lens capsule, covering surface of ovary Simple columnar Luminal surface epithelium of stomach, luminal surface epithelium of small intestine and colon, Striated border proximal convoluted tubule of kidney, Brush border distal convoluted tubule of kidney, Basal striations gallbladder, excretory ducts of glands, uterus, oviducts, Cilia small bronchi of lungs, Cilia some paranasal sinuses Cilia Pseudostratified columnar Large excretory ducts of glands, portions of male urethra, epididymis, Stereocilia trachea, bronchi, eustachian tube Cilia Stratified squamous Mouth interior, esophagus, epiglottis, conjunctiva, cornea, vagina, epidermis of skin, Keratin gingiva, hard palate, Keratin Stratified cuboidal Ducts of sweat glands, salivary glands Stratified columnar Ducts of salivary glands Transitional Urinary system Germinal Seminiferous tubules of testis

Cell Attachments Junctional Complexes

In addition to the intercellular matrix, several VOCABULARY: terminal bar, zonula attachment points or junctions occur between occludens (tight junction), zonula adherens, neighboring epithelial cells that aid in holding macula adherens (desmosome), attachment adjacent cell membranes close together. They also plaque, punctum adherens, hemidesmosome, serve as anchoring sites for microfilaments and focal adherens intermediate filaments of the cytoskeleton, which assists in stabilizing the cell shape. Terminal bars occur along the apical, lateral interfaces of most simple cuboidal and columnar epithelial cells that border on luminal surfaces. They are visible under the light microscope as short, dense lines. In electron micrographs, a terminal bar is seen to form an area of specialization called the junctional complex, which consists of three distinct regions.

34 Near the free surface, the external leaflet of membranes are associated with an actin filament adjacent cell membranes fuse, and the intercellular network. The plaques contain α-actinin and vinculin, space is obliterated. This region is the zonula two actin-binding proteins. Actin is linked to a occludens (tight junction), an area of transmembrane glycoprotein called E-cadherin specialization that forms a zone or belt around the (adherens cell adhesion molecule or A-CAM) that perimeter of each cell to create an occluding seal holds cells in this region together in the presence of between the apices of adjacent epithelial cells. As a calcium ions. result, materials that pass through the epithelial sheet must cross the cell membranes and usually cannot pass between cells through the intercellular spaces. The zonula occludens also is important in separating apical from basolateral cell membrane domains by preventing the lateral migration of specialized transmembrane proteins (e.g., receptors, ion transport proteins) in the plasmalemma. In most cases, membrane proteins in the apical plasmalemma function quite differently from those occupying the basolateral regions. The Fig. 3-1. A transmission electron micrograph illustrating a tripart junctional complex between two gastric lining epithelial plasmalemma of the apical domain is rich in cells of a human stomach. This complex is formed by a zonula cholesterol and glycolipids and contains ion occludens, a zonula adherens, and a macula adherens (X22,500). channels, transporter proteins, hydrolytic enzymes, and hydrogen-ATPase. Regulated secretory The third element of the junction complex is the products are released from this domain by macula adherens, also called the desmosome exocytosis. In contrast, the plasmalemma of the (Fig. 3-2). These small, elliptical discs form spot- basolateral domain is characterized by receptors for like points of attachment between cells that hormones, anion channels, and sodium-potassium function to link the intermediate filament network ATPase and is where most constitutive secretion of adjacent cells. The internal leaflet of the adjacent occurs. As a result of this distribution, the cell membranes appears dense and thick due to the plasmalemma of epithelial cells functions to presence of an amorphous material called an regulate the flow of ions and solutes between attachment plaque. Desmoplakin I and II are external environments and body fluids as well as located in the plaque. Intermediate filaments between compartments within the body. Tight (usually cytokeratin) extend into the plaque, where junctions also occur between simple squamous they appear to form hairpin loops. A thin, epithelial cells forming either an endothelium or a moderately dense lamina often is seen in the mesothelium. intercellular space of each desmosome. These The zonula occludens can be rapidly disassembled contain transmembrane linking glycoproteins and re-formed as occurs during leukocyte migration (desmoglein I and desmocollin I and II) which are across an endothelial barrier. Although the zonula calcium-binding proteins that mediate calcium- occludens usually prevents the diffusion of material dependent cell adhesion that holds adjacent cell across an epithelial barrier via the intercellular space membranes in close apposition. Desmosomes are (paracellular pathway), some epithelia are classified scattered along lateral epithelial surfaces and are not as tight or leaky based on the permeability of the restricted to regions of tight junctions. They are zonula occludens. Leaky zonula occludens allow especially abundant in stratified squamous epithelia, certain types of ions to pass through the epithelial where they are associated with numerous barrier via the paracellular pathway. intermediate filaments of the keratin type. The second part of the junctional complex, the Desmosome-like junctions that lack attachment zonula adherens, also forms an adhering belt or plaques and associated keratin filaments also occur zone that surrounds the apex of an epithelial cell, between epithelial cells. This type of cell junction immediately beneath the zonula occludens (Fig. 3- has been called a punctum adherens. 1). Adjacent cell membranes are separated by an intercellular space 15 to 20 nm wide, and in this region plaques along the internal leaflet of the cell

35 contiguous layer but also are anchored firmly to the underlying extracellular matrix. Desmosomes and hemidesmosomes serve as firm anchoring sites for intermediate (keratin) filaments of the cytoskeleton. In addition, desmosomes function to interconnect the keratin filament network between adjacent cells, and hemidesmosomes link this network to the extracellular matrix. Such an arrangement of Fig. 3-2. A transmission electron micrograph of desmosomes (maculae adherens) between two adjacent human intestinal interconnected intermediate filaments gives epithelial cells (X 30,000). mechanical stability to a group of cells so that they can maintain a specific three-dimensional shape and The basal cell membrane, although not function as a cohesive unit. Likewise, actin immediately adjacent to another cell, also shows an microfilaments are linked not only to plasmalemma attachment, the hemidesmosome, which closely at zonula adherens and focal adherens junctions but resembles one-half of a desmosome (Fig. 3-3). also are interconnected with actin filaments of Hemidesmosomes act as points of adhesion adjacent cells through these attachment points as between the basal plasmalemma and the underlying well as to elements of the extracellular matrix. This basal lamina and link the intermediate keratin arrangement provides epithelial cells with a degree filament network of the epithelial cells to the of mobility and is particularly important in extracellular matrix via transmembrane anchoring migrating epithelial cells (gastric and intestinal lining glycoproteins. These transmembrane glycoproteins epithelial cells) that move continuously along the act as laminin receptors and bind to laminin, a basal lamina. The integrity of desmosomes and glycoprotein within the lamina lucida of the basal hemidesmosomes appears to depend on the lamina. presence of calcium ions. Heparan sulfate-rich proteoglycans in the basal lamina give its surface a strong ionic (negative) charge that, acting with calcium ions, mediates the attachment of transmembrane linker glycoproteins to elements in the extracellular matrix.

Nexus Junction

VOCABULARY: nexus (gap junction), Fig. 3-3. A transmission electron micrograph of hemidesmosomes connexon, connexin, communicating junction along the basal cell membrane of cells from the stratum basale of the epidermis. Note the abundance of tonofilaments (keratin intermediate filaments) associated with the hemidesmosomes The nexus, or gap junction, like tight junctions (X 12,000). and desmosomes, forms a relatively firm point of Focal adherens type junctions also occur along attachment between adjacent cells. At a nexus, the the basal plasmalemma and link actin filaments to cell membranes are separated by a gap 2 nm wide the extra-cellular matrix. Intracellular attachment that is bridged by particles 8 nm in diameter. These proteins (α-actinin, vinculin, talin) link actin particles, the connexons, mainly consist of the filaments to another of the transmembrane protein connexin. Each connexon unit consists of glycoproteins (integrin) that serves as a fibronectin six identical transmembrane connexin proteins that receptor. The latter binds to fibronectin, a are arranged around a small aqueous channel or multifunctional extracellular glycoprotein of the pore. Connexon units extend through the basal lamina that interacts with various types of plasmalemma and cross the intercellular gap to join collagens and proteoglycans in the extracellular a corresponding unit from the opposing cell matrix. membrane. The hydrophilic channel within the Thus, through a series of transmembrane center of each connexon permits intercellular glycoproteins that constitute a large group of passage of ions, amino acids, and other small cellular adhesion molecules (CAMs) or integrins, molecules. epithelial cells are not only held tightly together in a

36 The nexus also provides an avenue whereby cyclic adenosine monophosphate (cAMP), released by one cell, can pass to adjacent cells. The closing and opening of the central pore appears to be regulated by intracellular levels of calcium. Thus, epithelial cells are electrically and metabolically coupled, and the response of a number of cells within an epithelial sheet to a specific stimulus can be Fig. 3-4. A transmission electron micrograph of the microvillus coordinated. Because nexus junctions act as sites of (striated) border of an intestinal epithelial cell. Individual microvilli communication between epithelial cells, they often can be visualized. (X20,000). are called communication junctions. In addition, nexus junctions appear to be capable of changing Microvilli also are numerous on cells forming the from low to high resistance and thereby inhibit cell proximal convoluted tubules of the kidney, where communication. they are somewhat longer and less uniform than Gap junctions also occur between cardiac and those in the small intestine and appear as tufts on smooth muscle cells, osteocytes, astrocytes, and the cell apices. Because of this paint-brush tip some endocrine cells. Interestingly, gap junctions appearance, they are called a brush border. have not been observed between cancer cells. Microvilli increase the surface area enormously, thereby Adjacent lateral cell membranes of epithelial cells enhancing the absorptive capacity of epithelial cells. often do not run parallel to one another but show The uniform shape of microvilli forming the striated and numerous interdigitations. Such interlocking of cell brush borders is maintained by a bundle of 25 to 40 membranes also is thought to aid in maintaining actin filaments linked together by the actin-binding cell-to-cell adhesion and stability in an epithelial proteins villin, fimbrin, and fascin. Minimyosin sheet. molecules attach the actin filament bundle within the microvillus core to the inner surface of the plasmalemma. The actin filaments extend from the Specializations of Epithelial tips to the microvilli to the apical cytoplasm, where Membranes just beneath the cell surface they join a transverse zone rich in actin filaments called the terminal web. The cells in an epithelial sheet often show special Epithelial cells that line the epididymis show adaptations of apical or basolateral surfaces that unusually long, slender, branching microvilli that increase the surface area of the cell or constitute have been called stereocilia. However, stereocilia motile "appendages" that move material across the are not cilia but are true microvilli and also serve to surface. The surface specializations include increase the surface area of a cell. microvilli, cilia, and basal-lateral infoldings. Cilia Microvilli VOCABULARY: microtubule, doublet, central VOCABULARY: striated border, brush border, pair, (9 + 2), dynein, basal body, triplet, (9 + 0), stereocilia ciliated simple columnar epithelium, ciliated pseudostratified columnar epithelium Microvilli are minute, finger-like evaginations of the apical cell membrane that enclose cores of Like microvilli, cilia represent appendages of the cytoplasm. Scattered microvilli occur on the apical cell membrane but are relatively large and surfaces of most cells of simple epithelia, but often are motile. They may be single or occur in individually they are usually too small to be seen large numbers and are seen readily with the light with the light microscope. In epithelia such as that microscope. The limiting plasmalemma of the lining the small intestine, however, microvilli are so cilium is continuous with that of the cell and numerous, uniform, and closely packed that they encloses a core of cytoplasm containing 20 form a striated border that is visible by microtubules. microscopy (Fig. 3-4).

37 Nine pairs of fused microtubules, the doublets, groups on cells, the cilia are nonmotile and are form a ring at the periphery of the cilium and believed to act as chemoreceptors. Cilia also are surround a central pair of individual microtubules. found in the maculae and cristae of the inner ear This arrangement has been called the (9 + 2) and act as mechanoreceptors. The most extreme configuration (Fig. 3-5). Within each doublet, the modification of cilia is seen in the formation of the microtubules are named the A and B microtubules. outer segments of the rods and cones of the retina, Microtubule A has two short, divergent arms that where the modified cilia serve as photoreceptors. project toward microtubule B of the adjacent The presence of cilia contributes to the doublet. The arms consist of the protein dynein, classification of epithelia. Where cilia are present which has adenosinetriphosphatase (ATPase) this term is incorporated into the nomenclature. activity that splits ATP to provide energy for ciliary Therefore, the simple columnar epithelium lining movement. Radial spokes project from the doublets the oviduct is referred to as a ciliated simple to the central pair of microtubules. columnar epithelium. When cilia are associated with pseudostratified columnar epithelium, as in the trachea, the epithelium is classified as a ciliated pseudostratified columnar epithelium. A rare type of immotile cilia syndrome (Kartagener's) occurs in which the dynein arms are absent. Patients with this congenital disorder are subject to respiratory infection because clearance from the lungs is impaired due to the absence of coordinated ciliary motion within the respiratory tree. Men with this syndrome are usually infertile because the Fig. 3-5. Cilia and microvilli from epithelial cells as seen in cross section. Compare the diameters and note the central axoneme of axonemes of sperm flagella also are defective. microtubules within each cilium. Basal bodies are seen in the apical Interestingly, women who are afflicted by cytoplasm (Transmission electron micrograph, X 4,000; inset, X 80,000). Kartagener’s syndrome may or may not be infertile even though the cilia associated with the epithelium lining the oviducts are defective. The microtubules of a cilium originate from a basal body in the apical cytoplasm of the cell. The basal Basolateral Infoldings body is similar in structure to the centriole and appears as a hollow cylinder, the wall of which VOCABULARY: basolateral infoldings, consists of nine triplet microtubules. Since there fluid/ion transport, basal striations are no central microtubules in a basal body, the arrangement of microtubules is expressed as (9 + In addition to the apical specializations, some 0). Each of the nine doublets in the cilium is epithelial cells show elaborate infoldings of the continuous with the two innermost microtubules of basolateral plasmalemma. These are called the triplets in the basal body. The two central basolateral infoldings and usually are prominent microtubules of the ciliary shaft terminate as they in epithelial cells active in fluid/ion transport, reach the basal body and do not join with it. Ciliary such as those forming the proximal and distal motion is thought to involve a sliding microtubule convoluted tubules of the kidneys and in those mechanism, dependent on the ability of the dynein lining the ducts of salivary glands. Like microvilli, protein arms to generate energy through its they greatly increase the surface area of the cell ATPase-like activity. membrane. The plasmalemma of these regions is Cilia are numerous on epithelial cells that line rich in ion channel proteins, in particular sodium- much of the respiratory tract and parts of the potassium-ATPase. Mitochondria, which often are female reproductive tract. Groups of cilia beat in a numerous in this area of the cell, tend to lie parallel coordinated, rhythmic fashion to move a thin layer to the infoldings of the plasmalemma and provide of fluid in one direction across the epithelial substrate ATP. Because of this association, the surface. In regions such as the distal tubules of the infoldings are visible with the light microscope and kidney, where cells bear a single cilium, or in the appear as basal striations. olfactory epithelium, where they occur in small

38 Glandular Epithelium Classification of Glands

In addition to performing functions such as VOCABULARY: unicellular, multicellular, protection and absorption, the cells of an epithelial exocrine, endocrine, type and mode of sheet also may secrete materials. Epithelial cells secretion adapted specifically for secretion constitute the glands. It should be understood that most complex Glands can be classified according to their glands originate in an epithelial surface and grow histologic organization, possession of ducts, type of out from this layer. Those that retain a connection material secreted, and manner in which material is with the epithelial surface of origin via ducts are secreted. Glands that consist of only a single cell are called exocrine glands. Those that loose the ductal called unicellular; aggregates of secreting cells connection to the point of origin are designated form multicellular glands. These can be divided endocrine glands. into exocrine and endocrine types according to whether or not they secrete onto a surface or into a ductal system (exocrine) or directly into the tissue fluid of the intercellular space (endocrine). Complex exocrine glands can be subdivided further on the basis of the branching of their ducts, the configuration of the secretory end piece, and by the type and mode of secretion. A classification of glands is shown in Table 3-2.

Table 3-2. Classification of glands.

Type Examples

Unicellular glands

Exocrine Goblet cells

Endocrine Gastrin-cells and other unicellular endocrine cells within the epithelial lining of the digestive and respiratory systems Multicellular exocrine glands

Secretory sheet Surface epithelium of gastric mucosa (gastric lining epithelium)

Intraepithelial glands Urethral (Littre’s) glands

Simple tubular glands Intestinal glands

Simple coiled tubular glands Eccrine and apocrine sweat glands

Simple branched tubular glands Esophageal glands, pyloric glands

Simple branched alveolar glands Meibomian glands, sebaceous glands

Compound tubular glands Brunner’s glands

Compound tubuloacinar glands Salivary glands, pancreas

Compound alveolar (saccular) glands Prostate

39 Unicellular Glands classified as simple tubular, simple coiled tubular, simple branched tubular, or simple branched acinar. VOCABULARY: goblet cell, mucin, mucus Similarly, compound glands are subdivided into compound tubular, compound saccular, and The classic example of a unicellular, exocrine gland compound tubuloacinar (tubuloalveolar). The initial is the goblet cell, found scattered among epithelial subdivision into simple and compound is made cells lining the trachea, small intestine, and colon. according to whether or not the ducts branch. The cell has a narrow base and an expanded apex Subsequent classification depends on the shape and filled with secretory granules. Goblet cells elaborate configuration of the secreting portion, the secretory mucin, which, on hydration, produces a viscous unit (Fig. 3-6). lubricating fluid called mucus. The mucin is It should be understood that the epithelial cells secreted onto an epithelial surface. Hence, a goblet forming the closely packed tubular structures of cell is classified as a unicellular exocrine gland. complex glands remain arranged as a single layer of Unicellular endocrine cells (glands) also occur. They are cells despite the complex appearance of the overall numerous in the epithelium lining the glandular structure. In this way a maximum number gastrointestinal tract and produce a number of of secretory cells can be organized into the smallest peptide hormones and/or amines. They also are area possible. found in the epithelium lining the respiratory system. These cells secret into the extracellular space the product of which may enter the adjacent vasculature.

Multicellular Glands

VOCABULARY: secretory sheet, intraepithelial gland

The simplest form of multicellular exocrine gland is the secretory sheet, exemplified by the gastric lining epithelium, in which the secreting cells form a continuous epithelial layer. Intraepithelial glands are small clusters of secretory cells that lie Fig. 3-6. Schematic illustration of several glandular forms. A. wholly within an epithelial sheet, clustered about a Unicellular glands. B. Secretory sheet. C. Intraepithelial glands. D. small lumen. Cells of both types of gland secrete Simple tubular gland. E. Simple acinar gland. F. Simple branched gland. G. Compound tubular gland. H. Compound acinar gland. I. their product onto the epithelial surface. Compound tubuloacinar gland.

Complex Exocrine Glands Endocrine Glands VOCABULARY: compound, simple, tubule, acinus, alveolus, saccule VOCABULARY: lack ducts, hormone

Exocrine glands secrete onto a luminal epithelial Like exocrine glands, the complex endocrine glands surface either directly, as in secretory sheets and arise from an epithelial sheet, but during their intraepithelial glands, or by a ductal system. If the formation, endocrine glands lose their connections with duct system of complex glands branches, the gland the surface and thus lack ducts. Cells of endocrine is said to be compound; if the duct system does glands secrete regulatory material (hormones), not branch, the gland is classified as simple. directly into tissue fluid of the extracellular space. The secretory cells may be organized into tubules, The hormones released may target and stimulate acini, alveoli (berry-like end pieces), or saccules the releasing cell itself (autocrine secretion) or (dilated, flasklike end pieces). Simple and adjacent cells (paracrine secretion) or enter the blood compound glands can be named by the shape of to target specific cells of distant organs. the secretory portion. Thus, simple glands can be

40 Scattered, solitary endocrine cells also occur in the of smooth muscle cells. These cells also contain the gastrointestinal tract and along the conducting intermediate filament, desmin. Long cytoplasmic portion of the respiratory tract. processes extend from the body of the cell to The structure of the complex endocrine glands is course around the secretory unit. The processes, by so diverse that the glands do not readily lend their contraction, aid in expressing products from themselves to histologic classification. However, the secretory units into the ductal system. some overall structural patterns can be made out, Myoepithelial cells occur in sweat, mammary, and and the cells may be arranged as clumps, cords, or salivary glands and in glands along the bronchi and hollow spherical structures called follicles. These will esophagus. be considered specifically in the discussion of Endocrine Glands. Histogenesis

Type and Mode of Secretion Epithelium can arise from ectoderm, endoderm, or

mesoderm. Sheets of epithelium derived from the VOCABULARY: mucous, serous, mixed gland, germ layers cover the surface of the developing merocrine, holocrine embryo and line the forming body cavities and

tubular structures. Glands form as outpockets of Glands also can be classified as mucous, serous, or epithelial sheets from which ductal systems and mixed according to the type of material secreted. secretory units develop as the ductal outgrowth Cells of mucous glands have pale, vacuolated expands. If ductal connections are maintained with cytoplasm and flattened, dense nuclei oriented the epithelial sheet of origin, an exocrine gland is toward the base of the cell. Serous cells show formed. If the connection is lost, an endocrine round nuclei surrounded by a basophilic cytoplasm gland results (Fig. 3-7). that contains discrete secretory granules. Mixed glands contain variable proportions of serous and mucous cells Two types of glands can be distinguished by their mode of secretion. Merocrine secretion refers to release of the product through fusion of secretory vesicles with the apical plasmalemma. This type of secretion is regulated/stimulated exocytosis and most exocrine secretion is by this mechanism. Holocrine secretion involves release of whole cells (such as sperm from the testes) or the breakdown of an entire cell to form the secretion (e.g., sebum in sebaceous glands). A third type of secretion, apocrine, has been described in which a portion of the apical cytoplasm is lost along with the secretory material. Electron microscopy has failed to support this mode of secretion except perhaps for the mammary glands during lactation. Even here, however, the existence of an apocrine secretion is dubious.

Myoepithelial Cells

The secretory units of some glands are associated with myoepithelial cells, which are specialized cells Fig. 3-7. Formation of exocrine and endocrine glands. All glands located between the glandular epithelial cells and form as an invagination from an epithelial sheet. Glands that retain the basal lamina. These cells are contractile and their a connection (duct) to the epithelial sheet of origin are classed as exocrine glands. Those that lose their connection are called cytoplasm contains the contractile proteins actin endocrine glands and usually secret into the blood vasculature. and myosin arranged in a similar manner to those

41 A summary of epithelial structures arising from the three embryonic germ layers is shown in Table 3-3.

Table 3-3. Epithelial structures derived from the three embryonic germ layers.

Epithelial Sheets

Ectoderm Epidermis, nails, hair; corneal covering; lining of nasal cavities, nasal sinuses, mouth, and anal canal

Endoderm Covering of posterior one-third of tongue; lining of pharynx, Eustachian tube, larynx, trachea, lungs, gastrointestinal tract, urinary bladder, and urethra

Mesoderm Lining of blood vessels, lymphatics, body cavities, joint cavities, ureter, and genital ducts

Glands

Ectoderm Sweat, sebaceous, mammary, and salivary glands; adenohypophysis

Endoderm Thymus, thyroid, parathyroids; esophageal, gastric, duodenal (Brunner’s) and intestinal glands; pancreas, liver

Mesoderm Kidneys, gonads, and adrenal cortex

Summary Desmosomes help maintain these cell-to-cell adhesions and also act as sites where keratin Epithelium covers the entire body surface and lines intermediate filaments of the cytoskeleton are the digestive, respiratory, cardiovascular, urogenital anchored. Hemidesmosomes anchor the keratin systems and body cavities. The epithelial intermediate network to the underlying basement sheets/layers control any substance that enters or membrane. The zonula occludens provides a tight leaves the body as it must first pass through an seal between cells so that the majority of material epithelial barrier. Epithelia protect the organism crossing an epithelium must pass through the from mechanical trauma and invasion by small plasmalemma (cell membrane) and not through organisms. They also limit fluid loss from intercellular spaces between cells. They are also underlying connective tissues and thus aid in instrumental is separating apical from basolateral preventing dehydration. Other functions they cell membrane domains. Nexus junctions provide contribute to are absorption, transport of useful an avenue for intercellular communication and materials, and excretion of wastes. In the form of exchange and coordinate epithelial cell responses to glands, they secrete products such as hormones, stimuli within the epithelial layer. which regulate and integrate the activities of many Thus, the ultimate barrier between the external tissues and organs. world and the interior substance of the body is the Surface specializations - microvilli, stereocilia, and plasmalemma. Incremental units of this basolateral infolding - provide a greatly increased plasmalemmal barrier are maintained by the cell surface area for the absorption and transport of individual cells within an epithelial barrier, each of materials into or out of the cells. Cilia move which can control and change a micro-portion of materials across the surface of an epithelium. The this vital barrier dependent on the physiological nonmotile cilia of olfactory epithelia and kidney needs of the body. tubules serve as chemoreceptors, the cilia of the All epithelia lie on a basal lamina that provides inner ear act as mechanoreceptors, and the physical support and serves as a surface for modified cilia of retinal rods and cones are involved attachment and migration of epithelial cells. in photoreception. It also acts as a barrier for some molecules, The attachment points between adjacent epithelial depending on their size, charge, and shape, and cells not only firmly unite cells together into a plays an important inductive role in differentiation. contiguous sheet but also aid component cells in The basal lamina also is very important in maintaining their three-dimensional configuration. regeneration and wound healing.

42 Loose connective tissues can be subdivided on the 4 General Connective Tissue basis of some special properties of their constituents, such as adipose (fatty) tissue, reticular Connective tissues form a diverse group of tissues tissue, and so forth. Dense connective tissues can derived from mesenchyme and, in keeping with the be subdivided according to whether the fibers are diversity of types, have a number of different randomly distributed or show an orderly functions. They provide structural elements, serve arrangement. Thus, dense connective tissues are as a foundation for the support of organs, form a classed as dense irregular or dense regular packing material for otherwise unoccupied space, connective tissues. A classification of connective provide an insulating layer (fat) that also acts as a tissues is given in Table 4-1. storage depot that can be used to provide energy, and play a vital role in the defense mechanisms of Table 4-1. Subdivisions of connective tissue. the body and in repair after injury. Some are Type Locations General connective tissue functions of ordinary (general) connective tissues; others are functions of specialized connective Loose connective tissue tissues. In organs, connective tissues form the stroma and epithelial (cellular) components make up Mesenchyme Primarily in the embryo and the parenchyma. developing fetus

Mucoid Umbilical cord Organization Areolar Most organs and associated connective tissue VOCABULARY: cell, intercellular substance, fiber, ground substance, tissue fluid, matrix Adipose Omentum, subcutaneous tissue

Like all basic tissues, connective tissues are made up Reticular Lymph nodes, spleen, and bone marrow of cells and intercellular substances. The latter consist of fibers, ground substance, and tissue Dense connective tissue fluid. Unlike epithelium, where the cells are closely apposed with little intercellular material, connective Irregular Dermis, capsules of organs, tissue cells are widely separated by the intercellular perichondrium, and periosteum fibers and ground substance that form the bulk of Regular these tissues. Together, the fibers and ground Collagenous Tendon, ligaments, aponeurosis, substance form the matrix of connective tissue. and cornea With some exceptions, connective tissue is generally well vascularized. Elastic Ligamentum nuchae, ligamenta flava

Classification Special connective tissue

Cartilage VOCABULARY: extracellular material, general, Hyaline Costal cartilages, trachea, majority loose, dense, irregular, regular of embryonic skeleton

Classification of connective tissues into various Elastic External ear, epiglottis subgroups is largely descriptive of extracellular Fibrous Symphysis pubis, intervertebral disc materials rather than features of the cellular components. Two large categories can be defined - Bone Adult skeleton general and special - from which further subdivisions are made. General connective tissues Blood Cardiovascular system are distinguished as loose or dense according to Hemopoietic Bone marrow, lymphatic tissue and whether the fibers are loosely or tightly packed. organs

43 Loose Connective Tissue also contain hydroxyproline and hydroxylysine. However, alpha units isolated from collagens taken Loose connective tissue is a common and simple from different sites vary somewhat in the form of connective tissue and can be considered composition and sequence of amino acids. Several the prototype of connective tissue. Essentially all distinct molecular types of collagen have been other types of connective tissues are variants of identified, all of which consist of three alpha units loose connective tissue in which one or more arranged in a right hand helix and differ mainly in components have been emphasized to serve the amino acid constituents of the alpha chain. Not specific functions. all unit fibrils of the various collagens present a banded, fibrillar appearance, nor are they arranged Fibers of Connective Tissue in a similar fashion. Thus, collagen represents a family (in excess of 12 VOCABULARY: collagen fiber, unit fibril, types) of closely related but genetically distinct tropocollagen, alpha unit, reticular fiber, elastic proteins. Some of these are listed in Table 4-2. fiber, microfibril, elastin, fibrillin, elastic Tropocollagen has been extracted from the ground laminae substance also but cannot be demonstrated histologically. Various types of collagen can be The fibrous intercellular substances in connective made by cells other than fibroblasts, and collagen is tissue consist primarily of collagenous, reticular, and synthesized by chondroblasts, osteoblasts, elastic fibers. Collagen fibers are present in all odontoblasts, epithelial cells, endothelial cells and connective tissues and run an irregular course with smooth muscle cells. Collagen molecules are much branching. The fibers vary in thickness from important components of basement membranes. 1 to 10 µm and are of undefined length. Although Reticular fibers do not gather into large bundles flexible, collagenous fibers have extremely high as do collagenous fibers but tend to form delicate tensile strength. Ultrastructurally, the fibers consist networks. They are not seen in routinely prepared of parallel fibrils, each of which represents a sections but can be demonstrated with silver stains structural unit (Fig. 4-1). These unit fibrils show or by the periodic acid-Schiff (PAS) reagent, which repeating transverse bands spaced at 64-nm reacts with proteoglycans that bind reticular unit intervals along their length and are composed of fibrils together. In electron micrographs, the unit macromolecules of tropocollagen that measure fibrils of reticular fibers show the same banding about 260 nm long by 1.5 nm wide. pattern as the unit fibrils of type I collagen fibers. They differ structurally in number, diameter, and in the arrangement of the unit fibrils. The unit fibrils of reticular fibers consist of type III collagen. Elastic fibers appear as thin, homogeneous strands that are similar and of more uniform size than collagen fibers. They cannot be distinguished readily in routine H&E sections and require special stains to make them easily visible. In electron

micrographs, elastic fibers are seen to consist of Fig. 4-1. Elastic and collagen fibers from the connective tissue bundles of microfibrils embedded around an component of the pleura as seen in the transmission electron amorphous component called elastin. The microscope. Note that each collagen fiber is made up of several unit fibrils that show cross striations. (X 11,000). microfibrils are about 11 nm in diameter and lack cross banding. During formation of elastic fibers, The tropocollagen molecules lie parallel to each microfibrils are laid down first and elastin is added other, overlapping by about one-fourth their secondarily but soon forms the bulk of the fiber. A lengths; the overlap is responsible for the banding precursor molecule, tropoelastin, is released by cells pattern. Each molecule of tropocollagen consists of and polymerizes extracellularly to form elastin. The three polypeptide chains called alpha units microfibrillar component tends to be peripherally arranged in a helix and linked by hydrogen bonds. located on the fiber. Elastin, like collagen, contains The polypeptides are rich in glycine and proline and the amino acids glycine and proline but has little hydroxyproline and lacks hydroxylysine.

44 Table 4-2. Types of collagen.*

Type Morphological features Distribution

I Broad, banded unit fibrils Widespread; tendon, bone, dermis, dentin, fascia

II Small-diameter, banded unit Hyaline cartilage, vitreous body, nucleus pulposus, notochord fibrils III Small-diameter, banded unit Corresponds to reticular fibers; prominent in organs with a major smooth fibrils muscle component; forms the supporting framework for the parenchyma of most glands and organs, fat, lymphoid tissue, uterus, and blood vessels

IV Feltwork of non-banded unit Basal laminae of epithelial cells, glomerular epithelium; lens capsule; fibrils external laminae

V Thick non-banded unit fibrils Widespread; pericellular laminae of smooth and striated muscle cells, tendon sheaths

VI Thin, banded unit fibrils Make up about 25% of collagen in cornea; small amounts where types I and III are found

VII Small-diameter, banded unit Form anchoring fibrils that link the basal lamina of many epithelia to the fibrils underlying connective tissue

VIII Unknown A major component of Descemet’s membrane; associated with and produced by endothelial cells

IX Unknown Found mainly in cartilage; links type II collagen in three-dimensional arrangement

X Unknown Cartilage matrix surrounding hypertrophic chondrocytes during endochondral bone formation

* Various types of collagen can be elaborated by cells other than fibroblasts. Collagen is known to be synthesized by epithelial cells, endothelial cells, chondroblasts, osteoblasts, odontoblasts, and smooth muscle cells.

It has a high content of valine and contains two Ground Substance amino acids, desmosine and isodesmosine, that are specific to elastin. Microfibrils consist of a VOCABULARY: gel, proteoglycan, nonsulfated glycoprotein called fibrillin. Similar glycosaminoglycan, bound water, adhesion microfibrils also exist that are associated with the glycoprotein, laminin, fibronectin, basal lamina of epithelia. Fibrillin mediates adhesion thrombospondin between elastin and the other components of the extracellular matrix. Elastic fibers can stretch more The fibers and cells of connective tissue are than 130% of their original length, and their embedded in an amorphous material called ground presence permits connective tissues to undergo substance that is present as a transparent gel of considerable expansion or stretching with return to variable viscosity. Ground substance consists of the original shape or size when the deforming force proteoglycans, glycosaminoglycans, and is removed. In addition to forming fibers, elastin glycoproteins that differ in amount and type in may be present in fenestrated sheets or layers called different connective tissues (Table 4-3). elastic laminae in the walls of blood vessels.

45

Table 4-3. Types of glycosaminoglycans.

Type of glycosaminoglycan Some locations

Hyaluronic acid Umbilical cord, vitreous humor, synovial fluid, loose connective tissue

Chondroitin Cornea

Chondroitin-4-sulfate Aorta, bone, cartilage, cornea

Chondroitin-6-sulfate Cartilage, nucleus pulposus, sclera, tendon, umbilical cord

Dermatan sulfate Aorta, heart valves, ligamentum nuchae, sclera, skin, tendon

Keratan sulfate Bone, cartilage, cornea, nucleus pulposus

Proteoglycans have a bottle brush configuration microorganisms and their toxic materials from the with long protein cores bound covalently to site of an infection. numerous glycosaminoglycan side chains. The primary glycoproteins of the matrix are Glycosaminoglycans are linear polymers of fibronectin, laminin, and thrombospondin. These repeating disaccharide units that contain are adhesion glycoproteins that function to link hexosamine (N-acetylglucosamine, N- cell membranes of various cell types to elements acetylgalactosamine), D-glucosamine, D- comprising the extracellular matrix. Laminin is the galactosamine, or uronic (glucuronic) acid as their largest of these adhesion glycoproteins and is most most constant feature. Because these abundant in the lamina lucida of epithelial basal macromolecules of the ground substance are laminae and in the external laminae of muscle cells. - commonly sulfated (SO3 ) or in the case of uronic It binds the plasmalemma via a transmembrane acid which exhibits a carboxyl group (Coo-) they glycoprotein laminin receptor of epithelial and muscle impart a net negative charge to the molecule. cells to type IV collagen and proteoglycans As a result, these molecules have a strong (heparan sulfate) in the adjacent matrix. hydrophilic behavior and are instrumental in the Fibronectin is a large, flexible glycoprotein of the retention of positive ions (Na+) together with water extracellular matrix synthesized by fibroblasts and in the extracellular matrix. Thus, these large, some epithelia. It also occurs in the external negatively charged molecules form and maintain a laminae of muscle cells and Schwann cells as well as large hydration space (an extracellular fluid on the surfaces of other cell types. Fibronectin compartment) within the extracellular matrix. mediates adhesion between cells and between cells As a result of these molecules, the ground and substrates in the matrix. It has several receptor- substance contains a high proportion of water, binding domains and links the plasmalemma of which is bound to long-chain carbohydrates and epithelial cells to collagen fibers and proteoglycans; most of the extravascular fluid glycosaminoglycans. Through fibronectin receptors (tissue fluid) is in this state. This bound water acts (integrins) in the basal epithelial plasmalemma, as a medium by which nutrients, gases, and fibronectin links actin filaments within the cell metabolites can be exchanged between blood and cytoplasm to components of the extracellular tissue cells. Hyaluronic acid is the chief matrix. Fibronectin is thought to be involved in cell glycosaminoglycan of loose connective tissue, and spreading and locomotion. Thrombospondin is because of its ability to bind water, it is primarily another adhesive glycoprotein that binds to responsible for changes in the permeability and collagen and some proteoglycans. It is produced by viscosity of loose connective tissue. In addition, fibroblasts, smooth muscle cells, and epithelial cells proteoglycans play an important role in maintaining and binds the plasmalemma of these cell types to the structural organization of the fibrous the extracellular matrix. constituents in the matrix and basal laminae and aid in preventing or retarding the spread of

46 Cells of Connective Tissue

VOCABULARY: fibroblast, myofibroblast, macrophage, foreign body giant cell, system of mononuclear phagocytes, mast cell, heparin, histamine, leukotriene C4 & D4, eosinophilic chemotactic factor, fat cell, plasma cell, leukocyte, neutrophil, eosinophil, major basic protein, lymphocyte, monocyte, antigen presenting cell

Connective tissue contains several different cell types. Some are indigenous to the tissues; others are transients derived from blood. The most common connective tissue cells are fibroblasts - large, Fig. 4-2. Formation of collagen by a fibroblast. flattened cells with elliptical nuclei that contain one or two nucleoli. The cell shape is irregular and often Myofibroblasts resemble fibroblasts but contain appears stellate with long cytoplasmic processes aggregates of the contractile microfilaments, actin extending along the connective tissue fibers. The and myosin. In contrast to smooth muscle cells, boundaries of the cell are not seen in most myofibroblasts lack a surrounding external lamina. preparations, and the morphology varies with the Normally found only in small numbers, they state of activity. In active cells the nuclei are plump increase following tissue injury. Myofibroblasts and stain lightly, whereas the nuclei of inactive cells produce collagen, and their contractile activity appear slender and dense. Ultrastructurally, active contributes to the retraction and shrinkage of early cells show increased amounts of granular scar tissue. endoplasmic reticulum (GER). Fibroblasts Macrophages (histiocytes) are almost as elaborate the precursors of collagen, reticular and abundant as fibroblasts in areolar connective tissue. elastic fibers, produce the ground substance, and They are actively phagocytic, ingesting a variety of maintain these extracellular materials that are materials from particulate matter to bacteria, tissue constantly being remodeled and renewed. This fact debris, and whole dead cells. The material ingested explains conditions such as scurvy that results from a is broken down by lysosomal digestion. deficiency of vitamin C in the diet. Vitamin C is Macrophages are activated by lipopolysaccharides necessary for proper cross-linking of the molecules (a surface component of gram-negative bacteria) that make up the collagen fibers and the lack of it and interferon-γ. The effectiveness of macrophages results in weakened collagen and connective tissue is enhanced by the binding of complement and throughout the body. antibodies to the surface of bacteria (opsonization). The peptides of collagen are formed on the Opsonized bacteria are more vulnerable to ribosomes of the GER, from which they are phagocytosis. Complement is a group of proteins transported to the Golgi complex. Procollagen, a circulating in the blood plasma that are synthesized molecular form of collagen is released at the and released by the liver. C3 (a component of maturing face of the Golgi complex within complement) receptors and Fc antibody receptors in the secretory vesicles into the surrounding cytoplasm macrophage plasmalemma binds to coated material and then released from the cell by exocytosis. and phagocytoses it for lysosomal digestion. Some An extracellular enzyme called procollagen peptidase invasive materials (asbestos, bacilli of tuberculosis, converts the procollagen molecules into Toxoplasma) do not undergo lysosomal digestion and tropocollagen, which then polymerize extracellularly to in response macrophages fuse together to form form the unit fibrils of collagen (Fig. 4-2). foreign body giant cells. Macrophages also interact with lymphocytes by releasing interleukin-6 (stimulates the differentiation of B lymphocytes into plasma cells) and interleukin-1 (stimulates T lymphocytes to divide) to combat infections.

47 Macrophages can synthesize and release a number foreign proteins and initiate a local inflammatory of other factors such as pyrogens (factors that response by rapidly discharging their secretory mediate fevers), granulocyte-macrophage colony- granules. The discharge of granules is unique in that stimulating factor, fibroblast growth factor, several fuse together and release their contents transforming growth factor, and tumor necrosis simultaneously. Mast cells also can promote factor, as well as connective tissue enzymes such as immediate hypersensitivity reactions (hay fever, collagenase and elastase. asthma, and anaphylaxis) following their release of Macrophages commonly are described as secretory granules, which act as chemical mediators. irregularly shaped cells with blunt cytoplasmic Fat cells are specialized for synthesis and storage processes and ovoid or indented nuclei that are of lipid. Individual fat cells may be scattered smaller and stain more deeply than those of throughout loose connective tissue or may fibroblasts. In fact, macrophages are difficult to accumulate to such an extent that other cells are distinguish morphologically from fibroblasts, crowded out and an adipose tissue is formed. Each fat especially active fibroblasts, unless the macrophages cell acquires so much lipid that the nucleus is show evidence of phagocytosis. flattened to one side of the cell and the cytoplasm The macrophages of loose connective tissue are forms only a thin rim around a large central droplet part of a widespread system of mononuclear of lipid. In ordinary H&E sections, fat cells appear phagocytes that includes phagocytes of the liver empty due to the loss of lipid during tissue (Kupffer cells), lung (alveolar macrophages), serous preparation, and groups of fat cells have the cavities (pleural and peritoneal macrophages), appearance of a honeycomb. nervous system (microglia), lymphatic tissue, and Plasma cells are not common in most connective bone marrow. Regardless of where they are found, tissues but may be numerous in the lamina propria macrophages have a common origin from of the gastrointestinal tract and are present in precursors in the bone marrow, and the monocytes of lymphatic tissues. Plasma cells appear somewhat blood represent a transit form of immature ovoid, with small, eccentrically placed nuclei in macrophages. which the heterochromatin is arranged in coarse Mast cells are present in variable numbers in blocks to form a "clock face" pattern. The loose connective tissue and often collect along cytoplasm is basophilic due to the abundance of small blood vessels. They are large, ovoid cells 20 to GER. A weakly staining area of cytoplasm called 30 µm in diameter with large granules that fill the the negative Golgi image often appears adjacent to one cytoplasm. Two populations of mast cells are side of the nucleus and corresponds to the site of known to exist: connective tissue mast cells and the Golgi complex. Plasma cells produce mucosal mast cells. In humans, the granules of immunoglobulins (antibodies) that form an important connective tissue mast cells are membrane-bound defense against infections. Various acidophilic and in electron micrographs show a characteristic inclusions may be found in the cytoplasm of some tubular pattern. The granules contain heparin, a plasma cells. Plasma cells are a differentiated form potent anticoagulant; histamine, an agent that of the B-lymphocyte. causes smooth muscle contraction in bronchi and Variable numbers of leukocytes constantly increased vascular permeability; leukotriene C4 migrate into the connective tissues from the blood and D4, which increase vascular permeability, (diapedesis). Neutrophils are one type of leukocyte increases vasodilation, and causes smooth muscle characterized by a lobed nucleus. In this cell type, contraction in bronchi; and eosinophil the granules are said to be neutrophilic, although in chemotactic factor. This factor attracts sections they stain faintly pink (ie, acidophilic). The eosinophils to the inflammation site. The granules cells are avidly phagocytic for small particles and of mucosal mast cells contain chondroitin sulfate bacteria and are especially numerous at sites of rather than heparin. Mast cells have highly specific infection. Neutrophils also contain antioxidants to membrane receptors for the Fc segment of IgE destroy potentially toxic peroxides generated during produced in response to allergens. Both mast cell lysosomal activity. Once neutrophils are activated in types arise from bone marrow stem cells. Mast cells tissues they do not live long and die. They form the are especially numerous along small blood vessels major cellular component of the substance called and beneath the epithelia of the intestinal tract and pus. respiratory system. Here, they detect the entry of

48 Eosinophils also are characterized by lobed nuclei, Subtypes of Loose Connective Tissue but their cytoplasmic granules are larger, more spherical, and more discretely visible than those of VOCABULARY: mesenchyme, mucoid tissue, neutrophils. The granules stain intensely with acid areolar connective tissue dyes such as eosin. All eosinophils have surface receptors for IgE. These cells are phagocytic for Mesenchyme is a loose, spongy tissue that serves specific molecules and have a special avidity for as packing between the developing structures of the antigen-antibody complexes. They increase in embryo. It consists of a loose network of stellate number as a result of allergic or parasitic types of and spindle-shaped cells embedded in an disease. Eosinophils are attracted to the sites of amorphous ground substance with thin, sparse histamine release by eosinophilic chemotactic factor fibers. The cells have multiple developmental of anaphylaxis (ECF-A), and their cytoplasmic potentials and can give rise to any of the connective granules contain enzymes capable of breaking down tissues. The presence of mesenchymal cells in the histamine. Eosinophils also produce major basic adult often has been proposed to explain the protein which enhances the antibody mediated expansion of adult connective tissue. However, destruction of the larvae of some helminthic fibroblasts, too, are capable of sequential divisions, parasites. Lymphocytes belong to the class of and may fulfill this role. leukocytes called mononuclear leukocytes and are Mucoid tissue occurs in many parts of the characterized by a single, round, nonlobed nucleus. embryo but is particularly prominent in the They are the smallest of the cells that migrate into umbilical cord, where it is called Wharton’s jelly. It connective tissues and measure about 7 µm in resembles mesenchyme in that the constituent cells diameter. These cells form part of the immunologic are stellate fibroblasts with long processes that defense system and may give rise to antibody- often make contact with those of neighboring cells, producing cells (B lymphocytes) or elaborate and the intercellular substance is soft and jelly-like nonspecific factors that destroy foreign cells (T and contains thin collagen fibers. The fiber content lymphocytes). B and T lymphocytes cannot be increases with the age of the fetus. Mucoid tissue distinguished morphologically using routine H&E does not have the developmental potential of preparations. They are few in number in normal mesenchyme. connective tissue but increase markedly in areas of Areolar connective tissue is a loosely arranged chronic inflammation. Monocytes are the blood- connective tissue that is widely distributed in the borne forerunners of tissue macrophages. Once body. It contains collagen fibers, reticular fibers, these cells have entered connective tissue, it is and a few elastic fibers embedded in a thin, almost difficult, if not impossible, to distinguish them from fluid-like ground substance. This kind of connective macrophages. As monocytes leave the vasculature tissue forms the stroma that binds organs and organ and become macrophages they receive further components together. It forms helices about the specific, molecular programing dependent on the long axes of expandable tubular structures such as microenvironment (spleen, lung, brain) within the ducts of glands, the gastrointestinal tract, and which they reside. Some macrophages have antigen- blood vessels. presenting functions and these specialized macrophages form a family of antigen presenting Adipose Tissue cells. This type of macrophage phagocytoses endogenous antigens that are degraded into antigen VOCABULARY: white fat, unilocular, leptin, peptide fragments. These fragments are then brown fat, multilocular presented on the macrophage surface together with class II major histocompatability complex (MCH). Adipose tissue differs in two respects from other The presented antigen peptide fragment is connective tissue: fat cells and not the intercellular recognized by CD4+ helper T lymphocytes which substances predominate, and unlike other are stimulated and in turn promote B lymphocyte connective tissue cells, each fat cell is surrounded differentiation. by its own basal lamina. Reticular and collagenous fibers also extend around each fat cell to provide a delicate supporting framework that contains numerous capillaries.

49 Adipose tissue can be subdivided into white and Reticular Connective Tissue brown fat. White fat is the more plentiful and is found mainly in the subcutaneous tissue (where it VOCABULARY: reticular cell, reticular fiber, forms the panniculus adiposus), omenta, fibroblast mesenteries, pararenal tissue, and bone marrow. White fat is an extremely vascular tissue and also Reticular connective tissue is characterized by a contains many nerve fibers from the autonomic cellular framework as seen in lymphatic tissues and nervous system. In white fat, the cells are filled by a bone marrow. Reticular cells are stellate, with single, large droplet of lipid; thus, it is often referred processes extending along the reticular fibers to to as unilocular fat. The lipid droplet is composed make contact with neighboring cells. The cytoplasm of glycerol esters and fatty acids. The materials stains lightly and is attenuated, and the large nucleus within the fat droplet are in a constant state of flux stains weakly. The reticular cell is equivalent to the and are not permanent entities. Adipocytes have fibroblast of other connective tissues and is receptors for insulin, thyroid hormone, responsible for the production and maintenance of glucocorticoids, and norepinephrine that modulates the reticular fibers, which are identical to those the release and uptake of lipid. White fat serves as a found in loose connective tissue. storage depot for calories taken in excess of the body's needs and can be used for production of Dense Connective Tissue energy as required or as a source of building materials as in the case of steroid hormone VOCABULARY: dense irregular, dense regular synthesis. It also acts as an insulating layer to conserve body heat, acts mechanically as a packing Dense connective tissue differs from the loose material, and forms shock-absorbing pads in the variety chiefly in the concentration of fibers and palms of the hands, on the soles of the feet, and reduction of the cellular and amorphous around the eyeballs. Adipocytes secrete a hormone constituents. Two types, dense irregular and dense called leptin the action of which is to decrease regular, are identified. appetite. This action is thought to be mediated Dense irregular connective tissue contains through satiety centers in the hypothalamic region abundant, thick, collagenous bundles that are of the brain where leptin receptors are found. woven into a compact network. Among the Brown fat is present in many species and is collagen fibers is an extensive network of elastic prominent in hibernating animals and newborn fibers. An example of dense irregular connective humans. Brown fat has a restricted distribution, tissue is the dermis of the skin. occurring mainly in the interscapular and inguinal Dense regular connective tissue also contains a regions. The brown or tan color is due to the high predominance of collagen fibers arranged in content of cytochrome enzymes. The cells show bundles, but these have a regular, precise round nuclei, and the cytoplasm is filled with arrangement. The organization of the collagen multiple small droplets of lipid; hence this type of bundles reflects the mechanical needs of the tissue. fat is called multilocular fat. Mitochondria here are In tendons, aponeuroses, and ligaments, for example, more numerous and larger than those in the cells of they are oriented in the direction of pull. white fat. Each cell of brown fat receives direct Fibroblasts are the primary cells present and occur sympathetic innervation. During arousal from in rows parallel to the bundles of collagen fibers. hibernation in animals or following birth in the case of humans, the lipid within the brown fat is rapidly Special Connective Tissue oxidized to produce heat and release substances such as glycerol that are used by other tissues. The Special connective tissues have functions and a energy produced by mitochondria in brown fat is histologic organization sufficiently unique to dissipated as heat rather than being stored as ATP. warrant their consideration as distinct and special Because brown fat is even more vascular than white forms of connective tissue. They are considered fat, the generated heat raises the temperature of the individually in succeeding discussions. blood significantly, thus increasing the general body temperature.

50 Histogenesis Connective tissues bind organs and unite organ components into a functioning unit. Adipose tissue All connective tissues originate from mesoderm, the forms an insulating blanket to limit heat loss and middle of the three primary germ layers forming the forms mechanically protective cushions in the early embryo. Mesoderm consists mainly of large, hands and feet and about the eyeball. It also stores stellate mesenchymal cells whose numerous excess calories in the form of fat that can be called processes are in contact with those of adjacent cells. on and used as needed. Initially, only mesenchymal cells are present, but as Collagenous fibers provide tissues with flexibility development proceeds, an interlacing network of and tensile strength; elastic fibers allow considerable fine collagenous fibers appears. Very early in deformity of an organ with recovery of the shape development the mesodermal layer is transformed and size when the force is removed. into a mesenchymal compartment separated from Collagen molecules form a major component of the surrounding epithelia by basal laminae. Collagen basal laminae, which determine vascular and molecules form a major component of such epithelial permeability. laminae, which are important in determining what Connective tissue also contributes significantly to substances enter or leave the established connective the defense of the body. This is the function of tissue compartment. With further development, the cellular components mainly, as in the phagocytic mesenchymal compartment becomes transformed activity of macrophages and granular leukocytes into numerous irregularly shaped connective tissue and the antibodies and cytotoxic agents elaborated passageways that surround neural and muscular by plasma cells and lymphocytes, respectively. The tissues or lie between epithelial tissues. Each of matrix also plays an important role in the body's these remains separated from the connective tissue defenses by preventing or slowing the spread of compartment by external or basal laminae, microorganisms and their toxic products from sites respectively. Glands develop from invaginations of of infection. epithelium that extend into the mesenchymal The various connective tissues together form a compartments. Interaction between epithelium and single vast compartment limited by sheets of mesenchyme is essential for normal development to epithelia. It is through this compartment that tissue occur. fluid must pass during exchange of nutrients and waste products between cells and tissues and the Summary blood vasculature. Most immune reactions take place in this compartment. Because the ground

substance of loose connective tissue binds water, it The functions of connective tissue are mainly acts as the medium in which these exchanges are passive, but mechanically they are indispensable. effected. The ground substance also is important in They provide the supporting framework for organs the spacial organization of components forming the and for the body itself and serve to connect distant connective tissues as well as contributing to their structures as, for example, in the connection of mechanical stability. muscle to bone by tendons.

51 away from the walls of the lacunae. Electron 5 Special Connective Tissue: microscopy reveals that each cell completely fills Cartilage, Bone, and Joints the lacunar space and sends short processes into the surrounding matrix, but neighboring cells do not touch one another. The nucleus is round or oval Cartilage and contains one or more nucleoli. The cytoplasm shows the usual organelles as well as lipid and Although adapted to provide support, cartilage glycogen inclusions. In growing chondrocytes the contains only the usual elements of connective Golgi complex and endoplasmic reticulum are well tissue - cells, fibers, and ground substance. It is the developed. ground substance that gives cartilage its firm In fresh cartilage and in routine histologic consistency and ability to withstand compression sections, the matrix appears homogeneous because and shearing forces. Collagen and elastic fibers the ground substance and the collagen fibers embedded in the ground substance impart tensile embedded within it have the same refractive index. strength and elasticity. Together, the fibers and The collagen in hyaline cartilage rarely forms ground substance form the matrix of cartilage. bundles but is present as a feltwork of slender unit Cartilage differs from other connective tissues in fibrils in which the banding pattern shows variable that it lacks nerves, blood and lymphatic vessels and is periodicities or even is lacking. Collagen in cartilage nourished entirely by diffusion of materials from appears to be less polymerized than in other tissues blood vessels in adjacent tissues. Although relatively and is classified as type II collagen. rigid, the cartilage matrix has a high water content The ground substance consists mainly of (about 75%) and is freely permeable, even to fairly proteoglycans, the specific glycosaminoglycans of large particles. cartilage being chondroitin-4- and chondroitin-6- Classification of cartilage into hyaline, elastic, and sulfate, keratan sulfate, and a small amount of fibrous types is based on differences in the hyaluronic acid. The proteoglycans are responsible abundance and type of fibers in the matrix. for the basophilic properties of the ground substance. Chondronectin, a glycoprotein within the Hyaline Cartilage ground substance, mediates the adhesion of type II collagen to chondrocytes. Chondrocalcin, a calcium- VOCABULARY: chondrocyte, lacuna, binding glycoprotein, also has been demonstrated in isogenous group, type II collagen, ground the ground substance of hyaline cartilage. The substance, territorial matrix, interterritorial matrix around each lacuna and isogenous group matrix, perichondrium stains more deeply than elsewhere, forming the territorial matrix. The less densely stained Hyaline cartilage is the most common type of intervening areas form the interterritorial matrix. cartilage and forms the costal cartilages, articular Except for the free surfaces of articular cartilages, cartilages of joints, and cartilages of the nose, hyaline cartilage is enclosed by a specialized sheath larynx, trachea, and bronchi. It is present in the of connective tissue called the perichondrium. growing ends of long bones. In the fetus, most of The outer layers of the perichondrium consist of a the skeleton is first laid down as hyaline cartilage. well vascularized, dense, irregular connective tissue The cells of cartilage are called chondrocytes and that contains elastic and collagen fibers and reside in small spaces called lacunae scattered fibroblasts. Where it lies against cartilage, the throughout the matrix. The cells generally conform perichondrium is more cellular and passes to the shape of the lacunae in which they are imperceptibly into cartilage. The slender collagen contained. Deep in the cartilage the cells and their unit fibrils of the cartilage matrix blend with the lacunae usually appear rounded in profile, whereas wider, type I collagen unit fibrils of the at the surface, they are elliptical, with the long axis perichondrium. Perichondrial cells adjacent to the parallel to the surface. Chondrocytes often occur in cartilage retain the capacity to form new cartilage. small clusters called isogenous groups that represent the offspring of a single cell. In the usual preparations, the cells show an irregular outline and appear shrunken and pulled

52 Elastic Cartilage Macroscopic Structure

VOCABULARY: elastic fibers VOCABULARY: cancellous (spongy) bone, trabecula, compact (dense) bone, diaphysis, Elastic cartilage is a variant of hyaline cartilage, epiphysis, periosteum, endosteum, diploë differing chiefly in that it contains branched elastic fibers in the matrix. Collagen fibers of the type Grossly, cancellous and compact forms of bone can found in hyaline cartilage also are present but are be identified. Cancellous (spongy) bone consists masked. Deep in the cartilage, elastic fibers form a of irregular bars or trabeculae of bone that branch dense, closely packed mesh that obscures the and unite to form a three-dimensional, interlacing ground substance, but beneath the perichondrium, network of boney rods, delineating a vast system of the fibers form a looser network and are small communicating spaces that in life are filled continuous with those of the perichondrium. The with bone marrow. Trabeculae of spongy bone chondrocytes are similar to those of hyaline from weight bearing lower limb bones do not form cartilage and elaborate the elastic and collagen a random network but occur in a precisely fibers. Elastic cartilage is more flexible than hyaline organized pattern along compression and tension cartilage and is found in the external ear, auditory stress lines. This strut-like arrangement of the tube, epiglottis, and in some of the smaller laryngeal trabeculae contributes to the strength of these cartilages. bones. Compact (dense) bone appears as a solid, continuous mass in which spaces cannot be seen Fibrous Cartilage with the naked eye. The two types of bone are not sharply delineated and merge into one another. Fibrous (or fibro-) cartilage represents a transition In a typical long bone the shaft or diaphysis between dense connective tissue and cartilage. It appears as a hollow cylinder of compact bone consists of typical cartilage cells enclosed in lacunae, enclosing a large central space called the marrow but only a small amount of ground substance is cavity. The ends of the long bones, the epiphyses, present in the immediate vicinity of the cells. The consist mainly of cancellous bone covered by a thin chondrocytes lie singly, in pairs, or in short rows layer of compact bone. The small inter- between bundles of dense collagen fibers the unit communicating spaces in the spongy bone are fibrils of which that show the 64-nm banding continuous with the marrow cavity of the shaft. pattern typical of type I collagen. Fibrous cartilage Except over articular surfaces and where tendons always lacks a perichondrium and merges into and ligaments insert, bone is covered by a dense hyaline cartilage, bone, or dense fibrous connective irregular fibroelastic connective tissue called the tissue. It occurs in the intervertebral discs, in some periosteum. The marrow cavity of the diaphysis articular cartilages, in the symphysis pubis, and at and the spaces within spongy bone are lined by sites of attachment of certain tendons to bone. endosteum, which is similar to periosteum but is thinner and not as fibrous. Both the periosteum and Bone endosteum have the ability to form new bone under appropriate stimulation. Bone also is a connective tissue specialized for Flat bones, such as those of the skull, also consist support. However, in bone the matrix is of compact and spongy bone. The inner and outer mineralized and forms a dense, hard, unyielding plates (often called tables) consist of thick layers of substance with high tensile, weight-bearing, and compact bone, while the space between the plates is compression strength. Despite its strength and bridged by spongy bone called the diploë. rigidity, bone is a dynamic, living tissue constantly turning over, constantly being renewed and reformed throughout life.

53 Microscopic Structure

VOCABULARY: lamella, lacuna, osteocyte, canaliculus, osteon (Haversian system), interstitial lamella, cement line, circumferential lamella, Haversian canal, Volkmann's canal

A fundamental characteristic of bone is the arrangement of its mineralized matrix into layers or plates called lamellae. Small, ovoid spaces, the lacunae, occur rather uniformly within and between the lamellae, each occupied by a single bone cell or osteocyte. Slender tubules called Fig. 5-1. Diagrammatic representation of a cross section through canaliculi radiate from each lacuna and penetrate compact bone. the lamellae to link up with the canaliculi of adjacent lacunae. Thus, lacunae are interconnected The longitudinally oriented channels at the center by an extensive system of fine canals. of osteons are the Haversian canals. These In compact bone, the lamellae show three communicate with one another by oblique branches arrangements. Most are arranged concentrically and by transverse connections called Volkmann's around a longitudinal space to form cylindrical units canals that penetrate the bone from the endosteal that run parallel to the long axis of the bone. These and periosteal surfaces. Unlike Haversian canals, are osteons or Haversian systems and make up Volkmann's canals are not surrounded by the structural units of bone. Osteons vary in size concentric lamellae. Volkmann's canals unite the and consist of 8 to 15 concentric lamellae that blood vessels within the Haversian canals with surround a wide space occupied by blood vessels. vessels of the marrow cavity and periosteum. Thus, In longitudinal sections, an osteon appears as plates the nutritional needs of compact bone are met by a of boney matrix running parallel to the slitlike space vast, continuous network of vascular channels. of the vascular channel. Throughout its thickness, Canaliculi adjacent to a Haversian canal open into compact bone contains a number of osteons the perivascular space, and the canalicular system running side by side. Osteons take an irregular brings all the lacunae within that osteon into course through the length of the bone and may communication with the canal. branch. Spongy bone shows a lamellar structure also but Other lamellae appear as angular, irregular bundles differs from compact bone in that it is not usually of lamellar bone that fill the spaces between traversed by blood vessels. Therefore, osteons are osteons. These are called interstitial lamellae. rare or lacking in the irregular rods of lamellar bone Osteons and interstitial lamellae are outlined by a that comprise spongy bone. It is thought that a refractile line, the cement line, which consists of critical distance (number of lamellae) exists beyond modified matrix. Cement lines are not traversed by which osteocytes cannot be maintained by the canaliculi. canalicular system. These needs are met in At the external surface of the bone, immediately trabeculae only a few lamellae in width as the beneath the periosteum, several lamellae run around canalicular system is linked to the perivascular the circumference of the entire bone. A similar but region of the marrow cavity that resides around all less well-developed system of lamellae is present on trabeculae. Trabeculae that exceed this critical the inner surface, just beneath the endosteum. distance, like compact bone, must contain These two systems of lamellae are the outer and inner Haversian systems to meet their nutritional needs. circumferential lamellae, respectively. The lamellar arrangement of compact bone is shown in Figure 5-1.

54 Periosteum and Endosteum calcium concentration in the body fluids, and is implicated in the resorption of bone. VOCABULARY: dense irregular connective In areas where bone is being resorbed, large tissue, Sharpey's fibers multinucleate giant cells are present. These are osteoclasts that often lie within shallow The outermost layer of the periosteum is a relatively depressions along the surface of the bone, most acellular dense irregular connective tissue with frequently on an endosteal surface. These cavities abundant collagen fibers, a few elastic fibers, and a are called Howship's lacunae. Although the cell network of blood vessels, branches of which enter may contain as many as 30 nuclei, the individual Volkmann's canals. The deeper layers of periosteum nuclei show no unusual features and usually are (those closest to the bone) are more cellular (with located in the part of the cell farthest from the osteogenetic potential) and consist of more loosely bone. The cytoplasm, usually weakly to moderately arranged connective tissue. During development of acidophilic, contains multiple Golgi complexes and bone, collagen fibers of the periosteum are trapped centriole pairs and numerous mitochondria and in the circumferential lamellae and form Sharpey's lysosomes. The cell surface adjacent to the bone fibers, which anchor the periosteum to the shows a ruffled border, a surface modification underlying bone. unique to osteoclasts; osteoclasts that lack a ruffled Endosteum lines all the cavities of bone, including border do not participate in bone resorption. The the marrow spaces of the diaphysis and epiphysis ruffled border consists of elaborate folds and clefts and the Haversian and Volkmann's canals. It that abut the surface of the bone; the plasmalemma consists of a single layer of squamous to cuboidal of the ruffled border is covered by small, bristle-like cells with a small and variable content of supporting projections. A clear zone containing many actin reticular and collagen fibers. As do the cells of the filaments lies adjacent to the ruffled border, and inner layer of the periosteum, the cells of the mitochondria tend to accumulate nearby. The cell endosteum retain osteogenic potential. surface farthest from the bone shows a smooth contour. The cytoplasm of active osteoclasts has a Cells of Bone frothy appearance due to numerous vacuoles that contain fragments of collagen and crystals of VOCABULARY: osteocyte, osteoblast, matrix. The plasmalemma of the ruffled border is osteoclast, Howship's lacunae, ruffled border rich in sodium/potassium ATPase and carbonic anhydrase that produces hydrogen ions. The area Osteocytes are the primary cells found in mature beneath the ruffled border is sealed off by the bone and take the shape of the lacunae in which osteoclast and an acidic environment created to aid in they are housed. They differ from osteoblasts, the digestion of the mineral component of the bone which are bone-forming cells. Osteocytes are not matrix. In addition, the osteoclasts secret lysosomal isolated in their lacunae but maintain extensive enzymes, primarily β-glucuronidase and aryl sulfatase, communication with nearby osteocytes. Delicate to breakdown the proteoglycans of the bone matrix cytoplasmic processes extend from the cell bodies and collagenase to digest type 1 collagen. and traverse the canaliculi to contact processes Osteoclasts arise by fusion of monocytes that have from neighboring cells. At the points of contact, emigrated from the blood. Osteoclast activity is apposed cells form nexus (communicating) junctions. A stimulated indirectly by parathyroid hormone and vitamin D thin layer of unmineralized matrix separates the cell (via osteoblasts stimulated to secrete Interleukin-1) and its processes from the walls of the lacunae and to increase bone resorption and elevate blood calcium canaliculi and provides the means for diffusion of levels. Osteoclasts have calcitonin receptors and their materials throughout the bony matrix. activity is reduced by the presence of calcitonin thereby Osteocytes contain the usual cell organelles, but reducing blood calcium levels. the Golgi apparatus is relatively inconspicuous, and ribosomes, mitochondria, and endoplasmic reticulum are scarce. Although the osteocyte appears to be not actively elaborating protein, it is not metabolically inert. It is responsible for maintaining bone, plays an active role in regulating

55 Matrix of Bone motion. Based on their structure, joints can be classified as fibrous, cartilaginous, or synovial. VOCABULARY: ground substance, collagen fiber (type 1), inorganic component Fibrous Joints

Bone matrix forms the bulk of bone and consists of VOCABULARY: suture, syndesmosis, collagen fibers, ground substance, and inorganic gomphosis components. The ground substance contains proteoglycans similar to those of cartilage but in a Fibrous joints are formed by dense fibrous lesser concentration. Glycosaminoglycans of bone connective tissue and permit little or no movement. include chondroitin sulfate, keratin sulfate, and Sutures are immovable fibrous joints found only in hyaluronic acid. Osteocalcin and osteopontin are the skull. The gap between the articulating bones is glycoproteins of bone that play a role in binding filled by a fibrovascular connective tissue and is calcium salts to the collagenous component of the bridged at the surfaces by periosteum. Sutures are matrix. An additional glycoprotein isolated from sites of osteogenic activity and allow for growth of bone, osteonectin, aids in binding cells of bone to the the skull. They are temporary joints; with aging the matrix. Bone has a relatively low degree of fibrous connective tissue gradually is replaced by hydration of about 7% water. bone to form a permanent boney union The collagen of bone is about 90% type I and (synostosis). The adjoining edges of the bones are forms the bulk of the bone matrix. When long highly irregular and interlock to create a firm union. bones such as the fibula are demineralized, they A syndesmosis, such as occurs at the inferior retain their shape and size due the large amount of tibiofibular joint, contains much more connective collagen, but now are extremely flexible. They can tissue than does a suture, allowing somewhat more even be tied in a knot. movement. A third type of fibrous joint is the The unit fibrils of the collagen form a gomphosis, a peg-and-socket joint restricted to the sophisticated internal network on which bone fixation of teeth in the alveolar bone of the jaws. mineral is deposited. The collagen of more mature bone is laid down in a highly organized fashion so Cartilaginous Joints that in any one lamella the unit fibrils are parallel to one other and run a helical course about the Haversian VOCABULARY: epiphyseal plate, canal. The pitch of the helix differs in adjacent synchondrosis, symphysis, intervertebral disc, lamellae so that the collagen unit fibrils of adjacent annulus fibrosus, nucleus pulposus lamellae lie at right angles to one another. It is this highly organized arrangement of the collagen unit Cartilaginous joints are present when bones are fibrils of bone that gives bone its strength. The united by a continuous plate of hyaline cartilage or a inorganic component is responsible for the disc of fibrocartilage. The hyaline cartilage of an rigidity of bone and consists of calcium phosphate, epiphyseal plate forms a temporary joint uniting citrate ions, and bicarbonate ions with small the shaft and epiphysis of a long bone during its amounts of calcium and magnesium fluoride. The development. This type of immovable joint, a minerals are deposited as crystals of calcium and synchondrosis, sometimes is classed as a primary phosphate hydroxides called hydroxyapatite cartilaginous joint. Permanent or secondary (Ca10(PO4)6(OH)2) and are present on and within cartilaginous joints are represented by symphyses, the osteoid collagen (type 1) at regular intervals which include unions such as those between the along the fibers. pubic bones (pubic symphysis) and between the vertebral bodies. The articulating surfaces are Joints covered by thin layers of hyaline cartilage that in turn are joined to a central disc of fibrous cartilage. Adjacent bones are united at joints, which show The bone, hyaline cartilage, and fibrous cartilage are considerable variation in their structure and intimately united, and the joint allows only limited function. At some joints, the articulating bones are movement (amphiarthrosis). The degree of so interlocked that they are immovable, while at movement possible depends on the thickness of the others, the joint structure permits a wide range of fibrous cartilage.

56 A slitlike cavity may appear in the fibrous cartilage of some symphyses (e.g., symphysis pubis), but the specialized sliding surfaces of a synovial joint are absent. In the spine, specialized discs of fibrous cartilage form the intervertebral discs that make up the major connections between adjacent vertebral bodies. The outer part of the disc, the annulus fibrosus, consists of a peripheral zone of collagen fibers and an inner zone of fibrous cartilage that is especially rich in collagen. The central part of the disc, the nucleus pulposus, is soft and gelatinous in the young but with aging is gradually replaced by fibrous cartilage. The articular surfaces of each vertebra are covered by thin plates of hyaline cartilage. The annulus fibrosus is firmly attached to the vertebral body by collagen fibers that pierce the cartilaginous end-plates and enter the compact bone of the vertebral body as Sharpey's fibers. Fig. 5-2. Diagrammatic representation of the synovial joint.

Synovial Joints The articular cartilages of most joints are hyaline in type but show a greater content of collagen than VOCABULARY: diarthrosis, capsule, articular usual. In some joints (e.g., temporomandibular), disc, meniscus, articular cartilage, tangential frank fibrous cartilage is present. Articular cartilage zone, transitional zone, radial zone, calcified lacks a perichondrium, and joint contact is made zone, lamina splendens between the free, uncovered surfaces of the opposing articular cartilages. Synovial joints, also called diarthroses, are The collagen fibers of articular cartilage have a complex, freely mobile joints capable of a wide unique arrangement. At the surface, the fibers are range of motion. Most of the joints of the limbs fall organized into tight bundles that tend to run at into this category. The bones involved in the joint right angles to one another and are parallel to the are contained within and linked by a capsule of surface. This forms the tangential zone. In the dense irregular connective tissue that is continuous deeper transitional and radial zones, the collagen with the periosteum over the articulating bones. fibers are randomly arranged. Where it abuts the Additional stability may be provided by intra- and underlying bone, the articular cartilage becomes extracapsular ligaments that also are dense irregular calcified and fuses with the bone, thus forming the connective tissue. The opposing boney surfaces are calcified zone in which the collagen fibers are covered by articular cartilages that provide low- perpendicular to the surface. The diameters of the friction gliding surfaces. Although it appears collagen fibers increase progressively as they pass smooth to the naked eye, the free surface of the from the tangential zone to the calcified zone. The articular cartilage is beset with various undulations, fibers of the calcified layer penetrate the troughs, and valleys. subchondral bone, where they are firmly embedded The inner surface of the capsule is lined by a like Sharpey's fibers. Overlying the tangential zone synovial membrane that elaborates the lubricating is the lamina splendens, a layer that contains synovial fluid. An articular disc may be interposed randomly arranged fine fibrils. completely or partially between the articular The junction between bone and calcified cartilage surfaces; a partial disc is called a meniscus. Discs is irregular and shows fine ridges, grooves, and and menisci consist of fibrous cartilage with an interdigitations that help unite the subchondral abundance of collagen fibers; at the periphery, they bone and articular cartilage. The junction between are connected to the capsule. The general structure the calcified and radial zones appears, in sections, as of a synovial joint is shown in Figure 5-2. a basophilic "tide line" that represents the advancing front of the calcification process.

57 The cells of the articular cartilage also show zonal Synovial Membrane differences. The tangential zone contains several layers of flattened, fibroblast-like cells whose long VOCABULARY: synovial cell, A-cell, B-cell, axes, like those of the collagen fibers, are parallel to areolar synovial membrane, adipose synovial the surface. Their oval or elongated nuclei usually membrane, fibrous synovial membrane, have smooth outlines, but some are irregular and synovial fluid show a variety of undulations, deep indentations, or clefts; the patchy, clumped chromatin stains deeply. The synovial membrane is one of the characteristic The cytoplasm contains short cisternae of granular features of a synovial joint. It is a loose-textured, endoplasmic reticulum (GER), a small Golgi highly vascular connective tissue that lines the complex, and small, round mitochondria. Inclusions fibrous capsule and extends onto all intraarticular such as lipids and glycogen are rare, but pinocytotic surfaces except those subjected to compression vesicles are plentiful. during movement of a joint. Thus, articular In the transitional zone, the cells are rounded and cartilages, articular discs, and menisci are not show many long cytoplasmic processes that often covered by synovial membranes. Occasional finger- bifurcate at their tips. The round, usually eccentric like projections, the synovial villi, and coarser folds of nuclei contain finely granular chromatin and the synovial membrane project into the joint cavity. frequently show one or more nucleoli. Granular The free surface (synovial intima) of the synovial endoplasmic reticulum is abundant, the Golgi membrane consists of one to three layers of apparatus is well developed, and secretory granules flattened synovial cells embedded in a granular, are prominent. The cells are scattered randomly fiber-free matrix. The surface cells do not form a throughout the transitional zone. continuous layer, and in places, neighboring cells The cells of the radial zone also are rounded but are separated by gaps through which the synovial tend to form short columns or isogenous groups. cavity communicates with tissue spaces in the The endoplasmic reticulum is less developed, the synovial membrane. Where the cells do make Golgi complex is sparse, and mitochondria are contact, their surfaces may be complex and small and dense. Intracellular filaments are interdigitated. Desmosomal junctions have been increased in number, and lipid droplets and described in rat synovial membranes, but their glycogen granules are common. presence in humans has not been confirmed. The calcified zone is characterized by short Ultrastructurally, two types of synovial cells have columns of enlarged, pale staining cells that are in been described. A-cells are predominant and the advanced state of degeneration. The nuclei are resemble macrophages (hence their alternate name, dense and pyknotic, the nuclear envelope is M-cells). The cell membrane shows numerous fragmented, and cytoplasmic organelles are lacking. invaginations, filopodia, and micropinocytotic The organization of the chondrocytes of articular vesicles, while the cytoplasm contains large Golgi cartilage into successive zones is reminiscent of the complexes and numerous mitochondria and arrangement of cells in an epiphyseal plate during lysosomes but only scant profiles of GER. B-cells endochondral bone formation. Indeed, during resemble fibroblasts and sometimes are called F- growth, articular cartilage does serve as a growth cells. They have a smooth outline and contain zone for the subchondral bone. When epiphyseal abundant GER and scattered free ribosomes. Both growth is complete, the deep zones of cell types contain much glycogen but only a few chondrocytes in the articular cartilage are converted lipid inclusions. The relationship between A- and B- to compact bone and incorporated into the cells is unknown, but the existence of intermediate subchondral bone layer. forms (AB-cells) suggests that the two may be Like other cartilage, articular cartilage is avascular merely different functional forms of a single type. and aneural. The central regions of the cartilage The subintimal tissue varies from place to place receive their nutrition by diffusion from the within the same joint, and based on the structure of synovial fluid, which bathes the cartilages, and, to a this tissue, the synovial membrane is classified as lesser extent, from vessels in the subchondral bone. areolar, adipose, or fibrous. In areolar synovial At their edges, the articular cartilages are well membranes, the underlying tissue is a loose nourished from blood vessels in the nearby synovial connective tissue with relatively few collagen fibers membrane. and an abundant matrix.

58 Cellular constituents include fibroblasts, mast cells, Interstitial growth results from proliferation of macrophages, and lymphocytes. Adipose synovial young chondrocytes that divide and lay down a new membranes line the articular fat pads, and the matrix to produce an internal expansion of cartilage. subintimal tissue mainly consists of fat cells. In Such growth occurs only in young cartilage that is fibrous synovial membranes, the underlying soft enough to allow daughter cells to move away tissue is a dense irregular connective tissue and is from each other after division. Appositional found in regions subjected to tension; where such growth occurs as a result of perichondrial forces are extremely high, fibrous cartilage may be chondrogenic activity. The cells called chondrogenic present. cells are recruited from the inner surface of the Synovial fluid is a clear or slightly yellow viscous perichondrium. These cells form a layer, continue fluid that bathes the joint surfaces. Normally, the to divide and differentiate into chondroblasts, volume of fluid is sufficient only to form a thin film secrete matrix about themselves, and add new over all the surfaces within a joint. In composition, cartilage to the surface. Once entrapped within their synovial fluid is an ultrafiltrate of plasma to which own matrix these cells are properly termed mucin has been added. Mucin is a product of the chondrocytes. Although adult perichondrium surface synovial cells and consists mainly of highly retains some potential to form new cartilage, the polymerized hyaluronic acid, which gives viscosity capacity for growth and repair is limited by its to the synovial fluid. Normal synovial fluid contains avascularity. Adult cartilage has the lowest mitotic rate of only 50 to 100 cells per milliliter, and these consist any skeletal tissue. of macrophages, free synovial cells, and an Where elastic cartilage forms, chondroblasts at occasional leukocyte. Amorphous particles and first elaborate fibers that are neither elastic nor fragments of cells and fibrous tissue may be present collagenous in character. These indifferent fibers and are believed to result from slow wearing of the subsequently mature into elastic fibers. Fibrous joint surface. cartilage develops much as does ordinary connective tissue, but some fibroblasts transform Histogenesis into chondroblasts and secrete a small amount of cartilage matrix about themselves. Cartilage Bone VOCABULARY: chondrogenic cells, chondroblast, chondrocyte, interstitial growth, Bone is mesenchymal in origin and develops by appositional growth replacement of a preexisting tissue. If bone is formed directly in a primitive connective tissue, the process All cartilage arises from mesenchyme, and at sites is called intramembranous ossification; when where cartilage is to form, the mesenchymal cells replacement of a preformed cartilaginous model is lose their processes, round up, proliferate, and involved, the process is known as endochondral crowd together in a dense aggregate of ossification. The essential process of bone formation chondrogenic cells (a center of chondrification). is the same in both, however. The cells in the interior of the mass are chondroblasts, which lay down fibers and ground Intramembranous Ossification substance. As the amount of intercellular material increases, the cells become isolated in individual VOCABULARY: osteoprogenitor cell, compartments (lacunae) and take on the osteoblast, osteocyte, osteoid, woven bone characteristics of chondrocytes. New chondroblasts are recruited from the more The flat bones of the cranium and part of the peripherally placed immature cells. The perichondrium mandible develop by intramembranous ossification; is derived by condensation of the mesenchyme that these bones frequently are called membrane bones. In surrounds the developing cartilage. Further growth the areas where bone is to form, the mesenchyme occurs by expansion of the mass of cartilage from becomes richly vascularized, and the cells within (interstitial growth) and by addition of new proliferate actively. Some cells undergo changes cartilage at the surface (appositional growth). that subsequently lead to bone formation and are regarded as osteoprogenitor cells.

59 Structurally, they closely resemble the mesenchymal lamellae are randomly arranged. These are called cells from which they arise, and their identification primary osteons. Between the plates of compact bone, depends mostly on their proximity to forming where spongy bone persists, thickening of the bone. trabeculae ceases and the intervening connective The osteoprogenitor cells enlarge and transform tissue becomes the blood-forming marrow. into osteoblasts that lay down layers of matrix. The connective tissue surrounding the developing The plasmalemma of osteoblasts has receptors for bone condenses to form the periosteum. The parathyroid hormone and 1, 25-dihydroxyvitamin osteoblasts on the outer surface of the bone assume D3 and can be stimulated by these molecules. The a fibroblast-like appearance and are incorporated osteoblasts remain in contact with one another by into the inner layer (cambium) of the periosteum, long tapering processes, and as more and more where they persist as potential bone-forming cells. matrix is deposited, the cells and their processes Similarly, osteoblasts on the inner surface and those become entrapped in the matrix. The cells then are covering the trabeculae of spongy bone are called osteocytes. Ultimately the cells and cell incorporated into the endosteum and also retain their processes are completely enclosed in the matrix, potential for producing bone. thus forming the lacunae and canaliculi. The first matrix consists of type I collagen fibers embedded Endochondral Bone Formation in a glycosaminoglycan gel that contains specific glycoproteins and, being unmineralized, is soft. VOCABULARY: primary ossification center, At this stage, the matrix is called osteoid; after a periosteal collar, periosteal bud, zone of reserve short lag period, it becomes mineralized to form cartilage, zone of proliferation, zone of true bone. Osteoblasts elaborate osteocalcin and maturation and hypertrophy, zone of alkaline phosphatase which hydrolyzes phosphate- calcification and cell death, zone of and calcium-containing substrates to release ossification, zone of resorption, epiphyseal phosphate and calcium. In addition, osteoblasts (secondary) ossification center, epiphyseal release matrix vesicles which concentrate calcium plate, primitive osteon, definitive osteon and phosphate during the mineralizing process. Blood alkaline phosphatase levels are good The bones of the vertebral column, pelvis, indicators to access not only osteogenesis but also extremities, face, and base of the skull develop by bone repair. As osteoblasts become trapped in the formation of bone in a cartilage model or precursor newly deposited matrix and become osteocytes, that must be removed before bone can be laid new osteoblasts are recruited from differentiation down. Endochondral bone formation involves of osteoprogenitor cells. simultaneous formation of the bone matrix and Bone development by intramembranous removal of the cartilage model. The cartilage does ossification occurs in several foci throughout the not contribute directly to the formation of bone, mesenchymal field and results in the formation of and much of the complex process involved in scattered, irregular trabeculae of bone that increase endochondral bone formation is concerned with in size by addition of new bone to their surfaces. removal of cartilage. The resulting bone is of the spongy type. In the The first indications of ossification of a long bone first formed bone, collagen fibers run in every appear in the center of the cartilage model in the direction; the bone is called woven bone and has region destined to become the shaft or diaphysis. In randomly scattered osteocytes and lacks a lamellar this area, called the primary ossification center, arrangement. Woven bone is gradually replaced by the chondrocytes hypertrophy and their cytoplasm lamellar bone. becomes vacuolated. Lacunae enlarge at the In areas that become compact bone, such as the expense of the surrounding matrix, and the cartilage inner and outer tables of the skull, the trabeculae between adjacent lacunae is reduced to thin, continue to thicken by appositional growth, and the fenestrated partitions. The matrix in the vicinity of spaces between them are gradually obliterated. As the hypertrophied chondrocytes becomes calcified bone encroaches on vascular spaces, the matrix is by deposition of calcium phosphate. Diffusion of laid down in irregular, concentric layers that nutrients through the calcified matrix is reduced, surround blood vessels and come to resemble and the cells undergo degenerative changes leading osteons, except that the collagen fibers in the to their death.

60 While these events take place in the cartilage, the Zone of proliferation. In this region the vascular perichondrium in this area assumes an chondrocytes actively proliferate, and as the cells osteogenic function. A layer of bone, the periosteal divide, the daughter cells become aligned in rows collar, forms around the altered cartilage and separated by a small amount of matrix. Within the provides a splint that helps maintain the strength of rows, the cells are separated only by thin plates of the shaft. The osteoblasts responsible for matrix. Growth occurs mainly in the regions development of the bone are derived from farthest from the reserve cartilage, and the length is perichondrial cells immediately adjacent to the increased more than the diameter. cartilage. In the region of the boney collar, the perichondrium now has become a periosteum. Blood Zone of maturation and hypertrophy. Cell vessels from the periosteum invade the altered division ceases and the cells mature and enlarge, cartilage forming the periosteal bud. Contained further increasing the length of the cartilage. within the connective tissue sheath that Lacunae expand at the expense of the intervening accompanies the blood vessels are cells with matrix, and the cartilage between adjacent cells in osteogenic properties. the rows becomes even thinner. With death of the chondrocytes there is no longer a means of maintaining the matrix, which softens. Zone of calcification and cell death. The matrix With the aid of the erosive action of invading blood between the rows of chondrocytes becomes vessels, the thin plates of matrix between lacunae calcified and the cells die, degenerate, and leave break down. As the adjoining lacunar spaces are empty spaces. The thin plates between the cells opened up, narrow tunnels are formed that are break down, and the lacunar spaces are opened up separated by thin bars of calcified cartilage. Blood to form irregular tunnels bounded by the remains vessels grow into the tunnels, bringing in of calcified cartilage that had been present between osteogenic cells that align themselves on the surface the rows of cells. of the cartilaginous trabeculae, differentiate into osteoblasts, and begin to elaborate bone matrix. At Zone of ossification. Vascular connective tissue first the matrix is laid down as osteoid, but it soon invades the tunnel-like spaces and provides becomes mineralized to form true bone. The early osteogenic cells that differentiate into osteoblasts. trabeculae thus consist of a core of calcified These gather on the surface of the calcified matrix cartilage covered by a shell of bone. These primitive and lay down bone. trabeculae soon are removed both by continued resorption of the calcified cartilage and through the Zone of resorption. Osteoclasts appear on the activity of osteoclasts that appear on the boney trabeculae and begin to resorb the bone. Ultimately shell. An expanding cavity is opened up within the all the calcified cartilages and their boney coverings developing shaft. Support for the bone is provided are resorbed, and the marrow cavity increases in by extension of the periosteal collar, which also size. becomes thicker as the periosteum lays down new As these changes occur within the cartilage, the bone at the surface. periosteal collar increases in length and thickness, This entire process continues as an orderly gradually extending toward the ends of the progression of activity that extends toward both developing bone to provide a continuous splint ends of the developing bone. Several areas or zones around the area of changing and weakening of activity can be distinguished in the cartilage. cartilage. Beginning at the ends farthest from the primary At about the time of birth, new centers of ossification center, these are the following: ossification, epiphyseal, or secondary ossification centers, appear in the ends of long Zone of reserve cartilage. This area consists of bones. The cartilage of the epiphyses passes hyaline cartilage and initially is a long zone but through the same sequences as in the diaphysis, but shortens as the process of ossification encroaches cartilage growth and subsequent ossification spread on it. The cells and their lacunae are randomly in all directions and result in a mass of spongy arranged throughout the matrix, and there is slow bone. Ultimately, all the cartilage of the epiphysis is growth in all directions. replaced by bone except for that at the free end, which remains as articular cartilage.

61 Cartilage also persists between the epiphysis and diaphysis and forms the epiphyseal plate, whose continued growth permits further elongation of the bone. Ossification occurs at the diaphyseal side, and for a time, formation of new cartilage and its replacement by bone take place at the same rate so that the thickness of the epiphyseal plate remains constant. Eventually, growth in the plate ceases, the cartilage is replaced by bone (closure of the epiphysis), and further increase in length of the bone is no longer possible. Increase in diameter of the bone results from deposition of bone at the outer surface as a result of the activity of the Fig. 5-3. Replacement of osteons during remodeling of compact bone. periosteum, while at the same time osteoclasts resorb lesser amounts of bone at the endosteal surface. Thus, not only is the gross diameter increased, the thickness of the compact bone and the size of the marrow cavity also increase. Bone formed by the periosteum represents intramembranous bone and results in a lattice of irregular, boney trabeculae. This is converted to compact bone by a gradual filling of the spaces between trabeculae. Bone is laid down in ill-defined layers, and where they surround and enclose blood vessels, the first Haversian systems or primitive osteons are formed, in the same way that they form during development of membranous bone. In the primitive osteons the collagen is laid down at random in each lamella. Constant remodeling and reconstruction of bone occurs throughout life, with the primitive osteons being replaced by definitive osteons; these in turn are replaced by successive generations of osteons. Replacement of osteons begins with formation of tunnels within the compact bone as a result of the osteolytic activity of osteocytes and osteoclasts. The Haversian canals widen and enlarge to reach considerable lengths. When osteolytic activity ceases, osteoblasts become active recruited from Fig. 5-4. Diagrammatic representation of the successive changes during the development of a long bone. osteoprogenitor cells within the endosteum, and bone is deposited on the walls of the formed cavity The successive changes occurring during until it is refilled as a definitive osteon. The process development of a long bone are summarized in continues throughout life; resorption cavities Figure 5-4. The cartilage model (A) becomes continue to form and to be filled with third, fourth, encircled by a periosteal collar in the region that will and higher generations of osteons. The resorption become the shaft (B), and the cartilage matrix in cavities do not always coincide with the osteon this area calcifies and degenerates (C). A vascular being replaced, and fragments of older osteons bud of connective tissue and blood vessels remain as interstitial lamellae. Replacement of osteons penetrates the altered cartilage, which is resorbed, and the remodeling of bone are shown in Figure 5- and the cartilage remnants become covered by bone 3. to form irregular trabeculae.

62 These in turn are soon resorbed, and an expanding Synovial Joints marrow-filled cavity develops as the process extends toward each end (D). Secondary The formation of appendicular bones is heralded by ossification centers appear in the epiphyses, development of cartilage in the center of each gradually expanding in all directions (E). Cartilage future skeletal element. As the cartilage models persists at the ends of the bone and, for a time, as expand, their ends approach one another, and the the epiphyseal plate from which all further intervening undifferentiated tissue condenses to lengthening will occur (F). Cartilage in the form the interzonal mesenchyme or midzone from epiphyseal plate ceases its growth and is replaced by which the joint will form. Simultaneously, the outer bone. The marrow cavity then becomes continuous layer of mesenchyme also condenses to form the throughout the length of the bone (G). primitive joint capsule, which is continuous with Cartilage, because of its capacity for rapid growth the mesenchyme that gave rise to the as compared with bone, is the ideal rigid, perichondrium over the adjacent cartilage model. lightweight tissue for the embryonic skeleton The mesenchyme enclosed by the primitive joint because it can keep pace with the rapidly growing capsule forms three distinct zones: a central, loose fetus. Likewise, it is growth of the hyaline cartilage layer of randomly arranged cells lying between two in the epiphyseal plate that allows for the rapid dense layers in which the cells are parallel to the growth of long bones during adolescence. surface of the underlying cartilage. The dense zones Ultimately, cartilage of the provisional skeleton, as give rise to the articular cartilages. well as that of the epiphyseal plate, is replaced by Development of the joint cavity begins in the the slower-growing bone. loose, central region of the interzone with the appearance of small, fluid-filled spaces between the Joints mesenchymal cells. The spaces coalesce and come to form a continuous cavity separating the dense Fibrous Joints chondrogenic layers and extending along the margins at the ends of the cartilage models. Fibrous joints form simply by the condensation and As the spaces unite, the primitive joint capsule differentiation of the mesenchyme that persists completes its differentiation and forms two layers. between developing bones, resulting in formation The outer layer becomes the dense, irregular of a dense irregular connective tissue that binds the connective tissue of the joint capsule proper, while bones together. the inner layer becomes cellular and gives rise to the synovial membrane. Menisci and articular discs Cartilaginous Joints develop from the mesenchyme that gave rise to the fibrous joint capsule. In the early stages of development, a symphysis is represented by a plate of mesenchyme between Repair of Cartilage and Bone separate cartilaginous rudiments of bone; the mesenchyme differentiates into fibrous cartilage. Cartilage When the adjacent cartilaginous bone models have ossified, thin layers of hyaline cartilage are left Because of its avascularity, mammalian cartilage has unossified and persist as laminae of hyaline cartilage a limited capacity to restore itself after injury. between the bone and fibrous cartilage. Damaged regions of cartilage become necrotic, and The nucleus pulposus of an intervertebral disc these areas then are filled in by connective tissue develops from the notochord. Early in embryonic from the perichondrium. Some of the connective life, notochordal cells in the regions between future tissue may slowly differentiate into cartilage, but vertebrae proliferate rapidly but later undergo most remains as dense irregular connective tissue mucoid degeneration to give rise to the gelatinous that may later calcify or even ossify. nucleus pulposus. The surrounding fibrocartilage of the annulus fibrosus is derived from the mesenchyme between adjacent vertebrae.

63 Bone fracture site the cells differentiate into chondroblasts rather than osteoblasts and lay down Repair of a broken bone is influenced by the size of a callus of hyaline cartilage. This also bridges the the bone, the thickness of its compact bone, and fracture gap to form a stabilizing splint around the the complexity of the fracture. However, the fracture. The cartilage is converted to bone by underlying process of repair remains the same and endochondral bone formation, but the process is in general recalls the events of bone formation. self-limiting, and all the cartilage disappears without A fracture ruptures blood vessels in the marrow, continuous formation of new cartilage as in an periosteum, and the bone itself, and bleeding may epiphyseal plate. Cartilage always appears during the be extensive. As a result of the vascular damage, repair of long bones, whereas flat bones heal bone dies for some distance back from the fracture without cartilage formation. site, as do the periosteum and marrow. However, Union of the compact cortical bone occurs from the latter have a greater blood supply than bone sources arising in the medullary cavity or from the tissue itself, so the area of cell death in these tissues periosteum. Since the ends of the bone at the is not as great. The blood clot that forms is fracture line consist of dead bone, direct union of removed by lysis and phagocytosis. Fibrovascular the fractured ends is very rare if it occurs at all. invasion of the clot immediately around and within Occasionally the gap is filled by formation of the fracture and its conversion to a fibroconnective hyaline cartilage, which then undergoes tissue are not prominent phenomena in humans. endochondral ossification to achieve cortical union. Repair occurs by activation of osteogenic cells in More frequently, the bone that initially unites the the viable endosteum and periosteum near the broken ends is a network of woven bone formed by fracture site. The cells proliferate and form new intramembranous bone formation. The last act in trabeculae of bone within the marrow cavity and repair is the resorption of excess bone and the beneath the periosteum. remodeling of newly formed and dead bone that is Repair occurring within the marrow cavity can be replaced by lamellar bone. Bone morphogenic proteins, very important in human fractures. Medullary which belong to the TGF-β superfamily of healing begins as foci of vascular and fibroblastic proteins, are involved in regulating bone formation proliferations in the viable tissue that borders the and repair. Bone morphogenic proteins act at all the damaged marrow, followed by osteogenic activity important steps in the cascade of events that form and bone formation. Ultimately, a network of fine new bone: chemotaxis of progenitor cells, mitosis, bony trabeculae extends across the marrow cavity differentiation and proliferation of chondrocytes from cortex to cortex on either side of the fracture and osteoblasts, stimulation of extracellular matrix and finally across the fracture line, providing an formation and binding to specific matrix molecules. internal scaffold until union of the fractured ends can be effected. All this new bone arises by Summary intramembranous bone formation and is woven bone. Medullary bone healing is particularly Cartilage serves as a rigid yet lightweight and important for the union of fractures in cancellous flexible supporting tissue. It forms the framework bones such as the vertebral bodies and lower end of for the respiratory passages to prevent their the radius and fractures through the metaphysis of collapse, provides smooth "bearings" at joints, and long bones. forms a cushion between the vertebrae, acting as a On the periosteal surface, the repair process arises shock absorber for the spine. Cartilage is important from the inner osteoblastic layer of the periosteum in determining the size and shape of bones and (osteogenetic layer) beginning a short distance from provides the growing areas in many bones. Its the fracture zone. Periosteal proliferation occurs on capacity for rapid growth while maintaining both sides of the fracture gap, resulting in collars of stiffness makes cartilage suitable for the embryonic bony trabeculae that grow outward and toward each skeleton. About 75% of the water in cartilage is other, ultimately fusing to span the gap in a bound to proteoglycans, and these compounds are continuous arch. The new trabeculae are firmly important in the transport of fluids, electrolytes, attached to the old bone surface, including the dead and nutrients throughout the cartilage matrix. bone. Growth of the osteogenic cells outstrips their vascular supply so that in the midzone of the

64 Bone forms the principal tissue of support and is paramount. Even the articular cartilages of synovial capable of bearing great weight. It provides joints serve as areas of growth for subchondral attachment for muscles of locomotion, carries the bone during development. The cartilaginous joints joints, serves as a covering to protect vital organs, of the spine, although permitting some degree of and houses the hemopoietic tissue. Bone is the motion, serve primarily as shock absorbers to major storehouse of calcium and phosphorus in the dampen and distribute mechanical forces acting on body. the spine. This is mainly the function of the nucleus Osteocytes are the dominant cells of mature pulposus, which, because of its high water content, bones and are responsible for maintaining the can act as a water cushion. matrix. They also aid in regulating the calcium and The joints specifically designed for movement are phosphorus levels of the body and play a role in the the synovial joints. The fibrous capsule and resorption of bone. Osteoclasts are specifically ligaments provide a tough, unyielding binding for implicated in bone resorption. They destroy the the articulating bones yet remain flexible enough to ground substance and collagen and release minerals allow movement of the joint. The synovial from the matrix. These cells elaborate lysosomal membrane mainly produces the lubricant of the enzymes and contain high concentrations of citrate, joint, the synovial fluid, but also may participate in which is involved in mobilizing calcium from bone. removing particulate matter from the joint cavity. The initial stage of bone resorption by osteoclasts is Synovial villi increase the surface area for secretion extracellular: glycosaminoglycans of the matrix are and absorption, while the larger folds serve as degraded, permitting fragmentation of the bone. flexible pads that can accommodate to the changing The fragments are phagocytosed by osteoclasts and shape and size of the joint space and its recesses digested intracellularly. The ruffled border of the during movement. The function of the articular osteoclasts increases the surface area and seals off discs and menisci is uncertain, but they may act as the area of resorption and allows a local shock absorbers, improve the fit between the joint environment conducive to the digestion of bone. surfaces, spread lubricant, distribute weight over a Longitudinal growth of a developing bone larger area, or protect the edges of the articular depends on the interstitial growth of cartilage in the cartilages. zone of proliferation of the epiphyseal plate and on Cartilages form the bearings on which the the enlargement of chondrocytes in the zone of articulating surfaces of the joint move and offer a maturation and hypertrophy nearby. Cartilage of the wear-resistant surface of low friction - about half epiphyseal plate is under the influence of growth that of Teflon, the most slippery of synthetic hormone secreted by cells in the anterior pituitary materials. The arrangement of fibers in the gland. Thyroid hormone modulates the activity of tangential zone places a dense mat of collagen growth hormone, and at puberty, androgens and fibers right at the articulating surface. The cartilage estrogens contribute to masculinization or is anchored to the underlying bone by collagen feminization of the skeleton, respectively. Increased fibers (Sharpey's fibers) that are embedded in width of the bone is achieved by appositional subchondral bone. The irregular and interlocking growth and the deposition of bone at the boney margin between the calcified zone and bone further collar. The first primitive trabeculae observed stabilizes the cartilage. The calcified zone also may during endochondral bone formation consist of help regulate diffusion of materials between bone cores of cartilage covered by bone. They are formed and cartilage. in the region where the boney collar is thinnest and Synovial fluid provides for the nutrition of serve as internal struts to support the developing articular cartilages, menisci, and articular discs and bone until the collar is thick enough to provide also lubricates the joint surfaces. Exactly how the support. As the collar increases in thickness, synovial fluid performs its lubricating function is osteoclasts remove bone at the endosteal surface, unknown. It has been suggested that the fluid thereby enlarging the marrow cavity. forms a thin film between the bearing surfaces and Although most joints serve to allow movement, that the lubricating activity depends on internal sutures are immobile and act only to unite bones. properties of the fluid such as cohesiveness, shear Sutures in the young provide areas where bone can rate, and flow rate. Another mechanism, "weeping grow, and it is only after they have been replaced by lubrication," has been proposed as a method of joint boney unions that their uniting functions become lubrication.

65 Because of the content of proteoglycans, articular As the cartilage is compressed, pools of synovial cartilage avidly binds water that, during fluid are trapped in the furrows and valleys of the compression, exudes ("weeps") from the cartilage articular surfaces. As compression increases, a to lubricate the surface. When compression is mobile component of the fluid containing small removed, fluid is reabsorbed into the cartilage. molecules is forced into the cartilage at the areas of Thus, weeping lubrication also could serve as a pressure. The fluid in the valleys thus becomes method for pumping nutrients and wastes into and increasingly rich in hyaluronidate and more viscous out of the cartilage. so that as the pressure increases, the synovial fluid A mechanism of "boosted lubrication" takes into progressively becomes more effective as a lubricant. account the irregularities normally seen on the surface of articular cartilage.

66 colloidal osmotic (oncotic) pressure, is formed in 6 Special Connective Tissue: the liver, as is fibrinogen, an essential component Blood for blood clotting. The globulins (alpha, beta, and gamma) include several proteins of different sizes. Blood often is defined as a special connective tissue Globulins participate in the transport of hormones, in which the intercellular substance is a fluid. Unlike ions, metals and lipids. The gamma globulins are other connective tissues, however, the intercellular immunoglobulins (antibodies) synthesized by cells substance of blood lacks a fibrous component, and of the lymphatic organs and tissues. The plasma most of the intercellular protein is produced by cells also contains hormones from various endocrine in other tissues (chiefly the liver) and not by the organs, metabolites, nutrients and several other blood cells. Many of the formed elements of blood substances. A partial list of normal values of other consist of anucleate elements (erythrocytes) and bits substances found in human blood is provided in of cytoplasm (platelets). True cells - leukocytes - Table 5-1. make up only a small part of the formed elements Plasma is obtained from blood after treatment with and are present only as transients that use the blood an anticoagulant and contains all the components for transportation to other organs and tissues into of the fluid portion of blood. It makes up about which they migrate to carry out their functions. 55% of the blood volume. In contrast, serum is Only erythrocytes and platelets function while in obtained from clotted or defibrinated blood and the bloodstream. None of the formed elements does not contain fibrinogen; it does contain other normally replicate within the blood; as they are lost, components elaborated during the process of blood new elements are added from special blood- clotting. forming tissues outside the circulation. Thus, blood can be considered the secretory product of several Table 5-1. A partial list of normal values organs and tissues rather than a tissue per se. of human peripheral blood. The total quantity of blood forms a rather constant proportion of the body mass. In humans pH (arterial) 7.35-7.45 the volume is about 5 liters, or approximately 7.5% Osmolality 280-296 of body weight. mOsm/kgH2O O2, saturated 96%-100% Components of Blood (arterial) CO2 24-30 mEq/liter PCO2 (arterial) 35-45 mmHg KEY TERMS: formed elements, plasma, Total protein 6.0-8.4 g/100 ml erythrocyte, platelet, leukocyte, albumin, Albumin 3.5-5.0 g/100 ml fibrinogen, globulin, serum Globin 2.3-3.5 g/100 ml

Blood consists of formed elements that include Cholesterol 120-220 mg/100 ml erythrocytes, platelets, leukocytes, and a fluid Triglycerides 40-150 mg/100 ml intercellular material called plasma. These elements Lactic acid 0.6-1.8 mEq/liter can be separated by centrifugation, and when done Ascorbic acid 0.4-1.5 mg/100 ml in calibrated tubes, the result (hematocrit) gives an Uric acid 3.0-7.0 mg/100 ml estimate of the volume of the formed elements. Glucose (fasting) 70-110 mg/100 ml The heaviest components, erythrocytes, form the Sodium 135-145 mEq/liter lower layer and make up about 45% of the blood Potassium 3.5-5.0 mEq/liter volume. Platelets and leukocytes are present in Chloride 100-106 mEq/liter the buffy coat, a grayish white layer immediately Calcium 8.5-10.5 mg/100 ml above the erythrocytes, and form about 1% of the Phosphorus 3.0-4.5 mg/100 ml total blood volume. The uppermost layer consists Magnesium 1.5-2.0 mEq/liter of plasma, a proteinaceous solution, which contains Iron 50-150 µg/100ml three main types of protein: albumin, globulin, and fibrinogen. Albumin, the most abundant and smallest of the plasma proteins maintains blood

67 Erythrocytes Shape and Size

At rest, the average human uses about 250 ml of VOCABULARY: biconcave disc, central oxygen and produces almost 200 ml of carbon depression, cell membrane, spectrin, ankyrin dioxide per minute. With activity, these quantities can increase 10- to 20-fold. Oxygen and carbon The human red cell usually is described as a dioxide are carried by the erythrocytes, which biconcave disc. Face on, the normal erythrocyte transport the gases with great efficiency. Exchange presents a smooth, rounded contour with a central of gases occurs in the blood capillaries of the lung, depression. In smears, human erythrocytes average through which the erythrocytes pass in less than 1 7.6 µm in diameter, are slightly smaller in tissue second, yet gaseous exchange is complete in sections (6.5-7.0 µm), and are slightly larger (about approximately the first third of this time. 8 µm ) in fresh blood. The thickness of the red cell is about 2 µm. Apparently this thickness allows Structure hemoglobin to maintain a distance from the cell surface that is optimal for its function. VOCABULARY: anucleate, hemoglobin, Erythrocytes are enclosed in a typical cell absence of organelles membrane, whose flexibility and elasticity allow the red cell to accommodate its passage through the In blood smears and tissue sections, erythrocytes small capillaries. In vivo, the red cells often assume a appear as anucleate, uniformly acidophilic bodies cup shape as they pass through small blood vessels. devoid of any internal structure. The pigment A subplasmalemmal network of protein (spectrin) hemoglobin makes up about 95% of the dry helps to maintain the biconcave shape and still weight. Erythroplastid more aptly describes these allows flexibility. Spectrin, linked by actin, appears elements, but custom and common use has given to form a web immediately beneath the the terms erythrocyte and red cell the status of proper plasmalemma and may act as a cytoskeleton. The terminology. Electron microscopy confirms the network is attached to the interior of the cell absence of organelles in the interior of mature red membrane by the protein ankyrin so that the cells, which are unable to synthesize protein or cytoskeleton and cell membrane are linked to act as renew constituents of their cell membrane. The a unit. Hemoglobin also may play a role in cholesterol of the plasmalemma, important for the maintaining cell shape, since marked changes in flexibility of the cell, is controlled by the plasma shape are associated with the abnormal hemoglobin concentration of cholesterol, not by cell of sickle cell anemia. metabolism. Carbohydrate chains extending from glycoproteins Number and Survival and glycolipids of the erythrocyte plasmalemma contain a number of antigenic determinants that are Consistent sex differences in the number of red the basis for the ABO blood-group system. The cell cells are seen, with lower values occurring in membrane limiting erythrocytes of some individuals women. Values of 4,500,000 to 6,000,000/mm3 in may have either antigen A (type A blood), antigen B men and 3,800,000 to 5,000,000/mm3 in women (type B blood), or both antigens A and B (type AB are considered normal. blood). Erythrocytes of other individuals may lack The survival time of red cells is about 120 days. both antigens, and such individuals are classified as Although red cells generally are removed as they having type O blood. Most individuals have age and wear out, a certain amount of random antibodies in their blood plasma against erythrocyte destruction also occurs; many red cells are antigens, with the exception of their own, and if a destroyed in the bone marrow without ever being different blood type is transfused an immune released. reaction will occur. Therefore, it is essential to As erythrocytes age, they use up most of the determine what antigens are present on the enzymes associated with adenosine triphosphate erythrocytes of donor blood and what antibodies (ATP) and are unable to maintain themselves, are present in the plasma of the recipient before synthesize protein, or renew their cell membranes. giving a transfusion. By the end of their life span, red cells have become rigid due to loss of cholesterol from the cell

68 membrane and to degradation of protein and its may be macrocytes (larger than normal) or cross-linkage with calcium. These aged red cells are microcytes (smaller than normal). In macrocytes trapped and destroyed by phagocytes mainly in the the central pale area is less marked than in normal spleen but also in the liver and bone marrow. cells, but the concentration of hemoglobin is not About 1% of circulating erythrocytes are destroyed increased. daily. Iron in hemoglobin is recovered by the liver, Irregularity in shape is called poikilocytosis; the stored, and recycled to new red cells. cells may show blunt, pointed, or hook-shaped projections from their surfaces. Under various Reticulocytes conditions, red cells in vitro may become shrunken and show numerous projections on their surfaces; VOCABULARY: polychromatophilic cell, these cells are said to be crenated and are called ribonucleoprotein echinocytes. Crenated cells can be produced by subjecting normal red cells to fatty acids, anionic Most erythrocytes stain an orange-red with the compounds, elevated pH, or hypertonic media. The usual blood stains, but a few take on a bluish or changes are reversible. One of the most severe slate gray tint. These polychromatophilic cells are changes in shape occurs during sickling of red cells erythrocytes that are not fully mature and contain a in sickle cell anemia, in which erythrocytes appear as small amount of ribonucleoprotein that, when crescents, holly leaves, or even tubes. stained with brilliant cresyl blue or new methylene Hypochromia denotes cells that stain poorly due blue, precipitates as a network or web. These cells to a decrease in hemoglobin; it frequently are called reticulocytes. Their numbers in peripheral accompanies microcytosis. In extreme forms, blood provide a rough index of erythrocyte staining may consist simply of a narrow peripheral production. Normally, reticulocytes make up only 1 band. Cells that are thinner than normal (leptocytes) to 2% of the red cells in peripheral blood. often appear as target cells in which the staining is disposed as a central disc and an external band Rouleaux separated by an unstained zone. Howell-Jolly bodies are nuclear fragments left Erythrocytes tend to adhere to each other by their over from the nucleated precursors of the red cell. broad surfaces to form stacks called rouleaux. They appear as one or two rounded or rod-shaped Rouleaux depend on changes in the blood plasma basophilic granules. rather than in the cell. Any condition that increases Siderocytes contain clusters of small, darkly the net positive charge in the plasma produces stained granules that react positively for iron. They changes in the surface charge of erythrocytes, are rare in normal peripheral blood but are quite allowing them to adhere to each other more readily. common in bone marrow. Normally the iron- Rouleaux are temporary phenomena, may occur containing granules are removed by the spleen intravascularly, and appear to do no harm to the red without harming the cell. cells. Increased rouleaux are reflected in an increase Allied to polychromatophilia is basophilic in the rate at which red cells settle out or sediment. stippling, which appears as blue-black dots that may be coarse or fine and consists of precipitated Abnormalities ribonucleoprotein. It is found occasionally in leukemia and severe anemias but is of diagnostic VOCABULARY: anisocytosis, macrocyte, importance in chronic lead poisoning. microcyte, poikilocytosis, crenated, sickling, In some pathologic conditions, globular rather hypochromia, target cell, Howell-Jolly bodies, than bi-concave red cells are produced. These are siderocyte, basophilic stippling, spherocyte called spherocytes and appear as small, deeply stained cells with a sharp, distinct outline. Departures from normal size, shape, or staining properties of erythrocytes can be important Hemoglobin indicators of disease, but to a much lesser degree, some of these abnormalities may be found in Erythrocytes contain both hemoglobin and the healthy individuals also. Anisocytosis describes enzyme, carbonic anhydrase. Oxygen is not soluble abnormal variations in the size of red cells, which and transport is carried out by hemoglobin, an iron-

69 containing respiratory pigment that gives the red Even minor modifications of the peptide chain color to human blood. In contrast, 90% of the can result in gross changes in the entire hemoglobin carbon dioxide is transported as bicarbonate in the molecule, as shown by the sickle hemoglobin plasma. Dissolved carbon dioxide (CO2) enters the (HbS) associated with sickle cell anemia. In this erythrocyte and reacts with water to form carbonic condition the only fault in the molecule is in the acid (H2CO3). Carbonic anhydrase within the sixth position of the beta chain, where the glutamic erythrocyte immediately converts the formed acid of normal hemoglobin has been replaced by carbonic acid to hydrogen ion (H+) and bicarbonate valine. - (HC03 ). Once formed the bicarbonate leaves the erythrocyte in exchange for chloride ion (Cl-). On Platelets reaching the lung bicarbonate passes back into the erythrocyte, combines with hydrogen ion carried Platelets are the second-most numerous of the and buffered by the hemoglobin to form carbonic formed elements of blood. Platelets are not cells acid which is catalyzed by carbonic anhydrase back but merely fragments of cytoplasm derived from a to carbon dioxide and water. The former then large precursor cell (megakaryocyte) in the bone diffuses from the erythrocyte into the alveoli of the marrow. lung to be exhaled. Structure Structure VOCABULARY: granulomere (chromomere), VOCABULARY: heme, globin, HbA, HbA2, hyalomere, actin, myosin, marginal bundle HbF, sickle hemoglobin (HbS) Platelets are small, anucleate bodies 2 to 5 µm in Hemoglobin is one of a group of catalytic diameter. In stained smears, platelets show a compounds that has an iron porphyrin group, granular part, the granulomere, and a pale, granule- heme, attached to a protein, globulin. The heme free hyalomere. group consists of four pyrrole rings combined with The granulomere (chromomere) often fills the iron through their N groups. Structurally, central region of the platelet and may be so hemoglobin is a symmetrical molecule formed by compact as to suggest a nucleus. Separation into two equal mirror-image halves. Each half molecule two zones is not seen in circulating platelets nor in contains two different peptide chains, each bearing those fixed immediately by drawing blood into a heme group around which the chain is coiled. The fixative. In these conditions, the granulomere heme groups form the functional component, and remains evenly distributed. Electron micrographs the peptide chains make up the globin part of the show that the granulomere consists of lysosomes, molecule. The amino acid composition and mitochondria, dense-cored granules that contain sequences in the globin confer species specificity to serotonin, adenosine diphosphate (ADP), ATP, the whole molecule and also determine the type of calcium, alpha particles that contain platelet-specific hemoglobin present. Humans synthesize four proteins, fibrinogen and other clotting factors, structurally different globin chains: alpha, beta, actin, myosin, adenosinetriphosphatase (ATPase), gamma, and delta. About 95% of normal adult and agents that increase vascular permeability. hemoglobin is hemoglobin A (HbA), which Mitochondria are small and few in number and contains two alpha and two beta chains: about 2% have few cristae. Variable numbers of glycogen is HbA2, which consists of two alpha and two delta granules also are present. chains. The remainder, fetal hemoglobin (HbF), is The hyalomere represents the cytoplasmic matrix composed of two alpha and two gamma chains. and appears as a homogeneous, finely granular Fetal hemoglobin predominates in fetal life but is background. A crisscross arrangement of actin and replaced by the adult type postnatally. Fetal myosin filaments is present just beneath the hemoglobin has a higher oxygen affinity than adult plasmalemma. A narrow peripheral zone remains hemoglobin and therefore has the capacity to pull free of granulomere elements and is the site of a oxygen from the erythrocytes of maternal blood system of microtubules that forms the marginal and into the fetal circulation during the pretnatal bundle. This structure has been described as a period.

70 single, coiled microtubule that acts as a stiffening leukocytes contain multilobed nuclei and thus the element to help maintain the discoidal shape. terms granular leukocyte and polymorphonuclear leukocyte Each platelet is bounded by a typical cell membrane denote the same class of cell. that limits the cytoplasm and becomes continuous Granular leukocytes (or granulocytes) can be with an elaborate system of channels. subdivided into neutrophils (heterophils), eosinophils, or basophils according to the color Number of their cytoplasmic granules after staining with standard blood stains. Heterophil granules do not The number of platelets varies widely depending on stain the same in all species, so the term is used the species and in individuals also. The variations inclusively for this type of granular leukocyte, reflect difficulties in obtaining accurate counts due regardless of staining reaction or species. The term to physiologic factors that affect the numbers and neutrophil refers to the heterophil of humans. to inherent properties of the platelets, such as their Agranular leukocytes consist of lymphocytes and tendency to stick to foreign surfaces or to each monocytes, which have inconspicuous or no other to form clumps. When in contact with a granules and a single, non-lobed nucleus. Thus, the foreign surface, platelets spread to cover an area terms mononuclear leukocytes and agranular leukocytes are several hundred times that of their initial surface. used to denote the same class of cells. The Although platelet numbers in humans are given as distinction between granular and agranular 250,000 to 400,000/mm3 of blood, some estimates leukocytes is not absolute: monocytes regularly extend the range to 900,000/mm3. Transient show fine, dustlike granules that are barely fluctuations in platelets have been associated with resolvable by light microscopy, and lymphocytes variation in oxygen concentration, exposure to occasionally may show a few coarse granules. The periods of cold, and food intake. Curiously, highly distinction is an old one based on appearances with spiced foods have been reported to cause a less refined dyes but remains useful because the significant reduction in circulating platelets. granules of granular leukocytes are specific, Emotional states such as fear and rage result in distinctive, and prominent. markedly elevated counts. A progressive decrease in the number of platelets occurs in women during the Neutrophil (Heterophil) Leukocytes two weeks prior to menstruation, with a rapid return to normal values thereafter. A severe VOCABULARY: lobe, filament, band form, reduction in the number of platelets in circulating drumstick, specific granule, lysozyme, blood is referred to as thrombocytopenia. lactoferrin, phagocytin, azurophil granule, lysosome Leukocytes The neutrophil granulocyte varies from 12 to 15 µm Leukocytes (white blood cells, WBCs) are true cells in diameter and is characterized by the shape of the that have nuclei and cytoplasmic organelles and are nucleus, which contains small lobes connected by capable of ameboid movement. They migrate from fine filaments. The nucleus stains deeply, and the the blood into tissues, where they perform their chromatin is aggregated into clumps that form a functions. patchy network. Nucleoli are absent. The number of lobes varies, but in humans, two to Classification four are usual. A single, elongated, non-lobed nucleus is seen in a small number of granulocytes; VOCABULARY: granular, agranular, called band forms, they represent cells recently polymorphonuclear, mononuclear, neutrophil released from the bone marrow. Excessive lobation (heterophil), eosinophil, basophil, lymphocyte, occurs in some diseases or as an inherited anomaly monocyte in humans. In addition to nuclear lobation, nuclear Leukocytes can be classed as granular or appendages in the shape of hooks, hand racquets, agranular on the basis of cytoplasmic granulation clubs, or drumsticks may be present. Only the or as polymorphonuclear or mononuclear drumstick is significant: it represents female sex according to the shape of the nucleus. Granular

71 chromatin and usually is found on a terminal lobe Neutrophils are the chief leukocytes in humans, in about 2 to 3% of the neutrophils from women. forming 55 to 70% of the circulating white blood The cytoplasm of neutrophils contains three types cells. Neutrophils circulate in a resting state. When of granules. Most numerous are the specific they are activated they are short lived and die when granules (secondary, type B granules), which are their lysosomal enzymes are released which also small and take on a pinkish hue. These granules causes liquefaction of adjacent tissues. The primary contain lysozyme, an enzyme complex that acts function of neutrophils is to engulf and/or destroy against components of bacterial cell walls, alkaline bacteria. phosphatase, collagenase, and lactoferrin, another antibacterial substance. Several other rather poorly Eosinophil Leukocytes characterized basic proteins (phagocytins) are present and also have antibacterial properties. VOCABULARY: diurnal variation, nuclear Azurophil granules (primary, type A granules) are lobation, crystalloid, lysosome, less numerous, somewhat larger, and stain a reddish myeloperoxidase, major basic protein purple. They contain myeloperoxidase as well as a battery of acid hydrolases and are considered to be Eosinophils form only a small proportion of the modified lysosomes. Azurophil granules are large white cells and in humans normally makes up 1 to and homogeneous, whereas the smaller, less dense- 3% of the circulating leukocytes. Their numbers in specific granules may contain a crystalloid body. blood bear some relation to the activity of the Myeloperoxidase complexes with peroxide to adrenal glands; a decrease in eosinophils is seen in produce activated oxygen, a potent bactericide. the alarm reaction. They increase in number during Tertiary granules of neutrophils are secreted and parasitic infestations. This relationship may account promote cell adhesion. These factors also may be for the wide range of values reported. Distinct involved in the mechanics of phagocytosis. diurnal variations related to activity cycles also Cytokines activate both white cells and the occur with highest numbers in the circulation endothelium to produce adhesion molecules that during morning hours and minimal numbers allows the leukocytes to stick tightly to the reported in the afternoon. endothelium. The activated white cells become The eosinophil is about the same size as the motile, cross the endothelial cell barrier, and neutrophil and also is characterized by nuclear actively migrate by ameboid movement into lobation. Although often described as having a adjacent tissues. It is estimated that about 40% of bilobed nucleus, eosinophils with three or four the neutrophils in the peripheral circulation are lobes are not uncommon. Conditions that produce marginated (adhering to the endothelium of small hypersegmentation in neutrophils also affect the blood vessels) under normal conditions. The eosinophil granulocyte. leukocytes move in the direction of the highest The distinctive feature of eosinophils is the closely concentration of chemotaxins. The crossing of the packed, uniform, spherical granules that stain a endothelial lining is referred to as diapedesis. brilliant red or orange-red. The granules are The granules are scattered throughout the membrane-bound and show an internal structure neutrophil cytoplasm, except in a narrow peripheral called a crystalloid, or internus. In humans, the zone, that contains fine filaments and microtubules crystalloid assumes various forms and may be that appear to function in cell movement. A small multiple. The crystalloid is embedded in a finely Golgi complex and a pair of centrioles are located granular matrix. centrally in the cell. Rare profiles of endoplasmic Eosinophil granules are lysosomes and contain reticulum, a few mitochondria, and ribosomes are the usual lysosomal enzymes. They show a higher present also. content of myeloperoxidase than do the azurophil Neutrophils produce a protein that converts granules of neutrophils and lack lysozyme and plasminogen to plasmin, the proteolytic enzyme phagocytin. The granules also contain lipid and responsible for fibrinolysis. Whereas the release of major basic protein (MBP) that is responsible for granule-associated factors is associated with death the eosinophilia. Eosinophils respond of the cell, release of plasminogen-activating chemotactically to bacterial products, complement substance is a secretory activity directly related to components, and substances released by mast cells cell viability. (histamine and eosinophilic chemotactic factor of

72 anaphylaxis). Eosinophils have a special affinity for glycosaminoglycans, leukotriene 3, eosinophilic antigen-antibody complexes, which tend to bind chemotactic factors, prostaglandins, and a platelet- complement and induce cell lysis. Phagocytosis of activating factor. The basophilia of the granules is the complexes may suppress cell destruction in due to the presence of heparin. There is no normal tissues. Factors such as histaminase released evidence that the granules have lysosomal activity. by eosinophils may dampen allergic responses by Basophils have surface receptors for IgE produced degrading histamine and histamine-like substances in response to allergens. Basophil granules also have released from mast cells and basophils. a high content of heparin (an anticoagulant), Prostaglandins E1 and E2 released by eosinophils histamine, vasodilating agents, and slow-reacting inhibit mast cell secretion. Eosinophils also have a substance (SRS). Histamine induces a prompt and major role in controlling parasitic infections transient vasodilation, whereas that induced by SRS (ascariosis, trichinosis, and schistosomiasis): is more sustained and occurs after a latent period. eosinophils selectively interact with the larvae of Both increase vascular permeability. Antigens that some parasites and damage them by oxidative bind to specific sites on the basophil cell membrane mechanisms. The major basic protein (MBP) in the cause degranulation with release of agents that crystalloid core of the eosinophil granules enhances cause smooth muscle spasm, mucous secretion, the antibody-mediated destruction of parasites. hives, itching, and rhinitis. Basophils play a role in Eosinophils have surface receptors for IgE immediate types of hypersensitivity reactions, antibody. The primary function of eosinophils is to anaphylactic reactions (hay fever), and some forms dampen or terminate allergic reactions and parasitic of asthma. infections. Lymphocytes Basophil Leukocytes VOCABULARY: agranulocyte, small VOCABULARY: nuclear lobe, filament, lymphocyte, mononuclear leukocyte, B- metachromatic lymphocyte (B-cell), humoral immunity, antibody, T-lymphocyte (T-cell), cell-mediated Basophils make up only about 0.5% of the total immunity, T-helper cell, T-suppressor cell, leukocytes. They are slightly smaller than neutrophil lymphokine, cytotoxic T-cell, null cell, large granulocytes and in blood smears measure 10 to 12 granular lymphocyte, natural killer cell, µm in diameter. mitogen, blastogenic transformation, Nuclear lobes are less distinct than in the recirculation, memory cell, motility neutrophil or eosinophil, and nuclei with more than two or three lobes are rare. Filaments between Lymphocytes are the predominant agranulocyte lobes tend to be short and broad and rarely form and in the adult human account for 20 to 35% of the threadlike structures seen in neutrophils. The the circulating leukocytes. chromatin is relatively homogeneous and stains less deeply than in the other granulocyte types. Nucleoli Classification are absent. The cytoplasm of basophils contains prominent, Lymphocytes comprise a family of cells, each rather coarse granules that stain a deep violet with differing in function, life span, background, and the standard blood stains and metachromatically size. They are found in blood, lymph, lymphatic with toluidine blue or thionin. Well-preserved tissues, and organs. The most common type is the granules are spherical and uniform but, being small lymphocyte. In blood smears this cell has a soluble in water or glycerin, frequently appear diameter of 6 to 8 µm and appears to be little more irregular in size and shape in fixed preparations. than a nucleus surrounded by a thin rim of pale The granules are scattered unevenly throughout the blue cytoplasm. At one aspect of the cell, the cytoplasm, often overlie and obscure the nucleus cytoplasm may be somewhat expanded. The round and are neither as numerous nor as densely packed or slightly indented nucleus is heterochromatic and as the granules of eosinophil leukocytes. Electron stains deeply. Except in living cells or in electron microscopy shows an internal striated pattern to the micrographs, nucleoli usually are not visible. granules. The granules contain myeloperoxidase,

73 Because of the round nucleus, lymphocytes are growth, and differentiation of small lymphocytes; classified as mononuclear leukocytes. Electron factors that attract neutrophil, eosinophil, or micrographs reveal a small Golgi complex and a basophil granulocytes to the sites of antigens (i.e., few mitochondria and lysosomes; a small amount of chemotactic agents); and factors that increase SER is present, but GER is very scarce. While vascular permeability. Thus, lymphokines represent lymphocytes are classified as agranular leukocytes, a a large class of lymphocyte-derived substances that few nonspecific azurophil granules may be present have a role in controlling immunologic and and represent lysosomes. inflammatory responses or have cytotoxic activities. Although morphologically indistinguishable by Some activated T-cells, the cytotoxic T-cell or killer ordinary means, small lymphocytes are a mixture of (K) lymphocytes, take part in antibody-dependent, functional types that can be divided into three major cell-to-cell interactions with foreign cells that result categories, B-lymphocytes, T-lymphocytes, and in lysis of the target cell. Only target cells that share natural killer (NK) cells, by means of distinctive antigens with the foreign cell that originally inciting surface markers. the appearance of cytotoxic cells are killed. B-lymphocytes (B-cells) play a central role in Cytolysis of the foreign cell requires intimate humoral immunity and represent cells that have contact between the effector and its target, the been conditioned to transform into plasma cells and latter then undergoing a series of membrane secrete antibody when stimulated by foreign changes that result in rupture of the cell. The antigens. The antibody reacts specifically with the cytotoxic T-lymphocyte is unharmed by the antigen that induced its formation. The organ interaction and remains free to react with other responsible for conditioning B-cells in mammals target cells. appears to be the bone marrow. In birds, B-cells are T-lymphocytes express cell surface markers called processed in the bursa of Fabricius, a CDs (cluster designation molecules) that help lymphoreticular, appendix-like diverticulum of the lymphocytes perform their specific effector cloaca. B-cells form about 5 to 10% of the functions. T-helper cells express the CD4 molecule. circulating blood lymphocytes in humans. They are necessary to induce B-lymphocytes to T-lymphocytes (T-cells) have been processed by produce antibody and to activate the macrophage the thymus, where they acquire distinctive T-cell defense system. The T-helper cells recognize markers and become immunologically competent. antigen when it is presented on cells that also However, immunologic capacities are not fully express class II major histocompatability (MHC) developed until the cells are exported to peripheral antigens. Cytotoxic T-cells express CD8 molecules lymphatic organs, especially the spleen. T-cells and recognize and kill target cells. T-suppressor contribute to cell-mediated immunity and cells can express either CD4 or CD8 molecules and perform a variety of functions within the immune are capable of inhibiting the response to helper T- system. T-lymphocytes are generally organized into three cells and modulate the immune response. functional subcategories. When appropriately An additional form of small lymphocyte, the null stimulated, some T-cells interact with activated B- cell, also has been described. These cells lack B- or cells and, as T-helper cells, amplify the production T-cell surface markers. Null cells also appear to be a of antibody; as T-suppressor cells, they dampen diverse group among which may be circulating antibody formation; and as cytotoxic T-cells, hemopoietic stem cells. some have the capacity to kill target cells. Some T- Larger forms of lymphocytes, 10 to 14 µm in cells produce lymphokines that have important diameter, also are present in peripheral blood in roles in body defenses. Of the many lymphokines small numbers. These cells have been called large described, the best characterized are agents that granular lymphocytes. The cytoplasm of these damage or destroy foreign cells or interfere with cells is more abundant and more basophilic than their replication, such as lymphotoxin, interleukin, that of the small lymphocyte. Electron micrographs interferon, and tumor necrosis factor; agents that show an increased number of ribosomes and inhibit migration of macrophages (MIF) and thus mitochondria, a greater amount of endoplasmic concentrate macrophages at the site of an antigen; reticulum, and an increase in the size of the Golgi macrophage-activating factor, which stimulates apparatus. Scattered azurophil granules are present. macrophage activity; lymphocyte blastogenic factor These cells are directly cytotoxic and also lyse target (LBF), which induces blastic transformation, cells by contact, but they appear to function in the

74 absence of antibody and without known prior 85%) are long-lived cells of T-cell origin, with a life exposure to antigen. Thus, they have been named span measured in years. A small proportion (12- natural killer (NK) cells. Commonly considered 15%) of the circulating cells are short-lived B-cells to be null cells, there is growing evidence that they with a life expectancy of several months. Until the are a subset of T-cells and may be related to lymphocytes make contact with an antigen, the cells immune-dependent cytotoxic T-cells. Natural killer continue to recirculate. A small number of long- cells form the third main type of lymphocyte and have lived cells, of T- and B-origin, are present as the ability to kill other cells. The primary role of this memory cells. These are small lymphocytes that form of lymphocyte is the elimination of virus-infected have been stimulated by antigen but have not cells and some tumor cells. responded with production of antibody or lymphokines. They remain for years as conditioned Stimulation by Antigens and Mitogens cells that can react immediately to the same antigen on subsequent exposure to it. Small lymphocytes are not simply terminal effector Lymphocytes are highly motile cells that leave the cells engaged in T- or B-cell activities; they also bloodstream to enter lymphatic organs, migrate have the properties of "stem" cells and can through connective tissues of the body, or propagate additional small lymphocytes when penetrate into epithelia. They have little ability to stimulated by antigens or nonspecific agents called adhere to or to spread on surfaces of any kind. mitogens. The best studied mitogens are plant extracts - phytohemagglutinin (PHA), concanavalin Monocytes A (con A), pokeweed mitogen (PWM) - but other agents such as liposaccharide from Escherichia coli, VOCABULARY: mononuclear leukocyte, antilymphocyte serum, and various pollens also azurophil granule, primary lysosome, have mitogenic activity. These agents have a mononuclear phagocyte system common ability to bind to surface groups on small lymphocytes, thereby triggering blastogenic Monocytes make up the second type of transformation. mononuclear leukocyte normally found in the During transformation, small lymphocytes blood. Their proportions are fairly consistent, and progressively enlarge; the nuclei increase in size and they regularly form 3 to 8% of the circulating become euchromatic, nucleoli become visible, the leukocytes. Monocyte numbers are depressed by Golgi complex enlarges, and the cell synthesizes corticosteroid administration. They vary in size deoxyribonucleic acid (DNA) and ribonucleic acid from 12 to 20 µm in diameter and contain a fairly (RNA). Within 36 to 48 hours, the small large nucleus that may be rounded, kidney-shaped, lymphocyte has assumed the appearance of a or horseshoe-shaped. Segmentation of the nucleus lymphoblast that divides and gives rise to new T- or and formation of filaments never occur, but coarse B-cells. The new cells can produce lymphokines (T- constrictions with blunt, broad lobes may be cells) or become plasma cells and synthesize present. The chromatin is more loosely dispersed antibody (B-cells). than in lymphocytes and therefore stains much less The reactions to mitogens show a degree of densely. Two or three nucleoli may be seen. The specificity: PHA and con A stimulate T-cells but blue-gray cytoplasm is abundant and contains not B-cells; E. coli lipopolysaccharide stimulates numerous fine azurophilic granules that impart only B-cells; PWM activates both types of small an opacity to the cytoplasm, giving it a "ground lymphocyte. The mechanism of the reaction is not glass" or "dusty" appearance. understood, but the reaction is specific to Ultrastructurally, the nucleus shows one or more lymphocytes in that although these agents bind to nucleoli and the cytoplasm contains a small amount the surface of other cells, they do not induce their of GER and polyribosomes. Small, elongated transformation. The response of lymphocytes to mitochondria are present, and the Golgi apparatus mitogens resembles that after exposure to antigen is well formed. The azurophil granules represent or lymphoblastic lymphokine. primary lysosomes and in electron micrographs Lymphocytes recirculate repeatedly between the appear as dense, homogeneous structures. blood, lymphoid organs, and lymph. Only small Monocytes are part of the mononuclear lymphocytes re-circulate, and most of these (80- phagocyte system and represent the cells of this

75 system in transit. They serve little function while in Fluctuations in leukocyte numbers usually involve the blood but migrate into various organs and neutrophil granulocytes and lymphocytes. If tissues throughout the body, where they differentiate neutrophil granulocytes are in excess, a into macrophages. Monocytes respond chemotactically neutrophilia is said to be present; if decreased, a to the presence of invading microorganisms and neutropenia is present. An increase or decrease in necrotic material. They express high levels of MHC lymphocytes is a lymphocytosis or lymphopenia, class II molecules on their surface and are an respectively. Increases in eosinophils, basophils, and important site for production of cytokine IL-1 monocytes are referred to as an eosinophilia, which plays an important role in systemic responses basophilia, and monocytosis, respectively. Decreases in to acute inflammation. In addition to serving as these cells are difficult to establish from smears tissue scavengers, monocytes also have a role in because of their normally low numbers, but they do processing antigen in the immune response and are exist. able to fuse with one another to form various Various cytologic abnormalities may occur, phagocytic giant cells. Some macrophages have especially among the granulocytes. Döhle's bodies antigen-presenting functions and these specialized are small, irregular blue clumps or patches, 0.5 to macrophages form a family of antigen presenting cells. 2.0 µm in diameter that may appear in the This type of macrophage phagocytoses endogenous cytoplasm of neutrophils during some infections, antigens that are degraded into antigen peptide after burns, in cancer, or after treatment with some fragments. These fragments are then presented on oncolytic drugs. They appear to consist of altered the macrophage surface together with class II major ribonucleoprotein. Toxic granulation is seen as histocompatability complex (MCH). The presented coarse, black or purple granules scattered in the antigen peptide fragment is recognized by CD4+ cytoplasm of neutrophils and represents altered helper T lymphocytes which are stimulated and in azurophil granules. Toxic granulation often turn promote B lymphocyte differentiation. accompanies Döhle's bodies and, with cytoplasmic Monocytes do not possess true storage granules vacuolation, may occur during severe bacterial for their enzymes as do granulocytes, and the cell infections. Alder's anomaly is a rare abnormality behaves mostly as a secretory cell. They contain characterized by large, coarse granules in the acid hydrolases, peroxidase, acid phosphatase, and cytoplasm of neutrophils. An apparently inherited aryl sulfatase. anomaly, it has no pathologic significance. Hypersegmentation of granulocyte nuclei occurs Abnormalities as an inherited anomaly of no significance and also accompanies anemia resulting from deficiency of VOCABULARY: leukocytosis, leukopenia, vitamin B12. In such anemia, the cells are neutrophilia, neutropenia, lymphocytosis, abnormally large and have been called lymphopenia, Döhle's bodies, toxic macropolycytes. At the other extreme, poorly granulation, Alder's anomaly, segmented granulocytes are present in the Pelger- hypersegmentation, macropolycyte, Pelger- Huët anomaly, an inherited condition. All types of Huët anomaly granulocyte are involved, and the mature cells rarely show more than two lobes; more frequently, the As with erythrocytes, the total number of cells have round nuclei with no filaments. The circulating leukocytes tends to fall within a relatively anomaly is without significance. narrow range in normal individuals. In humans, this Atypical lymphocytes may show vacuolated range is 5,000 to 12,000/mm3. An increase above cytoplasm, folded (monocytoid) nuclei, nuclear normal is called a leukocytosis and may be due to vacuolation, or prominent nucleoli. Most of the disease or emotional or physical stress. A atypical cells are medium lymphocytes. Dark, leukocytosis with counts of 25,000 to 35,000/mm3 basophilic cytoplasmic granules have been reported has been reported after severe exercise. The in lymphocytes in cases of gargoylism (Hurler's increase represents the flushing of leukocytes syndrome), and inclusions similar to Alder's sequestered in capillary beds and marginated at the granules occur in the lymphocytes of children as a edges of the bloodstream. A decrease in leukocytes familial disorder. is called leukopenia. Diurnal variations of variable degree are associated with activity cycles.

76 Blood in Children Summary

During infancy and childhood, the composition of Blood is important in the transport of materials blood differs significantly from that of adults. throughout the body, maintaining the acid-base Hematologically, a child is not merely a small adult, balance, and providing defense mechanisms. and children and infants have their own specific Transport functions include carriage of oxygen to blood pictures. In general, the younger the all cells of the body, transport of nutrients, and individual, the greater is the deviation from normal removal of waste products of cell metabolism. It adult values and the greater the instability of the aids in regulating body temperature by dissipating blood picture during disease. At birth, the heat formed during metabolism and distributes hemoglobin content of red cells is higher than at hormones, thus integrating the functions of the any subsequent period, and compared with adult endocrine system. Through its buffering capacities, values, the number of red cells is increased (a blood helps maintain the acid-base balance and polycythemia) and the cells are macrocytic. An ensures an environment in which cells may function increase in reticulocytes and the presence of normally. nucleated red cells (normoblasts) are characteristic Transport of oxygen and carbon dioxide is a of the normal neonate. Within a few weeks after function of erythrocytes and their hemoglobin. birth, macrocytes disappear and normal, adult-sized Packaging hemoglobin into a cell allows for a local red cells are present. By about the third month of environment in which hemoglobin can function postnatal life, hemoglobin levels drop; accompanied most effectively and not be converted to useless by a small decrease in the number of red cells so forms. The red cell plasmalemma provides that the red cells (as judged against adult standards) mechanical protection as well as enzymes, which are hypochromic. This picture is maintained until prevent irreversible oxidation of hemoglobin. about the second year, after which the values for Retention of hemoglobin in a cell permits increased red cells and hemoglobin gradually rise to reach concentration of the pigment without increasing the adult levels at puberty. It is only after puberty that viscosity of blood. The absence of a nucleus allows the sex differences in red cells and hemoglobin for a smaller cell and a more efficient distribution become apparent. of hemoglobin within the cell. The biconcave shape The number of leukocytes is much greater in the gives hemoglobin optimal access to the surface for neonate than in the adult, and neutrophil gas exchange and allows the cell to undergo granulocytes predominate. Most of these cells have considerable increase in volume without rupturing. only two lobes, many are band forms, and An iron atom on each of the four polypeptide myelocytes and even promyelocytes (immature chains of the hemoglobin molecule serves as the forms normally found only in bone marrow) may binding site for oxygen molecules. In contrast, the be present. The leukocyte count drops quickly and majority of carbon dioxide, after entering the reaches adult levels by about the third month. From erythrocyte and combining with water is catalyzed the second week to about the second year, by carbonic anhydrase to form hydrogen ion and lymphocytes dominate the blood picture rather than bicarbonate. The latter leaves the erythrocyte in neutrophils as in the adult. exchange for chloride ion and is transported to the In disease states, a child's blood is apt to show far lungs in the blood plasma. At the lungs bicarbonate greater deviations and variations than occur in adult passes back into the erythrocyte, combines with blood. An infection that in the adult would produce hydrogen ion carried and buffered by the a mild leukocytosis and an increase in band form hemoglobin molecule, and is catalyzed by carbonic leukocytes in a child may evoke a leukocytosis of anhydrase back to water and carbon dioxide. The leukemic proportions (leukemoid reaction) with the latter diffuses into the alveoli of the lung to be appearance of myelocytes and promyelocytes. exhaled. Similarly, anemia in a child may be marked by the Platelets have several diverse functions concerned presence of nucleated red cells where in the adult with maintaining the integrity of the blood only polychromatophilia would occur. vasculature. Because of their ability to stick to one Lymphocytosis develops much more readily in another and to foreign surfaces, platelets can cover children, and platelets are apt to fluctuate wildly. and temporarily plug small gaps in blood vessels. They play an important part in blood clotting,

77 releasing factors that initiate the clotting process; and the differentiation of the lymphocytes into they also contain fibrinolytic factors that dissolve antibody-producing cells. Some of the stimulated clots and thus aid in maintaining the fluidity of cells do not go on to produce immunoglobulins but blood. They are necessary for clot retraction, which remain as memory lymphocytes. The process of results in a firm, dense clot and reduces its bulk to humoral immunity is highly specific: only those prevent obstruction of the vessel. Platelets also are antigens that fit receptors on the surface of B- important in maintaining the integrity of lymphocytes elicit an immune response, and the endothelium, which becomes more permeable antibody produced will react only with the antigen when platelets are decreased. Although not that induced its formation. For most antigens that synthesized by platelets, the vasoconstrictors stimulate a humoral response, cooperation between epinephrine and 5-hydroxytryptamine (serotonin) T-lymphocytes and B-lymphocytes is essential. are contained within platelets. While probably When appropriately stimulated, some T- contributing little to the body's overall defenses, lymphocytes interact with activated B-lymphocytes platelets can phagocytize small particles, viruses, and, as T-helper lymphocytes, amplify the and bacteria. production of antibody; as T-suppressor Neutrophils are avidly phagocytic and part of the lymphocytes, they dampen antibody formation. first line of defense against bacterial infections. Cellular immunity depends on T-lymphocytes. As Azurophil granules are lysosomes, and phagocytosis cytotoxic T-lymphocytes, some have the capacity to by granulocytes is associated with fusion of the kill target cells directly. Some T-lymphocytes granules with the ingested material in the usual produce lymphokines that have additional manner of lysosomal digestion. The cells also important roles in body defenses. In response to an contain lysozyme, which hydrolyzes glycosides in antigen, T-lymphocytes proliferate and release a bacterial cell walls, and peroxidase, which variety of lymphokines that may inhibit macrophage complexes with hydrogen peroxide to release migration from the site of the antigen, increase the activated oxygen, an antibacterial agent. Specific activity of macrophages, interfere with virus granules contain a number of antibacterial agents replication, or lyse bacterial walls. including lysozyme; lactoferrin, which binds iron Natural killer cells (lymphocytes) function in the (required by bacteria for growth); and cationic absence of antibody and without prior exposure to compounds with bactericidal properties. antigen and destroy virus-infected cells and tumor Eosinophils have a special affinity for antigen- cells. antibody complexes, which tend to bind Monocytes leave the blood and differentiate into complement and induce cell lysis. Enzymes such as tissue macrophages. They not only serve as tissue histaminase released by eosinophils may dampen scavengers, ingesting and removing particulate allergic responses by degrading histamine and matter, tissue debris, and infective agents, they also histamine-like substances. Eosinophils also have a play a role in the immune response. Some major role in controlling parasitic infections. macrophages function as antigen-presenting cells Basophils are involved in immediate and retain antigen on their surfaces to build up a hypersensitivity reactions such as hives, itching, critical amount that can stimulate T-helper rhinitis, hay fever, asthma and anaphylaxis. lymphocytes to release cytokine signals that Lymphocytes are concerned primarily with the influence B-lymphocytes which have antibody on two major types of immune responses, humoral and their surfaces that matches the challenging antigen. cellular. The basis of humoral immunity is the These specific B-lymphocytes then divide to form a production of antibodies and their diffusion clone of plasma cells which elaborate an antibody throughout the body fluids. As an antigen enters specific to the antigen originally presented by the the body, it is complexed on the surface of B- macrophage. Macrophages also liberate antiviral lymphocytes, whose surface antibodies fit agents and a number of enzymes that digest determinants on the surface of the antigen. The collagen, elastin, and fibrin. antigen is internalized and triggers cell proliferation

78 access to the embryonic circulation via vitelline and 7 Special Connective Tissue: allantoic vessels. Hemopoietic Tissue Hepatic Hemopoiesis The formed elements of blood are not self- replicating; their numbers in the circulation are VOCABULARY: erythropoiesis, definitive maintained by a continuous replacement from erythroblast specialized blood-forming (hemopoietic) tissues located outside the blood vascular system. These The liver is the second important blood-forming consist of the bone marrow, spleen, lymph nodes, organ of the embryo. Hepatic hemopoiesis begins and thymus. during the sixth week of gestation. At this time, hemopoietic cells already make up about 10% of Embryonic and Fetal Hemopoiesis the total cells present in the liver. Hemopoiesis reaches a peak at 7 to 8 weeks and remains steady

until about the 15th week, after which it slowly In the adult blood formation is restricted to the declines. Erythropoiesis dominates throughout the bone marrow and lymphatic tissues, but in entire period of hepatic hemopoiesis, and while embryonic and fetal life, hemopoiesis occurs first in there is significant production of monocytes and the yolk sac and then successively in the liver, platelets during the first 2 ½ weeks, granulocyte spleen, and bone marrow. In some pathologic formation is minor. Some neutrophils and conditions, the liver and spleen may resume a role eosinophils are produced, almost always in the in hemopoiesis. connective tissue around portal spaces.

The liver remains the principal site of red cell Yolk Sac formation from the third to the sixth month of

gestation; erythropoiesis continues, in decreasing VOCABULARY: blood island, primitive amounts, until birth. Hemopoiesis in the liver erythroblast resembles that in bone marrow, and the red cell

precursors in the liver have been called definitive Blood development in the embryo begins soon erythroblasts because they give rise to nonnucleated red after formation of the germ layers. It first occurs in cells. The developing cells form islands of the walls of the yolk sac with the appearance of hemopoietic tissue between the cords of developing blood islands. Discrete foci of mesenchymal cells hepatic cells, but there is no basement membrane in the yolk sac proliferate to form solid masses of between the hepatocytes and adjacent blood- cells that soon differentiate along two lines. The forming cells. peripheral cells flatten and become primitive endothelial cells; the central cells round up, acquire Splenic Hemopoiesis a deeply basophilic cytoplasm, and detach from the peripheral cells to become the first hemopoietic A low level of splenic hemopoiesis overlaps that of precursors. Most of the first blood-forming cells the liver, contributing mainly to the production of differentiate into primitive erythroblasts that erythrocytes, although some granulocytes and synthesize hemoglobin and become nucleated red platelets are formed also. Erythroblastic islands and cells characteristic of the embryo. The isolated some megakaryocytes are present by the twelfth blood islands eventually coalesce to form a network week of gestation. Splenic hemopoiesis wanes as of vessels that ultimately join with intraembryonic the bone marrow becomes active, but the spleen vessels. Few or no hemopoietic foci develop in the produces lymphocytes throughout life. vessels of the embryo proper.

Yolk sac hemopoiesis begins 19 days after fertilization and continues until the end of the Myeloid Phase twelfth week. Subsequent hemopoiesis in the liver, spleen, and bone marrow develops as the result of The myeloid phase of hemopoiesis begins when migration of stem cells from the yolk sac. The cells gain ossification centers develop in the cartilaginous models of the long bones.

79 A few hemopoietic cells can be found in the clavicle marrow itself, but if demands for blood remain of the human fetus at 2 ½ months. Foci of high or are increased, red marrow gradually erythropoietic cells are present in many bones by 4 encroaches into the areas of fatty marrow. months, and by 6 months the bone marrow is an Prolonged or increased demands then are met by important source of circulating blood cells. During expansion of hemopoiesis into other organs or the last 3 months of pregnancy, the bone marrow is tissues such as the spleen, liver, or lymph nodes. the main blood-forming organ of the fetus. Blood formation in tissues other than the bone marrow is called extramedullary hemopoiesis and Bone Marrow occurs in some pathologic states.

In the adult, bone marrow is the major organ for Structure production of erythrocytes, platelets, granular leukocytes, and monocytes. Many lymphocytes also VOCABULARY: reticular connective tissue, are produced in the marrow and reside in the reticular fiber, hemopoietic cell, free cell, lymphatic tissues secondarily. In toto, the bone reticular cell, fixed cell marrow constitutes an organ that rivals the liver in weight and in humans is estimated to account for 4 Bone marrow is a specialized connective tissue that, to 5% of body weight. on the basis of its fiber content, can be classed as a reticular connective tissue. A loose, spongy Types network of reticular fibers and associated cells fills the medullary cavities of bone and provides a VOCABULARY: red marrow, active supporting framework (stroma) for the hemopoietic marrow, yellow (fatty) marrow, hemopoietic cells. The network of fibers and cells extramedullary hemopoiesis is continuous with the endosteum of the bone and is intimately associated with blood vessels that Bone marrow is an extremely cellular connective pervade the marrow. Within the meshes of the tissue that fills the medullary cavities of bone. On reticular fiber network are all the cell types normally gross inspection, it may have a red or yellow color. found in blood, their precursors, fat cells, plasma Red marrow is actively engaged in the production cells, and mast cells. These constitute the free cells of blood cells and represents the active or of the marrow. The reticular cells are fixed cells hemopoietic marrow. The red color is due to the that have no special phagocytic powers and do not content of red cells and their pigmented precursors. give rise to precursors of hemopoietic cells. They Yellow (fatty) marrow is inactive, and its principal are modified fibroblasts responsible for the cellular components are fat cells. Fat cells also are formation and maintenance of reticular fibers. scattered sparingly throughout the red marrow. Reticular cells have large, palely stained nuclei and The amount and distribution of fatty marrow vary irregularly branched cytoplasm that extend long with age and the need for blood cells. All bones slender processes along the reticular fibers. contain active marrow in the late fetus and neonate, but active marrow gradually is replaced by fatty Vasculature marrow during postnatal development and aging. Beginning in the shafts of long bones, fatty marrow VOCABULARY: nutrient artery, central gradually replaces red marrow until, by adulthood, nutrient (longitudinal) artery, sinusoid (venous almost all the marrow in the limbs is the fatty type. sinus), adventitial reticular cell, central Active marrow remains at the ends of the long longitudinal vein bones and in the ribs, sternum, clavicles, vertebrae, pelvis, and skull. Fatty marrow forms about 50% of In long bones, the nutrient artery enters the the total marrow. marrow cavity through the nutrient canal and gives Fatty marrow is very labile and easily replaced by off central nutrient (longitudinal) branches that active marrow. Yellow marrow serves as a reserve run centrally in the marrow cavity toward the ends space for the expansion of active marrow to meet of the bone. These are supplemented by branches increased demands for blood. Active marrow first from epiphyseal and periosteal arteries. replaces the fat cells scattered within the red

80 Along its course, the central nutrient artery smears, marrow cells appear to be randomly provides numerous branches that pass toward the distributed, but in serial sections, some ordering of boney wall. Some of these branches enter the bone the cells can be seen. Fat cells, though generally to supply osteons; others turn back to the marrow scattered, tend to concentrate toward the center of and join the venous system directly by uniting with the marrow, where they cluster about the larger sinusoids. blood vessels. The vascular arrangement of the Sinusoids (venous sinuses) are thin-walled marrow results in a concentration of small vessels, vessels, 15 to 100 µm in diameter, that form an especially sinusoids, at the periphery, and the extensive and complex network throughout the hemopoietic cells are largely confined to the area marrow. They are lined by thin, flattened near the vessels, close to the endosteum. endothelial cells that are closely apposed and joined Erythrocytes develop in small islets close to the by zonula adherens and gap junctions. A basal sinusoids with some ordering of the cells within a lamina is absent or discontinuous, with only cluster according to their stage of development. scattered patches present on the basal surface. The The most mature cells occupy the outer rim of the endothelium is incompletely wrapped by a loose islet. Macrophages are commonly associated with coat of adventitial reticular cells whose broad, developing red cells and often lie in the center of an branching processes merge with the reticular mesh erythroblastic islet surrounded by several layers of that supports the hemopoietic cells. The adventitial red cells. Cells that give rise to platelets reticular cells contain ribosomes, granular (megakaryocytes) are closely applied to the walls of endoplasmic reticulum, and bundles of sinusoids and frequently project small cytoplasmic microfilaments located just beneath the processes through apertures in the sinusoidal wall. plasmalemma. The proportion of endothelium Developing granulocytes form nests of cells that covered by these cells varies with the functional tend to locate at some distance from the sinusoids. state of the marrow and may be reduced to one-third Formation of blood cells occurs extravascularly: when there is heavy cell traffic across the sinusoidal the cells form outside the blood vessels and enter wall. the circulation through the sinusoids. The blood Sinusoids are the first venous elements. They pass cells must penetrate the outer investment of toward the center of the marrow cavity and join the reticular cells and cross the endothelial lining to central longitudinal vein either directly or after gain access to the lumen. In areas of active passage union with other sinusoids to form collecting of cells, the reticular sheath is much reduced, and sinusoids. Thus, in the marrow, blood flows from the vessel appears to consist only of an the center to the periphery and back to the center. endothelium. Blood cells penetrate the endothelium The central longitudinal vein also is thin-walled and through apertures in the cytoplasm and not by consists of a low endothelium, a thin but complete insinuating themselves between adjacent endothelial basal lamina, and a thin outer layer of supporting cells. The openings are relatively large, 1 to 3 µm in reticular cells. The venous blood drains from the diameter. They are not permanent structures and marrow by a main vein that exits through the develop only in relation to and during the actual nutrient canal, where it narrows abruptly. The passage of cells. blood supply appears to be "closed" –that is, there is endothelial continuity between venous and arterial Stem Cells components. VOCABULARY: pluripotent stem cell, spleen Distribution of Marrow Elements colony, colony-forming unit (CFU), restricted stem cell, candidate stem cell VOCABULARY: hemopoietic compartment, vascular compartment, extravascular The hemopoietic cells of the marrow encompass various developmental stages, from a primitive stem The hemopoietic elements form irregular cords cell to the mature circulating elements in the blood. between the sinusoids, and collectively, these cords A stem cell can be defined as a cell that can of blood-forming cells make up the hemopoietic maintain itself through self-replication and also can compartment; the blood vascular components are differentiate into a more mature cell type. referred to as the vascular compartment. In

81 There is much evidence that bone marrow contains cells are committed to the development of one a pluripotent stem cell capable of giving rise to all specific cell line. Experimental evidence supports the different types of blood cells. Mice given a the existence of restricted stem cells for erythrocytes lethal dose of irradiation soon die from loss of (CFU-E), granular leukocytes (CFU-G), and blood cells due to destruction of the hemopoietic megakaryocytes (CFU-Meg). Most studies suggest a organs. However, transfusion of bone marrow soon common precursor for granulocytes and monocytes (CFU- after exposure to radiation averts death, and both GM). Restricted stem cells arise from pluripotent the marrow and the lymphatic tissues are stem cells, are rapidly proliferative, but have limited reconstituted by functional cells derived from the capacity for self-renewal. Pluripotent stem cells are transfused marrow. During recovery, macroscopic capable of extensive replication but are only slowly nodules appear in the spleen; these spleen proliferative and actually represent a reserve cell. It colonies represent islets of proliferating is the restricted stem cells that provide for the hemopoietic cells and contain undifferentiated and immediate, day-to-day replacement of blood cells. maturing blood cells. The cells in the marrow Based on size, sedimentation velocities, and inoculum that give rise to the spleen colonies are response to erythropoietin, three categories of called colony-forming units (CFU). restricted erythropoietic stem cells have been Some colonies consist only of cells of the defined, and by their growth rates, size, and erythrocyte line, others contain developing sedimentation characteristics, two CFU-GM are granulocytes or developing megakaryocytes, but known. Within a given cell line, the various many are mixed colonies that contain developing categories of committed stem cells represent cells of all three lines. The unicellular origin of progressions of development from more primitive, mixed colonies has been shown by transfusing but restricted, stem cells to cells that are the donor cells that have been irradiated just immediate precursors of the classic, cytologically sufficiently to form unique chromosome markers but recognizable precursor cells. not enough to destroy their abilities to replicate. While the existence of a pluripotent stem cell is When these cells form spleen colonies, all the certain, its morphology is in doubt; there is constituent cells bear the same marker as the evidence, however, that it may be similar in inoculated cells and therefore must have arisen appearance to a lymphocyte. Fractionation of bone from the same CFU. The unique chromosome marrow on sedimentation columns has shown that marker also appears in cells that are repopulating CFUs (stem cells) are contained in a fraction whose the thymus and lymph nodes, indicating that the cells have a size and weight comparable with those CFU has the potential for forming cells of lymphatic of lymphocytes. The number of marrow stem cells lineage as well. Cultures of cells from granulocytic can be increased by administering antimitotic drugs and mixed colonies produce monocytes and that destroy hemopoietic cells capable of division. macrophages that also carry the marker Since pluripotent stem cells are only slowly chromosome. The reconstituted marrow of these proliferative, they are spared, and their relative "rescued" animals, when injected into new recipient numbers in the marrow are greatly increased. Stem animals, produces spleen colonies that still carry the cells can be enriched further by density gradient marker chromosomes. centrifugation, and as shown by their ability to Cells from spleen colonies give rise to new spleen produce spleen colonies, stem cells increase in colonies when injected into irradiated animals. Even number in proportion to an increase in cells with colonies that are purely erythrocytic or granulocytic lymphoid appearance. These lymphocyte-like cells can give rise to colonies of all types. Thus, the CFU have been called candidate stem cells. While the is not only capable of differentiation but also of morphology generally is similar to that of a small self-renewal, therefore fulfilling the definition of a lymphocyte, the nucleus is more irregular and less stem cell. The bone marrow must contain deeply indented than in most lymphocytes, and the pluripotent stem cells capable of feeding into the chromatin is more finely dispersed. Mitochondria erythrocyte, granular leukocyte, megakaryocyte, are few in both types of cells but are smaller and monocyte, and lymphocyte lines. more numerous in the candidate stem cell. Free In addition to pluripotent stem cells, bone marrow ribosomes but no granular endoplasmic reticulum contains stem cells that have a more restricted or lysosomes are present. capacity for development. These restricted stem

82 The earliest hemopoietic cells are those in the its chromatin is finely and uniformly granular or blood islands of the yolk sac, and there is evidence stippled in appearance. Two or more nucleoli are that undifferentiated cells from the yolk sac present and may be prominent. In electron circulate in the fetus and successively seed the liver, micrographs the endoplasmic reticulum and Golgi spleen, and marrow. As hepatic hemopoiesis apparatus are poorly developed, but free ribosomes declines, the number of circulating stem cells are abundant and polysomes are scattered increases, suggesting a large-scale migration of stem throughout the cytoplasm. The proerythroblast cells into the marrow. Cells of blood islands do undergoes several divisions to give rise to contain CFUs and can restore all the hemopoietic basophilic erythroblasts. organs in an irradiated animal. The basophilic erythroblast (basophilic normoblast, prorubricyte) generally is smaller than Development of Erythrocytes the proerythroblast, ranging from 12 to 16 µm in diameter. The nucleus still occupies a large part of VOCABULARY: erythropoiesis, the cell, but the chromatin is more coarsely proerythroblast, basophilic erythroblast, clumped and deeply stained. Nucleoli usually are polychromatophilic erythroblast, acidophilic not visible. The cytoplasm is evenly and deeply erythroblast (normoblast), reticulocyte basophilic, sometimes more so than in the proerythroblast. Electron microscopy shows only a The process of red cell production is called few or no profiles of endoplasmic reticulum, but erythropoiesis, during which the erythrocyte free ribosomes and polyribosomes are abundant. undergoes progressive changes that involve the Hemoglobin can be recognized as fine particles of cytoplasm and nucleus. The cell progressively low electron density but, as in the proerythroblast, becomes smaller, and the cytoplasm stains is not always seen by light microscopy because of increasingly acidophilic as it accumulates the intense cytoplasmic basophilia. The basophilic hemoglobin and loses organelles. The nucleus erythroblast also is capable of several mitotic shrinks and becomes more heterochromatic and divisions, its progeny forming the condensed until ultimately it is lost from the cell. polychromatophilic erythroblasts. Although it is possible to describe various "stages" The nucleus of the polychromatophilic in the developmental sequence, the process of erythroblast (polychromatophilic normoblast, erythropoiesis does not occur in stepwise fashion. rubricyte) occupies a smaller part of the cell and The process is a continuous one in which, at several shows a dense chromatin network with scattered points, the cells show distinctive, recognizable coarse clumps of chromatin. Nucleoli are absent, features. Unfortunately, the nomenclature of the and the cell is incapable of division. Cytoplasmic red cell precursors is confused by the multiplicity of staining varies from bluish gray to light slate gray, names given by various investigators to stages in reflecting the changing proportions of ribosomes the maturational series. The terms used here are and hemoglobin. When nucleoli disappear, no new commonly used, but alternative nomenclature is ribosomes are formed, and the change in staining provided. characteristics is the result of a decrease in the The proerythroblast (pronormoblast, rubriblast) concentration of ribosomes (which stain blue) and a is the earliest recognizable precursor of the red cell progressive increase in hemoglobin (which stains line and is derived from the pluripotent stem cell red). Cell size varies considerably but generally is through a series of restricted stem cells. The less than that of the basophilic erythroblast. The proerythroblast is relatively large, with a diameter of polychromatophilic erythroblasts encompass several 15 to 20 µm. The nongranular, basophilic generations of cells, the size reflecting the number cytoplasm frequently stains unevenly, showing of previous divisions that have occurred in the patches that are relatively poorly stained, especially basophilic erythroblast. It is sometimes convenient in a zone close to the nucleus. The cytoplasmic to divide these cells into "early" and "late" stages on basophilia is an important point in the identification the basis of their size and on the intensity of the of this early form of red cell. Synthesis of cytoplasmic basophilia. hemoglobin has begun, but its presence is obscured Acidophilic erythroblasts (acidophilic by the basophilia of the cytoplasm. The nucleus normoblast, metarubricyte) are commonly called occupies almost three-fourths of the cell body, and normoblasts.

83 At this stage the cytoplasm is almost completely Development of Granular Leukocytes hemoglobinized and takes on a distinctly eosinophilic tint. Electron micrographs show a VOCABULARY: granulocytopoiesis, uniformly dense cytoplasm devoid of organelles myeloblast, promyelocyte, azurophil granule, except for a rare mitochondrion and widely myelocyte, specific granule, metamyelocyte, scattered ribosomes. The nucleus is small, densely neutrophil band, eosinophil granulocyte, stained, and pyknotic and often is eccentrically basophil granulocyte located. Ultimately, the nucleus is extruded from the cell along with a thin film of cytoplasm. The maturational process that leads to production Reticulocytes are newly formed erythrocytes that of mature granular leukocytes is called contain a few ribosomes, but only in a few cells granulocytopoiesis. During this process the cells (less than 2%) are they in sufficient number to accumulate granules and the nucleus becomes impart color to the cytoplasm. After the usual flattened and indented, finally assuming the blood stains, these cells have a grayish tint instead lobulated form seen in the mature cell. During of the clear pink of the more mature forms and maturation, several stages can be identified, but as hence are called polychromatophilic erythrocytes. When in red cell development, the maturational changes stained with brilliant cresyl blue, the residual form a continuum, and cells of intermediate ribosomal nucleoprotein appears as a web or morphology often can be found. The stages reticulum that decreases as the cell matures and commonly identified are myeloblast, promyelocyte, varies from a prominent network to a few granules myelocyte, metamyelocyte, band form, and or threads. Reticulocytes are about 20% larger in polymorphonuclear or segmented granulocyte. An volume than normal mature red cells. alternative nomenclature substitutes the stem granulo After loss of the nucleus the red cell is held in the for myelo, and the series of stages becomes marrow for 2 to 3 days until fully mature. Unless granuloblast, progranulocyte, granulocyte, there are urgent demands for new erythrocytes, the metagranulocyte, band form, and reticulocytes are not released except in very small polymorphonuclear granulocyte. numbers. These young red cells form a marrow Myeloblasts are the first recognizable precursors reserve equal to about 2% of the number of cells in of granular leukocytes and represent a restricted circulation. stem cell committed to granulocyte and monocyte production. It is present in bone marrow only in Loss of Nucleus low numbers. The myeloblast is relatively small, ranging from 10 to 13 µm in diameter. The Ordinarily, the nucleus assumes an eccentric cytoplasm is rather scant and distinctly basophilic, position in the cell (late normoblast) and is lost just but much less so than in the proerythroblast, and before the cell enters a marrow sinusoid. Active lacks granules. Electron microscopy reveals expulsion of the nucleus by the normoblast has abundant free ribosomes but relatively little been observed in vitro and may involve some granular endoplasmic reticulum; mitochondria are contractile protein, possibly spectrin. Enucleation numerous and small. The round or oval nucleus of erythrocytes also has been described as the cells occupies much of the cell, stains palely, and pass through the pores in the sinusoidal presents a somewhat vesicular appearance. Multiple endothelium. The flexible cytoplasm is able to nucleoli are present. squeeze through the pore, but the rigid pyknotic The promyelocyte is somewhat larger, measuring nucleus is held back and stripped from the cell 15 to 22 µm in diameter. The nucleus may be along with a small amount of cytoplasm. Free nuclei slightly flattened, show a small indentation, or retain are rapidly engulfed and destroyed by macrophages. the round or oval shape. Chromatin is dispersed and lightly stained, and multiple nucleoli still are present. The basophilic cytoplasm contains purplish red azurophil granules, which increase in number as the promyelocyte continues its development.

84 Electron micrographs show abundant granular forms a dense network with many well-defined endoplasmic reticulum, free ribosomes, numerous masses of chromatin. Specific granules make up mitochondria, and a well-developed Golgi more than 80 to 90% of the granules present; the apparatus. Azurophil granules are formed only remainder are azurophil. during the promyelocyte stage and are produced at The neutrophil band has the same general the inner (concave) face of the Golgi complex by morphology as the mature polymorphonuclear cell fusion of dense-cored vacuoles. except that the nucleus forms a variously curved or Divergence of granulocytes into three distinct twisted band. It may be irregularly segmented but lines occurs at the myelocyte stage with the not to the degree that definite nuclear lobes and appearance of specific granules. Thus neutrophil, filaments have formed. eosinophil, and basophil myelocytes can be distinguished. The stages of maturation of eosinophil Myelocytes are smaller than promyelocytes and granulocytes are the same as for the neutrophil. measure 12 to 18 µm in diameter. The nucleus may Specific eosinophil granules appear at the myelocyte be round or indented, and the chromatin is more stage and usually are identifiable soon after they condensed. Some myelocytes still may show a appear. Occasionally, the granules may have a nucleolus outlined by a condensation of chromatin, slightly purple-blue color, becoming progressively whereas in others the nucleolus is poorly defined. more orange as the cell matures. A rare eosinophil The myelocyte is the last stage capable of mitosis. granule may be present as early as the promyelocyte The cytoplasm of the neutrophil myelocyte stage. The granules are much larger than the contains three populations of granules: azurophil neutrophil type, stain brilliantly with eosin, and in granules produced at the promyelocyte stage and electron micrographs are only slightly less dense smaller specific granules formed by the myelocyte. and smaller than azurophil granules. Specific granules also arise by fusion of dense-cored As the cells mature, the cytoplasm becomes less vacuoles but are produced at the outer (convex) basophilic and the nucleus more and more indented face of the Golgi complex rather than the concave to form lobes. In the late myelocyte and the face. As the myelocyte undergoes successive metamyelocyte, the contents of the eosinophil divisions, the number of azurophil granules in each granules crystallize. Some granules show a crystal of cell is progressively reduced, and specific granules variable shape occupying the center of the granule, soon outnumber the azurophil type. The cytoplasm surrounded by a matrix of lower density; others becomes less basophilic, and free ribosomes and remain dense and homogeneous. Eosinophil granular endoplasmic reticulum are decreased. granules are lysosomes and contain the usual Specific neutrophil granules stain lightly in routine battery of lysosomal enzymes and myeloperoxidase. blood smears, taking up a delicate lilac-pink color, The crystalloid core is rich in major basic protein. and are too small to be resolved individually with The nuclei of a number of eosinophilic the light microscope. Azurophil granules are larger metamyelocytes segment into two portions that and stain purplish red with the usual blood stains remain interconnected by a thin nuclear filament. but are less numerous. With the electron These become eosinophils that exhibit bilobed nuclei. microscope, azurophil granules appear more dense The remainder of the eosinophilic metamyelocytes than neutrophil granules. The contents of the become band forms and their nuclei segment into granules differ: azurophil granules are lysosomes three or four lobes interconnected by nuclear and possess a complex of enzymes, among which filaments. aryl-sulfatase, acid phosphatase, beta-galactosidase, Basophil granulocytes also pass through the beta-glucuronidase, esterase, and nucleotidase have same maturational sequences. The definitive been identified. Specific neutrophil granules contain granules usually appear at the myelocyte stage and alkaline phosphatase and proteins with antibacterial rarely are seen in the promyelocyte. Initially, the properties. granules are truly basophilic but become Myelocytes eventually reach a state at which they metachromatic and with toluidine blue or no longer can divide and then mature into methylene blue stain violet rather than blue-black. metamyelocytes. These cells show most of the features of the myelocyte except that the nucleus is deeply indented to form a horseshoe or kidney shape, nucleoli are lacking, and the chromatin

85 In addition to chemotactic and platelet factors, the membranes form clefts that surround each future granules contain heparin, histamine, and several platelet; as they are shed, the platelets separate enzymes including diaphorase, dehydrogenases, along the narrow clefts. Demarcation membranes peroxidases, and histidine carboxylase, which are continuous with the plasmalemma, and thus converts histidine to histamine. each platelet is bounded by a typical cell membrane. The megakaryocyte delivers platelets through Development of Platelets openings in the walls of the sinusoids, either as individual platelets or as ribbons of platelets that VOCABULARY: thrombopoiesis, separate into individual elements within the megakaryocyte, demarcation membrane, sinusoidal lumen. After shedding its platelets, the megakaryoblast, endomitosis, polyploidy, megakaryocyte consists only of a nucleus promegakaryocyte surrounded by a thin rim of cytoplasm with an intact cell membrane. It generally is assumed that Thrombopoiesis (thrombocytopoiesis) refers to such megakaryocytes are unable to restore their the formation of platelets. Platelets are derived cytoplasm and degenerate, with new generations of from giant cells, the megakaryocytes, which megakaryocytes being formed to replace them. measure 100 µm or more in diameter. Degenerate megakaryocytes can be found in the Megakaryocytes normally are found only in bone circulation, especially in the capillaries of the lung, marrow. where they may remain for some time. The nucleus of the megakaryocyte is large and Megakaryocytes arise from stem cells, the first convoluted and contains multiple irregular lobes of recognizable precursor being a large cell, 25 to 45 variable size interconnected by constricted regions. µm in diameter, with a single round or oval nucleus. The coarsely patterned chromatin stains deeply. The chromatin has a finely granular pattern, and the The cytoplasm is abundant and irregularly outlined basophilic cytoplasm is free of granules. These cells, and often has blunt pseudopods projecting from which are rare, have been called megakaryoblasts. the surface. In smears, the cytoplasm appears The cell undergoes a series of divisions in which the homogeneous and contains numerous azurophil nucleus is replicated but the cytoplasm does not granules. Ultrastructurally, a variable degree of divide (endomitosis). At metaphase, the zoning is apparent. Immediately around the nucleus chromosomes become aligned in several planes on a narrow perinuclear zone contains a few an increasingly complex multipolar spindle. With mitochondria, the Golgi complex, granular subsequent reconstitution, groups of chromosomes endoplasmic reticulum, numerous polyribosomes, are incorporated into a huge lobulated nucleus. and some granules. A large intermediate zone is Thus, a series of polyploid cells arises that may indistinctly separated from the perinuclear zone and reach 64n in some megakaryocytes, although 16n contains granules, vesicles of different sizes and nuclei are the more common. In general, cell and shapes, mitochondria, ribosomes, and components nuclear size are proportional to the degree of of the Golgi element. Depending on the degree of ploidy. The intriguing suggestion has been made development of the megakaryocyte, the granules that segmentation of the cytoplasm by demarcation may be distributed uniformly or in small clusters membranes represents a delayed and modified outlined to a variable degree by a system of cytokinesis. Cells of 4n and 8n ploidy, measuring 30 membranes. The outer most marginal zone is finely to 40 µm in diameter, frequently are called granular, varies in width, and lacks organelles; it promegakaryocytes. The fully formed but not yet does contain packets of microfilaments. functional megakaryocyte consistently shows a clear Platelets are formed by segmentation of the marginal zone, whereas in platelet-forming megakaryocyte cytoplasm via a system of megakaryocytes this zone disappears. demarcation membranes. The azurophil granules form small clusters, and simultaneously, small vesicles appear that become aligned in rows between the groups of granules. The vesicles initially are discontinuous but subsequently elongate and fuse to create a three-dimensional system of paired membranes. The narrow spaces between the

86 Development of Monocytes differentiate into B-cells. The mammalian equivalent of the bursa has not been identified, but VOCABULARY: promonocyte, azurophil the bone marrow itself may be the organ for granule, macrophage differentiation of B-cells. These cells also appear to complete their development in the spleen. B- and Monocytes originate from a pool of precursors in T-cells circulate and recirculate through the blood the bone marrow, most likely the same restricted and lymphatic organs. In humans, B-cells have a life stem cells that give rise to granular leukocytes. In span of at least several months and T-cells may cultures, cells from granulocytic spleen colonies exist for several years. yield monocytes as well as granulocytes. Immature monocytes of the bone marrow (promonocytes) are rare and difficult to Control of Blood Formation distinguish. They range from 8 to 15 µm in diameter and possess large round to oval nuclei VOCABULARY: hypoxia, erythropoietin with evenly dispersed chromatin and several nucleoli. The cytoplasm is fairly abundant and Erythropoiesis is under humoral control and is contains numerous free ribosomes but only scant regulated mainly by a specific erythropoiesis- endoplasmic reticulum. The prominent Golgi stimulating factor called erythropoietin. However, complex is associated with numerous small granules the fundamental stimulus for red cell production is that represent the formative stages of azurophil hypoxia, since the rate of production of granules. erythropoietin is inversely related to the oxygen The mature monocytes of the bone marrow supply of the tissues. Erythropoietin is a closely resemble those of the blood, are somewhat glycoprotein hormone mainly produced by the smaller than promonocytes (9 to 11 µm in kidney, though this is not the sole source of diameter), and contain fewer ribosomes and larger erythropoietin, since it can be produced in animals and more abundant azurophil granules. Azurophil whose kidneys have been removed. The sites of granules contain a variety of hydrolytic enzymes extrarenal production of erythropoietin are not and are primary lysosomes. Following release into known, but small amounts of an erythropoietic- the blood, the monocyte continues its maturation in active substance have been detected in liver the circulation, and additional azurophil granules perfusates. The liver is the primary source of are formed. erythropoietin in the fetus. Monocytes migrate into various tissues where they Erythropoietin increases synthesis of ribonucleic complete their maturation by transforming into acid (RNA) and protein, followed by the initiation macrophages, such as those of the peritoneum, of hemoglobin synthesis and differentiation of the alveoli of the lungs, or Kupffer cells of the liver. At restricted stem cell. Erythropoietin also appears to these sites they receive further molecular accelerate hemoglobin synthesis in those more programing from the surrounding environment to differentiated cells which still are capable of carry out specialized functions. synthesizing RNA and promotes release of reticulocytes from the marrow. Hormones other Marrow Lymphocytes than erythropoietin have been implicated in blood formation. Testosterone stimulates red cell Lymphocytes arise from pluripotent stem cells in development and appears to be important for the marrow and then emigrate to populate the normal maintenance of red cell formation; estrogen lymphatic organs and tissues. Restricted stem cells has the opposite effect. that reach the thymus proliferate there and are Whether there are individual hemopoietic released as T-lymphocytes (T-cells). After being released hormones for each of the cell lines in bone marrow from the thymus, the T-cells migrate to the spleen, is unknown. There is evidence for circulating where they complete their maturation and are proteins that control the differentiation of released as long-lived lymphocytes. B-lymphocytes (B- granulocytes and megakaryocytes. These factors, cells) also originate in the marrow. In birds they granulopoietin and thrombopoietin, have not been migrate to the bursa of Fabricius, where they characterized fully, nor is their tissue of origin known.

87 Agents capable of controlling the proliferation and specific hemopoietic tissues allows the maturation of specific CFUs have been establishment of environments best suited to each demonstrated in T-lymphocytes. tissue. It ensures that replicating cells are not unduly influenced by the activities of other organs Interrelationships of Hemopoietic Cells and tissues, as would occur if replication occurred within the bloodstream. It also provides the blood A pluripotent stem cell, present in the marrow and with small cells that can more easily circulate capable of unlimited self-renewal, gives rise to through fine capillary beds. Since there is no need several types of unipotent (restricted) stem cells. to set aside, within the circulation, a population of The latter have limited capacity for self-renewal, cells concerned with replication, all the blood cells and each is committed to one line of development. are functional and can serve the body's needs. Thus, there is a committed stem cell for each of the Subdivision of hemopoietic tissue into lymphatic cell types seen in blood, and these give rise to the and bone marrow components allows separate morphologic stages that can be recognized in any of conditioning of cells (such as T-lymphocytes) without, the developmental lines. Several orders of restricted at the same time, exposing the remainder of the stem cells have been recognized, representing hemopoietic cells to the same conditioning factors. increasing stages of maturity. The morphologic Within bone marrow, yellow marrow fills in identity of the restricted and pluripotent stem cells spaces unoccupied by active marrow, but being very is undetermined. labile, this fatty marrow can quickly be replaced by hemopoietic marrow when demands for blood Summary become insistent. The sinusoids of the marrow provide sites where newly formed blood cells can The obvious function of hemopoietic tissues is to gain access to the circulation. Additional maturation provide blood with continuous replacement of cells. of cells occurs within the sinusoids and the cells Separation of the replicating cells from their end held here serve as a reserve that can be mobilized as products in the blood and sequestering them in needed.

88 to as "the muscles”. Skeletal muscle makes up about 8 Muscle Tissue 40% of the total body weight. It permits a wide range of voluntary activities from the intricate and Muscle is the basic tissue in which the property of fine movements involved in writing to the rapid, contractility is preeminent. The purposeful powerful forces associated with jumping or movements of the body and the maintenance of throwing a ball. The power that can be generated by posture are the result of contractions of muscles skeletal muscle is illustrated by the speed attained attached to the skeleton. Muscular contraction also by a thrown ball which, at the moment it leaves the is responsible for beating of the heart, breathing, fingertips, may reach speeds of over 90 mph. constriction of blood vessels, movements of the Glycogen associated with skeletal muscle intestines, emptying of the bladder, and other vital functions in the formation of ATP to provide processes. Muscle contraction also plays an energy for this activity through glycolysis. Glycogen important role in generating heat and aids in is synthesized from glucose using glycogen synthase maintaining a normal body temperature of about during hyperglycemia (following a high- 37˚C. The unit of structure of muscle tissue is the carbohydrate meal) and an increase in the insulin to muscle cell, which, because of its elongated shape, glucagon ratio. Glycogen is broken down by also is called a fiber. Functionally, the shape of the muscle glycogen phosphorylase in response to cell is important, because a greater unidimensional exercise due to a decrease in ATP, calcium release contraction can be achieved by an elongated cell during contraction, and secretion of epinephrin than by a globular cell of the same volume. Within a which binds to α- and β-adrenergic receptors on muscle mass, the fibers are oriented in the direction skeletal muscle cells. Glycogen is broken down to of movement. glucose-6-phosphate which enters glycolysis to produce ATP. The enzyme, glucose-6-phosphatase, is Classification absent in skeletal muscle and therefore glycogen is not degraded to free glucose. Thus, glucose remains VOCABULARY: striated muscle, skeletal in the phosphorylated form and cannot leave the muscle, cardiac muscle, smooth muscle, muscle cell. This provides a metabolic advantage visceral muscle, voluntary muscle, involuntary for this cell type in that glucose does not have to be muscle phosphorylated and is available to provide energy directly by glycolysis. Three types of muscle can be distinguished based on their morphology and function. Some muscle Organization cells show a regular pattern of alternating light and dark bands that extend across the width of the fiber VOCABULARY: fiber, multinucleated cell, and these have been called striated muscles. This fascicle, muscle, epimysium, perimysium, is the type of muscle associated with the skeleton endomysium and heart and thus forms skeletal and cardiac muscle. Smooth muscle cells lack cross-striations, The smallest independent unit of skeletal muscle occur in the walls of viscera, and often are referred visible with the light microscope is the fiber, which to as visceral muscle. Functionally, muscle is is a long multinucleated cell. Groups of parallel either voluntary (under control of the will) or fibers form fascicles that can be seen with the involuntary (not controlled by the will). Striated naked eye. In turn, groups of fascicles make up an skeletal muscle is voluntary; smooth muscle is entire muscle. Fascicles vary in size with different involuntary; and cardiac muscle is involuntary muscles and in general are small in muscles striated muscle. Muscle tissue makes up nearly half associated with fine movements and large in the total body mass. muscles that perform actions demanding greater power. Skeletal Muscle At all levels of organization, muscle is associated with connective tissue. The entire muscle is surrounded by a connective tissue sheath called the Skeletal muscle is the most abundant tissue in the epimysium. body and forms those structures generally referred

89 Septa pass from the deep surface of the epimysium diameter tending to be located more centrally. The to invest each fascicle as the perimysium; delicate size of the fibers changes with use, and fibers may extensions of the perimysium wrap each fiber as the hypertrophy (increase in size) in response to endomysium. Although given different names continued use or atrophy (decrease in size) with according to its association with different structural disuse. units of muscle, the connective tissue is continuous The most outstanding structural feature of skeletal from one part to another. It consists of muscle fibers is the presence of alternating light and collagenous, reticular, and elastic fibers and dark segments that result in the banding or cross- contains several cell types including fibroblasts, striations that are seen when the fiber is viewed in macrophages, and fat cells. The endomysium is longitudinal section. Under polarized light, the dark especially delicate and consists mainly of reticular bands are anisotropic and are called A bands, fibers and thin collagen fibers; it carries blood whereas the light bands are isotropic and thus are capillaries and small nerve branches. The larger called I bands. Running transversely through the blood vessels and nerves lie within the perimysium. center of the I band is a narrow, dense line, the Z The connective tissue not only binds the muscle line or Z disc. In electron micrographs, a pale units together but also acts as a harness and aids in narrow region, the H band, can be seen transecting integrating and transmitting the force of their the A band, with a dark M line within it (Figs. 8-1 contractions. The amount of connective tissue & 8-2). The relative length of each band depends varies from muscle to muscle, being relatively on the state of contraction of the fiber. The length greater in muscles capable of finely graded of the A band remains constant, whereas the I band movements. Elastic tissue is most abundant in is prominent in a stretched muscle and short in a muscles that attach to soft parts such as the muscles contracted muscle. of the tongue and face.

The Skeletal Muscle Fiber (Cell )

VOCABULARY: banding, cross-striation, A band, I band, Z line, H band, M line, sarcomere

Generally, skeletal muscle fibers do not branch, but the muscles of the face and tongue may do so where the fibers insert into mucous membranes or skin. The fibers of skeletal muscle are elongated tubes that vary in shape and length. Some are cylindrical with rounded ends and appear to extend throughout the length of a muscle; others are spindle-shaped with narrowly tapering ends and apparently do not run the length of the muscle. One end of the fiber may attach to a tendon, while the other unites with connective tissue in the belly of the muscle, or both ends may lie within the muscle mass. In transverse sections of fresh muscle Fig. 8-1. A transmission electron micrograph showing the details of the the fibers are round or ovoid, but in fixed material banding on each myofibril in a skeletal muscle cell. (X 26,000). Photo they appear as irregular polyhedrons. micrograph courtesy of H.E. Huxley, Cambridge, England. The fibers range between 10 and 100 µm in The contractile unit of the muscle fiber is a diameter and my reach a length of 10 cm. The size sarcomere, defined as the distance between two varies from muscle to muscle and even within the successive Z lines. Within the sarcomere, as the I same muscle. Longer fibers have the greater band becomes shorter, the Z lines approach the diameters and are associated with more powerful ends of the A bands. muscles. Some spacial arrangement of the fibers within a muscle has been noted, fibers of large

90 Closely associated with the muscle fiber are satellite cells that lie flattened against the fiber covered by the same external lamina that invests the muscle cell. Although their nuclei are difficult to distinguish from muscle cell nuclei by light microscopy, they are more fusiform, more condensed, the chromatin is less dispersed, and nucleoli are lacking. The cytoplasm is scanty but may form a clear boundary between the nucleus of Fig. 8-2. A diagrammatic sketch illustrating the banded the satellite cell and the muscle fiber. In electron pattern that occurs on an isolated myofibril from a skeletal muscle cell. micrographs, satellite cells show centrioles (not seen in muscle cells), scant endoplasmic reticulum, a small Golgi complex, and a few mitochondria near Structure of Skeletal Muscle Fibers the ends of the nuclei. Satellite cells may represent survivors of the primitive myoblasts; they account VOCABULARY: sarcolemma, external lamina, for less than 4% of the muscle-associated nuclei. multinucleated cell, peripheral nuclei, Myofibrils are elongated, threadlike structures in sarcoplasm, satellite cell, myofibril, desmin, the sarcoplasm and run the length of the muscle sarcoplasmic reticulum, sarcotubules, terminal fiber. At 1 to 2 µm in diameter, myofibrils are the cisternae, T-tubule, triad, T-system, junctional smallest units of contractile material that can be feet identified with the light microscope. In cross sections, myofibrils appear as small dots, while in Each skeletal muscle fiber is an elongated cell longitudinal sections they give a longitudinal invested by a delicate membrane that, by electron striation to the fiber. Each myofibril shows a microscopy, can be resolved into the plasmalemma banding pattern identical to that of the whole fiber. and an adherent outer layer of glycoproteins and Indeed, the banding of the fiber results from the glycosaminoglycans. The plasmalemma of a muscle bands on consecutive myofibrils being in register. cell is referred to as the sarcolemma but is no Cross-striations are restricted to the myofibrils and different from the limiting plasmalemma of any cell. do not extend across the sarcoplasm between The outer glycoprotein/glycosaminoglycan layer fibrils. The striations on adjacent myofibrils are corresponds to the basal lamina of epithelia and is kept in alignment by a system of intermediate called the external lamina. It is associated with filaments composed of the protein desmin which delicate reticular fibers that mingle with the reticular links adjacent myofibrils to each other and also fibers of the endomysium. links the myofibrils to the cell membrane. The striated muscle fiber is a multinucleated cell. Associated with each myofibril is the Large skeletal muscle cells may contain several sarcoplasmic reticulum, a modification of the hundred nuclei. The nuclei are elongated in the smooth endoplasmic reticulum seen in other cells. direction of the long axis of the fiber and, in adult In muscle, the sarcoplasmic reticulum serves as a muscle, are peripheral, located immediately store for calcium ions. The membrane of the beneath the sarcolemma. The chromatin tends to sarcoplasmic reticulum contains numerous be distributed along the inner surface of the nuclear transmembrane proteins, including calcium-ATPase, envelope, and one or two nucleoli usually are that actively transport calcium from the cytosol into present. The nuclei are fairly evenly spaced along its lumen. Here, calsequestrin and other proteins the fiber but become more numerous and bind and store the internalized calcium ion. This irregularly distributed in the area of attachment of organelle consists of an extensive and continuous the muscle to a tendon. Although the cytoplasm of system of membrane-bound tubules called the muscle cell is called the sarcoplasm, it sarcotubules that form a mesh around each corresponds to that of any cell. Many small Golgi myofibril. At each junction of A and I bands, a pair complexes are present near one pole of a nucleus; of dilated sarcotubules, the terminal cisternae, lysosomes also usually take up a juxtanuclear pass around each myofibril and are continuous with position. the terminal cisternae of adjacent myofibrils.

91 One of the pair of terminal cisternae serves an A to the sarcoplasmic reticulum results in release of band and the other serves an I band. From each of free calcium ions from the sarcoplasmic reticulum the cisternae, narrow, longitudinal sarcotubules into the neighboring contractile elements, bringing extend over the A and I bands, respectively. Over about their contraction. the A band, the tubules form an irregular network T-tubules and terminal cisternae are closely in the region of the H band, while in the I band a apposed. Junctional feet span the narrow gap similar confluence occurs at the region of the Z between T-tubules and terminal cisternae, forming line. Thus, each A and I band is covered by a "unit" areas of low resistance through which impulses pass or segment of sarcoplasmic reticulum. These units to the sarcoplasmic reticulum. In electron consist of the terminal cisternae at the A-I micrographs, the junctional feet appear as regularly junctions, joined by longitudinal sarcotubules that spaced densities extending from the T-tubules to anastomose in the region of the H and Z lines of the terminal cisternae. These are matched by evenly their respective A and I bands. The sarcoplasmic spaced dimples on the cisternal membranes, reticulum shows the same structure regardless of corresponding to sites where the feet are located. fiber type (Fig. 8-3). The junctional feet are believed to be calcium ion channel proteins that extend from the terminal cisternae to voltage-sensing calcium ion channel proteins of the T-tubules. When depolarization occurs, the T-tubule channel proteins undergo a conformational change and (because of their intimate association with calcium ion channel proteins of the sarcoplasmic reticulum), the latter open, releasing calcium ions into the cytosol, initiating contraction.

Structure of Myofibrils

VOCABULARY: myofilament, thick filament, myosin, thin filament, actin, tropomyosin, troponin complex, myomesin, titin, dystrophin

Fig. 8-3. Diagrammatic representation of the sarcoplasmic reticulum and system of T-tubules associated with skeletal Under the electron microscope, the myofibril is muscle. seen to consist of longitudinal, fine myofilaments,

of which two types have been identified, differing The pairs of cisternae at the A-I junctions are in size and chemical composition. The thick separated by a slender T-tubule, and the three filaments are 10 nm in diameter; thin filaments have structures - the T-tubule and two terminal cisternae a diameter of only 5 nm. - form the triad of skeletal muscle. T-tubules are The thick filaments consist largely of myosin inward extensions of the sarcolemma and penetrate into and are 1.5 µm long with a slightly thickened the muscle fiber to surround each myofibril. The T- midportion and tapering ends. The midportion is tubules of one myofibril communicate with those smooth, whereas the ends are studded with many of adjacent myofibrils to form a complete network short projections. Myosin filaments can be through the fiber. The lumen of the T-tubule does dissociated into their constituent molecules, of not open into the cisternae but does communicate which there are about 180 per filament, each with the extracellular space at the surface of the consisting of a head and a tail. The tails of the sarcolemma. T-tubules are quite distinct from the molecules lie parallel and are so arranged that the sarcoplasmic reticulum and collectively make up the smooth midportion of the filament consists of the T-system. tails only. The heads project laterally from the The T-system rapidly transmits impulses from filament along its tapered ends and form a helical the exterior of the fiber to all the myofibrils pattern along the filament. Most of the tail consists throughout the cell, thereby producing a of light meromyosin, whereas the heads, with parts of coordinated response. Passage of electrical impulses the tails, consist of heavy meromyosin.

92 Thin filaments are about 1 µm in length and are myosin-binding protein, C protein, runs parallel to composed of globular subunits of actin attached the M-line in the outer region of the A band and end to end to form two longitudinal strands wound also aids in holding the myosin myofilaments in about one another in a loose helix. A second register. Titin (connectin) is an exceptionally large protein, tropomyosin, lies within the groove accessory protein with elastic properties. It is the between the two actin strands and gives stability to largest molecule found in mammalian cells. The the actin filament. titin molecule spans the distance between the Z line A protein complex consisting of three and the M line and is thought to function as a polypeptides designated as troponin-T, troponin-I, molecular spring for the development of a reactive and troponin-C is bound to tropomyosin at regular force during stretch of a non-activated muscle. intervals of 40 nm. The troponin complex Titin, together with the connective harness regulates actin and myosin binding. Troponin-T binds enveloping the skeletal muscle cell, resist forces that the troponin complex to tropomyosin and positions would pull actin filaments out from between the the complex at a site on the actin filament where myosin filaments making up the A band. Recent actin can interact with myosin. Troponin-I prevents evidence also suggests that titin may be involved in actin from binding to myosin. Troponin-C binds to intracellular signal transduction pathways. Actin calcium ion. When this occurs, a conformational filaments extend into the A bands between the change occurs in the troponin complex, and myosin thick filaments; the extent to which they penetrate can now bind and interact with the actin filament. determines the width of the H band (which consists The energy needed for the cross-bridging between only of thick filaments) and depends on the degree myosin and actin is obtained from the breakdown of contraction of the muscle (Figs. 8-4 & 8-5). of adenosine triphosphate (ATP) by ATPase Each myosin filament is associated with six actin activity located primarily in the globular heads of filaments. At the ends of the A bands, where the the myosin molecules. As a result of forming and thick and thin filaments interdigitate, the narrow breaking bonds between the myosin and actin space between filaments is traversed by cross- filaments, actin filaments slide past the myosin bridges formed by the heads of the myosin filaments, and the length of each sarcomere is molecules. Muscle function depends on a precise reduced, resulting in an overall shortening of each alignment of actin and myosin within each myofibril. myofibril. This is achieved by accessory proteins The arrangement of the thick and thin filaments is which attach to different components of the responsible for the banded pattern on the contractile mechanism and holds them in register myofibrils (Fig. 8-4). The I band consists only of with each other. thin filaments, which extend in both directions from the Z line. The filaments on one side are offset from those of the other, and connecting elements appear to run obliquely across the Z line to create a zigzag pattern. As the actin filament nears the Z line, it becomes continuous with four slender threads, each of which appears to loop within the Z line and join a thread from an adjacent actin filament. Several accessory proteins (α-actinin, Z protein, filamin, and amorphin) have been identified in the Z line. α-actinin binds the actin filaments to the Z line. An additional accessory protein, nebulin, is associated with the actin- containing thin filaments. The A band consists chiefly of thick filaments Fig. 8-4. The relationship of the cross-banding observed on a (myosin) with slender cross-connections at their myofibril compared to the arrangement of actin and myosin midpoints, which give rise to the M line (Fig. 8-4). myofilaments. A protein, myomesin, in the region of the M line holds the myosin filaments in register and maintains a three-dimensional spacial arrangement. Another

93 The arrangement of the filaments also establishes The force of muscle contraction is transmitted to the ultrastructural basis of the contractile the extracellular matrix through a series of link mechanism. proteins. Actin filaments are attached to a protein During contraction, the A band remains constant in known as dystrophin. Dystrophin is linked to length, the length of the I and H bands decreases, several glycoproteins which form a complex that and the Z lines approach the ends of the A bands. passes through the plasmalemma. The outer surface The changes result from alterations in the relative of the glycoprotein complex is linked to a protein positions of the thick and thin filaments. No component in laminin known as laminin 2 (merosin) change in the lengths of the filaments is involved; found in the external laminae of skeletal muscle the actin filaments slide past the myosin filaments cells. It is by this linkage that the contractile forces to penetrate more deeply into the A band. As a generated inside the skeletal muscle cell are result, the I and H bands become shorter. The Z transferred to the external lamina which in turn is united lines are drawn closer to the ends of the A bands, via molecular bonding into the connective tissue thereby decreasing the length of each sarcomere harness formed by interconnections of (the distance between successive Z lines) to endomysium, perimysium and epimysium. produce an overall shortening of the myofibrils Dystrophin is concentrated beneath the (Fig. 8-5). plasmalemma at the ends of the skeletal muscle cells at or near muscle/tendinous junctions. An inherited muscle disease known as Duchenne’s muscular dystrophy, that affects male children, results from a defect in the gene coding for the linking protein, dystrophin.

Fiber Types in Skeletal Muscle

VOCABULARY: type I, red fiber, aerobic, slow twitch, type IIA, fast twitch, fatigue resistant, type IIB, white, anaerobic, fast twitch, type IIC

Some skeletal muscles appear redder than others, reflecting the type of fibers (cells) present. Muscle fibers have been classed as red, white, and Fig. 8-5. The relationship of the cross-banding observed on a myofibril compared to the arrangement of actin and myosin intermediate from their gross appearance or as type

myofilaments in stretched, relaxed, and contracted skeletal 1, IIA, IIB, and IIC on the basis of their muscle. histochemical staining reactions.

Type I fibers are red fibers. These contain The sliding action of the filaments results from abundant myoglobin (a pigment similar to repeated "make and break" attachments between hemoglobin) and fat and are surrounded by many the heads of the myosin molecules and the capillaries. They have numerous small neighboring actin filaments. The attachments are mitochondria, are rich in oxidative enzymes, are low made at sites progressively further along the actin in myophosphorylase and glycogen, and filaments, causing the filaments to slide past one metabolically are aerobic. Aerobic glycolysis another. The force resulting in movement of the involves the conversion of glucose to carbon filaments appears to be generated as the heads of dioxide and water. The mitochondria are aggregated the myosin change their angle of attachment to the beneath the sarcolemma and form rows between actin filaments. myofibrils; they possess many closely packed To translate the contraction of the sarcomere to cristae. The Z line is wide. The fibers do not stain contraction of an entire fiber, each sarcomere in the when reacted for adenosinetriphosphatase fiber must contract simultaneously and not in (ATPase) at pH 9.4 but show "reversal" and stain sequence. This simultaneous contraction - not only well (dark) when reacted at pH 4.6 or 4.3. in a single fiber but throughout an entire muscle - is Physiologically, type I fibers represent slow twitch brought about by the T-system. fibers adapted to slow, repetitive contractions and

94 are capable of long, continued activity. Type I fibers is determined by the number of muscle fibers tend to be located deep in a muscle, closer to bone, served by a single nerve fiber and varies between and are common in antigravity muscles such as muscles. Small motor units occur where precise longitudinal muscles of the back. control of muscle activity is required; large units are Type IIA fibers also are red due to abundant found in muscles used for coarser activities. In the myoglobin but contain less fat than type I fibers. extraocular muscles of the eye, every muscle fiber is They contain much glycogen and show both supplied by a nerve (forming small motor units), aerobic and anaerobic metabolism. Oxidative whereas in the major muscles of the limb, a single enzymes are present, but their activity is slightly nerve fiber may supply 1,500 to 2,000 muscle fibers lower than in type I fibers. Myophosphatase activity - a large motor unit. All the muscle fibers contained is high, mitochondria are large but few in number, in one motor unit appear to be of the same and Z lines are wide. ATPase activity is high (the histochemical type; thus, there are type I and type II fibers stain darkly) at pH 9.4 but is poor at pH 4.6 motor units. or 4.3. Type IIA fibers are considered fast twitch, Branches of a motor nerve end on the muscle fatigue resistant muscle cells. fiber at a specialized region called the motor end- Type IIB fibers are white in color and contain plate or myoneural junction. The junction moderate amounts of glycogen but little myoglobin appears as a slightly elevated plaque on the muscle or fat. They are distinguished by their large fiber associated with an accumulation of nuclei. As diameters, poorer blood supply, decreased number the nerve approaches the end-plate it loses its of large mitochondria, and narrow Z lines. The myelin sheath and ends in a number of bulbous fibers are rich in myophosphorylase but poor in expansions, the outer surfaces of which are covered oxidative enzymes. Metabolically, they are by a continuation of the Schwann's cells. The nerve anaerobic. Anaerobic glycolysis involves the endings lie in shallow recesses, the synaptic conversion of glucose to lactate. After troughs (primary synaptic clefts), on the surface preincubation at pH 9.4, type IIB fibers show high of the muscle fiber (Fig. 8-6). The sarcolemma that ATPase activity but fail to stain at pH 4.3 and show lines these clefts invaginates to form numerous only moderate reactivity at pH 4.6. Type IIB fibers secondary synaptic clefts or junctional folds. represent fast twitch fibers adapted for rapid, short-lived activity as in the extraocular muscles of the eye. In comparison with types I and IIA fibers, type IIB fibers fatigue quickly. Type IIC fibers also have been described. They appear to be similar to type IIB fibers except that their ATPase activity is inhibited only at a pH lower than 4.0. They have narrow Z lines and correspond to the intermediate fiber type. Type IIC fibers represent a fast twitch muscle fiber that is more resistant to fatigue than the type IIB fiber. Fig. 8-6. A diagrammatic sketch of a motor end-plate Type IID fibers also have been characterized. Although the nerve and muscle fibers are intimately Motor Nerve Endings related at the junctions, their cell membranes remain separated by a glycoprotein layer derived VOCABULARY: motor unit, motor end-plate from fusion of the external laminae of the muscle (myoneural junction), synaptic trough, fiber, nerve fiber, and Schwann's cells. This layer (primary synaptic cleft), secondary synaptic extends along the primary synaptic cleft and dips cleft (junctional fold), synaptic vesicle, active into each secondary cleft. Numerous large, vesicular zones nuclei and abundant mitochondria, ribosomes, and granular endoplasmic reticulum are present in the Each skeletal muscle is supplied by one or more sarcoplasm that underlies the nerve terminal. The nerves that contain both motor and sensory fibers. nerve terminals contain many mitochondria and a A motor neuron and the muscle fibers supplied by large number of synaptic vesicles that contain it make up a motor unit. The size of the motor unit acetylcholine, a neurotransmitter agent.

95 A nerve impulse results in depolarization of the Two distinct types of intrafusal fibers are present. Both axon terminal, which permits an influx of calcium are striated over most of their lengths, but midway ions. This event triggers the release of acetylcholine along the fibers the striations are replaced by into the synaptic cleft. The synaptic vesicles release aggregates of nuclei. One type of intrafusal fiber is acetylcholine by exocytosis at specific sites in the larger, and its nonstriated region is occupied by a presynaptic membrane called active zones. cluster of nuclei that produces a slight expansion of Acetylcholine receptors (nicotinic acetylcholine the fibers in this region; these form the nuclear receptors) are present in the sarcolemma on the bag fibers. The second type of fiber is thinner, and outer edges of the secondary clefts, while the nuclei in the central region form a single row, acetylcholinesterase is located along the inner surface hence their name, nuclear chain fiber. Nuclear bag deep within the secondary clefts and in the external fibers extend beyond the capsule to attach to the lamina of this area. The acetylcholine receptors endomysium of nearby extrafusal fibers. Nuclear contain ion channels made up of a complex of five chain fibers are the more numerous and shorter. similar subunits which when bound to acetylcholine They attach to the capsule at the poles of the undergo a conformational change opening the spindle or to the sheaths of nuclear bag fibers. Since channel and permitting the entry of sodium ions. the central areas of the intrafusal fibers lack This influx results in a depolarization of the myofibrils, these regions are noncontractile. sarcolemma, and the action potential created travels Each spindle receives a single, thick sensory nerve over the sarcolemma to the T-tubule system and fiber that gives off several nonmyelinated branches initiates contraction. that end in a complex system of rings and spirals at This postsynaptic receptor site can be blocked or the central regions of the intrafusal fibers. These interfered with by a variety of factors. For example, annulospiral endings run beneath the external curare (a plant toxin) can selectively bind to the lamina in grooves in the sarcolemma. Flower spray nicotinic acetylcholine receptor proteins within the endings are cluster- or spray-like terminations of postsynaptic muscle plasmalemma effectively smaller sensory nerves that occur mainly on nuclear blocking and preventing muscle contraction. An chain fibers (Fig. 8-7). Intrafusal fibers also receive autoimmune disease such as myasthenia gravis is small motor nerve fibers that end either as motor characterized by circulating antibodies that develop end-plates or as long, diffuse trail endings that against the nicotinic acetylcholine receptor proteins ramify over the intrafusal fibers, making several and interfere with normal muscle activity. contacts with them. Muscle spindles serve as stretch receptors and are Sensory Nerve Endings found mainly in slow-contracting extensor muscles involved in maintaining posture. They are present VOCABULARY: muscle spindle, intrafusal also in muscles used for fine movements. fiber, extrafusal fiber, nuclear bag fiber, nuclear Stimulation by motor nerves maintains tension on chain fiber, annulospiral ending, flower spray the intrafusal fibers, resulting in a stretching of their ending, motor endplate, trail ending nonstriated segments. The degree of stretch is sufficient to maintain the sensory nerve endings in Skeletal muscle contains many sensory endings, this region in a state of excitation that is close to some of which are simple spiral terminations of their thresholds. When a whole muscle is stretched, nonmyelinated nerves that wrap about the muscle it causes increased tension on the intrafusal fibers fibers and others that are highly organized and stretching of their nonstriated regions, resulting structures called muscle spindles or in the discharge of impulses by the sensory nerve. neuromuscular spindles. Each muscle spindle When the muscle contracts, tension on the spindle consists of an elongated, ovoid connective tissue is reduced and sensory nerve endings cease firing. capsule that encloses a few thin, modified skeletal The frequency of the discharges by the nerve muscle fibers associated with sensory and motor endings is proportional to the tension on the nerves (Fig. 8-7). The modified muscle fibers intrafusal fibers; together with the number of active traverse the capsule from end to end and form the spindles, a degree of muscle contraction appropriate intrafusal fibers to distinguish them from the to the stimulus is ensured. ordinary muscle fibers (extrafusal fibers) outside the capsule.

96 muscle side of the junction, the connective tissue fibers extend into indentations of the sarcolemma and are firmly attached to the external lamina, to which the sarcolemma also attaches. Within the muscle fiber, the actin filaments of the last sarcomere are anchored to the sarcolemma at the end of the fiber. Thus, contraction of the muscle fiber is passed to the sarcolemma, the external lamina, and, by way of the connective tissue sheaths, to the tendon. Tendon organs are encapsulated sensory receptors found at tendon-muscle junctions. The organ consists of a capsule of dense irregular connective tissue traversed by specialized collagen bundles that are continuous with collagen fibers outside the capsule. A single large sensory nerve pierces the capsule and gives rise to several nonmyelinated branches whose terminations entwine among the collagen bundles. The tendon organ senses stresses produced by muscle contractions, preventing them from becoming excessive.

Cardiac Muscle

Cardiac muscle occurs only in the heart, where it forms the muscle wall (myocardium). It resembles Fig. 8-7. A diagrammatic sketch of a muscle spindle. both smooth and skeletal muscle; both skeletal and cardiac muscle is striated, but like smooth muscle, Tendon and Tendon-Muscle Junctions cardiac muscle is involuntary. However, cardiac muscle is unlike either in many of its structural VOCABULARY: collagen bundle, details and is unique in that its contractions are endotendineum, fascicle, peritendineum, automatic and spontaneous, requiring no external epitendineum, fibroblast, tendon organ stimulus.

Tendons consist of thick, closely packed collagen Organization bundles. Surrounding each bundle is a small amount of loose, fibroelastic connective tissue, the VOCABULARY: branching fiber, endomysium, endotendineum. Variable numbers of collagen perimysium bundles are collected into poorly defined fascicles wrapped in a somewhat coarser connective tissue As in skeletal muscle, the histologic unit of called the peritendineum. Groups of fascicles structure of cardiac muscle is the cell (fiber), but in form the tendon itself, which is wrapped in a thick cardiac muscle the cells are associated end to end to layer of dense irregular connective tissue, the form long tracts. The cells may divide at their ends epitendineum. The only cells present in the before joining to adjacent fibers and thus form a tendon are fibroblasts arranged in columns network of branching fibers. Individual cardiac between the collagen fibers. Tendons are classic muscle cells are small in comparison to skeletal examples of dense regular connective tissue. muscle cells measuring about 120 µm in length and Where muscle and tendon join, the connective 20 µm in width. tissue of the endomysium, perimysium, and epimysium becomes strongly fibrous and blends with the connective tissue of the tendon. On the

97 A web of collagenous and reticular fibers is present between the muscle fibers and corresponds to an endomysium, but because of the branching fibers, it is more irregular than in skeletal muscle. Large bundles of cardiac muscle fibers are wrapped in a coarser connective tissue of collagen and reticular fibers, forming a perimysium, as in skeletal muscle.

The Cardiac Muscle Fiber (Cell )

VOCABULARY: sarcolemma; central nuclei; A,

I, M, H, and Z bands; actin and myosin filaments; sarcoplasmic reticulum; T-tubule; Fig. 8-8. Diagrammatic representation of the sarcoplasmic diad; Z line; peripheral coupling reticulum and the system of T-tubules in cardiac muscle. (subsarcolemmal cisternae) The T-tubules of cardiac muscle are located at Z lines rather than at A-I junctions, and their lumina Each cell (fiber) is enclosed in a sarcolemma are much wider than the T-tubules of skeletal muscle. external to which is an external lamina associated The luminal surfaces of the cardiac T-tubules are with fine reticular fibers. Each fiber contains one or coated by a continuation of the external lamina of on occasion two central nuclei. The same banding the sarcolemma. Sarcotubules at the periphery of pattern seen in skeletal muscle is present on cardiac the cell may be closely applied to the sarcolemma, fibers, and A, I, M, H, and Z bands can be and these couplings have been called peripheral distinguished but are not as conspicuous as in couplings or subsarcolemmal cisternae. As in skeletal muscle. The banding pattern is due to the skeletal muscle, the functional unit of cardiac arrangement of actin and myosin filaments, but muscle is the sarcomere, demarcated by two the aggregation of filaments into myofibrils is not as successive Z lines. well defined, and bundles of myofibrils often become confluent with those of adjacent bundles or Intercalated Discs are separated by rows of mitochondria. Mitochondria make up as much as 30% of the VOCABULARY: Z line, desmosome, fascia cytoplasmic volume of cardiac myocytes making adherens, nexus (gap) junction then fatigue resistant. The bundles of myofibrils diverge around the nuclei to leave a fusiform area At their end-to-end associations, cardiac muscle of sarcoplasm at each nuclear pole. These areas are cells are united by special junctions that are visible occupied by small Golgi complexes and numerous under the light microscope as dark lines. These are large mitochondria that have closely packed cristae. the intercalated discs that cross the fibers in Glycogen is a prominent inclusion of cardiac muscle. stepwise fashion at the level of Z lines and thus The sarcoplasmic reticulum is neither as have transverse and horizontal parts. Along the extensive nor as well developed as that of skeletal transverse parts, the opposing cells are extensively muscle. The sarcotubules are continuous over the interdigitated and at points are united by length of the sarcomere and anastomose freely to desmosomes. The desmosomal junctions function give a plexiform pattern with no special to anchor intermediate (desmin) filaments of the anastomosis at the H band. There are no terminal cytoskeleton to the plasmalemma. For the most cisternae, and the sarcoplasmic reticulum makes part, the junction between cells is more extensive contact with the T-tubules via irregular expansions than that of a desmosome and thus is called a of the sarcotubules. Since most T-tubules are fascia adherens (Fig. 8-9). apposed to only one cisterna at any point, the couplings of T-tubule to sarcoplasmic reticulum have been called diads (Fig. 8-8).

98 The sarcoplasm immediately adjacent to the portion of the walls of blood vessels and of hollow cytoplasmic surface of the cell membrane at these viscera such as the digestive, respiratory, and locations contains a dense mat of filaments into reproductive tracts. Smooth muscle cells also are which the actin filaments insert. The fascia adherens present in the skin, where they form small muscles contains the actin-binding proteins, α-actinin and attached to hair follicles, and are in the iris and vinculin, which function to anchor actin to the ciliary body of the eye, in the erectile tissue of the plasmalemma. Transmembrane glycoproteins A-CA penis and clitoris, and in the stroma of the ovary and plakoglobin function as cell-adhesion molecules and prostate. and tightly unite cardiac muscle cells end to end at the intercalated disc. The large titin molecules also The Smooth Muscle Fiber (Cell ) are present in cardiac myocytes and, as in skeletal muscle, have an important role in passive force VOCABULARY: cell, single central nucleus, acting together with connective tissue surrounding filaments, dense body, external lamina, gap myocytes to resist forces that pull actin junction (nexus), attachment plaque, caveolae myofilaments from between the myosin filaments of the A band. Each smooth muscle fiber is an elongated cell with At irregular sites along the junctions, the an expanded central region and tapering ends. The membranes of opposing cells are united by nexus cells vary in length in different organs, from 20 µm (gap) junctions (Fig. 8-9). The longitudinal part of in small blood vessels to 500 to 600 µm in the the intercalated disc is continuous with the pregnant uterus. A single central nucleus transverse portion and also shows gap junctions occupies the wide portion of the cell about midway that, however, are much more extensive. The gap along its length, elongated in the long axis of the junctions represent areas of low electrical resistance cell. In a contracted cell, the nucleus has a wrinkled and allow rapid spread of excitation impulses from or pleated outline. The cells lack cross-striations cell to cell resulting in the synchronization of muscle and in the usual histologic preparations appear contraction. homogeneous. However, longitudinal striations can be seen after maceration of the cells in acid; these striations may represent the myofibrils. In electron micrographs, the sarcoplasm in the region of the nucleus shows long, slender mitochondria, a few tubules of granular endoplasmic reticulum, clusters of free ribosomes, and a small Golgi complex at one pole of the nucleus. Unlike cardiac and skeletal muscle cells, only a rudimentary sarcoplasmic reticulum is present and a system of T-tubules is absent. The bulk of the cytoplasm contains closely packed, fine filaments arranged in bundles that run in the long axis of the cell. Mitochondria and glycogen granules are interspersed between the myofilaments. Scattered throughout the sarcoplasm are a number of oval dense bodies into which the myofilaments (actin) appear to insert. The cytoplasmic dense Fig. 8-9. A transmission electron micrograph of an bodies contain α-actinin, an actin-binding protein. intercalated disc uniting ends of two cardiac muscle cells. (X 52,800). Desmin, the most abundant intermediate filament, and vimentin also insert into these anchoring points. Smooth Muscle The sarcolemma is covered externally by a thick external lamina consisting of proteoglycans Smooth muscle cells are widely distributed associated with numerous fine collagenous and throughout the body and play an essential role in reticular fibers that blend with the surrounding the functions of organs. They form the contractile connective tissue.

99 In some regions the external lamina is lacking, and myosin, and permits it to bind to actin. Contraction the cell membranes of adjacent smooth muscle cells then occurs. Myosin and actin myofilaments of are closely apposed in gap junctions (nexuses) smooth muscle interact as in striated muscle, and a through which excitation impulses spread from one sliding filament mechanism appears to account for cell to another. Areas of increased density, similar contraction of smooth muscle cells also. to the dense bodies, occur at intervals along the Contraction of smooth muscle cells is associated inner aspect of the sarcolemma, becoming more with formation of blebs of the plasmalemma; the numerous along the ends of the cells. These dense blebs are lost as the cells relax. The evaginations regions appear to be sites of attachment of increase in size and number as the cell continues to myofilaments and intermediate filaments to the cell shorten, and when the cell has reached 55% of its membrane and have been called attachment initial length, it is completely covered by blebs. In plaques. The subplasmalemmal attachment plaques electron micrographs, the blebs are free of have been shown to contain the actin-binding myofilaments, but filaments are prominent in the proteins, vinculin and talin. Between these areas the nonevaginated areas between attachment plaques. sarcolemma may show numerous caveolae that In the contracted cell, the thick and thin filaments form as a result of invaginations of the cell and the cytoplasmic dense bodies are oriented membrane. The caveolae are thought to be the obliquely to the long axis, criss-crossing the cell, functional equivalent to the T-tubule system of whereas in the relaxed cell these components striated muscle. Caveolae of smooth muscle cells generally are parallel to the cell axis. The contractile contain calcium ion and are intimately related to elements unite the attachment plaques with the terminal sacs of endoplasmic reticulum that lie dense bodies. During contraction, the contractile beneath the plasmalemma which also contain elements become obliquely oriented. As the cell calcium ion. shortens, its width must increase to accommodate the displaced volume, but this is opposed by the Myofilaments contraction pulling inward at the site of the attachment plaques. Those areas not directly VOCABULARY: thick filament, thin filament, subjected to the inward force bulge outward as actin, myosin blebs. The filaments between the dense bodies represent a contractile unit, several being strung Thick and thin filaments have been demonstrated together and anchored at either end in an in smooth muscle cells by electron microscopy, and attachment plaque. In addition to the actin actin and myosin have been identified myofilaments, the dense bodies and biochemically. The actin filaments of smooth subplasmalemmal attachment plaques serve as muscle cells are associated with tropomyosin but attachment sites for the intermediate filaments (desmin, lack troponin. However, myosin filaments have been vimentin), which provide strong cytoskeletal difficult to identify in standard electron microscopic support for the smooth muscle cell. preparations, and it has been suggested that in Smooth muscle is richly innervated by the smooth muscle, myosin is labile, aggregating into autonomic nervous system which generally acts to filaments only on initiation of contraction. increase or decrease the level of spontaneous Filamentous myosin has been shown in unfixed, contraction. Two physiologically different types of rapidly frozen smooth muscle cells where regular smooth muscle are recognized. Tonic smooth muscle, arrays of thick myosin filaments have been seen which is characterized by the lack of action surrounded by rosettes of actin. The myosin potentials and slow contraction. Tonic smooth identified is smooth muscle is different from that of muscle tends to generate its own level of rhythmic skeletal muscle in that it will bind to actin only if its contraction which can be initiated by stretch or light chain is phosphorylated. As in striated muscle, hormones. The autonomic nervous system acts to release of calcium ion from the sarcoplasmic increase or decrease levels of contraction rather reticulum in smooth muscle initiates contraction. In than actually initiating contraction. Only a few smooth muscle calcium ion is complexed to smooth muscle cells of the muscle sheet are calmodulin (a calcium-binding protein), and this innervated and the impulse spreads through gap complex activates myosin light-chain kinase, an junctions (single unit innervation). enzyme necessary for the phosphorylation of

100 Examples of this type of smooth muscle are the Initially the precursor muscle cells are rounded, limiting wall (muscularis externa) of the noncontractile cells with single centrally placed gastrointestinal tract, urinary bladder and uterus. nuclei. As the cells develop, they elongate and begin The other type of smooth muscle called phasic to synthesize myofilaments, creatinine kinase, and smooth muscle is characterized by distinct action acetylcholine receptors, thus becoming identifiable potentials and rapid contraction controlled precisely as myoblasts. The spindle-shaped myoblasts contain by the autonomic nervous system. In phasic numerous ribosomes, prominent Golgi complexes, smooth muscle almost every smooth muscle cell is and rather short mitochondria, as well as a few innervated (multiunit innervation). This type of myofilaments. The cells are actively mitotic. smooth muscle is found in the wall of the vas By seven weeks’ gestation the myoblasts begin to deferens, iris of eye, erector pili, and some arteries. fuse with one another, end-to-end, to form primary myotubes. Primary myotubules are long cells, 8 to 15 Organization µm wide, with multiple large, central nuclei, each with a prominent nucleolus. The myoblasts that Smooth muscle cells may be present as isolated undergo fusion have ceased to synthesize units or small bundles in ordinary connective tissue, deoxyribonucleic acid (DNA) and are postmitotic as in intestinal villi, the tunica dartos of the cells. Bulk synthesis of contractile proteins occurs in the scrotum, and the capsule and trabeculae of the myotubule, and organization of sarcomeres with spleen. They also form prominent sheets in the recognizable A and I bands and Z lines takes place. walls of the intestines and arteries. In any one sheet Actin filaments appear, closely associated with of muscle, the fibers tend to be oriented in the myosin filaments. Clusters of myofibrils appear, same direction but are offset so that the thick aggregate, increase in length and thicken, and soon portions of the cells lie adjacent to the tapering show banding. Shortly after the appearance of ends of neighboring cells. Thus, in transverse myofibrils, transverse tubules appear at intervals of sections, the outlines of the cells vary in diameter 1 to 5 µm. Fusion of Z bodies to which actin according to where along their lengths the cells filaments are joined results in the appearance of Z were cut. Nuclei are few and present only in the lines, aligned with the T-tubule system and units of largest profiles, which represent sections through the sarcoplasmic reticulum. the expanded, central areas of the cells. New generations of myoblasts continue to A thin connective tissue consisting of fine develop and either form new myotubules or fuse to collagenous, reticular, and elastic fibers extends the ends of older myotubules to form fibers that between the smooth muscle cells and becomes contain 50 to 100 nuclei. It is likely that new continuous with a more dense connective tissue myotubules continue to form until their number that binds the muscle cells into bundles or sheets. equals that of the cells of a given mature muscle. By The latter connective tissue contains more the thirteenth week, the nuclei begin to move from the abundant fibroblasts, collagenous and elastic fibers, center of the tube to the periphery, and the cell and a network of blood vessels and nerves. The becomes known as a myofiber. However, traction produced by contracting smooth muscle development of the cell is not synchronous along cells is transmitted to the reticuloelastic sheath its length, and the same structure may show about the cells and then to the denser connective features of a myotubule and a myofiber. The most tissue, permitting a uniform contraction throughout mature regions are those in the middle of the cell, the muscle sheet. The reticular and elastic fibers of where fusion of myoblasts first occurred. The least the reticuloelastic sheath are products of the smooth mature areas are those at the ends. Thus, for a time muscle cells. the middle of the cell might show peripheral nuclei, while at the ends, central nuclei are present as Histogenesis fusion of myoblasts continues. As the myotubule develops, the number of mitochondria increases Skeletal muscle arises from mesenchyme in various and the cristae increase in number and are more parts of the embryo. Muscles of the limbs and trunk tightly packed. develop from myotomal mesenchyme; those of the head and neck arise in the mesenchyme of the branchial arches.

101 From the sixteenth week of gestation until birth, development the myoblasts resemble fibroblasts. muscle cells enlarge in length and width. The cytoplasm contains abundant free ribosomes, The muscle mass continues to increase after birth granular endoplasmic reticulum, and mitochondria, until puberty. There is no evidence that the number and Golgi complexes are prominent. Few filaments of cells increases postnatally, but the cells do are present initially, but as the smooth muscle cell increase in length by the addition of new develops, bundles of thin filaments appear in sarcomeres at the ends. Increase in girth is due to association with dense bodies. Myosin can be an increase in the number of myofibrils and the detected in the bundles of filaments. Concomitant amount of sarcoplasm. with the appearance of filaments, cytoplasmic Histochemical cell types can be recognized after the organelles decrease in number. Attachment plaques eighteenth or seventeenth week of gestation, and along the cell membrane, plasmalemmal vesicles, prior to this, all the myotubules react uniformly as and a external lamina appear at the same time or a type IIC cells. A few large type I cells appear at little later than the thick filaments. In the newborn, about the twentieth week, but it is not until after myofilaments are small and usually restricted to the the thirtieth week that type IIB cells, then IIA cells cytoplasm at the edge of the cell. Granular differentiate from the type IIC cells. At birth, about endoplasmic reticulum and Golgi elements are 10% of the type IIC cells persist. In adults, less than prominent and extend throughout the cell. 3% are type IIC cells. There is evidence that even in the adult (e.g., in Injured skeletal muscle has a limited capacity for the uterus), new smooth muscle cells arise by regeneration or repair. Mature skeletal muscle cells mitotic division. Generally, however, adult growth do not undergo mitosis following injury. Rather, of smooth muscle is the result of hypertrophy. satellite cells associated with the mature fibers Following injury smooth muscle has a high capacity proliferate and fuse to form new skeletal muscle to regenerate and heal itself. cells. The forming fibers differentiate in a similar The smooth muscle of the iris is commonly thought to way as observed during normal development. be derived from cells near the margin of the optic Cardiac muscle develops from mesenchyme (the cup and therefore is of ectodermal origin. myoepithelial mantle) that envelops the endocardial tube of the embryonic heart. The cells are stellate at Summary first and joined by desmosomes. Unlike skeletal muscle cells, primitive cardiac muscle cells divide Nerve impulses reaching the myoneural junctions repeatedly. With the synthesis of myofilaments, the of skeletal muscle cells cause release of cells elongate and become myoblasts, which soon acetylcholine contained within the synaptic vesicles. begin rhythmic contractions. Cell division continues This neurotransmitter diffuses across the synaptic into postnatal life but subsequently gives way to trough, the surface area of which is greatly hypertrophy as the means of further growth. The increased by the junctional folds. Acetylcholine sites of desmosomal unions between cells causes depolarization of the sarcolemma at the transform into the intercalated discs. Formation of T- motor end-plate, from which an excitation wave tubules characterizes the final stage of development (action potential) sweeps over the surface of the of mammalian cardiac muscle. The tubules arise muscle fiber and, by means of the T-system, is from invaginations of the sarcolemma late in the delivered to all the myofibrils throughout the fiber. prenatal period. Cardiac muscle has very limited At the triads, the impulse is passed to the capacity for regeneration or repair after injury. sarcoplasmic reticulum, causing release of calcium Instead injured cardiac cells are replaced by ions from the terminal cisternae. Free calcium ions connective tissue. activate the myosin-actin interaction through the Smooth muscle arises from the mesenchyme at intervention of tropomyosin and troponin. various sites in the embryo where smooth muscle Troponin acts as a calcium receptor that, in the ordinarily will occur in the adult. The presence of increased calcium ion, is thought to undifferentiated cells in these areas continue to cause conformational changes in the tropomyosin, divide until they become confluent, after which bringing about the interaction of myosin and actin. individual cells, the myoblasts, differentiate. The The energy for the cross-bridging between myosin myoblasts elongate and take on the fusiform shape and actin is obtained from the breakdown of characteristic of adult smooth muscle cells; early in

102 adenosine triphosphate (ATP) by ATPase increase in the amount of sarcoplasm followed by contained in the heads of the myosin molecules. an increase in the number of myofibrils. As the actin filaments slide past the myosin Contraction of cardiac muscle occurs filaments to penetrate more deeply between them, spontaneously, with no need for an external the length of each sarcomere is reduced, resulting in stimulus. However, the mechanism of contraction an overall shortening of each myofibril and thus of is identical to that of skeletal muscle and involves each fiber. The tension generated is transmitted in interaction of actin and myosin filaments. Cardiac succession to the sarcolemma, the external lamina, muscle contraction depends on calcium ion but the connective tissue sheath, and then to a tendon shows a relatively slower contraction time than does or other muscle attachment. Muscle spindles serve skeletal muscle; this characteristic may relate to the as stretch receptors and coordinate the degree of relatively poorer development of the sarcoplasmic muscle contraction with the strength of the reticulum. The high content of mitochondria and stimulus. Tendon organs sense the stresses glycogen is related to the constant rhythmic produced by the muscle contractions and prevent expenditure of energy typical of cardiac muscle. them from becoming excessive. Because of the numerous gap junctions between The function of skeletal muscle depends on the cardiac muscle cells, the cells of cardiac muscle have precise alignment of actin and myosin filaments an inherent capacity to conduct impulses, and within each myofibril. A complex skeleton of electrically, the cells act as though there were no accessory proteins (myomesin, α-actinin, desmin, membranes between them. Rapid distribution of titin) supports the myofilaments, maintains their impulses throughout the cardiac muscle is aided by alignment, and holds them in register to each other. the many branchings that provide unions between Intermediate filaments of the cytoskeleton (desmin) adjacent cells. link adjacent myofibrils and maintain their register Thick and thin myofilaments have been identified as well as linking them to the lateral aspects of the in smooth muscle cells, but a sarcomere sarcolemma. Individual skeletal muscle fibers are organization has not been demonstrated clearly. organized and harnessed by several extracellular Contraction of smooth muscle is slower than in components (external lamina, endomysium, striated muscle, but the energy cost of an equivalent perimysium, and epimysium) to complete the force is much less. Contraction of smooth muscle is organization of each cell into a muscle. somewhat unique in that forceful contractions can The duration of the excitation wave at the be sustained for long periods without fatigue of the myoneural junction is limited by the rapid muscle. An example of such contractions is those breakdown of acetylcholine by the enzyme of the uterine muscle during parturition. This type acetylcholinesterase, which is associated with the of contraction differs from that of skeletal muscles, external lamina of the junctional folds. With the which are unable to sustain contraction without end of the excitation wave, calcium ion is rebound becoming fatigued, and cardiac muscle, which in the sarcoplasmic reticulum, and ATPase activity shows inherent intermittent contractions. Absence and actin-myosin interactions cease, resulting in of an organized sarcoplasmic reticulum is related to relaxation of the muscle. the slow contractions exhibited by smooth muscle. Skeletal muscle fibers have limited capacity to Nexus junctions permit free passage of impulses, regenerate but can repair minor injuries. It is and, as in cardiac muscle, smooth muscle frequently thought that repair occurs through activation of acts electrically as though there were no cell satellite cells. These cells, which separate from the boundaries. Thus, a few motor neurons can control parent muscle fiber, divide and fuse with other large numbers of smooth muscle cells. The satellite cells to form myotubules and new fibers connective tissue between and around smooth that bridge small defects in the parent fiber. Adult muscle bundles transmits the forces generated by muscle increases in mass by increase in the size of individual cells as a coordinated force throughout existing fibers, not by increase in the number of the entire tissue. fibers. A shift occurs from the small- and medium- Smooth muscle cells of different organs may sized fibers of normal muscle to the large fiber show differences with regard to size, organization groups, but few of the hypertrophied muscle fibers (individual cells, bundles, or sheets of cells), and become larger than the largest normal fibers. response to different stimuli. Physiologic hypertrophy at first consists of an

103 Individual smooth muscle cells are cemented junctions and link the contractile proteins and together into functional units by external laminae cytoskeleton to the external laminae by complexes linked to reticular fibers and glycosaminogylcans of of transmembrane glycoproteins that link these the surrounding extracellular matrix. Contractile elements to the extracellular matrix. The smooth proteins are linked to anchoring points along the muscle cells themselves secrete most elements plasmalemma (attachment plaques) as is desmin. forming the surrounding extracellular matrix within The attachment plaques function like adherent a sheet or bundle of smooth muscle.

104 inclusions. Bundles of neurofibrils form an

9 Nervous Tissue anastomosing network around the nucleus and extend into the dendrites and axon. Neurofibrils Nervous tissue, one of the four basic tissues, must be stained selectively to be seen with the light consists of nerve cells (neurons) and supporting cells microscope. Ultrastructurally, neurofibrils consist of (neuroglia). Nerve cells are highly specialized to react aggregates of slender neurofilaments, 10 nm in to stimuli and conduct the excitation from one diameter, and microtubules that, although called region of the body to another. Thus, the nervous neurotubules, appear to be identical to system is characterized by both irritability and microtubules found in other cells. Neurofilaments conductivity, properties that are essential to the act as an internal scaffold for the perikaryon and its functions of nervous tissue - to provide processes and function to maintain the shape of communication and to coordinate body activities. neurons. Specialized membrane proteins anchor the The nervous tissue of the brain and spinal cord neurofilaments to the plasmalemma. The perikarya makes up the central nervous system; all other nervous of most neurons contain characteristic tissue constitutes the peripheral nervous system. chromophilic (Nissl) substance, which, when Nervous tissue specialized to perceive an external stained with dyes such as cresyl violet or toluidine stimulus is called a receptor, from which sensory blue, appears as basophilic masses within the stimuli are carried by the peripheral nervous system cytoplasm of the perikaryon and dendrites. It is to the central nervous system, which acts as an absent from the axon and axon hillock, the region integration and communications center. Other of the perikaryon from which the axon originates. neurons, the effectors, conduct nerve impulses from In electron micrographs, Nissl substance is seen to the central nervous system to other tissues, where consist of several parallel cisternae of granular they elicit an effect. endoplasmic reticulum. The specialized contacts between neurons are Small slender or oval mitochondria are scattered called synapses; the impulses are transferred between throughout the cytoplasm of neurons and contain nerve cells by electrical couplings or chemical lamellar and tubular cristae. Well developed Golgi transmitters. Some neurons of the brain secrete complexes have a perinuclear position in the cell. substances (hormones) directly into the Although centrioles are seen occasionally, neurons bloodstream, and therefore, the brain may be in the adult usually do not divide. considered a neuroendocrine organ. Neurons also Inclusions also may be found in the perikarya. can stimulate or inhibit other neurons they contact. Dark brown granules of melanin pigment occur in neurons from specific regions of the brain, such as Neurons the substantia nigra, locus ceruleus, and dorsal motor nucleus of the vagus nerve, and in spinal and VOCABULARY: perikaryon, dendrite, axon, sympathetic ganglia. More common inclusions are neurofibril, neurofilament, neurotubule, lipofuscin granules, which increase with age and chromophilic (Nissl) substance, axon hillock, are the by-products of normal lysosomal activity. melanin pigment, lipofuscin granule Lysosomes are abundant in most neurons due to the high turnover of plasmalemma and other The nerve cell, or neuron, is the structural and cellular components. The lipid droplets seen in functional unit of nervous tissue. Usually large and many neurons represent storage material or may complex in shape, it consists of a cell body, the occur as the result of pathologic metabolism. perikaryon, and several cytoplasmic processes. Dendrites are processes that conduct impulses to the perikaryon and usually are multiple. The single process that carries the impulse from the perikaryon is the axon. The perikarya of different types of neurons vary markedly in size (4 to 140 µm) and shape and usually contain a large, central, spherical nucleus with a prominent nucleolus. The cytoplasm is rich in organelles and may contain a variety of

105 Nerve Processes and long, slender mitochondria. Since axoplasm does not contain Nissl substance, protein synthesis VOCABULARY: dendrite, dendritic spine does not occur in the axon. Protein metabolized by (gemmule), axon, axon hillock, collaterals, the axon during nerve transmission is replaced by telodendrion, axoplasm, antegrade transport, that synthesized in cisternae of granular kinesin, retrograde transport, dynein, action endoplasmic reticulum in the Nissl substance of the potential perikaryon. Neurotubules function to transport the new protein as well as other materials within the Nerve processes are cytoplasmic extensions of the axon. perikaryon and occur as dendrites and axons. Each Antegrade transport involves movement of neuron usually has several dendrites that extend material from the perikaryon to axon terminals and from the perikaryon, dividing repeatedly to form may be slow or fast. Fast transport (410 mm/day) branchlike extensions. Although tremendously involves transfer of membranous organelles, diverse in the number, size, and shape of dendrites, neurosecretory vesicles, calcium, sugars, amino each variety of neuron has a similar branching pattern. The acids, and nucleotides: Slow transport (1-6 mm/day) dendritic cytoplasm contains elongate includes movement of proteins, metabolic enzymes, mitochondria, Nissl substance, scattered and elements of the cytoskeleton. Fast antegrade neurofilaments, and parallel-running microtubules. transport is mediated by kinesin, a microtubule The cell membrane of most dendrites forms associated protein (MAP) which has ATPase numerous minute projections called dendritic activity. One end of the kinesin molecule attaches spines or gemmules that serve as areas for to a transport vesicle or organelle. The remaining synaptic contact between neurons; an important end undergoes cyclic interaction with the function of dendrites is to receive impulses from microtubule wall, resulting in movement toward the other neurons. Dendrites provide most of the axon terminal. Retrograde transport is the transfer surface area for receptive synaptic contact between of exhausted organelles, proteins, small molecules, neurons, although in some neurons the perikaryon and recycled membrane from the axonal endings to and initial segment of the axon also may act as the perikaryon and provides a route by which some receptor areas. The number of synaptic points on toxins (tetanus) and viruses (herpes, polio, rabies) the dendritic tree varies with the type of neuron but can invade the central nervous system. Retrograde may be in the hundreds of thousands. Dendrites rapid transport occurs at a rate of about 310 mm/day play an important role in integrating the many and depends on another microtubule associated incoming impulses. protein called dynein which also has ATPase A neuron has only one axon, which conducts activity. Dendritic transport also occurs but is less impulses away from the parent neuron to other well understood. functionally related neurons or effector organs. The Axons transmit responses as electrical impulses axon arises from the axon hillock, an elevation on called action potentials, which begin in the region the surface of the perikaryon that lacks Nissl of the axon hillock and the initial segment of the substance. Occasionally, axons may arise from the axon. An impulse behaves in an "all or none" fashion. base of a major dendrite. Axons usually are much Only when the threshold of activity is reached is the longer and more slender than dendrites and may or action potential transmitted along the axon to its may not give rise to side branches called terminations, which synapse with adjacent neurons collaterals, which, unlike dendritic branches, or end on effector cells. An axon always induces a usually leave the parent axon at right angles. Axons positive activity - for example, causing muscle cells also differ in that the diameter is constant to contract or stimulating epithelial cells to secrete. throughout most of the length and the external Impulses directed to another neuron may be surface generally is smooth. Axons end in several excitatory or inhibitory. branches called telodendria, which vary in number and shape and may form a network or basket-like arrangement around postsynaptic neurons. The cytoplasm of the axon, the axoplasm, contains numerous neurofilaments, neurotubules, elongate profiles of smooth endoplasmic reticulum,

106 Distribution of Neurons Individual neurons also can be classified by their number of dendrites. Unipolar neurons lack VOCABULARY: gray matter, white matter, dendrites and have only a single process, the axon. nuclei, ganglia Although common in the developing nervous system, they are rare in adults. Bipolar neurons The central nervous system is divided into gray matter have a single dendrite and an axon, usually located and white matter, reflecting the concentration of at opposite poles of the perikaryon. They are found perikarya. Gray matter contains the perikarya of in the retina, olfactory epithelium, and cochlear and neurons and their closely related processes, whereas vestibular ganglia. Neurons of all craniospinal white matter is composed chiefly of bundles or ganglia originate as bipolar neurons in the embryo, masses of axons and their surrounding sheaths. but during development, the dendrite and axon In the spinal cord, gray matter has a central location migrate to a common site in the cell body, where surrounded by white matter; in the cerebral and they unite to form a single process. In the adult, this cerebellar cortices, gray matter is at the periphery type of neuron is said to be pseudounipolar. The and covers the white matter. Aggregates of combined process, often called a dendraxon, may run perikarya occur within the gray matter of the central for a short distance and then divide into two nervous system and act as distinct functional units processes, one of which serves as a dendrite and called nuclei. Similar aggregates or individual receives stimuli from peripheral regions of the body neurons located outside the central nervous system and the other acts as an axon and enters the gray are called ganglia. matter of the central nervous system to synapse with other neurons. Although the process directed Types of Neurons toward the periphery acts as a dendrite, it is unusual because, morphologically, it resembles an axon. VOCABULARY: Golgi type I neuron, Golgi This functional dendrite is smooth and unbranched type II neuron, unipolar neuron, bipolar and usually receives nervous input from a receptor neuron, pseudounipolar neuron, multipolar organ. Multipolar neurons are the most common neuron type. They vary considerably in size and shape and are characterized by multiple dendrites. Internuncial Neurons may be divided into types according to and motor neurons are of this type. Of these, the their location, number of dendrites, and length and internuncial neuron is the most numerous. position of axons. In humans, most neurons lie within the gray matter of the central nervous Ganglia system. Some neurons have numerous, well- developed dendrites and very long axons that leave VOCABULARY: cranial ganglion, spinal the gray matter to enter the white matter of the ganglion, satellite cell, sensory ganglion, central nervous system and ascend or descend in pseudounipolar neuron, glomerulus, autonomic the major fiber tracts of the brain or spinal cord or ganglion, visceral motor efferent, multipolar leave the central nervous system and contribute to neuron the formation of peripheral nerves. Neurons of this type conduct impulses over long distances and are Ganglia are classified as cranial, spinal, and called Golgi type I neurons. Other neurons whose autonomic. Macroscopically, cranial and spinal axons are short and do not leave the region of the ganglia appear as globular swellings on the sensory perikaryon are called Golgi type II neurons. These roots of their respective nerves. Each ganglion is are especially numerous in the cerebellar and enveloped by a connective tissue capsule and may cerebral cortices and retina of the eye. They vary contain perikarya of only a few neurons or as many greatly in size and shape and have several branching as 55,000. A delicate network of collagenous and dendrites. Golgi type II neurons primarily serve as reticular fibers, accompanied by small blood vessels, association neurons - that is, they collect nerve extends between individual neurons and together impulses and disseminate them to surrounding with bundles of nerve processes often separates the neurons. perikarya into groups. The perikarya are surrounded by two distinct capsules.

107 The inner capsule is made up of a single layer of low Axons of peripheral nerves are enclosed by a cuboidal supporting cells called satellite cells sheath of flattened cells called Schwann cells. (amphicytes). The outer capsule lies immediately These cells are thin, attenuated cells with flattened, outside the basal lamina of the satellite cells and elongate nuclei located near the center of the cells. consists of a delicate vascular connective tissue. Their cytoplasm contains a small Golgi complex, a Cranial and spinal ganglia are sensory ganglia few scattered profiles of granular endoplasmic and contain pseudounipolar neurons. The reticulum, scattered mitochondria, microtubules, perikarya appear spherical or pear-shaped and are microfilaments, and at times, numerous lysosomes. 15 to 100 µm in diameter. They often show a large, A continuous external (basal) lamina covers successive central nucleus and a distinct nucleolus. The Schwann cells and the axonal surfaces of the nodes. dendraxon of the pseudounipolar neuron may The integrity of the external lamina is vital to become convoluted to form a glomerulus. This regenerating axons and Schwann cells after injury. nerve process then divides: One branch, a Schwann cells invest nerve fibers from near their functional dendrite, passes to a receptor organ; the beginnings almost to their terminations. The other, a functional axon, passes into the central resulting neurilemma (sheath of Schwann) is nervous system. The perikarya of these continuous with the capsule of satellite cells that pseudounipolar neurons do not receive synapses from surrounds the perikarya of neurons in craniospinal other neurons. ganglia. Schwann cells produce and maintain the myelin on Autonomic ganglia consist of collections of nerve fibers of the peripheral nervous system. perikarya of visceral efferent motor neurons and The neurilemmal and myelin sheaths are are located in swellings along the sympathetic chain interrupted by small gaps at regular intervals along or in the walls of organs supplied by the autonomic the extent of the nerve fiber. These interruptions nervous system. They generally have a connective represent regions of discontinuity between tissue capsule. The neurons usually are multipolar individual Schwann cells and are called nodes of and assume various sizes and shapes. Perikarya Ranvier. The neurilemmal and myelin sheaths are range from 15 to 60 µm in diameter; the nucleus is made up of a series of small individual segments, large, round, and often eccentrically placed in the each of which consists of the region between two cell; and binucleate cells are not uncommon. consecutive nodes of Ranvier and makes up one Lipofuscin granules are more frequent in neurons internode or internodal segment. An internode of autonomic ganglia than in craniospinal ganglia. represents the area occupied by a single Schwann cell and The ganglia of larger sympathetic chains are its myelin and is about 1 to 2 mm in length. Myelin encapsulated by satellite cells, but a capsule may be is not a secretory product of Schwann cells but a absent around ganglia in the walls of the viscera. mass of lipoprotein and glycolipid that includes Unlike craniospinal ganglia, neurons of autonomic galactocerebroside, which is abundant in myelin. ganglia receive numerous synapses and are influenced by Myelin results from a successive layering of the other neurons. Schwann cell plasmalemma as it wraps around the axon. The wraps of cell membrane that form Nerve Fibers myelin are tightly bound together by specialized proteins. Myelin may appear homogeneous, but VOCABULARY: myelin, Schwann cell, after some methods of preparation, it may be neurilemma (sheath of Schwann), node of represented by a network of residual protein called Ranvier, internode, internodal segment, neurokeratin. neurokeratin, major dense line, intraperiod line, Ultrastructurally, myelin appears as a series of mesaxon, action potential, saltatory conduction regular, repeating light and dark lines. The dark lines, called major dense lines, result from A nerve fiber consists of an axon (axis cylinder) and apposition of the inner surfaces of the Schwann cell a surrounding sheath of cells. Large nerve fibers are plasmalemma. The cytoplasmic surfaces are united enclosed by a lipoprotein material called myelin; by a protein called myelin basic protein (MBP). Po smaller nerve fibers may or may not be surrounded protein is associated with myelin basic protein to by myelin. Thus, nerve fibers may be classified as form the major dense line. Po protein constitutes myelinated or unmyelinated. about 50% of the protein in peripheral nervous system myelin and aids in holding the myelin

108 together. The less dense intraperiod lines many of the visceral sensory fibers. Type C fibers are represent the fusion of the outer surfaces of the small, unmyelinated nerve fibers that conduct Schwann cell plasmalemma and lie between the impulses at 0.5 to 2.0 m/second. repeating major dense lines. The outer surfaces of The first event in development of an action the Schwann cell plasmalemma are united by an potential along a nerve fiber is a sudden increase in adhesion molecule called proteolipid protein (PLP). permeability to sodium ion, resulting in depolarization Oblique discontinuities often are observed in the of the axon and development of a negative charge myelin of peripheral nerves and form the Schmidt- along the axon surface. The local current created Lantermann incisures or clefts. The clefts represent between the depolarized and resting surface local separations of the myelin lamellae. membranes causes an increased permeability of the Unmyelinated fibers (axons) also are surrounded by membrane to sodium ion. A cycle of membrane Schwann cells. In electron micrographs, 15 or more activation is established that results in the unmyelinated axons often are found in recesses in transmission of the depolarization process (a nerve the plasmalemma of a single Schwann cell. The cell impulse) along the nerve fiber. Once depolarization membrane of the Schwann cell closely surrounds occurs, it will pass along the entire cell membrane each axon and is intimately associated with it. of the axon. The nerve impulse lasts for only a As the cell membrane of the Schwann cell encircles short time, and repolarization is brought about by a the axon, it courses back to come in contact with sodium pump mechanism in the cell membrane of itself. This point of contact is called the mesaxon. the axon. The generation of a nerve impulse is an In myelinated nerves, the primary infoldings of the "all or nothing" phenomenon. Schwann cell membrane at the axon-myelin In the faster-conducting myelinated nerves, junction is called the internal mesaxon. The junction impulses travel from node to node; this method of between the superficial lamellae of the myelin impulse conduction is called saltatory conduction. sheath and the Schwann cell plasmalemma is the The node of Ranvier is the only segment of the external mesaxon (Fig. 9-1). axon not invested by the myelin sheath and is exposed to the extracellular fluid. It is at this location that action potentials are regenerated due to a high concentration of ion channels. Because depolarization occurs only at the nodes of Ranvier, nerve impulses jump from node to node across the intervening internodal segment. The axon at each node is slightly thicker than that associated with internodal segments, and the plasmalemma of the node region contains most of the sodium voltage- gated channel proteins held in place at this location by the link protein ankyrin, which attaches them to

Fig. 9-1. Diagrammatic representations of a myelinated nerve fiber the cytoskeleton. This explains, in part, the higher (A, B) and a nonmyelinated nerve fiber (C). velocity of nerve transmission in myelinated nerve fibers (Fig. 9-2). The speed with which an impulse is transmitted along a nerve fiber is proportional to the diameter of the fiber. The diameter of heavily myelinated nerve fibers is much greater than that of unmyelinated fibers, and therefore, conduction is faster in myelinated fibers. Nerve fibers in peripheral nerves can be classified according to the diameter and speed of conduction. Type A nerve fibers are large, myelinated motor and sensory fibers, 3 to 20 µm in diameter, that conduct impulses at 15 to 120 m/second. More finely myelinated fibers with diameters up to 3 µm that conduct impulses at Fig. 9-2. Schematic representation of an action potential in (A) nonmyelinated and (B) myelinated nerve fibers. 3 to 15 m/second are type B fibers. They represent

109 Peripheral Nerves small, oval expansions called motor end-plates. The skeletal muscle cells supplied by a single motor VOCABULARY: mixed nerve, epineurium, neuron constitute a motor unit. Muscles with a fascicle, perineurium, endoneurium large number of motor units can make more precise movements than those with fewer units for the Peripheral nerves consist of several nerve fibers same number of muscle cells. Visceral efferent united by surrounding connective tissue. They are nerve fibers stimulate smooth muscle, cardiac mixed nerves consisting of sensory (afferent) and muscle, and glandular epithelium. Visceral motor motor (efferent) nerve fibers that may be endings of smooth muscle terminate as two or myelinated or unmyelinated. The sheath of more swellings that pass between individual muscle connective tissue surrounding a peripheral nerve cells. In cardiac muscle, numerous thin nerve fibers makes up the epineurium and unites several end near the surface of individual muscle cells but bundles of nerve fibers into a single unit, a form no specialized contacts with them. Endings in peripheral nerve. The epineurium consists of glands are from unmyelinated nerves and form fibroblasts, longitudinally arranged collagen fibers, elaborate, delicate nets along the external surface of and scattered fat cells. Each bundle or fascicle of the basal lamina of the gland cells: fine branches nerve fibers is enclosed by concentric layers of penetrate the basal lamina and pass between flattened, fibroblast-like cells that form a dense individual epithelial cells. sheath called the perineurium. The perineurium is Terminal peripheral nerve fibers that are excitable limited externally and internally by a basal lamina. to stimuli are receptors and can transform Tight junctions occur between cells, and the tightly chemical and physical stimuli into nerve impulses. adherent cells form a perineurial compartment for Receptors vary in morphology, may be quite individual fascicles. Delicate collagenous fibers, complex, and often are grouped into free (naked), reticular fibers, fibroblasts, and macrophages lie diffuse, and encapsulated nerve endings. between individual nerve fibers within each fascicle Free (naked) nerve endings arise from and constitute the endoneurium. Small blood myelinated and unmyelinated fibers of relatively vessels are found primarily in the epineurium, but in small diameter. They are widely distributed and, thicker regions of the endoneurium they may occur although most numerous in the skin, as delicate capillary networks. The perineurium and are present in the connective tissue of visceral epineurium of peripheral nerves become organs, in deep fascia, in muscles, and in mucous continuous with the meninges of the central membranes. As myelinated fibers near their nervous system. terminations, they lose their myelin and form unmyelinated terminal arborizations. Unmyelinated Peripheral Nerve Endings nerve fibers end in numerous fibrils that terminate as small, knob-like thickenings. Many naked nerve VOCABULARY: somatic efferent, motor end- endings act as pain receptors. A diffuse type of naked plate, motor unit, visceral efferent, receptor, receptor encircles the base of hair follicles to form free (naked) nerve ending, encapsulated nerve peritrichal nerve endings that are sensitive to hair ending movement. In addition to the naked interepithelial nerve endings that terminate among epithelial cells, Nerve endings increase the surface contact area there are some endings that form concave between nervous and nonnervous structures and neurofibrillar discs applied to a single modified allow stimuli to be transmitted from nerve endings epithelial cell of stratified squamous epithelium. to muscles, causing them to contract, or to These more specialized nerve endings are called the epithelial cells, causing them to secrete. When tactile discs of Merkel. stimulated, dendrites at the periphery generate In encapsulated nerve endings, the terminals impulses that are transferred along the nerve fiber are enclosed in a capsule of connective tissue. They to sensory ganglia and ultimately to the central vary considerably in size, shape, location, and type nervous system. of stimuli perceived. Peripheral efferent (motor) nerve fibers can be divided into somatic and visceral efferent groups. Somatic efferent fibers end in skeletal muscle as

110 As the nerve fibers near their respective capsules, Synapse they generally lose their myelin and neurolemmal sheaths and enter the capsule as naked nerve VOCABULARY: terminal bouton, calyx endings (Fig. 9-3). (basket ending), bouton en passage, synaptic vesicle, presynaptic membrane, synaptic cleft, postsynaptic membrane, subsynaptic web, threshold of excitation, gap junction

The site where impulses are transmitted from one neuron to another is called a synapse and may occur between axon and dendrite (axodendritic), axon and cell body (axosomatic), axon and axon (axoaxonic), dendrite and dendrite (dendrodendritic), dendrite and perikaryon (somatodendritic), or cell bodies of adjacent neurons (somatosomatic). The number of synapses associated with a single neuron varies according to the type of neuron. Small Golgi type II neurons (granule cells of the cerebellum) may have only a few synaptic points, whereas other neurons, such as Golgi type I (Purkinje cells of the cerebellum), may have several hundred thousand. Physiologically, excitatory and inhibitory synapses occur. Synaptic endings of axons usually occur as small swellings at the tips of axon branches and are called terminal boutons. The axon may end in thin

Fig. 9-3. Diagrammatic sketches of types of sensory nerve endings. branches that surround the perikaryon or dendrites of another neuron; these endings are called calyces (basket endings). Synaptic contacts also may A number of encapsulated nerve endings, their occur at intervals along the terminal segment of an locations, and proposed functions are summarized axon, forming boutons en passage. in Table 9-1. Synaptic endings vary considerably in form from one type of neuron to another but generally show Table 9-1. Encapsulated sensory nerve endings. some common features. Each neuron is a distinct Name of receptor Locations Functions cellular unit, and there is no cytoplasmic continuity between cells. In some ways, synaptic points Corpuscles of Vater-Pacini Dermis of skin, Proprioception, mesenteries, pleura, deep pressure, resemble desmosomes and may aid in maintaining nipples, pancreas, vibration contact between nerve cells. Ultrastructurally, the tendons, joint capsules, presynaptic area (terminal bouton) contains clusters of ligaments, walls of viscera, penis, clitoris mitochondria and numerous membrane-bound vesicles with diameters of 40 to 60 nm. These Corpuscles of Meissner Dermal papillae of digits, Light touch synaptic vesicles contain neurotransmitter lips, genitalia, and nipples substances such as dopamine, norepinephrine, End-bulbs of Krause Dermis, conjunctiva, oral Proprioception, acetylcholine, serotonin, amino acids, and a number cavity pressure of peptides. Acetylcholine is the transmitter Genital corpuscles Penis, clitoris, nipples Touch, pressure substance found most commonly in the electron- lucent synaptic vesicles of the central nervous Corpuscles of Ruffini Dermis, joint capsules Proprioception, pressure system and motor end plates. Synaptic vesicles of most sympathetic postganglionic axons are Neurotendinous endings Tendons Proprioception characterized by electron-dense cores and contain (of Golgi) catecholamines. The vesicles congregate near the Neuromuscular spindles Skeletal muscle Proprioception presynaptic membrane, a slightly thickened area of the axon plasmalemma at the synaptic contact.

111 Pre-and post-synaptic membranes are separated by As a wave of depolarization reaches the a narrow space, 20 to 30 nm wide, called the presynaptic membrane, increased levels of calcium synaptic cleft, which may contain fine filaments ion result, triggering a release by exocytosis of and glycosaminoglycans. The postsynaptic transmitter substance into the synaptic cleft. The membrane shows increased electron density and is transmitter released interacts with receptors within slightly thickened, and its internal surface is the postsynaptic membrane to elicit one of three associated with a network of dense filamentous possible effects (depolarization, hyperpolarization, material called the subsynaptic web (Fig. 9-4). neuromodulation) in the target neuron. If the The cytoplasm of the postsynaptic region lacks transmitter substance (acetylcholine, glutamate) binds to synaptic vesicles and shows fewer mitochondria specific ion-channel receptor proteins, the ion- than are found in the presynaptic area. The channel receptor proteins in the postsynaptic postsynaptic plasmalemma contains several special membrane open that allows entry of positively transmembrane proteins and receptors involved in charged ions (sodium, potassium, calcium) resulting neurotransmission. in depolarization and excitation of the target neuron. If, on the other hand, the transmitter substance released is gamma-amino butyric acid (GABA), or glycine, which binds to ion-channel receptor proteins that allow entry of negatively charged ions (primarily chloride ion) into the cell, and then hyperpolarization occurs, resulting in inhibition. In some instances transmitters (opioid proteins, dopamine, small neuropeptides, epinephrine, and norepinephrine) bind to G-protein-linked receptors and not intrinsic ion channels. These receptors stimulate second-messenger molecule systems that through a cascade of intracellular events modify the sensitivity or modulate the target neuron to depolarization. Fig. 9-4. A diagrammatic sketch of a synapse. Two groups of synaptic vesicle are thought to exist within the axon terminal: a small group that is As an action potential reaches the presynaptic area released with the arrival of an action potential and a of an axon, the synaptic vesicles fuse at sites along larger reserve group that is linked to the the presynaptic membrane and release transmitter cytoskeleton by actin filaments. Synaptic vesicles of substance into the synaptic cleft. The transmitter the reserve group move to a releasable group as substance released reacts with the special receptors needed. Synapsin I, a phosphoprotein associated in the postsynaptic membrane, causing an with the cytoplasmic surface of synaptic vesicles, immediate increase in the permeability of the post- links actin to the vesicle surface, which results in neuronal cell membrane to sodium ion, thus the accumulation of reserve vesicles. changing the resting membrane potential. If the Phosphorylation of synapsin I, due to the influx of potential of the postsynaptic membrane rises above calcium ion triggered by the action potential, a certain level, the threshold of excitation, an disassociates this link, allowing these vesicles to join action potential is transmitted by the postsynaptic the releasable group. An additional phosphoprotein, neuron. The first parts of the postsynaptic neuron synapsin II, is thought to mediate the interaction of to initiate an action potential are the axon hillock and synaptic vesicles with the cytoskeleton. At rest a initial segment of the axon. The initial segment of an calcium-sensitive trigger protein (synaptotagmin) axon is a few microns long and lacks a myelin blocks membrane fusion and exocytosis. When an sheath. In most neurons, the cell membrane of this action potential reaches the terminal bouton, voltage- region has a lower threshold of excitation than does gated calcium ion channels open. The subsequent that of the perikaryon or dendrites. increase in calcium ion concentration releases a If transmitter substances decrease the permeability of trigger protein, allowing membrane fusion and the postsynaptic membrane, the threshold of exocytosis to occur. excitation rises and the net effect is inhibitory.

112 The membrane of the synaptic vesicle contains Depending on the location in the central nervous synaptobrevin, an anchoring protein that unites the system, neuroglia may outnumber neurons by as much as vesicle to docking proteins in the presynaptic 50:1. Together, neuroglia account for about 50% of membrane. The presynaptic membrane contains the weight of the brain and outnumber neurons additional proteins that promote membrane fusion about ten to one. It is estimated that about 1,000 with the synaptic vesicle and allows exocytosis to billion glia occur in the CNS as compared to about occur. 100 billion neurons. Neuroglia generally are smaller Following vesicle fusion with the presynaptic than neurons and in light microscopic preparations membrane and release of transmitter, equal can be identified by their small, round nuclei, which amounts of cell membrane are taken up by the are scattered among neurons and their processes. formation of clathrin-coated vesicles via Neuroglia includes ependymal cells, astrocytes, endocytosis. These vesicles may fuse with cisternae oligodendrocytes, and microglia. Astrocytes and of endoplasmic reticulum containing transmitter oligodendrocytes often are collectively referred to substances. New vesicles form by budding from as macroglia. these cisternae to return to the pool of reserve Morphologically, astrocytes are divided into synaptic vesicles. protoplasmic and fibrous types. Protoplasmic Acetylcholine released from the axon terminal is astrocytes are found mainly in the gray matter and rapidly hydrolyzed by acetylcholine esterase into are characterized by numerous, thick branched acetate and choline in the synaptic cleft. Choline is processes. Fibrous astrocytes occur mainly in then transported back through the presynaptic white matter and are distinguished by long, thin, membrane by a choline transporter, converted to generally unbranched processes. The two forms of acetylcholine by the enzyme choline acetyltransferase, astrocyte may represent a single type that varies in and repackaged into newly formed synaptic vesicles. morphology according to its location and metabolic Inhibitory transmitters such as GABA, on the other state. In electron micrographs, astrocytes usually hand, are taken up by specific GABA transporters show an electron-lucent, largely organelle-free located in the plasma membrane of surrounding cytoplasm and euchromatic nuclei. The cytoplasm astrocyte processes. In the astrocyte cytoplasm of the cell body and its processes contains bundles GABA is broken down to glutamine, which of intermediate filaments called glial filaments, reenters the nerve terminal. Here, glutamine is consisting of glial fibrillary acid protein (GFAP). enzymatically converted back into GABA and These filaments are particularly abundant in packaged into synaptic vesicles. protoplasmic astrocytes. Astrocytes provide a Neuropeptides, unlike many of the other framework of support for the neurons and transmitters, must be synthesized in the perikaryon contribute to their nutrition and metabolic activity. and transported to the axon terminal. Electrical The astrocyte processes vary considerably in coupling is another means of transferring impulses appearance. Some are thick and long; others are in some types of synapses and occurs at gap thin and sheetlike. The processes often expand to junctions. Gap junctions act as ion channels. form end feet (foot processes) that are aligned along the internal surface of the pia mater and Neuroglia around the walls of blood vessels. The largest concentration of end feet occurs beneath the pial VOCABULARY: astrocyte, protoplasmic surface, where they surround the brain and spinal astrocyte, fibrous astrocyte, glial fibrillary acid cord to form a membrane-like structure called the protein, end foot (foot process), glia limitans. oligodendrocyte, interfascicular Smaller neuroglial cells with fewer processes and oligodendrocyte, perineuronal satellite more deeply stained nuclei are oligodendrocytes, oligodendrocyte, microglia, ependyma, tela of which there are two types. Interfascicular choroidea, choroid plexus, choroid epithelial oligodendrocytes occur mainly in the fiber tracts cells, blood-brain barrier, tanycytes that form the white matter of the brain and spinal cord. Perineuronal satellite oligodendrocytes are Neurons form a relatively small proportion of the restricted to the gray matter and are closely cells in the central nervous system. Most of the cells associated with the cell bodies of neurons. are nonneuronal supporting cells called neuroglia.

113 The cytoplasm of both types of oligodendrocytes oligodendrocytes influence the metabolism and contains free ribosomes, short cisternae of granular nutrition of neurons. In addition, glia appear to be endoplasmic reticulum, an extensive Golgi involved in calcium homeostasis and in the complex, and numerous mitochondria. Large recycling of certain types of neurotransmitters. numbers of microtubules form parallel arrays that In the central nervous system, perineuronal run throughout the cytoplasm of the cell body and satellite and interfascicular oligodendrocytes have into its processes. the same relationship to perikarya and their axons Schwann cells are not present in the central as that which exists between satellite cells and nervous system; oligodendrocytes serve as the myelin- Schwann cells of peripheral nerve fibers. Schwann forming cells for this region. Myelinated nerve fibers of cells and satellite cells could be considered as the central nervous system have nodes of Ranvier, neuroglial elements of the peripheral nervous but unlike peripheral myelinated fibers, they do not system. show incisures. Nodes in the central nervous Small cells called microglia also are present in system are bare, while those of the peripheral the central nervous system. Microglia have nervous system are partly covered by extensions of extensive ramifying processes, a characteristic cytoplasm from adjacent Schwann cells into the phenotype that suggests they may be dendritic paranodal area. The myelin sheath begins just antigen-presenting cells. They also express class II beyond the initial segment of the axon, a few MCH molecules. In areas of damage or disease, microns from the axon hillock, and ensheathes the they are thought to proliferate and become rest of the axon to near its end. Each internodal phagocytic and are supplemented by monocytes segment of a nerve fiber in the central nervous from the blood that transform into additional system is formed by a single cytoplasmic process macrophages. As material is phagocytosed, the cells from a nearby oligodendrocyte, which wraps enlarge and then are called Gitterzellen, or compound around the axon. Unlike Schwann cells of the granular corpuscles. Monocytes appear to be the peripheral nervous system, a single oligodendrocyte precursors of microglial cells, which thus are part of provides the internodal segments of the myelin the mononuclear system of phagocytes with sheath for several separate, but adjacent axons (Fig. ultimate derivation from the bone marrow. 9-5). The ependyma lines the central canal of the spinal cord and ventricles of the brain. It consists of a simple epithelium in which the closely packed cells vary from cuboidal to columnar. The luminal surfaces of the cells show large numbers of microvilli and, depending on the location, may show cilia. Adjacent cells are united by desmosomes and zonula adherens; zonula occludens generally are not seen. Hence, cerebrospinal fluid in the central canal and ventricles can pass between ependymal cells to enter the parenchyma of the central nervous system. The bases of ependymal cells have long,

Fig. 9-5. A diagrammatic representation of myelination in the threadlike processes that branch, enter the central nervous system. substance of the brain and spinal cord, and may

extend to the external surface, where they In some regions, as in the optic nerve, a single contribute end feet to the glia limitans. oligodendrocyte may be responsible for the The ependyma forms a secretory epithelium in the internodal segments on 40 to 50 axons. In the CNS, ventricles of the brain, where it is in direct contact like the PNS, special adhesion molecules with a highly vascular region of the pia mater; the (proteolipid protein) link the external surfaces of tela choroidea. The modified ependyma called oligodendrocyte plasmalemma together. The choroid epithelial cells and the tela choroidea cytoplasmic surfaces of the plasmalemma are united form the choroid plexus, which produces by myelin basic protein. Because perineuronal cerebrospinal fluid. satellite oligodendrocytes lie close to the perikarya of neurons, it has been proposed that

114 Choroid epithelial cells of the choroid plexus are These regions lack the morphologic elements of a columnar and closely packed and bear numerous true blood-brain barrier and, except for the microvilli. Unlike those in other regions of the subcommissural organ, are able to accumulate vital ependyma, cell apices here are joined by zonula dyes. Large molecules are thought to traverse these occludens, which prevent passage of material regions, which provide areas of exchange between between cells. Choroid epithelial cells lie on a the central nervous system and blood. continuous basal lamina that separates them from a connective tissue that contains small bundles of Spinal Cord collagenous fibers, pia-arachnoid cells, and numerous blood vessels. Capillary endothelial cells VOCABULARY: gray matter, internuncial in this region, unlike those elsewhere in the brain, neuron, central canal, ependyma, white matter have numerous fenestrations. Fluid readily moves through the capillary wall but is prevented from The spinal cord is subdivided into a central H- entering the ventricles by the zonula occludens. shaped region of gray matter and a surrounding Choroid epithelial cells secrete sodium ions into the layer of white matter. Gray matter consists mainly ventricles, and chloride ions, water, and other of perikarya of neurons, their dendrites, and substances follow passively. Protein and glucose surrounding neuroglial cells and is arranged into concentrations are relatively low. Cerebrospinal two dorsal and two ventral horns. The dorsal horns fluid is formed continuously, moves slowly through contain perikarya of multipolar neurons receiving the ventricles of the brain, and enters the sensory impulses that enter the spinal cord from the subarachnoid space. It surrounds and protects the peripheral nervous system. Neurons of the dorsal central nervous system from mechanical injury and horns transmit the impulses to other neurons and in is important in the metabolic activities of the this and other areas of gray matter are referred to as central nervous system. internuncial neurons. The multipolar neurons in Certain substances in the blood are prevented the ventral horns are the largest in the spinal cord from entering the central nervous system, although and transmit motor impulses from the spinal cord they readily gain access to other tissues. Capillaries to the periphery. In the thoracic and upper lumbar deep in the spinal cord and brain are sheathed by regions of the spinal cord, small multipolar neurons the end feet of astrocytes, and the nonfenestrated form an intermediolateral horn that provides endothelial cells are united by occluding tight preganglionic sympathetic fibers for the autonomic junctions. The endothelial cells do not exhibit nervous system. The central canal of the spinal transendothelial transport vesicles typical of cord lies in the center of the crossbar of the H- endothelial cells found in capillaries elsewhere. shaped gray matter and is lined by ependyma. Additionally, the internal plasmalemma of the The white matter consists mainly of myelinated endothelial cells is thought to have special axons and lacks the perikarya and dendrites of properties that prevent passage of some substances. neurons. It is subdivided into anterior, lateral, and Together the tight junctions and internal posterior funiculi by the dorsal and ventral horns of plasmalemma of the endothelial cells form the the gray matter. A funiculus consists of several blood-brain barrier. tracts, each of which in turn contains several Highly vascularized areas - the circumventricular organs bundles of nerve fibers. Nerve fibers in each tract - are present in specific regions along the midline of carry similar impulses, motor or sensory, that either the walls of the ventricles. These organs include the ascend or descend along the long axis of the spinal organum vasculosum, lamina terminalis, subfornical cord. organ, subcommissural organ, and area postrema. Except for the subcommissural organ, the Cerebellar and Cerebral ependymal cells of these areas are modified and appear as stellate cells called tanycytes which are Hemispheres thought to transport cerebrospinal fluid to neurons in the hypothalamus. Such regions are devoid of an The cerebellar and cerebral hemispheres differ from internal (subependymal) layer and the external the spinal cord in that the gray matter is located at (subpial) layer of glial processes. The underlying the periphery and the white matter lies centrally. capillaries are lined by a fenestrated endothelium.

115 Both regions of the brain consist of an outer cortex Cerebellar Cortex of gray matter and a subcortical region of white matter. VOCABULARY: molecular layer, stellate cell, basket cell, Purkinje cell layer, granule cell Cerebral Cortex layer, mossy fiber, climbing fiber

VOCABULARY: laminated appearance, The cerebellar cortex is characterized by three pyramidal cell, apical dendrite, Betz cell, layers: an outer molecular layer, a middle Purkinje stellate (nonpyramidal) cell cell layer, and an inner granule cell layer. The molecular layer is mainly a synaptic area The cerebral cortex is 1.5 to 4.0 mm thick and with relatively few nerve cell bodies. It consists contains about 50 billion neurons plus nerve primarily of unmyelinated axons from granule cells, processes and supporting glial cells. In all but a few the axons running parallel to the cortical surface. It regions it is characterized by a laminated also contains large dendrites of the underlying appearance. Perikarya generally are organized into Purkinje cells and in its superficial portion contains five layers. Starting at the periphery of the cerebral small scattered neurons called stellate cells. Other cortex, the general organization of neurons is small neurons located deep in this layer and molecular layer (I), external granular layer (II), adjacent to Purkinje cells are called basket cells. external pyramidal layer (III), internal granular layer The Purkinje cell layer is formed by the cell (IV), internal pyramidal layer (V), and multiform bodies of Purkinje cells - large, pear-shaped layer (VI). neurons aligned in a single row and characterized by The molecular layer (I) is a largely perikaryon-free large branching dendrites that lie in the molecular zone just below the surface of the cortex. Neurons layer. They represent Golgi type I neurons and of similar type tend to occupy the same layer in the number about 15 million. Three-dimensionally, the cerebral cortex, although each cellular layer is large dendritic trees occupy a narrow plane, composed of several different cell types. For reminiscent of fan coral, and are so arranged that convenience of description, these neurons often are each dendritic tree is parallel to its neighbor. A placed in two major groups: pyramidal cells and single small axon from the Purkinje cell passes stellate or nonpyramidal cells. The perikarya of through the granule cell layer and synapses with pyramidal cells are pyramidal in shape and have neurons in the central cerebellar area. large apical dendrites that usually are oriented The granule cell layer consists of numerous toward the surface of the cerebral cortex and enter closely packed, small neurons whose axons enter the overlying layers; the single axons enter the the molecular layer to synapse with dendrites of subcortical white matter. They are found in layers Purkinje cells. The granule cells represent Golgi II, III, V, and, to a lesser extent, layer VI. Very large type II neurons. They have small, round nuclei with pyramid-shaped neurons (Betz cells) are present in coarse chromatin patterns and only scant the internal pyramidal layer (V) of the frontal lobe. cytoplasm; dendrites are short and clawlike. The Stellate (nonpyramidal) cells lack the pyramid- axons enter the molecular layer, bifurcate, and run shaped perikarya and the large apical dendrite. They parallel to the surface but perpendicular to the wide occur in all layers of the cerebral cortex but are plane of the Purkinje dendritic tree. The axons of concentrated in the internal granular layer (IV). granule cells synapse with about 450 Purkinje cells Impulses entering the cortex are relayed primarily in a relationship similar to that of wires coursing to stellate cells and then transmitted to pyramidal along telephone poles. Axons of granule cells also cells in the various layers by the vertical axons of synapse with stellate and basket neurons in the the stellate cells. Axons of pyramidal cells generally molecular layer. Another type of small neuron, the leave the cortex and extend to other regions of the Golgi cell, is found in the outer zone of the granule brain and spinal cord. cell layer. The cerebral cortex functions in vision, hearing, Two types of afferent nerve fibers enter the speech, voluntary motor activities, and learning. cerebellar cortex from outer regions of the central nervous system. These are the mossy fibers, which synapse with granule cells, and the climbing fibers,

116 which enter the molecular layer and wind about the squamous epithelium that also lines a large space, dendrites of Purkinje cells. the subarachnoid space, which contains The cerebellum functions primarily in the cerebrospinal fluid. modulation of skeletal muscle activity such as in the The pia mater is a thin, vascular membrane that coordination of limb movement and balance. However, closely invests the brain and spinal cord. Because there is some evidence that the cerebellar cortex the pia and arachnoid are closely related, they often also participates in cognitive functions. are considered together as the pia arachnoid. The pia The gray matter of the spinal cord, cerebrum, and consists of delicate collagenous and elastic fibers, cerebellum consists of a complex, highly ordered fibroblasts, macrophages, and scattered meshwork of dendritic, axonal, and glial processes mesenchymal cells. Blood vessels entering and that envelop the perikarya of associated neurons. leaving the brain substance are invested by a sheath This network, called the neuropil, provides a vast of pia mater. A perivascular space surrounds the area for synaptic contact and interaction between vessels as they extend into the substance of the nerve processes and forms an organizing central nervous system and is continuous with the framework. It is important for coordinating subarachnoid space. activities in the central nervous system. Autonomic Nervous System Meninges VOCABULARY: autonomic efferent pathway, VOCABULARY: dura mater, epidural space, sympathetic system, parasympathetic system, subdural space, arachnoid, trabeculae, thoracolumbar distribution, craniosacral subarachnoid space, pia mater distribution, acetylcholine, norepinephrine, dopamine In addition to the covering of bone formed by the skull and vertebral column, the central nervous Functionally, the nervous system can also be system is contained within three connective tissue subdivided into autonomic and somatic membranes called meninges. The outermost, the subdivisions. The somatic portion mediates dura mater, consists primarily of dense collagenous voluntary control over skeletal muscle whereas the connective tissue. Around the brain it forms two autonomic portion controls several organ functions layers and serves as a periosteum for the cranium that are not under direct conscious control. Both and as the covering of the brain. The periosteal are organized as reflex arcs composed of afferent layer is rich in blood vessels, nerves, and cells. The and efferent neurons with other neurons within the inner layer is less vascular, and its interior surface is CNS interposed between these two groups of lined by a simple squamous epithelium. Where it neurons. These internuncial neurons function to covers the spinal cord, the dura is a single layer of modulate the activity of the reflex arc. dense irregular connective tissue that contains The autonomic efferent pathway consists of scattered elastic fibers. The outer surface is covered both preganglionic and postganglionic neurons. Nerve by a simple squamous epithelium, separated from processes from preganglionic neurons emerge from the periosteum of the vertebrae by an epidural the CNS to synapse with postganglionic neurons space filled by loose connective tissue that is rich located outside the CNS. This autonomic outflow is in fat cells and veins. The inner surface of the dura further subdivided into either sympathetic or of the spinal cord is lined by a simple squamous parasympathetic. Sympathetic preganglionic neurons have epithelium. Between it and the next meninx, the a thoracolumbar distribution and their axons arachnoid, is a narrow, fluid-filled subdural space. leave the spinal cord with nerve fibers forming the The arachnoid is a thin, weblike, avascular ventral roots of the first thoracic through the fourth membrane made up of fine collagenous fibers and lumbar spinal nerves. These preganglionic nerve scattered elastic fibers. The outer region forms a fibers synapse with neurons located either within smooth sheet, while the inner surface gives rise to the paravertebral sympathetic chain ganglia or in numerous strands (trabeculae) that extend into and collateral ganglia close to the target organ. blend with the underlying pia mater. The inner In contrast, parasympathetic preganglionic neurons have surface of the arachnoid, the trabeculae, and the a craniosacral distribution whose axons exit with external surface of the pia are covered by a simple

117 cranial nerves (III, VII, IX, X) or with spinal nerves myelinization occurs after the differentiation of from the sacral region of the spinal cord. The oligodendrocytes. neurons of the parasympathetic ganglia are usually The regions of the neural tube destined to form located within the target organ or are in very close the cerebral and cerebellar cortices show three proximity. zones similar to those of the spinal cord. The neurotransmitter released by all Differentiating neurons in the mantle zone of the parasympathetic postganglionic nerve endings is cerebral cortex migrate into the marginal zone to acetylcholine. This neurotransmitter is also establish superficial cortical layers. The stratified released at neuromuscular junctions by the somatic appearance of the adult cerebral cortex results from nervous system as well as by nerve endings of the successive waves of migration of neuroblasts into the sympathetic nervous system associated with sweat marginal zone and the differentiation of different glands. Norepinephrine or dopamine are the types of neurons in the cortex. Sensory areas of the neurotransmitters released by most postganglionic cerebral cortex are typified by numerous granular sympathetic nerve endings. Generally, the sympathetic cells (neurons), whereas motor areas are system is associated with fight or flight responses whereas characterized by large numbers of pyramidal cells. those of the parasympathetic system are more closely Initially, the developing cerebellar cortex also is related with resting or digestive functions. arranged into ependymal, mantle, and marginal zones. As development progresses, some Histogenesis neuroblasts from the mantle zone migrate into the marginal zone of the cerebellar cortex to become Most of the nervous system arises from Golgi and granular cells, while others remain in the neuroectoderm on the dorsal side of the embryonic mantle zone to form cerebellar nuclei. As this takes disk, in the region called the neural plate. As a result place, a second migration occurs and these Purkinje of mitotic activity and cell growth, the lateral edges cells, together with cells from the first migration, of the neural plate fold upward to form a neural establish the definitive cerebellar cortex. groove. The edges of the fold converge, fuse, and As the neural tube closes, a second source of create a hollow neural tube that separates from the neurons, the neural crest, arises and gives origin to overlying ectoderm. Differential proliferation of neurons and supportive (glial) elements. Along the ectodermal cells along the internal aspect of the crest of each neural fold, ectodermal cells lose their neural tube ultimately gives rise to the brain and epithelial characteristics, detach, and migrate into spinal cord. Some of the proliferating cells the mesoderm that lies on either side of the neural differentiate into neurons; others become glial tube; they then acquire the characteristics of elements. mesenchymal cells. The neural crest is a major Three concentric zones become visible in the wall contributor to the peripheral nervous system and of the neural tube: an internal ependymal layer, a wide, gives rise to neurons and supporting cells (glia and intermediate mantle layer; and an outer marginal layer. satellite cells) of spinal root ganglia; autonomic Cells of the ependymal layer divide, migrate to the ganglia; Schwann cells; some neurons in the sensory mantle layer, and differentiate into neuroblasts or ganglia of cranial nerves V, VII, IX, and X; and glioblasts. Neurons differentiate first, followed by supporting cells, but not the neurons of the astrocytes and later by oligodendrocytes. geniculate, acoustic, petrosal, and nodose ganglia. In the spinal cord, the ependymal zone gives rise Other cells of the neural crest envelop the neural to the ependyma, the mantle zone becomes the gray tube and contribute to formation of the meninges. matter, and the white matter develops from the marginal zone. Presumptive neurons in the mantle Summary layer are apolar neuroblasts at first but become bipolar neurons as cytoplasmic processes develop at An essential function of nervous tissue is opposite ends of the cell body. As differentiation communication. Nervous tissue is able to fulfill this proceeds, one process (a primitive axon) elongates role because of its ability to react to various stimuli while the other is replaced by several small and transmit the impulse formed from one region processes (primordial dendrites), and multipolar to another. Thus, the individual is able to react to neurons develop. The axons enter into and form the external environment and internal events to the marginal layer. Depending on the region, integrate and coordinate body functions. In cortical

118 regions of the brain, complex relationships between internodal segment to adjacent nodes. This saltatory different types of neurons provide intellect, conduction partly explains the greater speed of memory, and conscious experience and serve for nerve impulses transmitted by myelinated nerve the interpretation of special impulses from the eye fibers. and ear into the sensations of sight and sound, Communication between neurons and effector respectively. All contribute to the personality and organs takes place at specific contact points called behavior of the individual. synapses, where nerve impulses are transferred Neurons are the structural and functional units of directly to other cells by chemical transmitters or nervous tissue. They generally are complex in shape electrical coupling. During chemical synaptic and usually have several cytoplasmic processes. transmission the following events are observed to Dendrites conduct nerve impulses to the occur: (a) neurotransmitter molecules are perikaryon, and a single axon conducts impulses synthesized and packaged into synaptic vesicles by from the perikaryon to other neurons or effector the presynaptic neuron, (b) an action potential organs. Receptor organs of nerve tissue convert arrives at the presynaptic membrane of the nerve mechanical, chemical, or other stimuli into nerve terminal, (c) voltage gated calcium channels open impulses that are transmitted along a physiologic and calcium ion enters the presynaptic terminal, (d) dendrite to its cell body located in spinal or cranial the rise in calcium ion triggers fusion of the ganglia. From ganglia the impulses are relayed to synaptic vesicles with the presynaptic membrane, other neurons in the central nervous system. Nerve (e) the transmitter molecules diffuse across the fibers of most somatic sensory neurons are large synaptic cleft and bind to specific receptors located and myelinated, and impulses are relayed quickly to on the postsynaptic neuron, (f) the bound receptors the central nervous system to elicit a response. activate the postsynaptic neuron, and (g) the Most effector and internuncial neurons are neurotransmitter substance is broken down and multipolar, and their numerous dendrites summate taken up by the presynaptic neuron terminal or glial the excitatory and inhibitory impulses that influence cells, or diffuses away from the synapse. The a specific neuron. Dendrites of this type of neuron terminal axons of some neurons, such as those in usually are unmyelinated. When the threshold of the hypothalamic and paraventricular regions, may activity is attained, an action potential is generated secrete peptides or other agents directly into the in the region of the axon hillock and/or initial bloodstream. These neurons influence cells that are segment of the axon. These regions of the neuron not in direct contact with them and thus act as have a lower threshold of excitation than the endocrine cells. dendrites or perikaryon. The impulse is conducted Neurons are surrounded by supporting cells. In the by the axon to other neurons or an effector organ peripheral nervous system, neurons are intimately to elicit a response. The speed of conduction associated with Schwann and satellite cells that, like depends on fiber diameter and usually reflects the astrocytes and oligodendrocytes of the central degree of myelinization, both in peripheral nerve nervous system, provide structural and metabolic fibers and those of the central nervous system. support to their associated neurons. The ependyma Myelin is formed by consecutive layering of the represents glial cells that form a simple epithelial plasmalemma of adjacent supporting cells. In the lining for the ventricles of the brain and the central peripheral nervous system, myelin is formed by canal of the spinal cord. In some areas of the Schwann cells scattered at intervals along a single ventricles, ependyma (choroid epithelial cells) axon. Oligodendrocytes are responsible for comes in direct contact with vascularized regions of myelinization in the central nervous system and the pia mater to form the choroid plexus. The form internodal segments on more than one axon. choroid plexus actively and continuously produces Myelin is thought to act as an insulating material to cerebrospinal fluid, which surrounds and cushions prevent the wave of depolarization (action the brain and spinal cord and plays an important potential) from traveling the length of the role in their metabolism. The meninges form axolemma as occurs in unmyelinated nerve fibers. protective coverings for the brain and spinal cord Depolarization occurs only at the nodes of Ranvier and contain numerous blood vessels that supply the in myelinated nerves, and the impulses jump the structures of the central nervous system.

119 micrographs show a few short microvilli, a thin 10 Cardiovascular and proteoglycan layer over the luminal surface, and the Lymph Vascular Systems usual organelles in the cytoplasm. The endothelial cells rest on a continuous layer of fine collagen Unicellular animals acquire their oxygen and fibers, separated from it by a basement membrane. nutrients directly from the external environment by The fibrous layer is called the subendothelial simple diffusion and other activities of their cell layer. Deep to it is a thick layer of denser membranes. However, in multicellular animals, connective tissue that forms the bulk of the most of the cells lie deep in the body, with no endocardium and contains elastic fibers and some access to the external environment, and materials smooth muscle cells. Loose connective tissue are carried to them through a closed system of constituting the subendocardial layer binds the branching tubes. Together with a muscular pump, endocardium to the underlying heart muscle and the tubular system makes up the cardiovascular (blood contains collagen fibers, elastic fibers, and blood vascular) system. The entire system consists of a heart vessels. In the ventricles it also contains the that acts as a pump, arteries that carry blood to specialized cardiac muscle fibers of the conducting organs and tissues, capillaries through which system. exchange of materials occurs, and veins that return the blood to the heart. Myocardium A second system of vessels, the lymph vascular system, drains interstitial (tissue) fluid from organs VOCABULARY: cardiac muscle, pectinate and tissues and returns it to the blood. It lacks a muscle, trabeculae carnea, atrial natriuretic pumping unit and is unidirectional, carrying lymph factor (ANF) toward the heart only. There is no lymphatic equivalent of blood arteries. The myocardium is the middle layer of the heart and consists primarily of cardiac muscle. The Heart myocardium is a very vascular tissue with a capillary density estimated to be about 2800 capillaries per

mm2 as compared to skeletal muscle which has a The heart is a modified blood vessel that serves as a capillary density of about 350 per mm2. The double pump and consists of four chambers. On capillaries completely surround individual cardiac the right side, the atrium receives blood from the myocytes and are held in close apposition to them body and the ventricle propels it to the lungs. The by the enveloping delicate connective tissue that left atrium receives blood from the lungs and passes occurs between individual muscle cells. The it to the left ventricle, from which it is distributed myocardial capillaries are normally all open to perfusion throughout the body. The wall of the heart consists unlike those of other tissues in which a certain of an inner lining layer, a middle muscular layer, and percentage are close to perfusion. In these other an external layer of connective tissue. tissues as activity increases so does the number of

capillaries open to perfusion. The myocardium is Endocardium the thinnest in the atria and thickest in the left

ventricle. The myocardium is arranged in layers that VOCABULARY: squamous (endothelial) cell, form complex spirals about the atria and ventricles. tight (occluding) junction, gap junction, In the atria, bundles of cardiac muscle form a subendothelial layer, subendocardial layer latticework and locally are prominent as the

pectinate muscles, while in the ventricles, isolated The endocardium forms the inner lining of the atria bundles of cardiac muscle form the trabeculae and ventricles and is continuous with and carnea. In the atria, the cardiac muscle cells are comparable to the inner lining of blood vessels. It smaller and contain a number of dense granules not consists of a single layer of polygonal squamous seen elsewhere in the heart. These myocytes have (endothelial) cells with oval or rounded nuclei. properties associated with endocrine cells. They are Tight (occluding) junctions unite the closely most numerous in the right atrium and release the apposed cells, and gap junctions permit the cells secretory granules when stretched. to communicate with one another. Electron

120 The granules contain atrial natriuretic factor passing through the surrounding plasmalemma and (ANF), released in response to increased blood volume that of T-tubules at the time the action potential is or increased venous pressure within the atria. Atrial generated. The large diameter of the T-tubules natriuretic factor acts on the kidneys causing allows the same extracellular fluid containing vasoconstriction of the afferent arteriole which calcium ion surrounding cardiac myocytes to enter increases both glomerular filtration pressure and the T-tubule system and be available in the cell filtration rate. ANF also acts on the distal and interior. The influx of calcium ion that occurs collecting tubules of the kidneys to decrease sodium during the development of the action potential is not and chloride resorption and inhibits the secretion of sufficient to cause contraction of the cardiac myocyte. antidiuretic hormone (ADH), aldosterone, and The entering calcium ion binds to channel proteins renin. These actions result in increased sodium chloride of the sarcoplasmic reticulum an event that triggers excretion (natriuresis) in a large volume of dilute urine. the release of stored calcium which in turn initiates Elastic fibers are scarce in the ventricular cardiac myocyte contraction. For this reason the myocardium but are plentiful in the atria, where calcium ion entering at the time the action potential they form an interlacing network between muscle is generated is referred to as trigger calcium. The fibers. The elastic fibers of the myocardium become mechanism of contraction of the cardiac myocyte is continuous with those of the endocardium and the similar to that of skeletal muscle. outer layer of the heart (epicardium). Cardiac muscle cells contract spontaneously (self- Epicardium excitation) in response to intrinsically generated action potentials which are then passed to VOCABULARY: visceral layer of pericardium, neighboring cardiac myocytes via gap mesothelial cell, subepicardial layer (communicating) junctions. The action potentials are generated by ion fluxes mediated by ion The epicardium is the visceral layer of the channels in the plasmalemma and T-tubule system pericardium, the fibrous sac that encloses the of the cardiac muscle cell. The prolonged plateau of heart. The free surface of the epicardium is covered the action potential observed during the contraction by a single layer of flat to cuboidal mesothelial of cardiac myocytes lasts up to 15 times longer than cells, beneath which is a layer of connective tissue that observed in skeletal muscle cells. The action that contains numerous elastic fibers. Where it lies potential, as in skeletal muscle, is caused in part by on the cardiac muscle, the epicardium contains the sudden opening of large numbers of fast sodium blood vessels, nerves, and a variable amount of fat. ion channels that allow sodium ions to enter the This portion is called the subepicardial layer. cardiac myocytes. These ion channels remain open The parietal layer of the epicardium consists of only for a few 10,000th of a second and then close. connective tissue lined by mesothelial cells that face In skeletal muscle, repolarization then occurs and those covering the visceral epicardium. The two the action potential is over within 10,000th of a epithelial lined layers are separated only by a thin second. In cardiac muscle, the action potential is film of fluid, produced by the mesothelial cells, that caused by the opening of two types of ion channels: (1) allows the layers to slide over each other during the fast sodium ion channels as in skeletal muscle contraction and relaxation of the heart. and (2) slow calcium channels (calcium-sodium channels). The latter are slower to open and remain Conducting System open for a much longer period of time. When open both calcium and sodium ions enter the cardiac VOCABULARY: specialized cardiac muscle, myocyte and maintain the prolonged period of sinoatrial node, nodal cell, atrioventricular depolarization resulting in the elongated plateau of node, atrioventricular bundle, Purkinje fiber the action potential. The permeability of the (cell) plasmalemma to potassium ions also decreases during this period as a result of the calcium influx The conducting system of the heart consists of and prevents an early return of the action potential specialized cardiac muscle fibers (cells) and is to a resting level as occurs in skeletal muscle. responsible for initiating and maintaining cardiac rhythm A considerable quantity of calcium ion enters the and for ensuring coordination of the atrial and myocyte sarcoplasm from the extracellular fluid by ventricular contractions.

121 The system consists of a sinoatrial node, an It is the cardiac muscle tissue of the atrioventricular node and atrioventricular bundle, atrioventricular node that delays the transmission of and Purkinje fibers. Cardiac myocytes, including the cardiac impulse and functions as “gate keeper” for those of the conducting system, have an initial resting the continued conduction of the impulse into the membrane potential of about – 90 millivolts that is ventricles. The cardiac myocytes of the established by the movement of potassium ions atrioventricular node are the slowest conducting fibers in from within the sarcoplasm of the myocyte to the the heart. From here, impulses travel rapidly along surrounding interstitial fluid. the atrioventricular bundle in the membranous Each cardiac cycle is initiated by the spontaneous part of the interventricular septum. The bundle generation of an action potential by cardiac muscle cells divides into two trunks that pass into the ventricles, forming the sinoatrial node. The sinoatrial node is where they break up into numerous twigs that located in the epicardium at the junction of the connect with the ordinary cardiac muscle fibers. superior vena cava and right atrium and forms an Thus, the impulse is carried to all parts of the ellipsoid strip about 13 mm long and 3 mm wide. ventricular myocardium. The specialized fibers of Nodal cells are smaller than ordinary cardiac these trunks and branches are called Purkinje muscle cells and contain fewer and more poorly fibers (cells) and differ from ordinary cardiac organized myofibrils. Intercalated discs are lacking muscle in several respects. Purkinje fibers are larger between cells of the sinoatrial nodes. Because of the and contain more sarcoplasm, but myofibrils are high sodium ion concentration in the surrounding less numerous and usually have a peripheral extracellular fluid, these ions normally tend to leak location. The fibers are rich in glycogen and into the sinus muscle fibers. It is the inherent leakiness mitochondria and often have two (or more) nuclei. of the plasmalemma of sinus nodal fibers to sodium Intercalated discs are uncommon, but numerous ions through special sodium ion channels (If desmosomes are scattered along the cell channels) that is related to the self-excitation boundaries. Purkinje fibers are the fastest conducting phenomenon. As a result of this leakage, the resting fibers in the heart. membrane potential of the sinus nodal fibers is It is the action potential of the cardiac myocytes lower (-55 to -60 millivolts) in comparison with throughout the heart that is traced in a normal normal cardiac myocytes of the ventricles (-85 to - electrocardiogram (ECG). In a normal ECG the P 90 millivolts). As a result of the less negativity of wave represents atrial depolarization, the QRS wave the resting potential, the fast sodium channels are complex represents ventricle depolarization and the T generally inactive and only the slow calcium-sodium wave represents repolarization of the ventricle. channels open resulting in the development of an The autonomic nervous system influences action potential. The ends of the cardiac muscle (modulates) heartbeat. Axons of postganglionic fibers constituting the sinoatrial node are linked parasympathetic neurons terminate in the tissue of directly to the adjacent ordinary atrial cardiac sinoatrial and atrioventricular nodes. Acetylcholine muscle cells. The spontaneously generated action potential released by these nerve terminals binds to initiated in the node cells then spreads throughout muscarinic acetylcholine receptors (G-protein- the entire atrial muscle mass to the atrioventricular linked receptors that inhibit adenylate cyclase and node located in the posterior wall of the right decrease adenosine monophosphate (c-AMP) atrium behind the tricuspid valve and adjacent to levels) and decreases heart rate. In contrast, axons of the opening of the coronary sinus. Here the action postganglionic sympathetic neurons distributed in the potential is delayed allowing enough time for the myocardium release norepinephrine which binds to atria to empty completely their contained blood β-adrenergic receptors (G-protein-linked receptors into the ventricles before ventricular contraction that stimulate adenylate cyclase and increase c-AMP begins. The sinoatrial node controls the heart beat levels) and increase heart rate. because its rate of rhythmical discharge is greater than any other region of the heart. Because of the faster discharge rate, nodal cell activity overrides all other potential pacemaker activity by other cells (cells of the atrioventricular node, Purkinje cells) in the heart. Thus, the sinoatrial node functions as the “pacemaker” of the heart.

122 Cardiac Skeleton side. Scattered smooth muscle cells are present on the atrial side of the valves, while on the ventricular VOCABULARY: annuli fibrosi, dense side elastic fibers are prominent. Thin tendinous connective tissue, trigona fibrosi, septum cords called chordae tendineae attach the membranaceum ventricular sides of the valves to projections of cardiac muscle called papillary muscles. Semilunar The ventricles of the heart are not emptied as other valves of the pulmonary arteries and aorta are hollow organs, but blood is wrung from their thinner but show the same general histologic cavities much like water from a soaked towel. The structure as the atrioventricular valves. heart has a fibrous skeleton organized in a complicated three-dimensional continuum of dense Blood Vessels connective tissue to which its musculature anchors. The main portion of the cardiac skeleton is formed The blood vessels are variously sized tubes arranged by the annuli fibrosi, rings of dense connective in a circuit, through which blood is delivered from tissue that surround the openings of the aorta, the heart to the tissues and back to the heart from pulmonary artery, and the atrioventricular orifices. all parts of the body. By this circulatory system, Of these, the aortic ring is the strongest. Also oxygen from the lungs, nutrients from the intestines contributing to the cardiac skeleton are triangular and liver, and regulatory substances such as thickenings of fibrous connective tissue, the hormones are distributed to all the organs and trigona fibrosi (left and right) that link the aortic tissues. Waste products also are emptied into the root to the atrioventricular annuli and the septum circulation and carried to organs such as the lungs membranaceum, which is the upper fibrous part and kidneys for elimination. of the interventricular septum. The trigona fibrosa in some instances may contain cartilage-like Structural Organization material and in old age may undergo calcification.

In addition to providing attachment for the VOCABULARY: tunica intima, tunica media, cardiac musculature and components of the valves, tunica adventitia the annuli fibrosi provide support for and maintain the integrity of all four orifices. Without these rings Although blood vessels differ in size, distribution, of support the orifices would stretch and the valves and function, structurally they share many common would be unable to function properly. The position features. As in the heart, the walls of blood vessels of the annuli fibrosi around the atrioventricular consist of three major coats or tunics. Differences orifices also provides a physical barrier separating in the appearance and functions of the various parts the myocardium of the atria and ventricles. In doing of the circulatory system are reflected by structural so, the only electrophysiological link between them is the changes in these tunics or by reduction and even specialized conducting tissue of the atrioventricular omission of some of the layers. From the lumen bundles, thus ensuring the orderly sequence of outward, the wall of a blood vessel consists of a events associated with the cardiac cycle. tunica intima, tunica media, and tunica adventitia.

The tunica intima corresponds to and is Valves continuous with the endocardium of the heart. It consists of an endothelium of flattened squamous VOCABULARY: chordae tendineae, papillary cells resting on a basal lamina and is supported by a muscles subendothelial connective tissue. The tunica media is the equivalent of the Valves are present between the atria and ventricles myocardium of the heart and is the layer most and at the openings to the aorta and pulmonary variable both in size and structure. Depending on vessels. Regardless of their location, the valves are the function of the vessel, this layer contains similar in histologic structure. The atrioventricular variable amounts of smooth muscle and elastic valves are attached to the annuli fibrosi, the tissue. connective tissue of which extends into each valve The tunica adventitia also varies in thickness in to form its core. The valves are covered on both sides different parts of the vascular circuit. by endocardium that is thicker on the ventricular

123 It consists mainly of collagenous connective tissue diameter. These transcytotic vesicles are thought and corresponds to the epicardium of the heart, but to form at one surface, detach, and cross to the it lacks mesothelial cells. opposite side, where they fuse with the plasmalemma to discharge their contents. In Arteries humans, about one-fourth of the total thickness of the intima is formed by the subendothelial layer, a layer of loose connective tissue that contains elastic As the arteries course away from the heart they fibers and a few smooth muscle cells. undergo successive divisions to provide numerous The tunica media is the thickest layer and branches whose calibers progressively decrease. The consists largely of elastic tissue that forms 50 to 70 changes in size and the corresponding changes in concentric, fenestrated sheets or elastic laminae, structure of the vessel wall are continuous, but each about 2 or 3 µm thick and spaced 5 to 20 µm three classes of arteries can be distinguished: large apart. Successive laminae are connected by elastic elastic or conducting arteries, medium-sized fibers. In the spaces between the laminae are thin muscular or distributing arteries, and small arteries layers of connective tissue that contain collagen and arterioles. A characteristic feature of the entire fibers and smooth muscle cells arranged arterial side of the blood vasculature system is the circumferentially. The smooth muscle cells are prominence of smooth muscle in the tunica media. flattened, irregular, and branched and are bound to adjacent elastic laminae by elastic microfibrils and Elastic or Conducting Arteries reticular fibers. An appreciable amount of amorphous ground substance also is present. VOCABULARY: tunica intima, endothelial Smooth muscle cells are the only cells present in the coat, caveolae, transcytotic vesicles, media of elastic arteries and synthesize and subendothelial layer, tunica media, elastic maintain the elastic fibers and collagen. lamina, tunica adventitia, vasa vasorum Elastic arteries are those nearest the heart and, because of the great content of elastic tissue, are The aorta is the main conducting artery, but others expandable. As blood is pumped from the heart in this class are the common iliacs and pulmonary, during its contraction, the walls of elastic arteries brachiocephalic, subclavian, and common carotid expand; when the heart relaxes, the elastic recoil of arteries. A major feature of this type of artery is the these vessels serves as an auxiliary pumping width of the lumen, which, by comparison, makes mechanism to force blood onward between beats. the wall of the vessel appear thin. The tunica adventitia is relatively thin and The tunica intima is relatively thick and is lined contains bundles of collagen fibers (type 1) and a by a single layer of flattened, polygonal endothelial few elastic fibers, both of which have a loose, cells that rest on a complete basal lamina. Adjacent helical arrangement about the vessel. Fibroblasts, endothelial cells may be interdigitated or overlap mast cells, and rare longitudinally oriented smooth and are extensively linked by occluding and muscle cells also are present. The tunica adventitia communicating (gap) junctions. The cells contain gradually blends into the surrounding loose the usual organelles, but these are few in number. connective tissue and functions to anchor the vessel Peculiar membrane-bound, rod-shaped granules, to adjacent structures. consisting of several tubules embedded in an The walls of large arteries are too thick to be amorphous matrix, are present and have been nourished only by diffusion from the lumen. shown to contain a factor (VIII) associated with Consequently, these vessels have their own small blood coagulation that is synthesized by these cells arteries, the vasa vasorum, that may arise as and released into the blood. A proteoglycan layer, branches of the main vessel itself or derive from the endothelial coat, covers the luminal surface of neighboring vessels. They form a plexus in the the cell membrane. On the basal surface, the tunica adventitia and generally do not penetrate endothelial cells are separated from the basal lamina deeply into the media. by an amorphous matrix. Numerous invaginations or caveolae are present at the luminal and basal surfaces of the plasmalemma, and the cytoplasm contains many small vesicles 50 to 70 nm in

124 Muscular or Distributing Arteries the junction of the tunica media and the tunica adventitia, the elastic tissue forms a second VOCABULARY: tunica intima, endothelium, prominent fenestrated membrane called the subendothelial layer, internal elastic lamina, external elastic lamina. tunica media, smooth muscle, external elastic The tunica adventitia is prominent in muscular lamina, tunica adventitia arteries and in some vessels may be as thick as the media. It consists of collagen and elastic fibers that The muscular or distributing arteries are the largest are longitudinal in orientation. This coat continues class, and except for the elastic arteries, the named into the surrounding loose connective tissue with arteries of gross anatomy fall into this category. All no clear line of demarcation. the small branches that originate from elastic trunks Muscular arteries are highly contractile with the are muscular arteries, and the transition is relatively degree of contraction controlled primarily by the abrupt, occurring at the openings of the branching autonomic nervous system and factors produced by arteries. Compared with the luminal diameter, the endothelial cells. For example, adrenergic walls of these arteries are thick and make up over sympathetic innervation of smooth muscle cells in one-fourth of the cross-sectional diameter. The the media of these vessels results in their thickness of the wall is due mainly to the large contraction and vasoconstriction of the vessel. amount of smooth muscle in the media, which, by contraction and relaxation, helps to regulate the Small Arteries and Arterioles blood supply to organs and tissues. The general organization of these vessels is similar to that of Small arteries and arterioles differ from large elastic arteries, but the proportion of cells and muscular arteries only in size and the thickness of fibers differs. their walls. The distinction between small arteries The tunica intima consists of an endothelium, a and arterioles is mainly one of definition: Arterioles subendothelial layer, and an internal elastic lamina. are those small arteries with a diameter less than The endothelium and subendothelial layers are 250 to 300 µm and in which the media contains similar to those of elastic arteries, but as the size of only one or two layers of smooth muscle cells. the vessel decreases, the subendothelial layer In the progression from distributing artery to becomes thinner. It contains fine collagen fibers, a arteriole, the subendothelial connective tissue few elastic fibers, and scattered smooth muscle cells progressively decreases until in arterioles, a that are longitudinal in orientation. The internal subendothelial layer is lacking and the tunica intima elastic lamina is a fenestrated sheet of elastin that consists only of endothelium and a fenestrated forms a prominent, scalloped boundary between internal elastic lamina. The endothelial cells are the tunica media and the tunica intima. The basal joined by occluding junctions. Basal processes make surface of the endothelial cells sends slender contact with smooth muscle cells through the processes through discontinuities in the elastic fenestrations in the elastic lamina. The thin basal lamina to make contact with cells in the tunica lamina becomes indistinct in the smallest arterioles. media. The layers of smooth muscle cells of the media The tunica media is the thickest coat and progressively decrease in number, and at a diameter consists mainly of smooth muscle cells arranged of about 30 µm, the muscle coat of arterioles in concentric, helical layers. The number of layers consists of a single layer of circumferentially varies from 3 to 4 in smaller arteries to 10 to 40 in oriented cells. The internal elastic membrane is thin the large muscular arteries. The muscle cells are and disappears in terminal arterioles; there is no surrounded by basal laminae and communicate with definite external elastic lamina. Tunica adventitia each other by gap junctions. Reticular fibers and also decreases in thickness, becoming extremely small bundles of collagen fibers interspersed with thin in the smallest arterioles. elastic fibers are present between the layers of Compared with the luminal size, the walls of smooth muscle. The number and distribution of arterioles are thick. Because of the smooth muscle elastic fibers correlate with the caliber of the artery. in the media, arterioles control the flow of blood to In the smaller vessels elastic fibers are scattered the capillary beds they feed and are the primary between the muscle cells, whereas in larger arteries regulators of blood pressure. the elastic tissue forms circular, loose networks. At

125 Because these vessels contain highly oxygenated Continuous Capillaries blood, they tend to shunt the blood flow towards the capillary beds in which the tissues have low VOCABULARY: continuous endothelium, oxygen. Arterioles are quite responsive to vascular continuous basal lamina, tunica adventitia, stimuli and as a consequence are major contributors pericytes to vascular resistance. Continuous capillaries are the common type of Metarterioles capillary found throughout the connective tissues, muscle, skin, and central nervous system. Their Metarterioles are intermediate between capillaries lumina range from 4 to 10 µm in diameter and in and arterioles and regulate the flow of blood through the smaller vessels may be encompassed by a single capillary beds. They also are called capillary sphincter endothelial cell. In larger capillaries, three or four areas or precapillary arterioles. Although poorly cells may enclose the lumen. The wall consists of a defined morphologically, their lumina generally are continuous endothelium and a thin tunica wider than those of the capillaries they serve, and adventitia. The endothelial cells rest on a circularly arranged smooth muscle cells are continuous basal lamina and show the same scattered in their walls. Here individual smooth features as endothelial cells elsewhere in the muscle cells spaced a short distance apart vascular tree. They contain the usual organelles, completely encircles the endothelial tube and forms caveolae and transcytotic vesicles, a proteoglycan a surrounding cork-screw configuration. layer covering a few short microvilli on the luminal surfaces, and a variable number of membrane- Capillaries bound granules. Multivesicular bodies also can be found. Adjacent cells may abut each other or Capillaries are the smallest functional units of the blood overlap obliquely or show sinuous interdigitations. vascular system and are inserted between arterial At intervals, adjacent cell membranes unite in and venous limbs of the circulation. They branch occluding junctions, but it is not certain that these extensively to form elaborate networks, the extent form complete belts as in other epithelia. and type of which reflects the specific organ or The tunica adventitia is thin and contains some tissue with which they are associated. The capillaries collagen and elastic fibers embedded in a small associated with some organs are considered tight amount of ground substance. Fibroblasts, and function as a barrier between the blood and the macrophages, and mast cells are present also. organ in question. Others exhibit variable degrees Pericytes are irregular, branched, isolated cells of “leakiness” and allow materials to pass from the that occur at intervals along capillaries, enclosed by blood into surrounding intersitial tissues. Based on the basal lamina of the endothelium. The cells the appearance of the endothelium and the basal resemble fibroblasts and characteristically contain a lamina in electron micrographs, capillaries are few dense bodies and numerous cytoplasmic classed as continuous, fenestrated, or as filaments. Actin and myosin have been discontinuous. Regardless of the type, the basic demonstrated in the cytoplasm by structure of capillaries is similar and represents an immunohistochemistry, indicating a contractile extreme simplification of the vessel wall. The tunica function. However, their exact function is intima consists of endothelium and a basal lamina; unknown. the tunica media is absent and the tunica adventitia is greatly reduced. Fenestrated Capillaries

VOCABULARY: fenestra, diaphragm, continuous basal lamina

Fenestrated capillaries have the same structure as the continuous type but differ in that the endothelial cells contain numerous fenestrae (pores) that appear as circular openings 70 to 100 nm in diameter.

126 Fenestrae may be distributed at random or in In the continuous capillaries, caveolae and groups and usually are closed by thin diaphragms transcytotic vesicles have been implicated in that show central, knoblike thickenings. Each transepithelial transport of water-soluble materials; diaphragm is a single-layered structure thinner than pinocytosis also may contribute to transfer of a single-unit membrane - so it would appear substances of high molecular weight. A subcategory unlikely that it is formed by apposition of two cell of continuous capillaries found at specific locations membranes. is referred to as tight capillaries. The endothelial cells The basal lamina is continuous across the of these capillaries are tightly united by occludens fenestrae on the basal side of the endothelium. The type junctions and the cytoplasm is devoid of fenestrated capillaries of the renal glomerulus differ caveolae and transcytotic vesicles. The endothelium in that the pores lack diaphragms and the basal lining of these capillaries contributes to the blood lamina is much thicker than in other capillaries. brain barrier, the blood testis barrier and the blood Fenestrated capillaries are generally associated with thymic barrier. the kidney, endocrine glands, and the The movement of fluid out of or into capillaries is gastrointestinal tract. controlled by pressures within the blood itself and in the surrounding interstitial (connective tissue) Discontinuous capillaries (Sinusoids) compartment. These forces are: capillary hydrostatic pressure, plasma solute osmotic VOCABULARY: gaps, discontinuous basal pressure, interstitial hydrostatic pressure and lamina, phagocyte interstitial solute osmotic pressure. If, for example, feeding arterioles dilate the hydrostatic pressure of Sinusoids are thin-walled vessels that are much the capillary blood increases and surpasses the wider and have lumina that are more irregular in plasma solute osmotic pressure as well as both outline than those of capillaries. Sinusoids are surrounding interstitial pressures the movement of abundant in the liver, spleen, and bone marrow. fluid will be out of the capillary and into the Their endothelia are attenuated and may be interstitial tissue. If, on the other hand, arterioles continuous as in the bone marrow, or the cells may contract thereby reducing the hydrostatic pressure be separated by gaps and rest on a discontinuous the net effect is to draw fluid into the capillary basal lamina. Sinusoids often are associated with lumen driven primarily buy the plasma solute phagocytes either as a component of the lining, as osmotic pressure. A similar situation occurs in the in the liver, or closely applied to the exterior of the distal (venous) portion of the capillary bed as wall, as in the spleen. The endothelial cells capillaries combine to form venules. Here the themselves show no greater capacity for plasma solute osmotic pressure is great enough to phagocytosis than do endothelial cells in other draw most of the tissue fluid back into the blood vessels. They do show more active endocytosis and except for about 10% which eventually is captured more numerous lysosomes than are found in and returned to the blood by the lymphatic system. endothelia elsewhere. Albumin, a highly negatively charged protein produced by the liver, plays an essential role in Diffusion maintaining the oncotic pressure to hold fluid within the vascular system. Exchange of nutrients and wastes between tissues Fluid exchange also may occur through and blood occurs across the thin endothelium of intercellular clefts between endothelial cells. For capillaries. It has been estimated that no active cell example, endothelial cells retract from one another lies more than 30 or 40 µm away from a capillary. in the presence of histamine. This allows fluid and The most important way materials cross the protein to diffuse out of capillaries into surrounding endothelium is by diffusion, and the total capillary tissues causing edema. This process is reversible surface available for exchange has been calculated and can occur in minutes. to be over 100 square meters. Lipid-soluble materials diffuse directly through the endothelial cells; water and water-soluble materials are transmitted by aquiporins, transcytotic vesicles, and fenestrae of fenestrated capillaries.

127 Endothelial Cell Secretion Veins

VOCABULARY: nitric oxide, endothelin I, The venous side of the circulatory system carries intercellular adhesion molecules blood from the capillary beds to the heart, and in their progression, the veins gradually increase in Endothelial cells are now known to secrete several size and their walls thicken. Their structure basically substances of the underlying extracellular matrix is the same as that of arteries, and the three coats - (types III, IV, and V collagens, fibronectin, tunica intima, media, and adventitia--can be laminin), as well as several factors involved with distinguished but are not as clearly defined. In maintenance of vascular tone, blood clotting, and general, veins are more numerous and larger than emigration of leukocytes. the arteries they accompany, but their walls are Endothelial cells synthesize and release nitric thinner because of a reduction of muscular and oxide, which causes relaxation of adjacent smooth elastic elements. Since their walls are less sturdy, muscle, thereby decreasing the vascular tone of veins tend to collapse when empty and in sections blood vessels. Nitric oxide is derived from arginine may appear flattened, with irregular, slitlike lumina. in a reaction catalyzed by nitric oxide synthase to Structurally, veins vary much more than arteries. produce nitric oxide and citrulline. Nitric oxide The thickness of the wall does not always relate to activates guanylate cyclase in smooth muscle cells the size of the vein, and the same vein may differ causing increased levels of cyclic guanosine structurally in different areas. Histologic monophosphate (cGMP) and vasodilation. The classification of veins is less satisfactory than for medicinal effects of nitroglycerin and amyl nitrates arteries, but several subdivisions usually are made - occurs as a result of the conversion of these namely, venules and small, medium, and large veins. compounds to nitric oxide whereas compounds such as Viagra inhibit cGMP phosphodiesterase Venules and thus maintain increased cGMP levels. Endothelial cells also can respond to anoxia by VOCABULARY: continuous endothelium, secreting a peptide called endothelin I, a potent pericyte, thin adventitia, incomplete tunica vasoconstrictor of long duration that binds to media smooth muscle cells of arteries, thereby elevating blood pressure. Endothelial cells at sites of Venules arise from the union of several capillaries inflammation in response to cytokines are capable to form vessels 10 to 50 µm in diameter. The of synthesizing and inserting intercellular junctions between venules and capillaries are adhesion molecules into their luminal important sites of fluid exchange between tissues plasmalemma. These factors bind to circulating and blood. leukocytes, allowing them to migrate through the The tunica intima consists of a thin, continuous endothelium to the site of inflammation. endothelium, the cells of which are loosely joined Endothelial cells also contain prostacyclin, an agent by poorly developed intercellular junctions. The that that causes vasodilation and inhibits platelet thin basal lamina is pierced by pericytes that adhesion, as well as thrombomodulin and tissue appear to make contact with the endothelial cells. plasminogen activator which exhibit anticoagulant The tunica media is missing in the smallest activity. Under some circumstances endothelial cells venules, and the relatively thin adventitia contains can release thromboplastin to promote blood a few collagen fibers, scattered fibroblasts, mast coagulation and other thromboplastic substances cells, macrophages, and plasma cells. As the vessels such as von Willebrand factor and antihemophilic increase in size to reach diameters of 50 µm, factor VIII of the blood coagulation system. Von circularly oriented, scattered smooth muscle cells Willebrand factor stored in Weibel-Palade granules begin to appear and form a somewhat of endothelial cells also promotes platelet adhesion discontinuous and incomplete tunica media. The to subendothelial collagen at the site of injuries. adventitia increases in thickness and consists of Thus, endothelial cells also are equipped to initiate longitudinally arranged collagen fibers that form an clotting and repair minor defects in the lining of open spiral around the vessel. Fibroblasts often are vessels to prevent the leakage of blood. irregular in shape and bear thin processes.

128 Small Veins Large Veins

VOCABULARY: continuous endothelium, VOCABULARY: thick tunica adventitia, vasa tunica media, tunica adventitia vasorum, thin tunica media, tunica intima

Small veins vary from about 0.2 to 1.0 mm in In large venous trunks (venae cavae; renal, external diameter. The tunica intima consists only of a iliac, splenic, portal, and mesenteric veins), the continuous endothelium that rests on a thin basal thick tunica adventitia forms the greater part of lamina. Smooth muscle cells make up a tunica the wall. It consists of a loosely knit connective media, which contains one to four layers of cells. tissue with thick, longitudinal bundles of collagen Between the smooth muscle cells is a thin network and elastic fibers. Smooth muscle bundles, also of elastic and collagen fibers. The tunica longitudinal in orientation, are present and are adventitia forms a relatively thick coat and especially well developed in the inferior vena cava. contains longitudinally oriented collagen fibers and Vasa vasorum are present and may extend into the some thin elastic fibers. media. A thin tunica media is poorly developed and may even be absent; otherwise it has the same Medium Veins organization as that in medium veins. The tunica intima is supported by a subendothelial layer that VOCABULARY: thin tunica intima, narrow may become prominent in larger trunks. subendothelial layer, tunica media, thick tunica adventitia, vasa vasorum, valve Veins with Special Features

The medium class of veins includes most of the Some veins lack a tunica media, such as the named veins of gross anatomy except for major trabecular veins of the spleen; veins of the retina, trunks. They vary in size from 1 to 10 mm in bone, and maternal placenta: most meningeal and diameter. The thin tunica intima consists of cerebral veins; and those of the nail bed. Veins in endothelial cells resting on a basal lamina, but a the pregnant uterus contain smooth muscle in all narrow subendothelial layer may be present and three coats; in the intima the fibers run contains fine collagen fibers and scattered thin longitudinally rather than circularly, as they do also elastic fibers. The elastic fibers may form a network in the tunica intima of the saphenous, popliteal, at the junction of tunica intima and media, but a femoral, umbilical, and internal jugular veins. At poorly defined internal elastic lamina is formed only their junctions with the heart, the adventitia of the in the larger vessels. In most medium veins, the pulmonary veins and venae cavae are provided with tunica media, though well developed, is thinner a coat of cardiac muscle; the fibers run than in corresponding arteries. longitudinally and circularly about the vessels for a The thick tunica adventitia forms the bulk of the short distance. wall and is larger than the tunica media. It consists of collagen and elastic fibers and frequently Arteriovenous Anastomoses contains longitudinally oriented smooth muscle cells. Vasa vasorum are present in the larger In addition to their capillary connections, arteries vessels of this class. and veins may unite by shunts called arteriovenous Most medium veins are equipped with valves, two anastomoses. Generally these arise from side branches pocket-like flaps of tunica intima that project into of arterioles that pass directly to venules. They are the lumen, their free edges oriented in the direction thick-walled, muscular vessels of small caliber that of flow. They consist of a core of connective tissue usually are coiled and surrounded by a connective covered on both sides by endothelium. A rich tissue sheath. They are plentiful in the plantar and network of elastic fibers is present in the connective palmar surfaces, fingertips, toes, lips, and nose and tissue beneath the endothelium on the down-stream also occur in the thyroid. When open, the side of the valves. As blood flows toward the heart, anastomoses shunt blood around the capillary bed the valves are forced against the vessel wall, but and thus regulate the blood supply to many tissues. with back-flow, the valves are forced outward In the skin they function primarily in the regulation against each other to occlude the vessel and prevent of body temperature. reversal of blood flow.

129 Carotid and Aortic Bodies Special Circulations

VOCABULARY: epithelioid (glomus) cells, Control of blood vasculature function associated neural crest with a number of specific tissues and/or organs differs dependent on the circumstance and needs at An expanded area at the bifurcation of the carotid a given point in time. For example, increased body artery known as the carotid sinus contains numerous temperature results in the sympathetic innervation baroreceptors that function in the control of blood of blood vessels of skin causing vasodilatation. This pressure by mediating changes in heart rate. Blood effect directs blood to the body surface to dissipate pressure is sensed by baroreceptors in specialized heat. In the respiratory system hypoxia causes regions located in the carotid sinus and along the vasoconstriction and blood is shunted away from aortic arch. Baroreceptors (mechanoreceptors) poorly ventilated to well-ventilated areas of the respond to stretch and consist of myelinated sensory lung. The circulation associated with the lung is the only nerve fibers the bipolar neuronal cell bodies of region of the vasculature that responds to hypoxia by which are located near the brain stem and project vasoconstriction. Other regions of the vasculature into the medulla of the brain. A decrease in blood respond to hypoxia by vasodilatation. pressure results in an increase in sympathetic Vasodilatation occurs in blood vessels supplying stimulation and a release of norepinephrine. The action skeletal muscle during exercise and increased of norepinephrine is to stimulate cardiac activity concentrations of lactate, adenosine, and potassium. and constrict blood vessels. Increase in blood During rest, sympathetic innervation of smooth pressure results in parasympathetic stimulation and muscle cells in the vasculature through release of acetylcholine. The action of acetylcholine is norepinephrine release stimulates both α-adrenergic to slow cardiac activity and cause vasodilation. and β-adrenergic receptors causing vasodilatation in A small group of cells and nerve endings adjacent skeletal muscle tissue. The vasculature of the to the carotid sinus known as the carotid body senses coronary circulation responds to both hypoxia and the partial pressure of respiratory gases and pH in adenosine (a byproduct of ATP metabolism from the arterial blood and also contributes to respiratory cardiac myocytes) with vasodilatation. Vessels of and cardiovascular reflex regulation. Carotid bodies the cerebral circulation respond to increased Pco2 are encapsulated structures that consist of masses or decreased pH with vasodilation. of large, polyhedral epithelioid (glomus) cells that are closely related to a rich network of Lymph Vascular System sinusoidal vessels. The cells have large, pale nuclei and light, finely granular cytoplasm. In electron The lymph vascular system consists of endothelial- micrographs, the granules show dense cores and lined tubes that recover intercellular (tissue) fluid contain catecholamines and 5-hydroxytryptamine. A not picked up by the blood vascular system and second cell type, the type II glomus cell, appears returns it to the blood. The fluid (lymph) carried by much the same as type I but lacks granules and are the lymphatics is a blood filtrate formed as fluid thought be supporting glial cells. crosses the blood capillaries into the tissues. Unlike Aortic bodies are similar structures that lie close to the blood vascular system, lymph flow is the aorta between the angle of the subclavian and unidirectional - from tissues to the union of the carotid arteries on the right and near the origin of lymphatic and blood vascular systems at the base of the subclavian artery on the left. They are smaller the neck. The lymphatic vascular system begins in than carotid bodies and consist of type I and type II the tissues as blindly ending capillaries that drain glomus cells. They are thought to have functions into larger collecting vessels and then into two main similar to those of carotid bodies. The aortic bodies lymphatic trunks. Lymph nodes occur along the are supplied by fibers of the vagus nerve (X) course of the vessels and filter the lymph. whereas the carotid body receives fibers primarily Lymphatics are present in most tissues but are from the glossopharyngeal nerve (IX). Both of absent from bone marrow, the central nervous carotid and aortic bodies are derived from the system, coats of the eye, internal ear, and fetal neural crest. placenta.

130 Lymph Capillaries Lymphatic Trunks

VOCABULARY: thin continuous endothelium, VOCABULARY: tunica intima, subendothelial discontinuous basal lamina, anchoring filament layer, internal elastic lamina, tunica media, tunica adventitia Lymph capillaries are thin-walled, blind tubes that branch and anastomose freely to form a rich Collecting lymphatic vessels ultimately gather into network in organs and tissues. They are wider and two main trunks, the thoracic and right lymphatic more irregular than blood capillaries. ducts. The thoracic duct is the longer and has a The wall of a lymph capillary consists only of a thin greater field of drainage. It begins in the abdomen, continuous endothelium and a discontinuous passes along the vertebral column, and opens into basal lamina that is present only in patches or may the venous system at the junction of the left jugular even be absent. Adjacent endothelial cells may and subclavian veins. It receives lymph from the overlap, but junctional complexes are few and clefts lower limbs, abdomen, left upper limb, and left side occur between the cells. Externally, the of the thorax, head, and neck. The right lymphatic endothelium is surrounded by a small amount of duct receives lymph only from the upper right part collagenous connective tissue. Fine filaments run of the body and empties into the brachiocephalic perpendicularly from the collagen bundles and vein. attach to the outer surfaces of the endothelium as The structure of the trunks is the same, generally anchoring filaments that maintain the patency of resembling that of a large vein. The tunica intima the vessel. consists of a continuous endothelium supported by a subendothelial layer of fibroelastic tissue with Collecting Lymph Vessels some smooth muscle. Near the junction with the tunica media, the elastic fibers condense into a thin VOCABULARY: tunica intima, tunica media, internal elastic lamina. The thickest layer is the tunica adventitia, valve tunica media, which contains more smooth muscle than does the media of large veins. The Collecting lymph vessels differ from lymph smooth muscle cells have a predominantly circular capillaries in size and the thickness of their walls. arrangement and are separated by abundant Although three coats - intima, media, and adventitia collagenous tissue and some elastic fibers. The - are described as in blood vessels, they are not tunica adventitia is poorly defined and merges clearly delineated. with the surrounding connective tissue. It contains The tunica intima consists of an endothelium bundles of longitudinal collagen fibers, elastic supported by a thin network of longitudinal elastic fibers, and occasional smooth muscle cells. The wall fibers. The tunica media is composed of smooth of the thoracic duct contains nutrient blood vessels muscle cells with a predominantly circular similar to the vasa vasorum of large blood vessels. arrangement, though some cells run longitudinally. Smooth muscle associated with the lymph vessels is Between the muscle cells are a few fine elastic phasic smooth muscle. Phasic smooth muscle of the fibers. The tunica adventitia is the thickest coat lymphatics, unlike visceral smooth muscle, and consists of bundles of collagen fibers, elastic generates action potentials and create waves of fibers, and some smooth muscle cells, all of which contraction that move lymph toward the heart and have a longitudinal orientation. its ultimate union with the blood. The valves within Valves are numerous along the course of the lymph vessels prevent backflow. Thus, the lymphatic vessels and occur at closer intervals than in lymph vascular system can be likened to a sump- veins. They arise in pairs as folds of the intima, as in pump system within the interstitial compartment veins. moving that tissue fluid not absorbed initially by blood vascular system back to the blood. The lymph vascular system may capture and move as much as 4 liters of fluid daily.

131 Because most lymph capillaries are considered carneae of the ventricles, which persist as remnants leakier than most capillaries, many larger molecules of the spongy layer. from a variety of organs (chylomicra, peptide The atrioventricular node is the first part of the hormones) gain access to the blood via this route. conducting system to develop, arising from cardiac muscle cells around the atrioventricular canal. From Organogenesis this the atrioventricular bundle arises. An alternative view suggests that the bundle arises first, Initially, the developing embryo is nourished by from muscle on the dorsal wall of the simple diffusion of nutrients, at first from the atrioventricular canal, then spreads to form the uterine fluids and later, after implantation, from atriventricular node and the right and left limbs of maternal blood lakes. With growth, these sources the bundle. The sinoatrial node is the last to appear, are unable to meet the nutritional needs of the mass from a narrow muscular band in the lateral surface of cells in the embryo, so a vascular system arises of the superior vena cava. Later in fetal life, the early in development. node is incorporated into the wall of the atrium.

Heart Vessels

The earliest cardiac structure appears as an The earliest blood vessels arise from aggregation of splanchnic mesoderm that forms extraembryonic mesenchyme, appearing first as two longitudinal strands. Each acquires a lumen and blood islands in the yolk sac and body stalk. Initially becomes a simple, endothelial-lined tube lying in a the islands form solid cords, but the outermost cells fold of mesoderm. The tubes fuse to form a single, flatten to become the first endothelial cells, while the saccular cardiac primordium, and the mesodermal innermost become free and form the first primitive folds merge into a single surrounding coat. The blood cells. This process of canalization occurs in endothelial tube gives rise to the endocardium; the several places, and by growth and union, the myocardium and epicardium arise from the outer layer isolated vascular spaces unite to form a network of of mesoderm. Myofibrils first appear as loosely vessels that, at this stage, consists of simple arranged groups of filaments within the cytoplasm endothelial tubes. The first vessels of the embryo of differentiating cardiac muscle cells of the form in a similar fashion. After circulation is myocardium. Later in development the myofibrils established, further extension of blood vessels acquire distinct Z bands prior to the appearance of occurs by endothelial budding and growth of pre- A and I bands. Following fusion of the two existing vessels. Formation of capillary-like vessels endocardial tubes, the single primitive heart tube precedes development of definite arteries and veins. grows more rapidly than the pericardial cavity. Smooth muscle and connective tissue coats are Because the two ends are anchored to surrounding acquired by differentiation of surrounding tissues, elongation of the cardiac tube cannot occur mesenchyme. and a cardiac loop is formed. The cephalic end Lymphatic vessels arise quite separately as isolated gives rise to the bulbus cordis, the caudal end to the spaces in the mesenchyme. Cells bordering the presumptive ventricle. spaces flatten to become endothelial cells. By At first the endothelial tube is separated from the progressive unions, the spaces link up to form myocardial layer by a jelly-like fluid that is invaded continuous, branching channels that ultimately lead by mesenchyme and transformed into the to a complete set of lymphatic vessels. In the larger subendocardial layer. The cells at the external lymphatics, the outer tunics are derived from the surface of the myocardium flatten into a surrounding mesenchyme. mesothelium and together with a sublayer of connective tissue form the epicardium. The Summary remainder of the myocardial coat forms an outer compact layer of cardiac muscle and an inner layer The myocardium of the heart provides the of loosely arranged trabeculae. The entire muscle propelling force that moves blood through the wall becomes compact except for the trabeculae systemic and pulmonary circulations. Cardiac muscle is characterized by its ability to contract spontaneously without external stimulus.

132 The heart has a system of specialized fibers that cells, also are important sites of fluid exchange. regulate the contractions into rhythmic beats Lipid-soluble materials readily diffuse through coordinated throughout the chambers of the heart. endothelial cells, whereas water-soluble materials The contraction impulse is initiated in the sinoatrial pass via transcytotic vesicles, through fenestrae (SA) node, the pacemaker of the heart. The cells of and/or intercellular clefts. Bulky material, including the SA node are unique in that they spontaneously whole cells, readily crosses the walls of sinusoids. reach the threshold at which an action potential is Blood is returned to the heart via the veins, which automatically triggered. The impulse passes through are able to constrict or enlarge and store large ordinary cardiac muscle fibers of the atria to reach quantities of blood. From 70-75% of the circulating the atrioventricular node, where it is delayed, allowing blood is contained in the veins, and much of it can the atria to complete their contractions and empty be made available to organs as needed. Blood is before the impulse is passed to the ventricles. The moved along the veins by the massaging action of impulse then is conducted rapidly along the muscles and fascial tissues and by the special atrioventricular bundle and by its branches is spread arrangements of vessels in the soles of the feet that throughout the ventricular myocardium. act as pumps when pressure is exerted on them. The granular cells of the atria secrete the peptide Blood flows away from the area of compression but hormone ANF, which plays a role in regulating is prevented from flowing backward by the valves. blood pressure and volume and in controlling the Negative intrathoracic pressure created during excretion of water and sodium. Thus, the heart inspiration also contributes in moving venous functions, to a limited extent, as an endocrine blood in the direction of the heart. organ. Although most of the fluid filtered from the Flow of blood from the heart is intermittent, and blood capillaries is resorbed by them, about 10% is the arteries immediately joining the heart are not recovered by the blood vascular system, and it subjected to pulses of high pressure. Because of the is this fluid that is recovered by the lymphatics. large amounts of elastic tissue in their walls, the Substances of high molecular weight such as large conducting vessels can expand and absorb the proteins pass into the blood capillaries with forces without damage. During the intervals difficulty but are recovered by lymphatic capillaries between contractions, the elastic recoil of the vessel almost unhindered. Lymphatic channels are one of wall serves as an additional propelling mechanism, the main routes by which nutrients, especially fats, helps smooth out the blood flow, and protects are absorbed from the gastrointestinal tract. delicate capillaries and venules from severe changes Blood vascular endothelium serves as a in pressure. permeability barrier - a moderator of selective Distribution of blood to organs is controlled transport – and has important functions in largely by muscular arteries. The caliber of these maintaining the fluidity of blood. The endothelium vessels is regulated by the degree of contraction of provides a physical surface to which platelets do the tunica media in response to changes in blood not normally stick, so it prevents their aggregation; pressure and the demands of organs. Arterioles are it also synthesizes prostacyclin, an agent that the primary regulators of blood pressure. inhibits platelet adhesion. However, endothelium Metarterioles (precapillary arterioles) regulate the also contains thromboplastic substances and flow of blood through capillary beds. antihemophilic factor VIII of the blood coagulation Exchange of materials between blood and tissues system. Thus, the endothelium is also equipped to occurs at the level of capillaries and venules. initiate clotting and repair minor defects to prevent Passage of fluid across capillary walls depends leakage of blood. The luminal surface bears a net partly on blood pressure and partly on colloidal negative charge, as do red cells, thus preventing osmotic pressure in the capillaries. Blood pressure their aggregation on the wall of the blood vessel promotes passage of fluid into tissues; osmotic and resulting eddying and turbulent flow. pressure promotes resorption. At the arterial end of Endothelial cells secrete several other substances capillary beds, blood pressure is greater than involved with the maintenance of vascular tone osmotic pressure and fluid is driven out of the (nitric oxide, endothelin I), as well as adhesion vessel; at the venous end, osmotic pressure is the factors concerned with the emigration of leukocytes greater and fluid is driven into the vessel. Venules, at the sites of inflammation. with their thin walls and loosely joined endothelial

133 and reticular fibers, the two elements of the reticular 11 Lymphatic Tissues and network, are intimately related, and the fibers often Organs reside in deep grooves in the cytoplasm of the reticular cells. By light microscopy, reticular cells Lymphatic Tissues appear as elongated or stellate elements with round or oval, palely stained nuclei and scant, lightly

basophilic cytoplasm. The cytoplasm is more Scattered throughout the body are various voluminous than is apparent under the light lymphatic elements that form part of the body's microscope, since the bulk of it is spread thinly defense system. Generally distributed so that along the reticular fibers. Electron microscopy noxious agents entering the body soon encounter reveals a variable amount of endoplasmic reticulum them, lymphatic tissues are prominent in the and a moderately well developed Golgi apparatus; connective tissue coats of the gastrointestinal, other organelles are relatively inconspicuous. As in respiratory, and urogenital tracts. Lymphatic bone marrow, lymphatic reticular cells are incapable structures also are inserted into the lymph drainage of giving rise to other cell types and show no and blood circulation, where they serve to filter special capacity for phagocytosis. lymph and blood. Eventually, all the body fluids are The remaining cells of lymphatic tissue are filtered through some form of lymphatic structure. contained within the spaces of the reticular network Lymphatic cells may be concentrated within the and constitute the free cells. The bulk of these are connective tissue and form the lymphatic tissues, or lymphocytes, but macrophages and plasma cells they may be organized into discrete structures, the also are present in variable numbers. The term lymphatic organs. lymphocyte encompasses a spectrum of cells that

possess some general common features. As a class Classification they show a rounded, centrally placed nucleus; their cytoplasm shows variable degrees of basophilia and VOCABULARY: reticular tissue, hemopoietic lacks specific granules. The lymphocyte population tissue, diffuse lymphatic tissue, nodular customarily is divided into small, medium, and large lymphatic tissue lymphocytes on the basis of their size, nuclear morphology, and intensity of cytoplasmic staining. Lymphatic tissue represents a specialization within Although useful for purposes of description, such a the connective tissue compartment and, as for all subdivision is somewhat artificial because between connective tissues, can be classified according to the small and large types there is a continuum of the type of fiber that is present. On this basis, cell sizes and morphologies. lymphatic tissue can be defined as a reticular Small lymphocytes are the most numerous. As tissue. It also contributes cells (lymphocytes) to the seen in lymphatic tissues, these cells are 4 to 8 µm blood, so it is a hemopoietic tissue. In common in diameter and contain a deeply stained nucleus with bone marrow, the cellular content exceeds the surrounded by a thin rim of cytoplasm that may be intercellular material. Subdivision of lymphatic expanded slightly at one side of the cell. Although tissue into diffuse and nodular depends on the rounded in shape in the blood or lymph, in tissues arrangement and concentration of the cells, not on the cells are crowded together and assume various differences in fiber types. polyhedral shapes due to compression. The rounded or slightly indented nucleus contains a Cells of Lymphatic Tissues nucleolus that is barely discernible unless thin sections are examined. The chromatin is present as VOCABULARY: fixed cell; reticular cell; free scattered masses of heterochromatin with some cell; small, medium, large lymphocyte; plasma small patches of euchromatin. Electron microscopy cell, macrophage shows a small Golgi complex, a few mitochondria and centrioles located in the region of the nuclear The cells of lymphatic tissue are present as fixed indentation, a moderate number of ribosomes, and and free cells. Fixed cells are the reticular cells a few lysosomes scattered throughout the responsible for the formation and maintenance of cytoplasm. Granular endoplasmic reticulum is reticular fibers. The fixed cells of lymphatic tissue sparse.

134 Medium lymphocytes (prolymphocytes) range material. Free ribosomes are numerous, and a well- from 8 to 12 µm in diameter. Their nuclei are developed Golgi apparatus and centrioles are somewhat larger and more palely stained because of present in the paranuclear area. Russell's bodies, greater dispersion of the chromatin, and the when present, are contained within distended nucleoli are larger and better defined. The cisternae of the granular endoplasmic reticulum. cytoplasm is more voluminous and shows a greater Plasma cells are sessile and ultimately degenerate. basophilia because of an increased number of Despite the massive amounts of endoplasmic ribosomes. reticulum, most antibody production is carried out Large lymphocytes (lymphoblasts) range in size by forms that are intermediate between from 15 to 20 µm. Occasionally even larger forms lymphocytes and plasma cells. The precursors of (25 to 30 µm) may be found, and some authors plasma cells are B-lymphocytes. reserve the term lymphoblast for these cells. The The macrophages of lymphatic tissue are part of nucleus is rounded, palely stained, and contains one a system of mononuclear phagocytes widely spread or two prominent nucleoli, and the fine chromatin throughout tissues and organs. The cells vary from is more uniformly dispersed. The basophilic 10 to 20 µm in diameter and have oval-, kidney-, or cytoplasm is abundant and contains free ribosomes horseshoe-shaped nuclei in which one or more and polysomes. The centrosomal region and Golgi nucleoli are present. The cytoplasm is abundant and complex are more highly developed and lightly basophilic and may be vacuolated or contain mitochondria are abundant, but granular ingested material. Unless the cytoplasm does endoplasmic reticulum remains scanty. contain phagocytosed matter, the macrophage is In the development of lymphocytes, it generally is difficult to distinguish from other large accepted that the direction of maturation is from large mononuclear cells including active fibroblasts. In (lymphoblast) to small. However, this progression is electron micrographs, the cytoplasm shows complicated by the ability of small lymphocytes to numerous folds, processes, and invaginations of the respond to antigenic agents, reassume a blastlike surface and contains the usual assembly of appearance and character, and reproduce additional organelles. The Golgi apparatus is conspicuous, small lymphocytes and plasma cells. lysosomes are numerous, and residual bodies may Plasma cells (plasmocytes) vary in size from 6 to be prominent. As are macrophages elsewhere, those 20 µm in diameter. By light microscopy, the cell in lymphatic tissue are derived from blood presents a rounded, somewhat elongated or monocytes. polyhedral form, depending on its location. The nucleus is round to oval and usually is eccentrically Diffuse Lymphatic Tissue placed. This is most obvious in elongated cells, but in rounded cells the nucleus may be only slightly VOCABULARY: reticular fiber, reticular cell, eccentric in its placement. The chromatin is lymphocyte dispersed in coarse heterochromatic blocks along the inside of the nuclear envelope, giving the Diffuse lymphatic tissue is a constituent of nucleus a clock face or spoke wheel appearance. lymphatic organs and also is widely dispersed along Binucleate cells may be seen. The cytoplasm is mucous membranes. It appears as a rather loose deeply basophilic except for a prominent pale area aggregate of cells and shows no distinct adjacent to the nucleus. This area represents a demarcation from surrounding tissue with which it negative image of the Golgi apparatus. gradually merges. Basically, diffuse lymphatic tissue Occasionally, plasma cells contain globular or consists of a three-dimensional array of reticular crystalline inclusions in their cytoplasm, the best fibers and their associated reticular cells. The two known of which are Russell's bodies. These are form a spongelike framework pervaded by large round or ovoid acidophilic inclusions of variable numbers of cells, chief of which are lymphocytes. size that represent incomplete antibody. Diffuse lymphatic tissue is particularly prominent In electron micrographs the characteristic feature in the connective tissue that underlies the of plasma cells is the extensive development of the epithelium of the intestine. Here the lymphatic granular endoplasmic reticulum, which almost tissue, in association with the lining epithelium, completely fills the cytoplasm. The cisternae may be produces antibody that bathes the luminal surface. flat and parallel or distended with flocculent

135 Any antigen that does penetrate the epithelial lining lymph nodes, red pulp in the spleen, and the induces an immune response in the lymphatic epithelium of a tonsil. tissue, the cellularity of which is related to the Germinal centers appear to be sites where bacterial content of the intestines. lymphocytes are formed, but many of the newly formed cells die there. They also are the sites of Nodular Lymphatic Tissue antibody formation, and each germinal center appears to represent a clone of cells derived from an VOCABULARY: solitary nodule, primary antigen-stimulated lymphocyte and active in the nodule, germinal center, secondary nodule, production of one specific antibody. Germinal centers light pole, dark pole, cap, follicular dendritic produce B-cells that can migrate through the cap to cell, confluent lymphatic nodule, Peyer's patch leave the center and eventually pass to other lymphatic tissues. Areas where germinal centers Nodular lymphatic tissue contains the same develop contain follicular dendritic cells, which structural elements as diffuse lymphatic tissue, show numerous complex cytoplasmic processes differing only in that the components are organized that interdigitate with follicular lymphocytes. The into compact, somewhat circumscribed structures. cytoplasm contains many mitochondria, but It may occur anywhere in connective tissue but is ribosomes, lysosomes, and secretory granules are prominent along the digestive and respiratory tracts. scarce. The nucleus is irregular and shows Lymphatic nodules (also called follicles) may be peripherally placed bars of heterochromatin and a present as solitary nodules or in masses forming distinct nucleolus. These cells are considered to be confluent nodules, as occur in the appendix and the antigen-presenting cells that retain antigen on their Peyer's patches of the ileum. Lymphatic nodules are surface for the stimulation of T-helper cells prominent in organs such as the tonsils, lymph (lymphocytes). nodes, and spleen but are absent from the thymus. Germinal centers develop only after birth and in In ordinary histologic sections, some lymphatic response to antigenic stimuli. They are absent in nodules appear as rounded collections of densely animals born and raised in a germ-free packed small lymphocytes; this type of nodule is environment. Following a first exposure to antigen, called a primary nodule. Other lymphatic nodules germinal centers form de novo and then regress in contain a lightly staining central area surrounded by the absence of antigen and eventually disappear. a deeply stained cuff or cap of closely packed small Upon a second stimulation by the same antigen, lymphocytes. The pale region has been called a there is a rapid and marked production of germinal germinal center and the whole structure a centers, which precedes a rise in circulating secondary nodule. antibody. However, germinal centers are not There are few reticular fibers within the germinal essential for antibody formation, and it has been center, and the free cells are supported by a cellular suggested that germinal centers arise only after framework consisting of stellate reticular cells. The repeated contact with antigen and may be involved numerous processes of the reticular cells are joined in long-term (memory) antibody responses. by desmosomes. Reticular fibers are present in the Aggregate or confluent lymphatic nodules occur cuff of cells at the periphery of the center, where in the appendix and are exemplified by Peyer's they form concentric layers around the structure. patches found in the lower half of the small Germinal centers have dark and light poles. The intestine. Peyer's patches consist of many solitary light pole is sparsely populated by scattered small nodules massed together to form grossly visible lymphocytes and reticular cells, whereas the dark structures that underlie the intestinal epithelium. pole is densely packed with large and medium The light poles of their germinal centers are lymphocytes, cells in transition to plasma cells, directed toward the epithelium. Individual nodules macrophages, mitotic cells, and the pyknotic nuclei may be quite discrete and well defined, but usually of degenerating lymphocytes. Surrounding the they coalesce and can be distinguished only at their center is a zone of small lymphocytes that usually is apices and by their germinal centers. Peyer's patches thicker at the light pole, where it forms the cap. A are prominent in children but undergo gradual thinner layer of lymphocytes covers the dark pole. regression with aging. The structure shows definite polarity: The light pole always is directed toward a surface - the capsule in

136 Lymphatic Organs many of which migrate through the covering epithelium and appear in the sputum as salivary Lymphatic organs are divided into primary (central) corpuscles. Bacteria that penetrate the lymphoid and secondary (peripheral) organs. Primary lymphatic tissue of the tonsils act as antigens to stimulate the organs are the first to develop and include the production of antibodies. thymus in mammals and the thymus and bursa of Fabricius in birds. The mammalian equivalent of Palatine Tonsils the bursa is the bone marrow. The secondary lymphatic organs are the lymph nodes, spleen, tonsils, VOCABULARY: stratified squamous and lymphatic tissues associated with the epithelium, crypt, partial capsule gastrointestinal tract. Lymphatic organs consist of accumulations of The palatine tonsils are paired, oval lymphatic diffuse and nodular lymphatic tissues that are organs located laterally at the junction of the oral organized into discrete structures. The lymphatic cavity and oropharynx. A wet stratified squamous tissues that compose the organs are isolated from epithelium continuous with that of the oral cavity the surrounding tissues by a well-defined capsule. and oropharynx covers the free surface of the tonsil and is very closely associated with the lymphatic tissue. Tonsils Deep invaginations of the epithelium form the tonsillar crypts that reach almost to the base of the

tonsil. Secondary crypts may branch from these Tonsils are aggregates of lymphatic nodules deep pockets and also are lined by stratified associated with the pharynx and oropharynx. A ring squamous epithelium that tends to thin out in the of lymphatic tissue (Waldeyer’s ring), in varying deeper parts of the crypts. Lymphatic nodules, degrees of completeness, is present at the entrance many with prominent germinal centers, usually are to the esophagus and respiratory tract. The arranged in a single layer beneath the epithelium, structures are spread through different areas - embedded in a mass of diffuse lymphatic tissue. oropharynx, nasopharynx, and tongue - and form Large numbers of lymphocytes infiltrate the the palatine, pharyngeal, and lingual tonsils, epithelium, especially that lining the lower ends of respectively. The general structure of a tonsil is the crypts, often to such an extent that the shown in Figure 11-1. demarcation between epithelium and lymphatic

tissue is obscure. A partial capsule beneath the basal surface of the tonsil separates it from surrounding structures to which the capsule adheres. Septa of loosely arranged collagen fibers extend from the capsule into the tonsillar tissue and partially divide the crypts and their associated lymphatic tissue from one another. Fibers from the septa spread out and become continuous with the reticular fibers of the lymphatic tissue. The connective tissue is infiltrated by lymphocytes of various sizes, plasma cells, and mast cells. Neutrophil granulocytes may be present and are numerous during inflammation of the Fig. 11-1. A diagrammatic sketch illustrating the general structure of a tonsil. tonsils. Small compound tubuloacinar mucoserous glands lie outside the capsule, and their ducts drain Tonsils do not filter lymph; they have no afferent to the free surface of the tonsil or, rarely, empty vessels leading to them, nor do they have an into a tonsillar crypt. internal system of sinuses for the filtering of lymph. However, at the inner surfaces of the tonsils, plexuses of blindly ending lymph capillaries form the beginnings of efferent lymphatic vessels. Tonsils contribute to the formation of lymphocytes,

137 Lingual Tonsils Structure

VOCABULARY: root of the tongue, crypt, VOCABULARY: hilus, afferent lymphatic, stratified squamous epithelium, mucous gland efferent lymphatic, diffuse lymphatic tissue, nodular lymphatic tissue, capsule, trabeculae, The lingual tonsils form nodular bulges in the root reticular network, cortex, outer cortex, deep of the tongue, and their general structure is similar (inner) cortex, thymus-dependent area, to that of the palatine tonsil. Crypts are deep, may follicular dendritic cell, medulla, medullary be branched, and are lined by a wet stratified cord squamous epithelium that invaginates from the surface. The associated lymphatic tissue consists of Grossly, lymph nodes appear as flattened, ovoid or diffuse and nodular types. Compound tubuloacinar bean-shaped structures with a slight indentation at mucous glands embedded in the underlying one side, the hilus, through which blood and muscle of the tongue drain by ducts, most of which lymphatic vessels enter or leave. Lymph nodes are open into the bases of the crypts. the only lymphatic structures that are set into the lymphatic circulation and thus are the only lymphatic organs to Pharyngeal Tonsil have afferent and efferent lymphatics. Afferent lymphatics enter the node at multiple sites, VOCABULARY: nasopharynx, ciliated anywhere over the convex surface; efferent pseudostratified columnar epithelium lymphatics leave the node at the hilus. Both sets of vessels have valves that allow unidirectional flow of The pharyngeal tonsil is located on the posterior lymph through a node. The valves of afferent wall of the nasopharynx. Its surface epithelium is a vessels open toward the node; those of efferent continuation of that lining the respiratory passages - vessels open away from the node. namely, ciliated pseudostratified columnar Essentially, lymph nodes consist of accumulations epithelium that contains goblet cells. Patches of of diffuse and nodular lymphatic tissue enclosed stratified squamous epithelium may be present, in a capsule that is greatly thickened at the hilus. however, and tend to become more common with The capsule consists of closely packed collagen aging and smoking. The crypts are not as deep as in fibers, scattered elastic fibers, and a few smooth the palatine tonsils, and the epithelium forms muscle cells that are concentrated about the numerous shallow folds. A thin capsule separates entrance and exit of lymphatic vessels. From the the pharyngeal tonsil from underlying tissues and inner surface of the capsule, branching trabeculae provides fine septa that extend into the substance of dense irregular connective tissue extend into the of the tonsil. Small compound tubuloacinar node and provide a kind of skeleton for the lymph mucoserous glands lie beneath the capsule and node. The spaces enclosed by the capsule and empty onto the surface of the folds. trabeculae are filled by an intricate, three- dimensional reticular network of reticular fibers Lymph Nodes and their associated reticular cells. The meshes of the network are crowded with lymphatic cells, so Lymph nodes are small encapsulated lymphatic disposed as to form an outer cortex and an inner organs set in the course of lymphatic vessels. They medulla. are prominent in the neck, axilla, groin, and The cortex forms a layer beneath the capsule and mesenteries and along the course of large blood extends for a variable distance toward the center of vessels in the thorax and abdomen. the node. It consists of lymphatic nodules, many with germinal centers, set in a bed of diffuse lymphatic tissue. Trabeculae are arranged fairly regularly and run perpendicular to the capsule, subdividing the cortex into several irregular "compartments." However, trabeculae do not form complete walls but rather represent bars or rods of connective tissue, and compartmentalization of the cortex is incomplete.

138 All the cortical areas communicate with each other sinuses that run between the medullary cords and laterally and above and below the plane of section trabeculae of the medulla. At the hilus, the where trabeculae are deficient. medullary and subcapsular sinuses unite, penetrate The cortex usually is divided into an outer cortex the capsule, and become continuous with the that lies immediately beneath the capsule and efferent lymphatics (Fig. 11-2). contains nodular and diffuse lymphatic tissue and a deep (inner) cortex that consists of diffuse lymphatic tissue only. There is no sharp boundary between the two zones, and their proportions differ from node to node and with the functional status of the node. In general, B-lymphocytes are concentrated in the nodular lymphatic tissue, while T-cells are present in the diffuse lymphatic tissue. The deep cortex becomes depleted of cells after thymectomy and has been called the thymus- dependent area. This area also contains follicular dendritic cells, a cell type with a pale-staining Fig. 11-2. A diagrammatic sketch illustrating the general structure of a nucleus, few cytoplasmic organelles, and numerous, lymph node. long processes that interdigitate with similar projections from nearby lymphocytes. They In sections, sinuses appear as regions in which the represent a form of antigen-presenting cell. The deep reticular net is coarser and more open. Stellate cells cortex continues into the medulla without supported by reticular fibers crisscross the lumen interruption or clear demarcation. and are joined to each other and to the cells The medulla appears as a paler area of variable bordering the lumen by slender processes. width, generally surrounding the hilus of the node. Numerous macrophages are present in the luminal It consists of diffuse lymphatic tissue arranged as network and also project from the boundaries of irregular medullary cords that branch and the sinus. The cells that form the margins of the anastomose freely. Medullary cords contain sinuses and extend through the sinus space abundant plasma cells, macrophages, and generally are regarded as flattened reticular cells. lymphocytes. The trabeculae of the medulla are However, they also have been considered to be more irregularly arranged than those of the cortex. attenuated endothelial cells akin to those which line the lymphatic vessels with which they are Lymph Sinuses continuous. Sinuses in the cortex are less numerous than in the

medulla and are relatively narrow. Those in the VOCABULARY: subcapsular (marginal) sinus, medulla are large and irregular and show repeated cortical (trabecular, intermediate) sinus, branchings and anastomoses. They run a tortuous medullary sinus course in the medullary parenchyma and account

for the irregular cordlike arrangement of the Within the lymph node is a system of channel-like lymphatic tissue in this area. spaces, the lymph sinuses, through which lymph percolates. Lymph enters the node through afferent lymphatic vessels that pierce the capsule anywhere Blood Vessels along the concave border and empty into the subcapsular (marginal) sinus, which separates VOCABULARY: postcapillary venule, high the cortex from the capsule. The sinus does not endothelium form a tubular structure but is present as a wide space extending beneath the capsule, interrupted at The major blood supply reaches the node through intervals by the trabeculae. The subcapsular sinus is the hilus, and only occasionally do smaller vessels continuous with similar structures, the cortical enter from the convex surface. The arteries (trabecular, intermediate) sinuses, which extend (arterioles) at first run within the trabeculae in the radially into the cortex, usually along the trabeculae. medulla but soon leave these and enter the These in turn become continuous with medullary medullary cords to pass to the cortex.

139 Here they break up into a rich capillary plexus that Broad bands of connective tissue, the trabeculae, distributes to the diffuse lymphatic tissue and also extend from the inner surface of the capsule and forms a network around the nodules. The capillaries pass deeply into the substance of the spleen to form regroup to form venules that run from the cortex a rich, branching and anastomosing framework. As and enter the medullary cords as small veins. These in lymph nodes, the trabeculae subdivide the organ in turn are tributaries of larger veins that pass out of into communicating compartments. The spaces the node at the hilus. In the deep cortex, the between trabeculae are filled by a reticular venules take on a special appearance and have been network of fibers and associated reticular cells. The called postcapillary venules. These vessels are meshes vary in size and tend to be smaller around characterized by a high endothelium that varies blood vessels and aggregates of lymphatic tissue. from cuboidal to columnar and at times appears to The substance of the spleen is called the splenic occlude the lumen. The walls of these vessels often pulp, and sections from a fresh spleen show a clear are infiltrated with small lymphocytes that generally separation of the tissue into rounded or elongated are presumed to be passing into the lymph node. grayish areas set in a greater mass of dark red tissue. The cells pass between adjacent endothelial cells, Collectively, the scattered gray areas form the white indenting their lateral walls as they cross through pulp and consist of diffuse and nodular lymphatic the endothelium. The significance of the tall tissue. The dark red tissue is the red pulp and endothelium is not known, but it has been consists of diffuse lymphatic tissue that is suffused suggested that as the lymphocytes sink deeper with blood. The large number of red cells present between them, the adjacent endothelial cells resume imparts the color to the red pulp. The red pulp their original relationship to each other above the contains large, branching, thin-walled blood vessels lymphocytes and seal off the interendothelial cleft, called splenic sinusoids (sinuses), and such thereby limiting the loss of plasma from the venule. spleens are said to be sinusal.

Spleen White Pulp

The spleen embodies the basic structure of a lymph VOCABULARY: periarterial lymphatic sheath node and can be regarded as a modified, enlarged (PALS), lymphatic nodule, splenic follicle, lymph node inserted into the blood flow. Unlike lymph marginal zone, interdigitating cell nodes, the spleen has no afferent lymphatics and no lymphatic sinus system, and the lymphatic tissue of The white pulp generally is associated with the the spleen is not arranged into a cortex and arterial supply of the spleen and forms the medulla. It does have a distinctive pattern of blood periarterial lymphatic sheaths (PALS) that circulation and specialized vascular channels that extend about the arteries, beginning as soon as the facilitate the filtering of blood. vessel leaves the trabeculae to enter the splenic pulp. The sheaths have the structure of diffuse Structure lymphatic tissue and contain the usual cellular elements of lymphatic tissue. Small lymphocytes, VOCABULARY: capsule, hilus, trabeculae, mostly T-cells, make up the bulk of the cells. Here reticular network, splenic pulp, white pulp, red and there along the course of the sheath, the pulp, splenic sinusoid lymphatic tissue expands to incorporate lymphatic nodules that resemble the cortical nodules of The spleen is enclosed in a well-developed capsule lymph nodes. They represent sites of B- of dense irregular connective tissue. Elastic fibers lymphocytes, and many contain germinal centers. are present between bundles of collagen fibers and The lymphatic nodules of the spleen are called are most abundant in the deeper layers of the splenic follicles. At the periphery of the lymphatic capsule. Smooth muscle fibers also may be present sheath, the reticular net is more closely meshed in small groups or cords, but the amount varies. On than elsewhere, and the reticular fibers and cells the medial surface of the spleen, the capsule is form concentric layers that tend to delimit the indented to form a cleft-like hilus through which lymphatic tissue from the red pulp. This forms the blood vessels, nerves, and lymphatics enter or leave marginal zone. the spleen.

140 Cells similar to the follicular dendritic cells of Blood Supply lymph nodes are present in the white pulp of the spleen. These dendritic-like cells are called VOCABULARY: trabecular artery, central interdigitating cells and also represent antigen- artery, penicillar artery, sheathed capillary, presenting cells. open circulation, closed circulation, pulp vein, trabecular vein Red Pulp The architecture of the spleen is best understood in VOCABULARY: splenic sinusoid, splenic cord relation to the blood circulation, which shows some special features. Branches of the splenic artery enter The red pulp is a diffuse lymphatic tissue associated the spleen at the hilus, divide, and pass within with the venous system of the spleen. The reticular trabeculae into the interior of the organ. These meshwork is continuous throughout the red pulp branches have an over-lapping, segmental and is filled with large numbers of free cells, arrangement, with each main branch serving a including all those usually found in the blood. Thus, defined area of the spleen. The trabecular arteries, in addition to the cells of lymphatic tissue, the red as they now are called, branch repeatedly, finally pulp is suffused with red cells, granular leukocytes, leaving the trabeculae as central arteries that and platelets. Occasionally, macrophages can be immediately become surrounded by the lymphatic seen that contain ingested red cells or granulocytes tissue of the periarterial lymphatic sheath. Where or that are laden with a yellowish brown pigment, the sheath expands to form nodules, the central hemosiderin, derived from the breakdown of artery (more appropriately called the follicular hemoglobin. arteriole) is displaced to one side and assumes an The red pulp is riddled with large, irregular blood eccentric position in the nodule. Only rarely does vessels, the splenic sinusoids, between which the the vessel retain a central position in the nodular red pulp assumes a branching, cordlike arrangement tissue. Throughout its course in the white pulp, the to form the splenic cords. The sinusoids have central artery provides numerous capillaries that wide lumina (20 to 40 µm diameter), and their walls supply the sheath and then pass into the marginal have a unique structure. Almost the entire wall is zone surrounding the white pulp. How these made up of elongated, fusiform endothelial cells capillaries end is not certain. Many arterial branches that lie parallel to the long axis of the vessel. The appear to open directly into the marginal zone, cells lie side by side around the lumen but are not in often ending in a funnel-shaped terminal part. contact and are separated by slitlike spaces. Outside There is no direct venous return, and the white pulp is the endothelium, the wall is supported by a associated only with the arterial supply. Although the basement membrane that is not continuous but term artery commonly is used for the different forms widely spaced, thick bars that encircle the levels of the splenic vasculature, once the vessel has sinusoid. The bars of basement membrane are joined to reached lymphatic tissue, it is actually an arteriole. each other by thinner strands of the same material The central artery continues to branch, and its and are continuous with the reticular network of attenuated stem passes into a splenic cord in the red the splenic cords (Fig. 11-3). pulp, where it divides into several short, straight penicillar arteries, some of which show a thickening of their wall and now form sheathed capillaries. The sheath consists of compact masses of concentrically arranged cells and fibers that become continuous with the reticular network of the red pulp. Close to the capillary the cells of the sheath are rounded, while at the periphery of the sheath the cells become stellate. The cells of the sheath are avidly phagocytic. Not all the capillaries are sheathed, and Fig. 11-3. A diagrammatic sketch illustrating the structure of a occasionally, a single sheath may enclose more than splenic sinusoid. one capillary.

141 Sheathed capillaries may continue as simple hilus. Areas of the spleen supplied by sinusoids (i.e., capillaries or may divide to produce two to four the red pulp) lack lymphatics. nonsheathed, terminal capillaries. How these end still is a matter of some debate and has led to the Thymus "open" and "closed" theories of circulation. According to the open circulation theory, the The thymus is a bilobed, encapsulated lymphatic capillaries empty into the meshwork of the red pulp organ situated in the superior mediastinum dorsal in the splenic cords. The blood then slowly seeps to the sternum and anterior to the great vessels that through the red pulp and finds its way into the emerge from the heart. The thymus is the only venous system through the walls of the splenic primary lymphatic organ and is the first organ of sinusoids. The closed circulation theory holds that the embryo to become lymphoid. Unlike the spleen the capillaries open directly into the lumina of the and lymph nodes, it is well developed and relatively venous sinusoids; blood enters the sinusoid at the large at birth. The greatest weight (30 to 40 g) is capillary end and leaves at the venous end because achieved at puberty, after which the organ of a decreasing pressure gradient between the two undergoes progressive involution and is partially ends of the sinusoid. A compromise between the replaced by fat and connective tissue. two views suggests that a closed circulation in a contracted spleen becomes open when the spleen is Structure distended. Splenic sinusoids drain into pulp veins, which are VOCABULARY: lobe, capsule, septum, lobule, supported by a thin muscle coat and, more cortex, medulla, reticular network, epithelial externally, are surrounded by reticular and elastic reticular cell, cytoreticulum fibers. A sphincter-like activity of the smooth muscle has been described at the junction of the The thymus consists of two lobes closely applied pulp veins and sinusoids. Pulp veins enter and joined by connective tissue. Each lobe arises trabeculae, where, as trabecular veins, they pass in from a separate primordium, and there is no company with the artery (Fig. 11-4). Trabecular continuity of thymic tissue from one lobe to the veins unite and leave at the hilus as the splenic vein. other. A thin capsule of loosely woven connective tissue surrounds each lobe and provides septa that extend into the thymus, subdividing each lobe into a number of irregular lobules. Each lobule consists of a cortex and medulla. In the usual sections, the medulla may appear as isolated, pale areas completely surrounded by denser cortical tissue. However, serial sections show that the lobules are not isolated by the septa; the medullary areas are continuous with one another throughout each lobule. The free cells of the thymus are contained within the meshes of a reticular network, which, however, differs from that of other lymphatic tissues. In the thymus the reticular cells take origin from endoderm rather than mesenchyme and are not Fig. 11-4. A diagrammatic sketch illustrating the blood supply of associated with reticular fibers. These epithelial the spleen. reticular cells are stellate in shape with greatly branched cytoplasm that contains few organelles. The spleen has no afferent lymphatics, but The large, round or oval euchromatic nuclei show efferents arise deep in the white pulp and converge one or more small but prominent nucleoli. The on the central arteries. The lymphatics enter cytoplasmic processes are in contact with the trabeculae and form large vessels that leave at the processes of other reticular cells and at their points of contact are united by desmosomes.

142 Thus, the stroma of the thymus consists of a Medulla cytoreticulum composed of epithelial cells. Reticular fibers are found only in relation to blood VOCABULARY: pleomorphic reticular cell, vessels. dendritic interdigitating cell, small lymphocyte, thymic corpuscle Cortex The medulla occupies the central region of the VOCABULARY: lymphocyte, no lymphatic thymus, where it forms a broad, pale band of tissue nodules, reticular cell, macrophage that is continuous throughout each lobule. Often, however, in histologic sections it appears to be The cortex consists of a thick, deeply stained layer isolated within a lobule, surrounded by a complete extending beneath the capsule and along the septa. layer of cortex. Lymphocytes are less numerous Most of the cells of the cortex are lymphocytes than in the cortex, and the epithelial reticular cells that are closely packed with little intervening are not as widely dispersed or as highly branched. material between them. Because of mutual The reticular cells tend to be pleomorphic and compression, the cells appear polyhedral in shape. vary from stellate cells with long processes to Large, medium, and small lymphocytes are present, rounded or flattened cells with many desmosomes the latter being the most abundant; they are and abundant tonofilaments. Dendritic indistinguishable from small lymphocytes found interdigitating cells are found in the medulla and elsewhere. Large lymphocytes tend to concentrate corticomedullary region. They act as antigen-presenting in the outer cortex beneath the capsule and cells for more mature T-lymphocytes. represent stem cells that have newly emigrated from The free cells of the medulla are mainly small the bone marrow. They form only a small part of lymphocytes, but a small and variable number of the cell population. Small lymphocytes become macrophages are present. Plasma cells, mast cells, increasingly more numerous toward the deeper and eosinophil granulocytes can be found. Mitotic cortex, where degenerating cells with pyknotic figures are rare. Collagen and reticular fibers extend nuclei also are found. Unlike lymph nodes, there are for short distances from blood vessels and wind no lymphatic nodules in the cortex of the thymus, between the epithelial cells. nor is there an internal sinus system. Rounded or ovoid epithelial structures, the Reticular cells in the cortex are highly branched, thymic corpuscles, are a prominent feature of the but their processes are obscured by the mass of medulla. These bodies vary in size from 10 to lymphocytes. They form a continuous layer at the 100µm or more in diameter and consist of flattened periphery of the cortex, separating it from the epithelial reticular cells, wrapped about one another capsule and septa. These epithelial reticular cells in concentric lamellations that are joined by contain tonofilaments and membrane-bound numerous desmosomes. Many contain granules of structures that appear to be secretion granules. keratohyalin. The cells at the center of the structure Macrophages are consistently present in small undergo hyalinization or necrosis and may become numbers, scattered throughout the cortex. They are lysed to leave a cystic structure. Some of the cells at difficult to distinguish from reticular cells by light the periphery of the corpuscle retain their microscopy unless phagocytosed material can be connections with the surrounding cytoreticulum. seen in their cytoplasm. In electron micrographs The function of the thymic corpuscles is unknown; they are distinguished from epithelial reticular cells they have been regarded purely as degenerated by the lack of desmosomes. Macrophages that have structures, but there is evidence that they may be engulfed degenerating cells can be found scattered secretory bodies. throughout the thymus and tend to increase in number toward the junction of the cortex and Blood Supply medulla. VOCABULARY: corticomedullary junction, cortical capillary, blood-thymic barrier

The arteries to the thymus penetrate the organ within the connective tissue of the septa.

143 Arteriolar branches from these vessels run along size and number of thymic corpuscles. The septa the corticomedullary junction and provide also increase in width. The cortical areas become arterioles and capillaries to the medulla and infiltrated by fat cells, and the replacement may capillaries to the cortex; vessels larger than become extensive. By the time involution capillaries are not found in the cortex. Within the commences, T-cells have disseminated into the cortex, the capillaries run toward the capsule, where secondary lymphatic tissue throughout the body. they form branching arcades before passing back The thymic parenchyma does not disappear through the cortex to drain into venules and thence completely even in old age, and the thymus into veins that accompany the arterioles in the maintains some activity in the adult. corticomedullary region and medulla. The veins leave via the septa and ultimately unite to form a Other Constituents single thymic vein. The cortical capillaries are enveloped by a collar The thymus frequently shows a number of peculiar of connective tissue that forms part of the blood- structural elements, the significance of which is not thymic barrier. This envelope in turn is known. Among these structures are cystlike spaces surrounded by a continuous layer of epithelial lined by cells with brush borders, cilia, or mucus- reticular cells. The perivascular connective tissue producing cells or by reticular cells that contain space varies in width and is traversed by reticular microvillus-lined vacuoles. fibers that accompany the vessel. Within the Most peculiar are the "myoid" cells, which have an perivascular space are granular leukocytes, plasma imperfect resemblance to striated muscle. These cells, macrophages, and lymphocytes. The blood- cells may resemble embryonal or adult muscle thymic barrier in the cortex thus consists of the fibers complete with typical banding patterns: Z capillary endothelium and its basal lamina, the lines and A, I, and M lines have been described. It perivascular connective tissue sheath, and the layer is not known whether these various inclusions have of epithelial reticular cells and their associated basal functional significance or represent aberrant lamina. There is little movement of macromolecules differentiation of embryonal elements. across this barrier, and cortical lymphocytes develop in relative isolation from antigens in a Dendritic (Antigen-Presenting) Cells privileged environment. Vessels of the medulla and corticomedullary junction are permeable to Dendritic (antigen-presenting) cells arise in the circulating macromolecules. As T-lymphocytes bone marrow but, unlike monocytes (macrophages), differentiate in the cortex of the thymus, they have low levels of lysosomal enzymes. In contrast, acquire surface marker molecules of major they have high levels of class II major histocompatibility complex (MHC I and MHC II) histocompatibility complex (MHC) molecules and synthesize receptors for recognition of foreign essential for presenting new antigens to T- antigens. Those lymphocytes which recognize self lymphocytes. Dendritic cells have the capacity to MHC are destroyed in the thymus, whereas those ingest foreign proteins, process (partially digest) the T-lymphocytes capable of reacting to nonself go on antigens, and insert selected portions of an antigen to form clones of cells that mature and are released into their cell membrane, thereby retaining the as immunocompetent T-lymphocytes. Lymphocytes product for long periods of time. leave the thymus by entering postcapillary venules Dendritic cells are found in lymph nodes, spleen, located in the medulla and near the thymus, mesenteries, and skin (Langerhans' cells) corticomedullary junction. and under mucosal surfaces. Despite the morphologic similarities of exhibiting fine ramifying Involution cytoplasmic processes and containing relatively few lysosomes, each dendritic antigen-presenting cell Growth of the thymus during fetal life is very rapid, appears to have developed specialized surface and the organ attains its greatest relative size by the receptors for a given specific microenvironment at time of birth. It continues to grow, at a lesser rate, the site in which it is found. After a period of time, until puberty, after which it undergoes progressive they leave the specific environment (skin, for involution. The organ decreases in weight, loses example), make their way to lymphoid tissue, and cortical lymphocytes, and shows an increase in the gradually present the antigens collected in a specific

144 environment to T-lymphocytes. Thus, the dendritic they fuse, the sinuses follow the courses of the antigen-presenting cells play a very important role developing trabeculae, subdivide, and develop into in the immune system by monitoring a number of the trabecular and medullary sinuses. The different environments. A short introduction as to mesenchymal aggregates give rise to the reticular the function of the lymphatic organs and their cells and fibrous network of the cortex and intricate interrelationships with one another are medulla. The entire mass is seeded by lymphocytes from presented in the summary. the bone marrow and thymus, and the cells take up their characteristic locations in the nodes. B-lymphocytes Organogenesis tend to migrate into lymphatic nodules of the outer cortex; T-lymphocytes tend to migrate into the deep The thymus arises as a pair of endodermal outgrowths cortex (paracortex) of the lymph node. from the third pharyngeal pouch, with some The spleen first appears as several thickenings in contribution from the fourth also. Each outgrowth the cranial end of the greater omentum. With extends caudally and medially and by the eight week growth, the primordia fuse to form a lobulated lies in the midline beneath the upper part of the spleen that extends into the body cavity. As the sternum. Initially, each outgrowth has a slitlike spleen expands, it loses its lobulated appearance. lumen surrounded by several layers of tall cuboidal Mesenchymal cells in the body of the cellular mass or columnar cells. differentiate into reticular cells that provide the As the cells proliferate, the lumina disappear. splenic reticular stroma. The outermost connective Centrally, the cells become stellate and loosely tissue gradually condenses to form the capsule from arranged but retain their cytoplasmic connections which mesenchymal projections condense to form with one another. These form the epithelial reticular the trabeculae. At this stage (eighth to ninth week) cells of the thymic cytoreticulum. the spleen consists of a meshwork of reticular cells At about 8 weeks, the thymic rudiment is seeded surrounded by a thin capsule. This spleen rudiment by blood-borne stem cells that originate in the yolk sac is seeded by lymphocytes from the bone marrow and and fetal liver; lymphoid stem cells capable of thymus. Enlargement of the white pulp to form repopulating the thymus are present in the bone nodules occurs late, and during much of marrow also. The migration of stem cells appears to development, lymphatic tissue mainly forms be guided by chemotactic agents liberated by the periarterial lymphatic sheaths. Germinal centers appear thymic reticular cells. The stem cells give rise to the only after birth. The fetal spleen is hemopoietic, thymic lymphocytes, which, by the fourteenth week producing erythrocytes and granular leukocytes, but of gestation, are arranged into cortex and medulla. in humans this function ceases just before birth. It is not known whether the lymphoid progenitor cells are already differentiated as T-cell precursors Summary at the time of seeding, or whether differentiation occurs in the thymus. The assembly of lymphatic tissue – lymphatic Mesenchyme surrounding the thymic primordium organs and lymphoid cells spread throughout the condenses to form the capsule and also invades the connective tissue compartment – constitutes the cellular mass to provide the septa. immune system that protects the body against Lymph nodes develop locally in vascular invading foreign macromolecules and constantly mesenchyme at sites normally occupied by lymph monitors the circulating fluids (blood and lymph) nodes, and their formation is related closely to for abnormal components. Antigenic material development of the lymph vascular system. evokes a specific defense reaction, the immune Aggregates of mesenchymal cells form around response, that gives rise either to B-cells that loops of lymphatic capillaries, in close association produce specific antibodies or to a variety of T-cells with lymphatic sacs. At the periphery of the that attack foreign cells directly, release nonspecific aggregates, lymphatic vessels form isolated spaces toxic agents, assist B-cells in the production of that are separated by slender bridges of connective antibody, or direct macrophage response and tissue. As the lymph spaces gradually fuse to form maturation. Antibodies may act by binding to the the marginal (subcapsular) sinus, the connective antigen to neutralize it, inhibit the entrance of the tissue outside the sinus and in the bridges antigen into a cell, enhance its phagocytosis, or condenses to form the capsule and trabeculae. As initiate its lysis.

145 Germinal centers are associated with production of disseminate throughout the body, and the node humoral antibody, especially during a secondary itself may become a focus of chronic infection. response, and may represent a clone of B-cells Lymph nodes are immunologic organs and geared to the production of a single antibody. participate in humoral and cellular immune Stimulation by antigen results in proliferation and responses. Antigen accumulates about and within differentiation of lymphocytes, some of which go primary nodules and at the junction of the deep and on to react against the antigen, while others remain outer cortex. Some of the antigen appears to be as committed memory cells. On a second exposure held on the surfaces of macrophages and follicular to the same antigen, memory cells react swiftly and dendritic cells and thus is available to react with T- with great efficiency. and B-cells as these move into the node and sort Tonsils are the major bulwark against oral and out within the parenchyma. pharyngeal routes of infection. They contribute to Antigens that evoke antibodies react with the formation of lymphocytes, are able to mount uncommitted B-cells to elicit their activation and immunologic responses, and take part in the proliferation. The first antibody-producing cells immunologic defenses of the body. Tonsils appear appear in the cortex and then migrate into to be especially valuable sources of interferon, an medullary cords, where they accumulate as plasma antiviral factor, and may be of greater importance in cells. After this cortical response, intense antibody fighting infection than has been recognized formation occurs in newly formed germinal centers. previously. This is the primary response, elicited on first Lymph nodes contribute to the production of exposure to an antigen, and takes several days to lymphocytes, as indicated by the mitotic activity, develop. Relatively few cells respond, and a low especially in germinal centers. The titer of antibody results. On a second exposure to lymphocytopoietic activity of quiescent nodes does the same antigen, a rapid and greatly enhanced not seem to be great, and the bulk of the cells in an production of antibody occurs. This is the unstimulated node are recirculating lymphocytes. B- secondary response, marked by an explosive and T-cells enter the node through the postcapillary development of germinal centers that expand venules, but some also enter through afferent rapidly because of the presence of conditioned lymphatics. Once in the parenchyma, B-cells home memory cells produced during the primary in on the lymphatic nodules of the outer cortex; T- response. cells make their way to the deep cortex and the Cell-mediated responses (rejection of foreign region of the thymus-dependent area. The cells grafts, reaction to molds, fungi, and some bacteria remain in the lymph node for variable periods of and viruses that invade cells) involve the T-cells. time and then leave to re-circulate in the blood and The deep cortex of the draining nodes become very other lymphatic organs. Only small lymphocytes thick, and lymphoblasts in this area divide rapidly. recirculate. After a short lag, small lymphocytes leave the Lymph nodes form an extensive filtration system nodes, infiltrate the affected region, and destroy the that removes foreign particles from the lymph, antigen. Committed T-cells also are disseminated preventing their spread through the body. A single throughout the body as memory cells and form part lymph node can remove 99% of the particulate of the recirculating pool of small lymphocytes. matter presented to it, and lymph usually passes Since an associated humoral response also through several nodes before entering the develops, newly formed germinal centers appear in bloodstream. Sinuses form a settling chamber the cortex. through which lymph flows slowly, while baffles of Normal humans have no significant splenic crisscrossing reticular fibers form a mechanical reservoir of erythrocytes (only 30 to 40 ml), as in filter and produce an eddying of the lymph. Most of some species, but there are indications that even the lymph that enters a node flows centrally this small volume can be mobilized. About one- through the sinuses, where it comes into contact third of the platelets in the body are sequestered in with numerous phagocytes. Filtration by a lymph the spleen, where they form a reserve available on node may be impaired if the number of particles is demand. excessive or if the organism is exceptionally The spleen serves as a filter for blood, removing virulent. Agents not destroyed in a lymph node may particulate matter that is taken up by macrophages in the marginal zone, splenic cords, and sheathed

146 capillaries. The sinusoidal endothelium and the spleen are the same as those occurring in lymph reticular cells of the reticular net have no special nodes and include primary and secondary phagocytic powers and contribute little to the responses. In the primary response, clusters of clearing of foreign materials from the blood. antibody-forming cells first appear in the periarterial The spleen also functions as the graveyard for lymphatic sheaths, then increase in number and worn out red cells and platelets and possibly for concentrate at the edge of the sheath. Immature granular leukocytes as well. As the blood filters and mature plasma cells appear, and germinal through the splenic cords, it comes under constant centers develop in the splenic nodules. Ultimately, scrutiny and monitoring by macrophages. Viable plasma cells become numerous in the marginal zone cells are allowed to pass through the spleen, but between white and red pulp and in the red pulp damaged, aged, or infected cells are retained and cords, either by direct emigration from the white phagocytosed. Several factors are probably involved pulp or indirectly via the circulation. During a in the recognition of old cells. As the cell ages, secondary response, germinal center activity changes in the surface may permit antigenic dominates, occurs almost immediately, and is of reaction with opsonizing antibodies that enhance large scale. phagocytosis. The sinusoidal wall is a barrier to the The spleen has important hemopoietic functions re-entrance of cells into the circulation, since the in all vertebrates and in lower forms is the primary cells must insinuate themselves through the narrow blood-forming organ, producing all types of blood slits between sinusoidal endothelial cells. Normal cells. The spleen produces red cells, platelets, and cells are pliant and able to squeeze through the granulocytes only during embryonic life, but interendothelial clefts, but cells such as spherocytes production of lymphoid cells continues throughout or sickled red cells are unable to pass through the life. In some conditions (leukemia and some sinusoidal barrier. As red cells age, their membranes anemias) the red pulp contains islets of hemopoietic become more permeable to water, and the relatively tissue, even in the adult. This probably results from slow passage through the red pulp may allow the sequestration of circulating stem cells from the cells to imbibe fluid, swell, and become too rigid to blood rather than from activation of an indigenous pass into the sinusoids. Red cells that have been population of potential stem cells in the spleen. excluded are phagocytosed, and the cells appear to Removal of the spleen (splenectomy) emphasizes be taken up intact without prior lysis or break its primary functions. The peripheral blood shows down. Components of the red cells that can be an increase in the number of platelets and abnormal reused in production of new blood cells are erythrocytes. Older erythrocytes often contain recovered by the spleen; it is especially efficient in Howell-Jolly bodies that normally would have been conserving iron freed from hemoglobin and removed by the splenic cord-sinus system. returning it to accessible stores. Following splenectomy, individuals are at risk of A "pitting function" has been described for the developing bacterial septicemia. Under normal spleen. Red cells that contain rigid inclusions circumstances, blood-borne vectors would (malarial parasites or the iron-containing granules of stimulate the immune component of the spleen and siderocytes, for example) but that are otherwise prevent infection via the blood. normal are not destroyed, but the inclusions are The thymus is essential for production of T-cells, removed at the wall of the sinusoid. The flexible lymphocytes that are involved in cell-mediated part of the erythrocyte passes through the immune responses such as rejection of foreign sinusoidal wall, but the rigid inclusion is held back grafts and immunologic responses to fungi, viruses, by the narrow intercellular clefts and is stripped and certain bacteria. from the cell. The rigid portion remains in the Although not directly involved in elaborating splenic pulp and is phagocytosed; the rest of the cell conventional antibodies, T-cells do cooperate with enters the lumen of the sinusoid. B-cells in producing antibodies against antigens The spleen has great importance in the immune such as foreign red cells. All these responses are system, mounting a large scale production of impaired in animals that have been thymectomized antibody against blood-borne antigens. However, at birth. There is a marked decrease in the number antigen introduced by other routes also evokes a of circulating small lymphocytes, and the deep response in the spleen, since an antigen soon finds cortex of lymph nodes and the periarterial its way into the bloodstream. The reactions in the lymphatic sheaths of the spleen fail to develop.

147 Most of the lymphocytes in the thymus seem to thymopoietin, which induces T-cell maturation; be inert and acquire immunologic capabilities after thymic humoral factor, which enhances T-cell passage through the spleen. T-cells have a long life mediated graft rejection; and serum thymic factor, span and re-circulate between lymphatic organs which induces development of surface markers on (except for the thymus), lymph, and blood. Thymic T-cells. The agents appear to be produced locally in lymphocytes show a high rate of mitotic activity, the thymus, possibly by the reticular epithelial cells, but many die within the thymus or emigrate to and to have local effects on the lymphocyte other organs. population in the thymus. Thymic tissue contained in diffusion chambers In addition to encapsulated lymphatic tissue, an and transplanted into thymectomized newborn equally large amount of non-encapsulated lymphoid animals partly prevents or reverses the effects of tissue occurs in the walls of the respiratory, thymectomy. Permeable molecules but not whole urogenital, and gastrointestinal tracts that provides cells diffuse from the chamber, indicating the immunologic protection along these mucosal presence of a thymic-produced humoral factor. surfaces. Here it is generally referred to as mucosa- Several peptides have been isolated from thymic associated lymphoid tissue (MALT); if specifically extracts and appear to have some regulatory and associated with the gastrointestinal tract it is termed stimulating effects on the thymus. The best gut-associated lymphoid tissue (GALT); if characterized of these is thymosin, an agent that associated with intrapulmonary bronchi it is restores T-cell deficiencies in thymectomized mice; referred to as bronchi associated lymphatic tissue it is regarded as a hormone that induces T-cell (BALT). The T-lymphocytes found in the mucosal differentiation. Other agents have been described: tissues are primarily of the TCR-1+ type.

148 12 Skin Epidermis

The skin covers the exterior of the body and VOCABULARY: keratinized stratified constitutes a large organ with several functions. In squamous epithelium, keratinocyte, humans, the skin accounts for about 7% of the total keratinization, melanocyte, pigmentation, body weight and has a total surface area of about layers (strata) 1.8 square meters. It protects the body from mechanical injury and loss of fluid, acts as a barrier The epidermis forms the surface layer of the skin against noxious agents, aids in temperature and consists of keratinized stratified squamous regulation, excretes various waste products, and epithelium. The cells, keratinocytes, undergo an through its receptors for sensations of heat, cold, orderly progression of maturation and touch, and pain, provides information about the keratinization to produce a superficial layer of external environment. In addition, the ultraviolet dense, flattened, dead cells at the surface. A smaller rays of sun light stimulate the deep epidermis to population of cells, the melanocytes, is associated produce vitamin D (calciferol). with pigmentation of the skin. The epidermis lacks blood vessels and is nourished by diffusion of material from vessels in the underlying dermis. The Structure epidermis is innervated, however. The epidermis can be divided into several layers VOCABULARY: epidermis, dermis, (strata) that reflect the sequential differentiation of appendage, integument, thick skin, thin skin, keratinocytes as they progress from the base of the epidermal ridge, dermal papilla epidermis to the surface, where they are exfoliated. In thick skin, the layers are stratum basale, stratum The skin consists of a surface layer of epithelium spinosum, stratum granulosum, stratum lucidum, called the epidermis and an underlying layer of and stratum corneum. In thin skin the layers are connective tissue, the dermis (corium). A loose narrower and less well defined and stratum lucidum layer of connective tissue, the hypodermis, attaches is absent. the skin to underlying structures but is not considered a component of skin. The appendages Stratum Basale of the skin – hair, nails, and sweat and sebaceous glands – are local specializations of the epidermis. VOCABULARY: columnar epithelial cell, Together, the skin and its appendages form the growing layer, stratum germinativum integument.

The thickness of the skin varies in different parts Stratum basale abuts the underlying dermis, of the body, and the proportions of dermis and separated from it only by a basement membrane. It epidermis also differ. Between the scapula, the consists of a single row of columnar epithelial dermis is especially thick, whereas on the palms of cells that follows the contours of the ridges and the hands and soles of the feet, the epidermis is papillae. Stratum basale is the growing layer of the thickened. The terms thick skin and thin skin epidermis, and most, but not all, of the mitotic refer to the thickness of the epidermis and not to the activity occurs in this layer. For this reason, stratum thickness of the skin as a whole. In thick skin the basale often is called the stratum germinativum. epidermis is especially well developed, whereas in Each cell of stratum basale is limited by a typical thin skin it forms a relatively narrow layer. trilaminar cell membrane and is joined to adjacent At its junction with the dermis, the epidermis cells by numerous desmosomes. At the basal forms numerous ridgelike extensions, the surface, hemidesmosomes aid in attaching the cells epidermal ridges that project into the underlying to the underlying basement membrane. The dermis. Complementary projections of the dermis basement membrane at the dermoepidermal fit between the epidermal ridges and form the junction, as elsewhere, consists of three strata: a dermal papillae. The surface patterns of the skin, superficial lamina lucida, a lamina densa, and an such as the fingerprints, reflect the pattern of the underlying fibroreticular lamina. dermal papillae.

149 On the dermal side of the basement membrane, keratohyalin granules that vary in size and shape. anchoring filaments of type VII collagen link the The granules are not limited by membranes and are undersurface of the lamina densa to collagen fibers associated closely with bundles of keratin filaments. of the papillary dermis. Fibrillin microfibrils attach Chemically, they contain large amounts of sulfur- the lamina densa to elastic fibers. The cytoplasm of containing amino acids. The granules increase in the basal keratinocytes is fairly dense and contains number and size in the outermost layers of stratum many scattered keratin intermediate filaments. granulosum, and the cells show evidence of Clusters of ribosomes are prominent, and the cells degenerative changes. The nuclei stain more palely, contain a moderate number of mitochondria, some and the contacts between adjacent cells become less profiles of granular endoplasmic reticulum, and a distinct. The cells of the granular layer are viable few Golgi saccules. but undergo programmed death as they pass into the succeeding horny layer. Membrane-coating Stratum Spinosum granules increase in number in the cells of stratum granulosum. These rod-shaped granules fuse with VOCABULARY: prickle cell, membrane- the plasmalemma and empty their contents into the coating granule, stratum malpighii intercellular space. The lipid-rich contents act as a barrier between cells of this layer and those toward Stratum spinosum consists of several strata of the surface and contribute to the sealing effect of irregular, polyhedral cells that become somewhat skin, preventing water loss and entrance by foreign flattened in the outermost layers. The cells are substances between cells. Direct evidence of this closely applied to each other and joined at all can be observed during a deep abrasion or scrape (a surfaces by numerous desmosomes. During tissue strawberry) of the epidermis deep to the stratum preparation, the cells tend to shrink and pull apart granulosum. When this occurs an amber color fluid except at these points of attachment. Thus, the cells seeps to the surface that continues to ooze for appear to have numerous short, spiny projections some time. This is tissue fluid that has passed that extend between adjacent cells and commonly between keratinocytes toward the surface to meet are called prickle cells. The projections are not the nutritional needs of cells deep to stratum areas of cytoplasmic continuity between cells but granulosum. are sites of typical desmosomes. In addition to the organelles seen in the basal cells, Stratum Lucidum prickle cells in the upper layers of stratum spinosum contain ovoid granules, 0.1 to 0.5 µm in diameter, VOCABULARY: loss of nuclei, thickened called membrane-coating granules. These consist plasmalemma, thick skin of parallel laminae bounded by a double membrane and are rich in glycols and phospholipids that help Stratum lucidum forms a narrow, undulating, lightly maintain the barrier function of the skin. Keratin stained zone at the surface of stratum granulosum. intermediate filaments are numerous and may form It consists of several layers of cells so compacted dense bundles that extend into the spinous together that outlines of individual cells cannot processes, ending in the dense plaques of always be made out. Traces of flattened nuclei may desmosomes. Stratum spinosum and stratum basale be seen, but generally this layer is characterized by often are grouped together as stratum malpighii. the loss of nuclei. Only a few remnants of organelles are present, and the main constituent of Stratum Granulosum the cytoplasm is aggregates of keratin intermediate filaments that now have a more regular VOCABULARY: keratohyalin granules arrangement, generally parallel to the skin surface. The plasmalemma is thickened and more Stratum granulosum of thick skin consists of three convoluted, and the amount of intercellular material to five layers of flattened cells whose long axes are is increased. Stratum lucidum is prominent in the parallel to the surface of the epidermis. In thin skin thick skin of the palms and soles but is absent stratum granulosum consists of a single layer of from the epidermis in other parts of the body. cells. The cells are distinguished by the presence of amorphous, densely packed dark granules, the

150 Stratum Corneum found in the keratohyaline granules interact with the keratin intermediate filaments as well as the VOCABULARY: squame, keratin, soft keratin, plasmalemma to produce a compact mass of desquamation, stratum disjunctum keratin. During this migration keratinocytes also produce a complex hydrophobic glycophospholipid Stratum corneum forms the outermost layer of the which is released as the superficial keratinocytes die. epidermis and is composed of scalelike cells, often This material acts to glue the keratin filled squames called squames that become increasingly flattened together as well as making the epidermal surface as they approach the surface. Squames are enclosed water proof for the short term. This water proofing by a thickened, modified cell membrane due to the breaks down after prolonged exposure to water as continued deposition of an intracellular protein evidenced by placing hands in water for prolonged known as involucrin that initially began being periods. When this occurs the keratin absorbs expressed in cells occupying the upper layers of water, swells and softens. stratum spinosum. The squames represent the The keratin layer acts as the main barrier to remains of cells that have lost their nuclei, all their mechanical damage, desiccation, invasion by organelles, and their desmosomal attachments to bacteria, is inert chemically and relatively adjacent cells. The cells are filled with keratin, impermeable to water. which consists of tightly packed bundles of keratin The life span of keratinocytes in their passage intermediate filaments embedded in an opaque, from the basal layer to desquamation is between 40 structureless material rich in the protein filaggrin. and 55 days. As the keratinocytes progress toward The keratin intermediate filaments of stratum the surface, they become much broader and flatter corneum consist of "soft" keratin as distinct from so that, ultimately, only 4 surface squames are the "hard" keratin of nails and hair. Soft keratin needed to cover the same area as 100 columnar has a lower sulfur content and is somewhat more basal cells. Thus, a low rate of mitosis (less than 7 elastic than hard keratin. The outermost cells of per 1000 basal cells) can maintain the surface layer. stratum corneum are constantly shed or desquamated; this region often is referred to as Pigmentation stratum disjunctum (Fig. 12-1). VOCABULARY: melanin, melanocyte, epidermal-melanin unit, melanosome

The majority of keratinocytes forming the epidermis are relatively transparent during life and on exposure to sunlight ultraviolet rays readily pass through the various strata of the epidermis to reach the underlying dermis. As a result, prolonged exposure can not only damage the genetic makeup of nuclei within cells of the stratum basale but also damage elements of the underlying dermis resulting in premature aging and wrinkling of the skin. Skin color is determined by the pigments carotene and melanin and by the blood in the capillaries of the dermis. Carotene is a yellowish pigment found in cells of the stratum corneum and in fat cells of the dermis and hypodermis. Melanin, a brown pigment, Fig. 12-1. A diagrammatic sketch illustrating the structure of is present mainly in the cells of stratum basale and thick skin. the deeper layers of stratum spinosum. Melanin is formed in specialized cells of the Thus, as keratinocytes move toward the surface epidermis, the melanocytes, which originate from they are tightly united by desmosomes. As cells pass cells that emigrated from the neural crest during through the stratum granulosum and approach the development. surface, specialized proteins some of which are

151 Melanocytes are scattered throughout the basal represent local areas of pigmentation resulting from layer and send out numerous finger-like cytoplasmic aberrant clones of melanocytes. Pigmentation of processes that extend between the cells of stratum freckles and the areolae and nipples may be spinosum. Each melanocyte maintains a functional intensified with pregnancy. A general increase in relationship with about 36 keratinocytes in stratum melanization occurs from exposure to sunlight, at spinosum and basale. The combination of first by darkening of existing melanin and later by melanocytes and keratinocytes forms an increased formation of new melanin. The activated epidermal-melanin unit. In electron micrographs, melanocytes produce larger melanosomes. epidermal melanocytes can be distinguished by the A deficiency of melanin also may occur. The white lack of keratin filaments and the content of melanin spotted patterns of piebaldism in humans result granules (melanosomes) in various stages of from abnormalities in the morphology of development. Melanocytes also contain the usual melanocytes in which the length and number of the array of cytoplasmic organelles. While melanocytes cell processes is deficient. Complete albinism results lack desmosomal junctions with neighboring cells, from an inability of the melanocytes to synthesize they are affixed to the basement membrane by melanin, not from an absence of melanocytes. hemidesmosome-like junctions. Mature melanosomes are transferred from Other Cell Types melanocytes via their cytoplasmic processes to keratinocytes of the basal layer and stratum VOCABULARY: Langerhans' cells, Merkel's spinosum. In humans, melanosomes frequently cells aggregate above the nuclei of basal cells, but in the superficial layers of the epidermis the granules are Two other cell types present in the epidermis are more evenly dispersed and become progressively Langerhans' and Merkel's cells. Langerhans' cells finer. Since epidermal cells constantly are shed from are present throughout the epidermis but are the surface, melanocytes must provide for especially numerous in the upper layers of stratum continued renewal of melanin. Although malpighii. In routine sections, the cells have darkly melanocytes may die and be replaced by division of stained, large, convoluted nuclei surrounded by a neighboring melanocytes, this is a random event, clear cytoplasm. With special stains they are seen to and there does not appear to be a regular cycle of be stellate cells that extend long cytoplasmic replacement of melanocytes. processes into stratum spinosum. In electron The number of melanocytes and the size of the micrographs, the nucleus shows a highly irregular melanosomes are somewhat greater in Negroid and outline, and the abundant cytoplasm lacks keratin Mongolian skin than in Caucasian, but not filaments, desmosomes, and melanosomes but is sufficiently to account for the differences in skin characterized by membrane-bound, rod-shaped color. Caucasoid melanosomes are less completely granules with a regular, granular interior. Some of melanized and are packaged in groups, the these granules appear to be continuous with the cell melanosome complexes, surrounded by a membrane. Langerhans' cells trap antigens that membrane. The larger Negroid melanosomes are penetrate the skin and transport them to regional more widely dispersed within cells and do not form lymph nodes. These antigen-presenting cells originate in membrane-bound complexes. The distribution of the bone marrow from a monocyte precursor and pigment within the epidermis also differs, being participate in a type IV delayed-type reaction concentrated in the lower zones of the Malpighian mediated by T lymphocytes. layer in Caucasian skin and distributed throughout Merkel's cells tend to lie close to naked sensory most keratinocytes of this layer in Negroid skin. nerve endings in stratum basale and form Merkel The smaller melanosomes of Caucasians are cell-neurite complexes. Like melanocytes, Merkel’s degraded by lysosomes before they reach the upper cells originate from neural crest. Morphologically, strata. Merkel's cells resemble cells of stratum basale, but The degree of pigmentation varies in different in electron micrographs, these lighter-staining cells parts of the body. The axilla, circumanal region, are distinguished by indented nuclei, prominent scrotum, penis, labia majora, nipples, and areola are Golgi complexes, and numerous dense-cored areas of increased pigmentation. In contrast, the vesicles. palms and soles contain little or no pigment. Freckles

152 The vesicles tend to concentrate basally at the site corresponding pits on the undersurface of the where nerve endings approach. The function of epidermis. They are especially prominent on the Merkel's cells is unknown, but they may have a palmar surface of the hands and fingers, where they mechanoreceptor function. They may play a role in are closely aggregated and arranged in parallel lines sensory processes and have been compared with that correspond to the surface ridges of the polypeptide- and hormone-containing cells, but a epidermis. Capillary loops are present in the definite relationship with these has not been papillae, and in some, especially those in the palms established. and fingers, nerve endings and tactile corpuscles are present. The epidermis, like all epithelia, is avascular Dermis and must rely on diffusion through the basement membrane to meet its nutritional requirements. VOCABULARY: reticular layer, papillary layer, Capillaries within the dermal papillae facilitate this dermal papilla, arrectores pilorum muscle exchange deep within the epidermis. Smooth muscle cells are present in the deeper The dermis (corium) varies in thickness in different parts of the reticular layer in the penis, scrotum, regions of the body. It is especially thin and delicate perineum, and areola. On contraction, the smooth in the eyelids, scrotum, and prepuce; very thick in muscle cells produce wrinkling of the skin in these the palms and soles; thicker on the posterior than areas. The arrectores pilorum muscles are small anterior aspects of the body; and thicker in men bundles of smooth muscle cells associated with hair than in women. The dermis is tough, flexible, and follicles. Their contraction results in “goose highly elastic and consists of a feltwork of collagen bumps”. fibers with scattered elastic fibers in a glycosaminoglycan-containing matrix. The Dermatoglyphics connective tissue is arranged into deep reticular and superficial papillary layers. Skin ridges are normal features on the volar surface The reticular layer is the thicker, denser part of of the fingers, hands, feet, and toes. They act as the dermis and consists of interwoven bundles of anti-slip devices and also are thought to improve collagen fibers that mostly run parallel to the the sense of touch. The ridges form three main surface. Below, the reticular layer merges patterns: arches, loops, and whorls. Loops are the indistinctly into the subcutaneous tissue most common pattern on the fingers and arches the (hypodermis), which generally contains many fat rarest. The ridge patterns on the toes are similar to cells. Collagen in the reticular layer is mostly type I. those on the fingers, but arches are more numerous The reticular layer of the dermis represents a classic and whorls are fewer. The ridge pattern is example of dense irregular connective tissue. In animals established early in life and is permanent. In any this is the layer used for making leather products individual the pattern varies from digit to digit, and (shoes, belts, coats). The reticular layer of the it is unlikely that in a group of unrelated persons dermis functions as fibroelastic whole-body- the sequence of patterns would be identical. stocking that firmly surrounds and encloses the contents of the limbs, trunk, neck and head. Appendages of the Skin The papillary layer lies immediately below the epidermis and extends into it in the form of the The appendages of the skin are derived from the dermal papillae. The papillary layer is not clearly epidermis and include hair, nails, and sweat and demarcated from the reticular layer, but its fibers sebaceous glands. and bundles of collagen tend to be thinner and more loosely arranged. This layer also contains more ground substance and has a more cellular appearance. Most of the collagen is type III. Just beneath the epidermis, the reticular fibers present have a vertical orientation. Elastic fibers are found deeper in the papillary layer. Dermal papillae are small, conical projections with rounded or blunt apices that fit into

153 Nails content than the keratin of the epidermis and is called hard keratin. VOCABULARY: body (nail plate), root, proximal nail fold, lateral nail fold, eponychium, hyponychium, lunule, nail bed, nail matrix, hard keratin

The nails are hard, keratinized structures that cover the dorsal surfaces of the tips of the fingers and toes. Each nail consists of a visible body (nail plate) and a proximal part, the root, which is implanted into a groove in the skin. The root is Fig. 12-2. A diagrammatic sketch illustrating a longitudinal overlapped by the proximal nail fold, a fold of profile through a nail. skin that continues along the lateral borders of the nail, where it forms the lateral nail folds. Stratum Hair corneum of the proximal nail fold extends over the upper surface of the nail root and for a short VOCABULARY: lanugo, vellus hair, terminal distance onto the surface of the body of the nail, hair where it forms a thin cuticular fold called the eponychium. At the free border of the nail, the Hairs are present on almost all surfaces of the skin skin is attached to the underside of the nail, except for the palmar surfaces of the hands, plantar forming the hyponychium. surfaces of the feet, margin of the lip, prepuce, The nail is a modification of the cornified zone of glans penis, clitoris, labia minora, and inner surfaces the epidermis and consists of several layers of of the labia majora. They consist of flexible, flattened cells with shrunken, degenerate nuclei. keratinized threads that vary in length and thickness The cells are hard, tightly adherent, and throughout in different regions of the body and in different most of the body of the nail, clear and translucent. races. From the middle of fetal life, the skin is The pink color of the nails is due to transmission of covered by fine hair called lanugo; this mostly is color from the underlying capillary bed. Near the shed by birth and is replaced by downy vellus hair. root, the nail is more opaque and forms a crescentic Vellus hairs are retained in most regions, where area, the lunule, which is most visible on the they appear as short, soft, colorless hair such as that thumb, becoming smaller and more hidden by the on the forehead. In the scalp and eyebrows, vellus proximal nail fold toward the little finger. The hairs are replaced by coarser terminal hair that also lunule represents the region from which nail forms the axillary and pubic hair and, in men, the formation occurs. hair of the beard and chest. Beneath the nail lies the nail bed, which corresponds to the stratum malpighii of the skin. It Structure of Hair consists of prickle cells and a stratum basale resting on a basement membrane. The underlying dermis is VOCABULARY: shaft, medulla, cortex, cuticle, thrown into numerous longitudinal ridges that are root, hair bulb, papilla very vascular. Near the root, the ridges are smaller and less vascular. The nail bed beneath the root and Each hair consists of a root embedded in the skin lunule is thicker, actively proliferative, and and a hair shaft projecting for a variable distance concerned with growth of the nail; it is called the above the surface of the epidermis. In cross section nail matrix (Fig. 12-2). The nail bed beneath the the shaft appears round or oval and is made up of rest of the nail is thinner and not involved with nail three concentric layers. The medulla (core) is growth. Cells in the deepest layer of the matrix are composed of flattened, cornified, polyhedral cells in cylindrical and show frequent mitoses, while above which the nuclei are pyknotic or missing. There is them are several layers of polyhedral cells and no medulla in thin fine hair (lanugo), and a medulla flattened squames that represent the differentiating may be absent in hairs of the scalp or extend only cells of the nail. Nail keratin has higher sulfur part of the way along the shaft.

154 The bulk of the hair consists of the cortex, which is cuticle. Immediately surrounding the cuticle of the composed of several layers of intensely cornified, inner epithelial root sheath are several layers of elongated cells tightly compacted together. elongated cells that form Huxley's layer. These Most of the pigment of colored hair is found in the cells contain granules that are similar to cortex and is present in the cells and the keratohyalin granules but differ chemically; they are intercellular spaces. Variable accumulations of air called trichohyalin granules. Huxley's layer is spaces are present between and within the cells of surrounded by Henle's layer, a single row of clear, the cortex. Together with fading of pigment, flattened cells that contain keratin filaments. The increase in the number of air spaces is responsible cells of these three layers are nucleated in the distal for graying of the hair. The outermost layer is thin parts, but as the inner epithelial root sheath and forms the cuticle. It consists of a single layer approaches the surface, the nuclei are lost. of clear, flattened, squamous cells that overlap each The outer epithelial root sheath is a direct other, shingle fashion, from below upwards. continuation of stratum malpighii. The cells of the The root of the hair is embedded in the dermis. At outermost layer are columnar and arranged in a the lower end, the root expands to form the hair single row and at the surface become continuous bulb, which is indented at its deep surface by a with stratum basale of the epidermis. The inner conical projection of the dermis and called a layers of cells are identical to and continuous with papilla. Papillae contain blood vessels that provide the prickle cells of stratum spinosum. nourishment for the growing and differentiating The connective tissue portion of the follicle cells of the hair bulb. The structure of the root consists of three layers. A narrow clear band, the differs somewhat from that of the shaft. In the glassy (vitreous) membrane, is closely applied to lower part of the root, the cells of the medulla and the columnar cells of the outer epithelial root cortex tend to be cuboidal in shape and contain sheath and is equivalent to the basement nuclei of normal appearance. At higher levels in the membrane. The middle layer consists of fine, root, nuclei become indistinct and finally are lost. circularly arranged collagen fibers. The outermost The cells of the cortex become progressively layer is poorly defined and contains collagen fibers flattened toward the surface of the skin. arranged in loose, longitudinal bundles interspersed with some elastic fibers. Hair Follicle A bulbous expansion of the hair root surrounds the papilla. The cells of the hair bulb are not VOCABULARY: inner epithelial root sheath, arranged in layers but form a matrix of growing cuticle of root sheath, Huxley's layer, cells. Matrix cells at the tip of the papilla trichohyalin granules, Henle's layer, outer differentiate into the medulla of a hair. Those on epithelial root sheath, glassy (vitreous) the slopes develop into the cortex and medulla. membrane, matrix Laterally, cells of the bulb form the inner epithelial root sheath. Pigmentation occurs from the activity The root of each hair is enclosed within a tubular of melanocytes present in the matrix. sheath called the hair follicle, which consists of an The structure of a hair and hair follicle are shown inner epithelial component and an outer connective in Figure 12-3. tissue portion. The epithelial component is derived from the epidermis and consists of inner and outer root sheaths. The connective tissue sheath is derived from the dermis. The inner epithelial root sheath corresponds to the superficial layers of the epidermis that have invaginated from the epidermal surface and undergone specialization to produce three layers. The innermost layer is the cuticle of the root sheath, which abuts the cuticle of the hair shaft. The cells are thin and scalelike and are overlapped from above downward; the free edges of the cells interlock with the free edges of the cells of the hair Fig. 12-3. A diagrammatic sketch illustrating the structure of a hair and a hair follicle.

155 Hair Cycle holocrine type, meaning the entire cell breaks down, and cellular debris, along with the secretory No hair grows forever; each cycle through a product (triglycerides, cholesterol, and wax esters), proliferative phase (anagen), a period of decreasing is released as sebum. Myoepithelial cells are not growth (catagen), and a resting phase (telogen). The observed with sebaceous glands, but the glands are cyclic activity continues throughout life, but the closely related to the arrectores pilorum muscle (Fig. 12- phases of the cycle change with age. At about the 3). Contraction of this smooth muscle bundle helps fifth month of gestation, all the hairs are in anagen, in the expression of secretory product from the a uniformity of growth not seen again. Between 8 sebaceous glands. In the nipple, smooth muscle and 10 weeks before birth, some hair sites have bundles are present in the connective tissue reached catagen and telogen phases. The frontal between the alveoli of these glands. and parietal scalp areas show the first shedding The short duct of the sebaceous gland is lined by events; in the occipital region, hairs remain in stratified squamous epithelium that is a anagen until after birth. From about 6 weeks before continuation of the outer epithelial root sheath of birth, telogen hairs again appear in the frontal and the hair follicle. Replacement of secretory cells of parietal scalp, indicating a second cycle of hair the alveolus comes mostly from division of cells growth. close to the walls of the ducts, near their junctions All hairs usually enter telogen immediately after with the alveoli. Some replacement comes from birth, giving rise to a second period of shedding. cells at the periphery of the alveoli. After this, the phases are more irregular. About 18 Collectively, the hair follicle, hair shaft, sebaceous weeks after birth, cycles are associated with gland, and erector pili muscle are referred to as the individual hairs or groups of hairs. In adults, hair pilosebaceous apparatus. The pilosebaceous apparatus cycles vary with body region. The total hair cycle in produces hair and sebum, the latter of which the scalp extends over 300 weeks, with telogen protects the hair and acts as a lubricant for the occupying 18 to 19 weeks. epidermis to protect it from the drying effects of the environment. Sebaceous Glands Sebaceous glands become more active at puberty and are under endocrine control: androgens VOCABULARY: alveolus, holocrine, sebum increase activity, estrogens decrease activity.

Sebaceous glands occur in most parts of the skin Sweat Glands and are especially numerous in the scalp and face and around the anus, mouth, and nose. They are VOCABULARY: eccrine sweat gland, simple absent from the palms of the hands and soles of the tubular, coiled, dark cell, clear cell, feet. Generally, sebaceous glands are associated myoepithelial cell, apocrine sweat gland with hairs and drain into the upper part of the hair follicle, but on the lips, glans penis, inner surface of Two classes of sweat glands are distinguished: the prepuce, and labia minora, the glands open ordinary or eccrine sweat glands and apocrine directly onto the surface of the skin, unrelated to glands, such as those of the axilla and perineum. hairs. The glands vary in size and consist of a Eccrine sweat glands are distributed throughout cluster of two to five oval alveoli drained by a single the skin except in the lip margins, glans penis, inner duct. surface of the prepuce, clitoris, and labia minora. The secretory alveoli lie within the dermis and are Elsewhere the numbers vary, being plentiful in the composed of epithelial cells enclosed in a well- palms and soles and least numerous in the neck and defined basement membrane and supported by a back. Each gland is a simple tubular structure. The thin connective tissue capsule. Cells abutting the deep part is tightly coiled and forms the secretory basement membrane are small and cuboidal and unit located in the deep dermis. The secretory unit contain round nuclei. The entire alveolus is filled with consists of a simple columnar epithelium resting on cells that, centrally, become larger and polyhedral a thick basement membrane. and gradually accumulate fatty material in their Two types of cells, clear cells and dark cells are cytoplasm. Nuclei become compressed and present. pyknotic and finally disappear. Secretion is of the

156 The dark cells are narrow at their bases and broad of the eccrine glands. The ducts are similar to those at the luminal surface; in electron micrographs they of ordinary sweat glands but empty into a hair show numerous ribosomes, secretory vacuoles, and follicle rather than onto the surface of the few mitochondria. These cells secrete glycoproteins, epidermis. Secretion by apocrine glands is of the which have been identified in secretory vacuoles. merocrine type and involves no loss of cellular Clear cells are broad at their bases and narrow at structure. Although retained, the term apocrine is the apices. Intercellular canaliculi extend between misapplied to these glands. adjacent clear cells, which contain glycogen, Apocrine sweat glands become more active at considerable smooth endoplasmic reticulum, and puberty and are under endocrine control by numerous mitochondria but few ribosomes. The androgens and estrogens. They also receive basolateral plasmalemma of the cell shows innervation from postganglionic sympathetic extensive, complex infoldings. Clear cells are neurons that release norepinephrine (adrenergic thought to secrete sodium, chloride, potassium, innervation). urea, uric acid, ammonia, and water. The ceruminous (wax) glands of the external auditory Myoepithelial cells are present around the canal are simple coiled tubular apocrine glands in secretory portion, located between the basal lamina which the secretory portion and the duct may and the bases of the secretory cells. These stellate branch. Glands in the margins of the eyelids (Moll's cells are contractile and are believed to aid in the glands) also are simple coiled tubular apocrine discharge of secretions. Eccrine sweat glands are glands but differ in that the terminal portions show drained by a narrow duct that at first is coiled and less coiling and have wider lumina. then straightens as it passes through the dermis to reach the epidermis. The lining of the duct consists Organogenesis of two layers of cuboidal cells. At their luminal surfaces, the cells of the inner layer show Skin has a dual origin: the epidermis and its aggregations of filaments organized into a terminal appendages arise from ectoderm, and the dermis is web. The epithelium lies on a basal lamina, and formed from mesoderm. Initially, the surface of the myoepithelial cells are lacking. In the epidermis, the embryo is covered by a single layer of cuboidal cells duct consists of a spiral channel that is simply a resting on a basal lamina. Only a few desmosomes cleft between the epidermal cells; those cells unite the loosely arranged cells. With proliferation, immediately adjacent to the duct lumen are two layers are formed: an outer periderm and an circularly arranged. inner basal layer. When eccrine sweat glands function to regulate body The periderm is the outermost layer in the 2 to 6- temperature they are regulated by postganglionic month-old fetus and covers the “epidermis proper” sympathetic neurons that release acetylcholine as prior to keratinization. It consists of a single layer the neurotransmitter (cholinergic innervation). In of flattened, polygonal cells that in humans bear contrast, when eccrine sweat glands are involved in microvilli on their free surfaces. The periderm emotional sweating they are controlled by protects the epidermis and may serve in some postganglionic sympathetic neurons that release exchange capacity between the fetus and amniotic norepinephrine (adrenergic innervation). fluid. Bleb-like projections form on the surfaces of Apocrine sweat glands are enlarged, modified the periderm cells, reaching maximum development eccrine sweat glands. Their secretions are thicker at the beginning of the sixth week and thereafter than those of the ordinary eccrine sweat glands and decreasing in size. At the same time blebbing contain glycoproteins, lipids, glycolipids and occurs, the cell membrane shows alterations similar pheromones. Their histologic structure also differs to those that occur in the keratinization of adult from eccrine sweat glands in several respects. epidermal cells. When keratinization of the Apocrine sweat glands also are simple coiled, epidermis is complete, the periderm sloughs and the tubular glands the secretory units of which are lined cells become mixed with sebaceous secretions to by columnar cells. The secretory tubules of these form part of the vernix caseosa, a greasy coating glands are of much wider diameter than those of found on the surface of the newborn’s skin. eccrine sweat glands. Their myoepithelial cells are The basal layer is the only portion of epidermis larger and more numerous, and there is only one proper in the 1- and 2-month-old fetus. type of secreting cell, which resembles the dark cells

157 It forms the germinative layer from which all the All the epidermal appendages (e.g., nails, hair, sweat epidermal cells arise. During the first two months, glands, and sebaceous glands) arise as outgrowths peridermal and epidermal cells are filled with of the primitive epidermis. Hairs arise from solid glycogen and have few organelles. In the second cords of epidermis (hair buds) that grow into the month, bundles of filaments develop in the dermis at an angle. Each hair bud has an outer layer epidermal cells, along with desmosomes and of columnar cells and an inner core of polygonal hemidesmosomes. More extensive intermediate cells. The deepest part of the hair bud swells to filaments, presumed to be keratin, spread form the hair bulb, which comes to sit, cuplike, throughout the cytoplasm. Basal cells are the first to over a small mound of mesenchyme that forms the lose glycogen and attain adult morphology, and as papilla of the hair. The primitive dermal connective they proliferate, additional layers are added between tissue along the length of the hair bud condenses to the periderm and the basal layer. The first form the connective tissue sheath of the hair intermediate cells appear at 9 to 10 weeks and the follicle. Cells at the periphery of the hair bud give layer is fully developed by 11 weeks. Second and rise to the epidermal portions of the root sheath: third layers are added in the forth month. the inner cell mass forms the substance of the hair. The cytoplasm of the intermediate cells is filled Sebaceous glands arise from thickenings of the with glycogen, but desmosomes and filaments also outer epithelial root sheath. Solid buds of cells grow are prominent. As the number of intracellular into the surrounding mesenchyme, branching into keratin filaments increases, the amount of glycogen several flask-shaped alveoli. The lining cells, derived diminishes. The outermost intermediate cells from stratum germinativum, proliferate, pushing accumulate laminar granules at their upper borders, older cells toward the center where they elaborate then differentiate into cells with cornified cell fats, degenerate, and form sebum. A cellular bulge membranes, bundles of keratin filaments, and small on the root sheath, below the origin of the amounts of glycogen in their cytoplasm. Once the sebaceous glands, provides for the attachment of stratum granulosum has formed, true cornified cells the arrectores pilorum muscles, which arise appear and a thin stratum corneum becomes independently from mesenchyme. evident by the end of 6 months. Thus, at the end of Eccrine sweat glands also begin as solid the second trimester, all the layers of the epidermis downgrowths of epidermis. As these elongate, the are established. deeper portions become coiled to form the Melanocytes migrate into the epidermis from the secretory units of the glands. A lumen develops in neural crest during the second month and each cord, which now forms a tube that opens to synthesize melanin in the forth month. Langerhans’ the surface. The ductal connection is lined by a cells arrive in the epidermis from the bone marrow double layer of epithelium. Myoepithelial cells during the third month, and Merkel’s cells are first associated with the secretory portions of the glands recognized in special regions (nail bed, fingertips) in arise locally from the same epidermal bud. Some the sixteenth week. Merkel’s cells originate from the sweat glands (apocrine sweat glands) develop from neural crest. superficial epidermal swellings on hair follicles; their The first surface ridges are formed on the palmar ducts empty into the uppermost part of the follicle. and plantar surfaces of the tips of the digits at about the thirteenth week. The ridges later extend Summary over the entire volar surfaces of the hands and feet. The pattern that develops at this time remains for The outer, keratin layer of the epidermis is highly life. impermeable to water and rather inert chemically. It Dermis arises as a condensation of adjacent is this portion of the epidermis that acts as the main mesenchyme. The fibroblasts differentiate and barrier to mechanical damage, desiccation, and collagen and elastic fibers appear. The dermis soon invasion by bacteria. Contents of membrane- is well defined and organized into papillary and coating granules contribute to or constitute the reticular layers. Initially, the dermoepidermal primary intercellular barrier to water. The epidermis junction is smooth, but dermal ridges soon appear has a high capacity for self-renewal, new cells being and can be identified by the third to fourth month derived mainly from stratum basale. of gestation.

158 The main function of melanin is to protect the Although the reasons for variations in skin color germinal cells from the effects of ultraviolet of various human populations is not fully irradiation. It has been suggested that melanin may understood, it may be related to the fact that capture harmful free radicals generated in the ultraviolet rays from the sun are both essential and epidermis by ultraviolet light. Accumulation of potentially dangerous. Production of Vitamin D by melanin above the nuclei of basal cells affords the epidermis as a result of ultraviolet exposure is maximum protection of the genome from the important, as this hormone is required for calcium effects of irradiation. Poorly melanized skin is more absorption in the intestinal tract. If one accepts the subject to sunburn, degenerative changes, and skin premise that the family of man originated in Africa cancer after chronic exposure to sunlight than is under the intense rays of the equatorial sun, darker skin. inhabitants at this location acquired a thick screen The dermis provides mechanical strength for the of melanin pigment to reduce the danger posed by skin, due to the high content of collagen fibers; ultraviolet light exposure yet allowed enough to elastic fibers provide elasticity. The vascular supply pass through to generate vitamin D, essential for is contained within the dermis, and the epidermis is the uptake of calcium from the diet. As humanity nourished by diffusion from this vascular bed. The expanded northward out of Africa and through the area of contact between dermis and epidermis is Middle East and eastern Asia and finally into increased by the dermal papillae, which facilitate northern Europe, skin color lightened to exchange of nutrients from the capillaries in the accommodate for less daylight exposure. Thus, the papillary layer of the dermis. They also provide an light almost colorless epidermis of Caucasian skin interlocking surface to more tightly secure the due to less melanin ensured enough ultraviolet light epidermis to the dermis. penetration for vitamin D production. People Hairs in humans mainly are concerned with inhabiting intermediate latitudes became cutaneous sensations of touch. Each hair follicle is characterized by brown skin tones of intermediate elaborately innervated, and some of the nerve character between the two extremes providing endings are associated with tactile discs of the some protection yet allowing enough ultraviolet Merkel type. Where hairs are absent (volar surface penetration for vitamin D production. The darker of hands, fingers, feet, and toes) Meissner’s skin of people located closer to the equator likewise corpuscles function as light touch receptors. reduced the danger of long exposure to ultraviolet Regulation of body temperature occurs in part by light. The production of vitamin D and other the evaporation of sweat from eccrine sweat glands. factors due to sunlight exposure are critical to the Sebum from sebaceous glands acts as a lubricant developing fetus and to the health of the young. for the skin, protects it from drying, and aids in The production of vitamin D and these factors are waterproofing. It also has some slight antibacterial thought to be directly related to the reproductive activity, but the effect appears to be minimal. Nails health of these populations as they moved across not only have a protective function; they also serve the globe. as rigid bases for the support of the pads at the ends of the digits and thus may have a role in tactile mechanisms.

159 The surface of the epithelial lining is bathed by a 13 Respiratory System thin film of mucus that constantly is moved toward the pharynx by the action of the ciliated epithelial The respiratory system consists of respiratory and cells. The mucus is derived from surface goblet cells conducting portions. Respiratory tissue, located in and secretions from mucoserous glands in the the lungs, is that portion of this system where lamina propria. The layer of mucus helps to exchange of gases between air and blood occurs moisten the air and trap particulate matter. The and is characterized by a close relationship between mucous layer contains IgA and other capillary blood and air chambers. The conducting immunoglobulins that protect against local portion delivers air to the respiratory tissue and is infection. IgA is produced by plasma cells within characterized by rigid walls that keep the airways the lamina propria and is taken up by secretory cells open. This part of the tract consists of nasal of adjacent glands. The IgA is then coupled to the cavities, pharynx, larynx, trachea, and various secretory component of these cells, transported, subdivisions of the bronchial tree. Parts of the and secreted onto the surface of the nasal mucosa. conducting system are within the lungs The same type of mucosa extends into the (intrapulmonary) and parts are outside the lungs paranasal sinuses, but here the epithelium is thinner, (extrapulmonary). there are fewer goblet cells, the lamina propria is thinner and contains fewer glands, and venous Nose sinuses are absent. The paranasal sinuses (maxillary, sphenoid, frontal, ethmoid) lie within bones of the VOCABULARY: ciliated pseudostratified same name surrounding the nasal cavity and are columnar epithelium, goblet cell, olfactory continuous with it through small openings. epithelium, lamina propria, venous sinus, Although cilia of the lining epithelium within the mucoperiosteum, mucoperichondrium paranasal sinuses generally beat toward the nasal cavity, the ciliated epithelial cells are coordinated in The skin that covers the external surface of the such a way that the pathway ciliary motion follows nose extends for a short distance into the vestibule of is a large, open spiral, the pitch of which narrows at the nasal cavity, where stiff hairs and their the opening to the nasal cavity. associated sebaceous glands aid in filtering the inspired air. More posteriorly, the vestibule is lined Olfactory Epithelium by nonkeratinized stratified squamous epithelium. Throughout most of the remainder of the nasal VOCABULARY: pseudostratified columnar cavity, the respiratory passage is lined by ciliated epithelium, supporting (sustentacular) cell, pseudostratified columnar epithelium that olfactory cell, bipolar nerve cell, olfactory contains goblet cells, but a specialized olfactory vesicle (knob), olfactory hair, fila olfactoria, epithelium is present in the roof of the nasal basal cell, olfactory gland cavities. A layer of connective tissue, the lamina propria, Olfactory epithelium occurs in the roof of the nasal underlies the epithelium and is separated from it by cavities, where it extends over the superior conchae a basement membrane. Contained within the lamina and for a short distance on either side of the nasal propria are mucous glands, serous glands, and thin- septum. It also is a pseudostratified columnar walled venous sinuses. The venous sinuses warm epithelium, but it lacks goblet cells and is much incoming air and are especially prominent in the thicker than the respiratory lining epithelium. lamina propria that covers the middle and inferior Three primary cell types are present: supporting conchae. Lymphatic tissue is present and becomes cells, basal cells, and sensory or olfactory cells. prominent near the nasopharynx. The deep layers Supporting (sustentacular) cells are tall with of the lamina propria fuse with the periosteum or narrow bases and broad apical surfaces that bear perichondrium of the nasal bones and cartilages, long, slender microvilli. An oval nucleus is located and at these sites, the nasal mucosa forms a just above the center of the cell. Apically, well- mucoperiosteum or mucoperichondrium, developed junctional complexes join the supporting respectively. cells to adjacent olfactory cells.

160 Olfactory cells are the sensory component and are bipolar nerve cells evenly distributed among the supporting cells. They are spindle-shaped with rounded nuclei located centrally in an expanded area of cytoplasm. Apically, the cell tapers to a single slender process (a modified dendrite) that extends between the supporting cells to reach to the surface, where it expands into a bulblike olfactory vesicle (knob). Six to eight long olfactory hairs extend from the olfactory vesicle and pass parallel to the surface of the epithelium, embedded in a film of fluid. The olfactory hairs are modified, nonmotile cilia that act as the excitable component of these Fig. 13-1. A diagrammatic sketch of olfactory epithelium. cells. For a short distance from their origins, the olfactory hairs have a typical ciliary structure but then narrow abruptly, and the microtubules Rich plexuses of blood vessels and branched, decrease in number and change from doublets to tubuloacinar serous glands are present in the lamina singlets. Basally, the olfactory cells narrow to a thin propria. The glands are the olfactory glands (of process, an axon, that passes into the lamina propria, Bowman), whose watery secretions flush the where, with similar fibers, it forms small nerve surface of the olfactory epithelium to prevent bundles. These collect as the fila olfactoria that continuous stimulation by a single odor and also pass directly to the olfactory bulb. There are about serve as a solvent for odorant materials. 50 million olfactory receptor neurons in the nasal In contrast to reports in the early literature, cavity. vomeronasal organs (VNO) do exist in the nasal Odorants (airborne substances which stimulate cavities of most adult humans. In humans, the olfactory sensory neurons) bind with a nasal- VNO consist of two small sacs about 2 mm deep specific protein called olfactory binding protein which that open into shallow depressions on either side of aids in binding the odorant molecules to receptors the nasal septum. The epithelium lining these sacs located in the plasmalemma of olfactory cilia. appears similar to olfactory epithelium and contains Odorant receptors are G-protein coupled seven supporting cells and cells that resemble olfactory transmembrane molecules that are capable of receptor bipolar neurons. These bipolar neurons detecting and discriminating among thousands of bind to antibodies against neuron-specific enolase specific molecules. and protein gene product 9.5, as do most neurons. Basal cells are short, pyramidal cells crowded The neuronal connection of these cells to the brain between the bases of sustentacular and olfactory is unknown in humans. Likewise, the function of cells (Fig. 13-1). They are undifferentiated cells that the vomeronasal organ in humans is unknown. In give rise to either of the other two types of cells. other mammals that have been studied in detail, the The olfactory cells are unique among neurons in VNO has neural connections to an accessory that they are in direct contact with the external olfactory bulb, which, in turn, is linked to the limbic environment and, unlike other neurons, constantly are system of the brain. In these mammals, the VNO replaced every 30 to 60 days. New olfactory cells arise functions in the detection of pheromones that from differentiation of the basal cells. Between the mediate sexual and territorial behaviors. luminal surface and the basal cells lie variable numbers of progressively more differentiated receptor neurons, accounting in part for the thickness of the olfactory epithelium.

161 Pharynx and Larynx contains mucous and some serous or mucoserous glands. VOCABULARY: nasopharynx, ciliated pseudostratified columnar epithelium, goblet Trachea and Extrapulmonary cell Bronchi

The nasal cavities continue posteriorly into the VOCABULARY: rings of hyaline cartilage, pharynx, which has nasal, oral, and laryngeal parts. smooth muscle, ciliated pseudostratified The most superior part, the nasopharynx, is columnar epithelium, goblet cell, brush cell, directly continuous with the nasal cavities and is pulmonary neuroendocrine cell, short cell, lined by the same respiratory passage epithelium - adventitia that is, ciliated pseudostratified columnar epithelium with goblet cells. In areas subject to The walls of the trachea and its terminal branches abrasion, a nonkeratinizing stratified squamous (the primary bronchi) are characterized by the epithelium may occur, such as on the edge of the presence of 15 to 20 C-shaped rings of hyaline soft palate and posterior wall of the pharynx, where cartilage that maintain patency of the airway. The these surfaces make contact during swallowing. The spaces between successive rings are filled with underlying connective tissue contains mucous, dense fibroelastic connective tissue, and the gaps serous, and mixed mucoserous glands and abundant between the arms of the cartilages are filled with lymphatic tissue. The lymphatic tissue is irregularly bundles of smooth muscle referred to as the scattered throughout the connective tissue and also trachealis muscle. The gaps are directed posteriorly, forms tonsillar structures: the pharyngeal tonsils in the and in section, the trachea and extrapulmonary posterior wall of the nasopharynx, the tubal tonsils bronchi appear flattened. The lining epithelium around the openings of the eustachian tubes into consists of a ciliated pseudostratified columnar the nasopharynx, and the palatine and lingual tonsils at epithelium with numerous goblet cells that rests the junctions of the oral cavity and oropharynx. on a thick basal lamina. Cilia beat toward the This lymphatic tissue forms part of the mucosa- pharynx. associated lymphoid tissue (MALT) and constitutes In addition to the ciliated and goblet cells, several Waldeyer's ring. Occasional, scattered other cell types have been described in the accumulations of lymphoid tissue are found tracheobronchial epithelium. Brush cells contain scattered in the connective tissue underlying the glycogen granules, show long, straight microvilli on epithelium of the trachea and bronchi to the level their apical surfaces, and basally, make contact with of true bronchioles. Lymphoid tissue within the nerve processes. They are rare in humans and their bronchi is referred bronchus associated lymphoid tissue function is unknown, but they may be sensory cells, (BALT). exhausted goblet cells, or intermediates between The larynx connects the pharynx and trachea. Its short cells and mature ciliated cells. Small framework consists of several cartilages, of which granulated cells known as pulmonary the thyroid, cricoid, and arytenoid are hyaline, while neuroendocrine cells are few in number and the epiglottis, corniculate, and tips of the arytenoid scattered. They are part of a pulmonary neuroendocrine are elastic. This scaffold of cartilages is held system that extends throughout the respiratory tree. together and to the hyoid bone by sheets of dense These cells are part of a diffuse system of cells that fibrous connective tissue that form the cricothyroid contain amines (serotonin (5-HT)) and amine and thyroid membranes. The lining epithelium precursors as well as a number of peptides that are varies with location. The anterior surface and about thought to act locally on adjacent cells and on half the posterior surface of the epiglottis are tracheal and bronchial smooth muscle cells. The covered by nonkeratinized stratified squamous diffuse pulmonary neuroendocrine cells in the epithelium that may contain a few taste buds. The epithelium lining the trachea and bronchi are vocal cords also are covered by a wet stratified thought to be excitable cells capable of detecting squamous epithelium, but elsewhere the larynx is hypoxia and can release powerful relaxing and lined by the respiratory passage type of epithelium. contracting factors in response to different oxygen The lamina propria of the larynx is thick and concentrations to modulate the contraction of

162 adjacent airway smooth muscle cells. Short cells submucosa, their ducts penetrating the muscle layer occur in the depths of the epithelium between the to open onto the epithelial surface. Secretions from bases of the other cell types and are these glands, as well as those throughout the undifferentiated cells that give rise to goblet and respiratory tree, contain IgA, acquired from ciliated cells. A network of dendritic cells lies along adjacent lymphoid tissue, which provides the larger the base of the tracheal epithelium. They function respiratory passages with a degree of immunologic as antigen-presenting cells capable of binding and protection. presenting antigens to T-lymphocytes. The lamina With successive divisions, the intrapulmonary propria contains many small seromucous tracheal bronchi progressively decrease in size, and although glands and occasional accumulations of lymphatic they retain the basic structure outlined above, the tissue. The serous units of these compound layers of their walls become thinner. The number of tubuloacinar glands produce a thin, watery secretion bronchial generations (branches) may be as many as rich in glycoproteins, lysozyme and IgA acquired 25 or as few as 10 prior to reaching the respiratory from adjacent lymphoid cells. Myoepithelial cells are portion of the lung. The smallest of the associated with the secretory units of the tracheal intrapulmonary bronchi show only isolated cartilage glands and aid in expression of secretory product. plates and no longer are surrounded by cartilages. External to the cartilage is a fibroconnective tissue The epithelium is reduced to ciliated simple coat, the adventitia, which contains elastic, columnar with goblet cells. Bronchial glands are collagenous and reticular fibers, blood vessels, and present as far down as the cartilages extend. nerves. Medium sized bronchi are the primary locations for airway resistance during breathing. This Intrapulmonary Air Passages resistance can either be increased or decreased through the contraction of bronchial smooth VOCABULARY: intrapulmonary bronchus, muscle. Parasympathetic stimulation of bronchial plates of hyaline cartilage, ciliated smooth muscle constricts airway passages and increases pseudostratified columnar, goblet cell, airway resistance. Sympathetic stimulation of bronchial submucosa, ciliated simple columnar, smooth muscle dilates airways and decreases airway bronchiole, ciliated cuboidal, terminal resistance. bronchiole, Clara cell, neuroepithelial body When the diameter of the tube reaches about 1 mm, cartilage disappears from the wall and the The primary bronchi divide to give rise to several structure becomes a bronchiole. Glands and orders of intrapulmonary bronchi in which the C- lymphatic tissue also disappear, but smooth muscle is shaped cartilaginous rings of the primary bronchi fairly prominent and becomes the major supporting are replaced by irregular plates of hyaline element. The lining epithelium varies from ciliated cartilage that completely surround the structure. columnar with goblet cells in the large bronchioles Thus, the intrapulmonary bronchi are cylindrical to ciliated cuboidal with no goblet cells in the and are not flattened on one side as are the main terminal bronchioles. Terminal bronchioles are bronchi and trachea. Internally, the large the smallest branches of the purely conducting intrapulmonary bronchi are lined by a mucous system. Scattered among the ciliated cells are a few membrane that is continuous with and identical to nonciliated cells whose apical surfaces bulge into that of the trachea and primary bronchi: a ciliated the lumen and bear a few microvilli. These are the pseudostratified columnar epithelium with Clara cells, also called bronchiolar secretory cells. goblet cells. The lamina propria contains some They secrete protease inhibitors, oxidases, diffuse lymphatic tissue and is separated from the glycosaminoglycans, and proteins with epithelium by a prominent basal lamina. Beneath immunosuppressive properties. Some of these the lamina propria, a sheet of irregularly arranged factors (cytochrome P450) are thought to be smooth muscle cells runs around the bronchus in protective against inhaled toxins and carcinogens open left- or right-handed spirals. The muscle layer and also may act in preventing conditions such as separates the lamina propria from the connective emphysema. The Clara cell may also play a role in tissue of the submucosa, which lies immediately surfactant elimination or production in this region. internal to the plates of hyaline cartilage. Mucous and mucoserous bronchial glands are present in the

163 Neuroepithelial bodies are innervated epithelial oxygen-sensing protein (NADPH oxidase) and an corpuscles scattered throughout the intrapulmonary oxygen-sensitive potassium channel. Thus, a natural airways and even may extend into alveoli. They are stimulus (oxygen concentration) acting on oxygen not present in the trachea. The corpuscles appear as sensors in both the solitary pulmonary ovoid or triangular bodies, 20 to 40 µm wide, set neuroepithelial cells and innervated clusters of into the respiratory epithelium, where they extend pulmonary neuroepithelial cells (neuroepithelial from the lumen to the basement membrane. bodies) may modulate pulmonary processes such as Basally, the neuroepithelial body is closely related to airway tone, pulmonary circulation, and even a capillary. The whole structure is richly innervated, influences the control of breathing. There is also and many of the nerve endings make contact with considerable evidence that the solitary cells of the corpuscle. neuroepithelial cells play a primary role in lung The neuroepithelial bodies contain 4 to 10 tall, development based on the presence of bombesin nonciliated neuroendocrine cells with a slightly and other peptides with growth factor-like acidophilic cytoplasm that gives a positive properties expressed in human fetal lung tissue. argyrophil (silver) reaction. The oval nuclei are basally located and oriented in the long axis of the Respiratory Tissue cells, which contain numerous mitochondria, moderately well-developed granular endoplasmic VOCABULARY: respiratory bronchiole, reticulum, small Golgi complexes, and some alveolar duct, alveolar sac, alveolus, glycogen and multivesicular bodies. Characteristic interalveolar septum, alveolar pore, blood-air of these cells are numerous dense-cored vesicles. barrier, pulmonary epithelial cell (type I The neuroepithelial bodies are believed to be pneumonocyte), septal cell (type II chemoreceptors that react to the composition of pneumonocyte), surfactant, alveolar inhaled air. When the oxygen tension of the macrophage, bronchial vascular system, inspired air decreases, the neuroepithelial body pulmonary vascular system releases dense-cored vesicles that contain serotonin, a potent vasoconstrictor as well as additional Exchange of gases between air and blood occurs peptides that may modulate airway tone. Thus, only when air and blood are in close relation. Such during local hypoxia, blood is shunted from poor to a condition occurs first in the respiratory better oxygenated and ventilated areas of the lungs. bronchioles, which form a transition between the During conditions of reduced oxygen availability, purely conducting and purely respiratory regions of three events occur to increased ventilation and to the lung. The walls of respiratory bronchioles direct pulmonary blood flow to better ventilated consist of collagenous connective tissue with regions of the lung. These events are (a) an interlacing bundles of smooth muscle and elastic increased signaling rate from glomus cells of the fibers. The larger respiratory bronchioles are lined carotid body to respiratory centers in the brain, (b) by simple cuboidal epithelium with only a few intrinsic hypoxic vasoconstriction of pulmonary ciliated cells; goblet cells are lacking. Many of the arterioles, and (c) local and central modulation by cuboidal cells are Clara cells. In the smaller pulmonary neuroendocrine cells within the respiratory bronchioles, the epithelium becomes respiratory epithelium of the major airways. Each of low cuboidal without cilia. Alveoli bud from the these tissues has the ability to sense acute changes walls of the respiratory bronchioles and represent in oxygen, produce a signal and to initiate the respiratory portions of these airways. The compensatory mechanisms. The heavily innervated alveoli become more numerous distally. Respiratory neuroepithelial bodies act as transducers of the bronchioles end by branching into alveolar ducts. hypoxia stimulus via neural input, affecting control Alveolar ducts are thin walled tubes from which of breathing and other pulmonary functions. As in numerous alveoli or clusters of alveoli open around glomus cells of the carotid body, neuroepithelial its circumference so that the wall becomes little bodies release serotonin in response to hypoxia. more than a succession of alveolar openings. The plasmalemma of cells forming the Appearances of a tube persist only in a few places, neuroepithelial bodies has membrane properties of where small groups of cuboidal cells intervene excitable cells. The plasmalemma of these cells for between successive alveoli and cover underlying oxygen sensing depends on interaction between the bundles of fibroelastic tissue and smooth muscle.

164 The alveolar ducts end in irregular spaces lumen between pulmonary (type I) epithelial cells. surrounded by clusters of alveoli called alveolar Their free surfaces have short microvilli, and sacs. laterally, the cells are united to pulmonary epithelial Alveoli are thin-walled, polyhedral structures of cells by junctional complexes (Figs. 13-3 & 13-4). varying size that are open at one side to allow air The most distinctive feature of the septal cells is the into their cavities. They number between 300 and presence of multilamellar bodies in their cytoplasm 800 million (both lungs) and provide a surface area (Fig. 13-4). of 70 to 95 square meters for gaseous exchange. Adjacent alveoli are separated by a common interalveolar septum, whose most conspicuous feature is a rich network of capillaries that bulge from within the septal wall. Reticular and elastic fibers form a tenuous framework for the septa. Small openings in the septal wall 1 to 10 µm in diameter, alveolar pores of Kohn, permit communication and equalization of air pressure Fig. 13-3. A scanning electron micrograph of the interior of an between alveoli. The pores can play a significant alveolus demonstrating type I and type II pneumocytes. (X 2,000). role in obstructive lung disease by serving as a bypass mechanism to aerate alveoli distal to the blockage. On each side, the alveolar wall is covered by an attenuated epithelium beneath which is a basal lamina. In many areas the epithelial basal lamina is separated from the basal lamina of the capillary by a space of only 15 to 20 nm; in other regions the two laminae are fused. Thus, at its thinnest, the blood-air barrier consists of a thin film of fluid, the attenuated epithelium of the alveolar lining cell, the fused basal laminae, and the endothelium of the capillaries within the septal wall (Fig. 13-2). The blood-air barrier measures between 0.2 to 0.5 µm in thickness.

Fig. 13-4. A transmission electron micrograph of a type II pneumocyte within the limiting wall of an alveolus. Note the dark staining multilamellar bodies which contain surfactant. (X 5,000). Fig. 13-2. A transmission electron micrograph of the alveolar wall illustrates the close association between the type I These bodies consist of thin, concentric lamellae pneumocyte and the endothelial cell of an adjacent capillary. Note the fused basal laminae. (X 4,000). and are the storage sites of surfactant. It is synthesized by septal cells and released by Several types of cells are present in the septa. The exocytosis. The released surfactant forms a attenuated squamous cells that form a continuous monolayer over the thin film of fluid that coats lining for the alveolar wall are called pulmonary alveoli and acts as a detergent thereby lowering surface epithelial cells or type I pneumonocytes. In tension at the liquid-air interphase. The lowering of addition to these are septal cells, alveolar surface tension aids in preventing the collapse of macrophages and endothelial cells that line blood alveoli at the end of expiration. Thus, pulmonary capillaries. Septal cells (type II pneumonocytes) surfactant forms the surface-active film that is are rounded or cuboidal cells that may lie deep in crucial for normal lung function. the surface epithelium or bulge into the alveolar

165 It consists of a complex mixture of various capillaries that envelop alveoli are referred to as phospholipids (primarily dipalmitoyl phosphatidyl septal or alveolar capillaries; those positioned at the choline) and four proteinaceous components corners between alveoli are often referred to as known as surfactant-associated proteins: SP-A, SP- extra-alveolar or corner capillaries. Capillaries of the B, SP-C, and SP-D. SP-A is the most abundant and lung form one of the largest surface areas for is a water-soluble oligomeric glycoprotein that exchange in the body (about 90 M2). Following facilitates the formation of the surface-active film. gaseous exchange in the pulmonary capillaries the It also has a role in anti-microbial defense. re-oxygenated blood is collected by pulmonary The type I pneumocytes make up about 40% of the venules and veins to be distributed back to the alveolar lining cell population but account for over body tissues via the left side of the heart. The 90% of the alveolar surface area. In contrast, type II arterioles of the pulmonary vascular circuit are unusual in pneumocytes make up about 60% of the alveolar comparison to the systemic arterioles in that in cell population but account for only 8 to 10% of ischemic regions of the lung these arterioles tend to the alveolar surface area. Type I pneumocytes have shunt blood away from such regions rather than little capacity to divide, however, type II towards an area of low oxygen tension as would be pneumocytes have retained their mitotic capacity true of systemic vessels. and serve as a pool of replacement cells for type I Lymphatic capillaries begin at about the level of the and type II pneumocytes respiratory bronchioles and gradually merge to form Alveolar macrophages are present within the larger vessels that follow the bronchial tree to the interalveolar septa and alveolar lumina. Many hilum and exit from the lung. A second set of contain particles of inhaled material and have been lymph capillaries are located in the visceral pleura called "dust" cells. As with all macrophages, the and connective tissue septa that subdivide each lung alveolar macrophages originate from blood into individual lobes. These are thought to drain monocytes. Most of these cells ultimately are into the pleural cavity. eliminated via the air passages and appear in the If pressure within the pulmonary capillaries sputum; a few may migrate into lymphatics and increases the fluid component of the blood readily escape the lungs by this route. passes through the extremely thin alveolar wall and It is important to realize that the lungs are into the alveolar lumen. Such an event occurs provided with a dual vascular supply. A bronchial during pulmonary edema resulting from left heart failure. In vascular system provides oxygenated blood to the this situation the left ventricle of the heart fails to major components of the bronchial tree. The empty completely and pressure builds in the left bronchial artery arises directly from the thoracic atrium as well as the pulmonary veins. The aorta and the bronchial veins of the major increased pulmonary venous pressure extends into subcomponents of bronchial region drain into the the extensive pulmonary capillary network forcing azygos and hemiazygos veins. The distal branches fluid from the capillaries into the alveolar spaces. of the bronchial veins drain into the pulmonary The fluid flow overburdens the lymphatics, which veins. begin within the distal bronchial tree, and the In contrast, it is the capillaries of the pulmonary respiratory portion of the lung fills with fluid vascular system that are the site of gaseous preventing gaseous exchange. exchange. The pulmonary arteries supply the lung with relatively deoxygenated blood rich in carbon Pleura dioxide coming from the body tissues via the right side of the heart. Pulmonary vessels are generally The pleura are thin membranes made up of short and wide and because less smooth muscle collagenous and elastic fibers, covered by a single occurs in their walls they are more distensible. layer of mesothelial cells. The layer lining the wall Pulmonary vessels are further characterized by low of the thoracic cavity is the parietal pleura, which pressure, low resistance and because all cardiac output reflects from the thoracic wall onto the surface of from the right side of the heart goes through the the lungs, where it becomes the visceral pleura. The lungs have a high rate of flow. Pulmonary capillaries pleura secrete a small amount of fluid between the are arranged in a sheet-like fashion (in contrast to two layers of covering/lining mesothelium to systemic vessels arranged more like roots of a plant) permit a friction-free movement. positioned between and around alveoli. The

166 Although the two pleurae easily glide over one The chemoreceptors sensitive to these changes are another, their separation is strongly resisted. As a result separated into two major categories: central the lungs adhere tightly to the interior of the chemoreceptors, located primarily in the medulla of the thoracic wall and expand and shrink as the volume brain; and peripheral chemoreceptors located in the of the thoracic cavity increases or decreases during carotid and aortic bodies. Carotid bodies send their respiration. Contraction of the diaphragm increases sensory information to the medulla primarily the vertical dimension (superior inferior diameter) through the glossopharyngeal nerves whereas the of the thoracic cavity and contraction of the aortic bodies convey their information through the intercostal and other accessory respiratory muscles vagus nerves. raise and elevate the ribs expanding the anterior- posterior diameter of the thorax. Contraction of Organogenesis intercostal muscles results in the rotation of the second to the seventh rib arches at the costosternal During the fourth week of gestation, the respiratory and costovertebral joints. This movement referred passages first appear as an endodermal bud arising to as the bucket-handle movement describes the from the caudal end of the laryngotracheal groove. elevation of the ribs and the eversion of their lower As the lung bud grows, it becomes covered by borders which results in an increase in the mesenchyme that later differentiates into blood vessels transverse diameter of the thorax. The sternal ends and the cartilage, smooth muscle, and connective of the ribs and costal cartilages also are elevated tissues that support the walls. The endoderm provides thrusting the sternum forward to increase the the lining epithelium for the entire respiratory tract. anterior posterior diameter of the thorax. This The lung bud grows caudally and undergoes movement is described as the pump-handle movement. successive divisions, the first branches representing In addition, the intercostal muscles also act to the main bronchi. Subsequent divisions provide stiffen the thoracic wall to prevent changes in the finer and finer branches of a complex respiratory shape of the thorax as a result of negative pressure tree, ultimately ending in small expansions called created by contraction of the diaphragm. As the infundibula. These minute pouches eventually volume within the thoracic cavity enlarges the become the definitive alveoli. pressure within the pleural cavity and lungs decreases. During their development, the lungs undergo As a result, air flows into the respiratory system considerable change in appearance, and from the surrounding atmosphere. Expiration is histologically, three phases can be identified. The primarily a passive process. As the relaxed first or glandular phase extends from the fifth to diaphragm moves superiorly and intercostal the seventeenth week and consists mainly of muscles relax, the rib cage is compressed decreasing development of the branching air-way passages. the volume of the thorax and lungs. Forced The repeated divisions of the endodermal tubes expiration utilized the abdominal muscles. within a bed of mesenchyme give the lung the Simultaneously the elastic tissue within the lungs appearance of a gland. The tubes are lined by recoils increasing the pressure within the lungs undifferentiated columnar cells that have few forcing air out of the lungs. organelles but are rich in glycogen and contain fat. Nuclei are variable in size and show one or two Control of Respiratory Rate nucleoli. Mitoses are common. The cells are united at their apices by tight junctions and desmosomes. The primary respiratory center for regulating Even in the earliest stages, the epithelial cells are respiration is located in the reticular formation of separated from the mesenchyme by a delicate the medulla (rostral ventrolateral medulla) of the basement membrane. The cells later become brain. This region functions as a pacemaker, the cuboidal, and by the thirteenth week, cilia and neurons of which generate a basic respiratory goblet cells appear, first in the trachea and then rhythm. This basic rhythm can be modified by progressively at more distal levels. By sixteen weeks input from other regions of the brain as well as Clara cells are differentiating and endocrine cells can input from receptors that sense changes in the be found. chemistry of the arterial blood. The chemoreceptors respond to rising levels of carbon dioxide, falling levels of oxygen and changes in pH.

167 The canalicular phase extends from the beat of the cilia moves the fluid film toward the seventeenth to the twenty-sixth week and is marked mouth, in both the nose and lower respiratory tract. by rapid growth of the tubular elements to provide The large venous plexuses in the nasal cavities the finer twigs of the respiratory tree, and by warm the incoming air. In several species, including delineation of the respiratory parts. The terminal humans, periodic engorgement of the venous buds (infundibula) become less spherical and are plexus occurs, alternating on either side of the nasal intimately related to capillary networks. cavity. This results in temporary obstruction to flow Transformation of tubules to canaliculi is on the one side, allowing the mucosa to recover accompanied by flattening of the epithelium and from the drying effects of air. In humans the considerable cytodifferentiation. Osmiophilic periods of occlusion occur approximately hourly. lamellar bodies associated with production of The olfactory epithelium functions in the sense of surfactant appear in some of the epithelial cells, smell, the sensory component being the olfactory which now have fairly well developed endoplasmic cells. The olfactory hairs borne by these cells form reticulum and plentiful ribosomes. These cells the excitable element of the sense organ. Glands in identifiable as type II pneumocytes and surfactant first this area secrete a watery fluid that serves as a appear at the 28th week of gestation. During this solvent for odorant molecules and flushes the phase, the cells reduce their height, and their olfactory epithelium to prevent continuous intercellular junctions lie at the lower half of the cell stimulation by a single odor. rather than at the apex as in the glandular phase. Patency of the conducting portion, from the These low cells are the precursors of type I trachea to the smallest intrapulmonary bronchus, is pneumocytes of saccules and alveoli. maintained by cartilage in the walls. In the trachea The alveolar (terminal sac) phase extends from and main bronchi, the fibroelastic tissue between the twenty-sixth week until term. By this time cartilages gives pliability and longitudinal respiratory bronchioles can be recognized as tubes extensibility to these structures. This is essential for lined on one side by flattened epithelium and on the the accommodations these structures make for the other by a ciliated cuboidal epithelium. Branches incursions of the lung during breathing. that had been buds lined by cuboidal epithelium Beginning with the small bronchi, the epithelium now are clusters of five to six generations of progressively simplifies and becomes thinner. saccules lined by flattened epithelial cells closely Ciliated cells persist farther along the respiratory associated with capillaries. The septa between tree than do mucus-secreting cells, and this ensures capillaries are thick, but gradually thin. that the sites of gaseous exchange do not become At birth, alveoli are small and shallow, and only a coated with a viscid, mucoid material that would few solitary alveoli are present in respiratory impede or prevent the passage of gases. bronchioles. Inflation of alveoli occurs after birth with Medium-sized bronchi are the main sites of airway the onset of respiration, first in the hilar region, resistance through the contraction or relaxation of then in the central portion of the lungs. Only after bronchial smooth muscle. Parasympathetic several hours of breathing are the distal alveoli innervation generally constricts airways, whereas inflated. New alveoli continue to be formed after sympathetic innervation (or a sympathetic agonist birth, and it is not until about the eighth postnatal such as isoproterenol) dilates airways. year that the adult number of alveoli is attained. The pulmonary neuroendocrine cell system consists of solitary cells and innervated clusters Summary called neuroepithelial bodies. The pulmonary neuroendocrine cells have the ability to detect The stiff hairs in the vestibule of the nose help hypoxia and can release contracting and relaxing prevent particulate matter in the inspired air from factors in response to different oxygen reaching the rest of the respiratory passages. Mucus concentrations. These factors modulate adjacent secreted by goblet cells in the respiratory lining airway smooth muscle contraction and also may epithelium and by glands in the lamina propria of influence adjacent epithelial cell function. The the conducting portion provides a sticky film that neuroepithelial bodies appear to be specifically traps particles and, along with the serous secretions adapted to respond to oxygen tension and adjust from these glands, prevents drying of the mucosal blood flow to areas of high oxygen content. surface. The secretions also humidify the air. The

168 The mucous secretions and cilia of the respiratory their role as phagocytes, provide the main defense tree serve the same protective functions as in the against microorganisms. During phagocytosis the nasal passages. Additionally, the mucosubstances cells release hydrogen peroxide and other peroxide have antiviral and antibacterial properties. The radicals to destroy foreign organisms. Cigarette lymphoid tissue associated with the larger smoking triggers an inappropriate release of respiratory passages give some immunologic peroxide by alveolar macrophages, resulting in protection by producing immunoglobulin A (IgA) damage to normal respiratory and connective antibodies. The serous units of the bronchial glands tissues. Phagocytosis by macrophages is greatly produce a thin, watery secretion rich in depressed in smokers, and the cells show twice the glycoproteins, lysozyme and IgA acquired from normal volume with a marked reduction in surface- adjacent lymphoid cells. Secretory IgA is assembled to-volume ratios. The density of lysosomes also by serous cells of the bronchial glands and is decreases. complexed to a protein (secretory piece) that Each lung is enclosed in a sac formed by the provide this molecular complex with some degree pleural membranes. The outer (parietal) layer lines of protection from intra- and extracellular the inner surface of the thoracic wall; the inner breakdown. (visceral) layer covers the surface of the lungs. The Exchange of gases occurs where air and blood are space between the two layers is under negative closely approximated - in the alveoli. The barrier pressure, and the two layers are separated only by a between air in the alveoli and blood in the thin film of fluid. As the thoracic wall moves out to capillaries of the alveolar septa consists only of a increase the size of the thoracic cage, the parietal thin film of fluid, an extremely attenuated pleura is taken along, passively. The combined cytoplasm of the alveolar lining cell, the conjoined effects of surface tension and negative pressure basal laminae plus the endothelium of the capillary. result in the visceral pleura being drawn outward Surfactant-secreting cells also are present in the also. Since the visceral pleura is firmly attached to alveolar lining. Without surfactant, the alveolus the lungs, they are expanded, the air pressure within tends to collapse because of the surface tension of the lungs decreases, and air flows into the lungs. the fluid that bathes the alveolar epithelium. Expiration of air occurs as the result of elastic recoil Particulate matter that reaches the alveoli is of the lungs when the thoracic wall relaxes. removed by alveolar macrophages that, through

169 14 Digestive System Lip

VOCABULARY: vermilion border, papilla, The digestive system or alimentary tract consists of labial gland the oral cavity, pharynx, esophagus, stomach, small and large intestines, rectum, and anal canal. In The external surface of the lip is covered by thin addition to the intrinsic glands within the wall of skin and contains sweat glands, hair follicles, and the digestive tract proper, there are associated sebaceous glands. At the vermilion border, the lip glands that lie outside the tract but that are is covered by a nonkeratinized, translucent, connected to it by ducts. stratified squamous epithelium that lacks hair

follicles and glands. Its lamina propria projects into General Structure the overlying epithelium to form numerous tall connective tissue papillae that contain abundant VOCABULARY: mucous membrane, mucosa, capillaries. The combination of tall vascular papillae lining epithelium, lamina propria, muscularis and translucent epithelium accounts for the red hue mucosae, submucosa, muscularis externa, of this part of the lip. The red portion is continuous adventitia, serosa with the oral mucosa internally and with thin skin externally. Because glands are lacking, the red Throughout its extent, the alimentary tract shows portion of the lips is kept moist by licking. several features that represent common structural The inner surface of the lip is lined by components. The innermost tissue layer of the nonkeratinized (wet) stratified squamous epithelium digestive tract is called a mucous membrane or that overlies a compact lamina propria with mucosa and consists of a lining epithelium and numerous connective tissue papillae. Between the an underlying layer of fine, interlacing connective oral mucosa and the skin are the skeletal muscle fibers tissue fibers that form the lamina propria. A of the orbicularis oris muscle. Coarse fibers of muscular component of the mucosa, the connective tissue in the submucosa connect the muscularis mucosae, is found only in the tubular mucosa to the adjacent muscle. The ducts of portion of the tract. Immediately beneath the numerous mixed mucoserous labial glands in the mucosa is a layer of connective tissue consisting of submucosa empty onto the internal surface of the coarse, loosely woven collagen fibers and scattered lip and provide fluid and lubrication for this region. elastic fibers. This layer is the submucosa, which They can be identified as small bumps when the houses larger blood vessels and nerves and provides tongue is pressed firmly against the interior of the the mucosa with considerable mobility. Where it is lip. absent, the mucosa is immobile and firmly attached to surrounding structures. In the tubular portion of Cheek, Palate, and Gingiva the tract, a thick layer of smooth muscle, the muscularis externa, forms an outer supporting A similar mucous membrane lines the interior of wall that in turn is surrounded either by connective the cheek, soft palate, floor of the mouth, and tissue, an adventitia, or a layer of connective tissue underside of the tongue. The mucosa of the cheek covered by mesothelium called a serosa. and soft palate, like that of the lips, is attached to underlying skeletal muscle fibers by coarse Oral Cavity connective tissue elements of the submucosa, allowing considerable mobility to the oral mucosa The irregularly shaped oral cavity consists of the in these regions. Mucoserous glands, adipose tissue, lips, cheeks, tongue, gingiva, teeth, and palate. The larger blood vessels, and nerves are present in the mucosa consists of a stratified squamous epithelium submucosa. The cheek and soft palate contain a and a rather dense lamina propria. A submucosa core of skeletal muscle, the buccinator and palatine may be present in some regions of the oral cavity. muscles, respectively. Externally, the cheek is covered by thin skin.

170 Where the noncornified, stratified squamous tongue is smooth, whereas the anterior two-thirds epithelium of the oral cavity reflects over the of the dorsal surface shows numerous small posterior aspect of the soft palate to enter the protuberances called papillae. The posterior one- nasopharynx, it changes to a typical respiratory third lacks papillae but shows mucosal ridges and passage epithelium. mound-like elevations. The latter results from the The hard palate, gingiva, and dorsum of the underlying lymphatic tissue of the lingual tonsils. tongue, which are subjected to mechanical trauma Four types of papillae are associated with the during chewing, lack a submucosa and are covered anterior portion of the tongue. Filiform papillae by cornified, stratified squamous epithelium. In are 2 to 3 mm long and contain a conical core of these areas, the lamina propria is immobilized by its connective tissue that is continuous with the attachment to underlying structures. Where the underlying lamina propria. The covering stratified mucosa attaches directly to bone, it forms a squamous epithelium shows variable degrees of mucoperiosteum. The mucosa of the gingiva (gums) is cornification. They are the most numerous type of attached directly to the periosteum of alveolar bone, papillae. The mushroom-shaped fungiform and a similar attachment occurs in the midline papillae are scattered singly between the filiform raphe of the hard palate and where the hard palate type. They are most numerous near the tip of the joins the gingiva. A submucosa is present under the tongue, where they appear as small red dots. The rest of the hard palate and contains abundant color is due to the richness of capillaries in their adipose tissue anteriorly and small mucous glands connective tissue cores. Taste buds may be posteriorly. associated with this type of papilla. Circumvallate The presence or absence of a submucosa as well papillae, the largest of the papillae, are located as the texture of the mucous membrane lining along the V-shaped sulcus terminalis that divides the different regions can be determined by self tongue into anterior and posterior regions. They examination of the oral cavity with the tip of the number 10 to 14 in humans. The circumvallate tongue. The interior lining of the upper and lower papillae appear to have sunk into the mucosa of the lips, cheek, covering of the mouth floor and soft tongue and are surrounded by a wall of lingual palate feels slippery and wet; a typical mucous tissue, from which they are separated by a deep membrane lined by nonkeratinized stratified furrow. Each contains a large core of connective squamous epithelium. Likewise, the mucosa of all tissue with numerous vessels and small nerves. of these regions can either be sucked between the Occasionally, small serous glands may be present. teeth (lips and cheek region) or feel pliable when The epithelium covering the lateral surfaces of poked by the tongue (mouth floor and soft palate) these papillae contains numerous taste buds that indicating a submucosa which gives mobility to these may number 250 or more per papilla. Serous (von regions. This contrasts to the hard palate, gingiva Ebner's) glands, lingual minor salivary glands, lie and dorsum of the tongue which lack a submucosa, in the lamina propria and open into the bottom of and the mucosa is anchored directly to bone or the furrows. Their thin serous secretions muscle, respectively. The texture of these regions continuously flush the furrow and provide an also differs indicating a degree of epithelial environment suitable for sensory reception by the cornification. taste buds. Foliate papillae are rudimentary in humans but in species such as the rabbit are well Tongue developed and contain many taste buds. They form oval bulges on the posterior, dorsolateral aspect of VOCABULARY: papilla, lingual tonsil, filiform the tongue and consist of parallel ridges and papilla, fungiform papilla, taste buds, furrows. Taste buds lie in the epithelium on the circumvallate papilla, serous glands (of von lateral surfaces of the ridges, and small serous Ebner), foliate papilla glands drain into the bottoms of the adjacent furrows. The mucous membrane of the tongue is firmly bound to a core of skeletal muscle, and there is no submucosa. The tongue musculature is formed by interwoven fascicles that run in three planes, each perpendicular to the other. The undersurface of the

171 Taste Buds Salivary Glands

VOCABULARY: taste pore, supporting VOCABULARY: major salivary glands, minor (sustentacular) cell, neuroepithelial cell, taste salivary glands, saliva, lysozyme, IgA, secretory hair piece

The normal human tongue contains about nine Salivary glands of the oral cavity can be divided into thousand taste buds. major and minor glands; all are compound tubuloacinar Taste buds are present in fungiform, circumvallate, glands. The major salivary glands are the parotid, and foliate papillae and may be scattered in the submandibular, and sublingual glands and lie epithelium of the soft palate, glossopharyngeal outside the oral cavity. Numerous smaller, intrinsic arches, pharynx, and epiglottis. They appear as salivary glands are present in the limiting wall of the lightly stained, oval structures that extend from the oral cavity and tongue and make up the minor basement membrane almost to the surface of the salivary glands (Table 14-1), which secrete lining epithelium. A small taste pore allows for continuously to lubricate the mucosa of the oral communication with the external environment. cavity, vestibule, and lips. Taste buds consist of supporting (sustentacular) cells, between which are neuroepithelial cells; the Table 14-1. Minor salivary glands. cells are arranged somewhat like the segments of a peeled orange. Both types of cells have large Name Location Type of microvilli called taste hairs that project into the gland taste pore and are embedded in an amorphous, Labial glands Upper and lower Mixed polysaccharide material. Neuroepithelial cells are lips stimulated by tastant molecules that enter the taste pore. Binding of tastants with taste binding receptor Buccal glands Cheeks Mixed proteins occurs on the microvilli of the neuroepithelial cells. The taste sensation is Anterior lingual glands Tip of tongue Mixed transmitted to club-shaped nerve endings that pass von Ebner’s gland Near circumvallate Serous between both cell types of the taste bud but papillae apparently make synaptic contact only with neuroepithelial cells. Depolarization of the Posterior lingual glands Root of tongue Mucous neuroepithelial cells as a result of receptor binding Palatine glands Palate Mucous stimulates the release of glutamate, which then generates an action potential in adjacent afferent nerve terminals. Peripheral and basal cells, also Saliva is a mixture of secretions from both groups associated with taste buds, are thought to represent of salivary glands and contains water, ions, mucins, undifferentiated progenitors of the supporting and immunoglobulins, desquamated epithelial cells, and neuroepithelial cells. degenerating granulocytes and lymphocytes (salivary Five taste sensations are perceived: bitter, sweet, corpuscles). Amylase, maltase, and salivary lipase salty, sour, and umami. The sensations may be also are present to begin digestion of some detected regionally in the tongue -sweet and salty at carbohydrates and fats. Saliva moistens the oral the tip of the tongue, sour at the sides of the cavity, softens ingested materials, cleanses the oral tongue, and bitter in the area of the circumvallate cavity, and acts as a solvent to permit materials to papillae - but structural differences in taste buds be tasted. Some heavy metals are eliminated in the from these areas have not been seen. Stimulation of salivary secretions, and decreased secretion during the umami receptor results in foods that are dehydration helps initiate the sensation of thirst. ingested to “taste good”. Salivary mucins, particularly those of the submandibular gland are largely monomeric in molecular form and have the capacity to bind to and form aggregates with the microflora within the oral cavity thereby keeping the number of

172 microorganisms in check. The glycoproteins plus basement membrane and the bases of the secretory the bound microbes are then swallowed and cells and may aid in expressing secretions out of the removed from the oral cavity. This is one secretory units and into the duct system. Acini are mechanism by which the bacterial flora of the oral composed of pyramidal-shaped, serous cells with cavity is kept in check. basally placed, oval nuclei, basophilic cytoplasm, Other components within the saliva also are and discrete, apical secretory granules. Small important in controlling the bacterial population in channels, the intercellular secretory canaliculi, the mouth. In addition to producing lysozyme, the are found between serous cells and provide an serous cells of the salivary glands participate in the additional route secretory products can take to production of immunoglobulin to suppress reach the lumen. bacterial growth. Immunoglobulin A (IgA) The initial segment of the duct system is the synthesized and released by B-lymphocytes and intercalated duct, which is especially prominent in plasma cells is taken up by the serous cells and the parotid gland. It is lined by a simple squamous complexed to a protein known as secretory piece or low cuboidal epithelium and may be associated before being released into the oral cavity. Secretory with surrounding myoepithelial cells. Intercalated piece is synthesized by the epithelial cells and ducts are continuous with striated ducts, which are prevents the IgA from being broken down. The lined by columnar cells that show numerous basal submandibular gland also secretes epidermal growth striations. These elaborate infoldings of the factor (EGF) and nerve growth factor (NGF) into the basolateral plasmalemma increase the surface area, oral cavity. About 1000 ml of fluid is secreted daily while the mitochondria associated with the by the salivary glands. infoldings provide energy (ATP) at the base of the cell and together play a significant role in fluid/ion Major Salivary Glands transport during secretion. The intralobular ducts of all the major salivary glands are intimately related to The major salivary glands - parotid, submandibular, a surrounding network of capillaries that aid in this and sublingual - secrete only in response to nervous function. The intercalated and striated ducts stimulation. The response is reflexive and can be constitute the duct system within the lobule and stimulated by the smell, sight, or even the thought collectively form the intralobular duct system. of food. The remaining ducts are found in the connective tissue between lobules and are referred to as Parotid Gland interlobular ducts. They are continuous with the intralobular ducts and at first are lined by a simple VOCABULARY: compound tubuloacinar, columnar epithelium that becomes pseudostratified serous, myoepithelial cell, intercellular and then stratified as the diameter of the duct secretory canaliculi, intercalated duct, striated increases. The surrounding connective tissue duct, basal striation, intralobular duct system, becomes more abundant as these ducts join to form interlobular duct the major excretory duct. The distal part of the major excretory duct is lined by nonkeratinized The parotid is the largest of the major salivary stratified squamous epithelium that becomes glands and is located below and in front of the continuous with the interior lining epithelium of the external ear. The main excretory duct passes cheek. through the cheek to open into the vestibule of the mouth opposite the upper second molar tooth. It is Submandibular Gland a compound tubuloacinar gland and in humans is entirely serous. The adult human parotid VOCABULARY: compound tubuloacinar contains abundant, scattered fat cells. gland, mixed gland, serous demilune The parotid is enclosed in a fibrous capsule and (crescent), intercellular canaliculi, striated duct subdivided into lobes and lobules by connective tissue septa. A delicate stroma surrounds the The submandibular gland, like the parotid, is a secretory units and ducts and contains numerous compound tubuloacinar gland and has a fibrous blood capillaries and scattered nerve fibers. capsule with septa, lobes, lobules, myoepithelial Myoepithelial cells lie between the limiting cells, and a prominent duct system.

173 It is a mixed gland, with the majority of the Structure secretory units being serous in humans. The mucous tubules present usually show serous VOCABULARY: crown, root, neck, socket demilunes (crescents) at their blind ends. Small (alveolus), pulp cavity, apical foramen, enamel, channels, the intercellular secretory canaliculi, dentin, cementum, pulp, periodontal pass between the mucous cells and extend between membrane, gingiva the serous cells of the demilune. Thus, the secretory product of the demilunes has direct access to the All teeth show the same histologic organization. lumen of the mucous tubule. Myoepithelial cells lie Each has a crown and a root, and the point where between the secretory cells and the basement the two meet is called the neck. The root fits into a membrane and invest the secretory units as well as socket (or alveolus) of the mandible or maxilla. the initial portions of the ductal system. Generally, Each tooth contains a small pulp cavity that the duct system is similar to that of the parotid, but corresponds in shape to the external form of the the striated ducts are much longer and hence more tooth. The pulp cavity communicates with the conspicuous in sections of the submandibular alveolar cavity and periodontal membrane through gland. The major excretory duct of each the apical foramen, a small opening at the tip of submandibular gland empties onto the floor of the the root. The hard tissues of the tooth consist of oral cavity on either side of the lingual frenulum. enamel, dentin, and cementum. Soft tissues associated with the tooth are the pulp, periodontal Sublingual Glands membrane, and gingiva (Fig. 14-1).

VOCABULARY: composite gland, mixed Dentin compound tubuloacinar gland VOCABULARY: dentinal tubule, dentinal The sublingual is a composite gland, formed by a (Tomes') fibers, odontoblast, Neumann's number of individual glands of variable size. Each sheath gland opens independently onto the floor of the mouth or into the excretory ducts of the Dentin forms the bulk of the tooth and surrounds submandibular gland. The capsule is not well the pulp chamber. It is harder than compact bone defined, although septa are present. It is a mixed and consists of 80% inorganic material and 20% compound tubuloacinar gland with most of the organic substance. Most of the organic material is secretory units being mucous. Serous cells are, for collagen, while the inorganic component is in the the most part, arranged as demilunes. Myoepithelial form of hydroxyapatite crystals. Dentin has a cells are found in relation to the secretory units. radially striated appearance due to numerous Segments of the intralobular duct system are short minute canals called dentinal tubules that pursue and not as readily seen as they are in the other two an S-shaped course as they extend from the pulp major salivary glands. chamber to the dentinoenamel junction. The dentinal tubules contain fine cytoplasmic processes, Teeth called dentinal (Tomes') fibers, from a cell type known as the odontoblast. The thin layer of dentin In humans, the teeth appear as two distinct sets: that immediately surrounds each dentinal tubule primary (deciduous) and permanent. Primary teeth shows greater refringence than the remaining dentin erupt 6 to 8 months after birth and form a and is called Neumann's sheath. The cell body of complete set of 20 by 2 years of age. They are shed the odontoblast lies within the pulp cavity adjacent between the sixth and thirteenth year and gradually to the dentin. The odontoblast lays down the are replaced by the permanent set of 32. organic matrix of dentin (predentin) and is active throughout life so that there is a progressive narrowing of the pulp cavity with age. Predentin is rich in collagen, contains glycosaminoglycans, and is unmineralized. After its extracellular formation, predentin becomes mineralized.

174 Some areas of dentin remain incompletely calcified as Sharpey's fibers, which anchor the root of the and form the interglobular spaces. Dentin is tooth to the surrounding alveolar bone of the sensitive to cold, pain, touch, and hydrogen ion socket. concentration. Sensation is thought to be perceived by the processes of odontoblasts, which in turn Pulp transmit the sensory stimulation to adjacent nerves in the pulp chamber. VOCABULARY: connective tissue, apical foramen Enamel Pulp is the connective tissue that fills the pulp VOCABULARY: enamel prism, interprismatic cavity. It contains numerous thin, collagenous substance, ameloblast fibers embedded in an abundant gelatinous ground substance. Stellate fibroblasts are the most Enamel covers the dentin of the crown and is the prominent cells of the pulp, although mesenchymal hardest substance of the body. It is acellular and cells, macrophages, and lymphocytes are found in consists primarily of calcium salts in the form of limited numbers. The cell bodies of the apatite crystals. Only 1% of the enamel substance is odontoblasts also are found in the pulp, lining the organic material. Enamel consists of thin rods perimeter of the pulp cavity immediately adjacent to called enamel prisms that lie perpendicular to the the dentin. Blood vessels, lymphatics, and nerves surface of the dentin and extend from the enter and exit the pulp cavity through the apical dentinoenamel junction to the surface of the tooth. foramen. Each prism, 6 to 8 µm in diameter, follows a spiraling, irregular course to the surface of the Periodontal Membrane tooth. A small amount of organic matrix surrounds each enamel prism and is called the prismatic rod VOCABULARY: bundle of collagen fibers, sheath. The organic matrix of enamel consists Sharpey's fibers primarily of proteins called enamelins that bind to crystallites of the enamel prisms. Between the The periodontal membrane consists of thick enamel prisms is the interprismatic substance, bundles of collagen fibers that run between the which also consists of apatite crystals in a small cementum covering the root of the tooth and the amount of organic matrix. Each enamel prism is the surrounding alveolar bone (Fig. 14-1). The fibers product of a single ameloblast, the enamel- extend into the bone and cementum as Sharpey's producing cells that are lost during eruption of the fibers. The orientation of the fibers in the tooth. Thus, new enamel cannot be formed after periodontal membrane varies at different levels in the tooth has erupted. the alveolar socket. Although firmly attached to the surrounding alveolar bone, the fibers are not taut, Cementum and the tooth is able to move slightly in each direction. The periodontal membrane forms a VOCABULARY: acellular cementum, suspensory ligament for the tooth. In addition to cementocyte, cellular cementum, Sharpey's typical connective tissue cells, osteoblasts and fibers osteoclasts may be found where the periodontal membrane enters the alveolar bone. The Cementum covers the dentin of the tooth root. periodontal membrane has a rich vascular supply Nearest the neck of the tooth, the cementum is thin and is sensitive to pressure changes. Sensory nerve and lacks cells, forming the acellular cementum. fibers linked to the mesencephalic nucleus of the The remainder, which covers the apex of the tooth fifth cranial nerve prevent excessive pressures from root, contains cells, the cementocytes, that lie in being exerted on the teeth during chewing and lacunae and are surrounded by a calcified matrix thereby prevents the crushing of one’s own teeth. similar to that of bone. This type of cementum is referred to as cellular cementum. The organic matrix of cementum increases with age. Coarse bundles of collagen fibers penetrate the cementum

175 Oropharynx

VOCABULARY: nonkeratinized stratified squamous epithelium, lamina propria, muscularis, skeletal muscle, adventitia

The oropharynx is lined by a nonkeratinized stratified squamous epithelium supported by a dense, fibroelastic lamina propria. Immediately beneath the lamina propria is a well-developed layer of elastic fibers that is continuous with the muscularis mucosae of the esophagus. Proximally, the elastic layer blends with the connective tissue between the muscle bundles of the pharyngeal wall. A submucosa is present only where the pharynx is continuous with the esophagus and in the lateral walls of the nasopharynx. The muscularis of the Fig. 14-1. A diagrammatic representation of the structures pharyngeal wall consists of the skeletal muscle of associated with a tooth. the three pharyngeal constrictor muscles, which, in turn, Gingiva (Gum) are covered by connective tissue of the adventitia.

VOCABULARY: gingival ligament, epithelial Tubular Digestive Tract attachment cuff The tubular digestive tract consists of the The gingiva (gum) surrounds each tooth like a esophagus, stomach, small intestine, colon, and collar and is attached to the periosteum of the rectum. Each region of the digestive tube consists of the underlying alveolar bone. Near the tooth, four basic layers, but the components of these layers collagenous fibers of the gingival lamina propria vary according to the functions of the region. blend with the uppermost fibers of the periodontal membrane. Some collagenous fibers extend from Esophagus the lamina propria into the cervical (upper) cementum and constitute the gingival ligament, The esophagus is a relatively straight muscular tube which provides a firm attachment to the tooth. about 25 cm long. It is continuous above with the The keratinized stratified squamous epithelium of oropharynx at the inferior border of the cricoid the gingiva also is attached to the surface of the cartilage and below becomes continuous with the tooth and at this point forms the epithelial cardia of the stomach. attachment cuff. Attachment of the cuff to the tooth is maintained by a thickened basal lamina and Mucosa hemidesmosomes that seal off the dentogingival junction. VOCABULARY: nonkeratinized stratified squamous epithelium, lamina propria, Pharynx muscularis mucosae, smooth muscle

The oral cavity continues posteriorly into the The mucosa of the esophagus consists of a lining pharynx, which extends from the base of the skull epithelium, a lamina propria, and a muscularis to the level of the cricoid cartilage, where it is mucosae. The epithelial lining is thick and consists continuous with the esophagus. The digestive and of nonkeratinized stratified squamous respiratory systems merge briefly in the pharynx, epithelium that is continuous with a similar which is subdivided into nasal, oral, and laryngeal epithelium lining the oropharynx. Antigen-presenting parts. The pharyngeal walls basically consist of (Langerhans) cells have been identified in the three strata: a mucosa, a muscularis, and an adventitia. esophageal epithelium.

176 At the junction with the stomach, the epithelium anatomic sphincters, and normally, the intraluminal shows an abrupt transition to a simple columnar pressure generated by the LES is sufficient to epithelium. The lamina propria is a loose areolar prevent reflux of gastric contents into the connective tissue with diffuse and nodular esophageal lumen. In some individuals, the LES lymphatic tissue scattered throughout its length. fails, and the mucosa of the lower esophagus is The muscularis mucosae consists of chronically subjected to gastric acid and pepsin. A longitudinally arranged smooth muscle cells in a chemically induced chronic esophagitis results, and fine, elastic network. Circularly arranged smooth the esophageal epithelium undergoes metaplasia, muscle cells also may be present. The muscularis changing from wet stratified squamous to a simple mucosae is continuous with the elastic layer of the columnar epithelium similar to gastric lining pharynx at the level of the cricoid cartilage. epithelium. Such a condition (Barrett's esophagus) may result in severe ulceration of this area. This Submucosa condition is known as gastroesophageal reflux disease (GERD). The upper esophageal sphincter VOCABULARY: collagenous fiber, elastic functions to keep air from entering the proximal fiber, longitudinal fold esophagus. The hormone gastrin acts to stimulate the constriction of the lower esophageal sphincter. The submucosa consists primarily of coarse collagenous fibers and elastic fibers arranged in a Adventitia ground substance. Within it are larger blood vessels, lymphatics, nerve fibers, occasional autonomic A layer of loosely arranged connective tissue, the ganglia, and glands. Extensive longitudinal folds adventitia, covers the outer surface of the of the submucosa give the mucosa of the non- esophagus and binds it to surrounding structures. distended esophagus a characteristic pleated Below the diaphragm a short segment of the appearance. During swallowing the bolus of food esophagus is covered by a serosa. smoothes out these folds and allows the lumen to increase in size temporarily to accommodate the Glands material swallowed. VOCABULARY: esophageal cardiac gland, Muscularis Externa lamina propria, esophageal glands proper, submucosa VOCABULARY: skeletal muscle, mixed smooth and skeletal muscle, smooth muscle Two types of glands are present in the esophagus: esophageal cardiac glands and esophageal glands The upper quarter of the muscularis externa proper. Two groups of esophageal cardiac consists of skeletal muscle only. From the lower glands are present: one group located in the border of the inferior constrictor muscle of the proximal esophagus near the pharynx, the other in pharynx, the muscle progressively becomes more the distal esophagus near the cardia of the stomach. regularly arranged into inner circular and outer Both are compound tubuloacinar glands of mucous longitudinal layers. In the second quarter of the type and occur only in the lamina propria. The esophagus, mixed skeletal and smooth muscle is esophageal glands proper also are mucous found, while in the distal one-half, only smooth producing compound tubuloacinar glands but are muscle is present. Histochemical studies suggest confined to the submucosa and scattered that the skeletal muscle fibers of the esophageal wall throughout the remaining length of the esophagus. are type IIa, fast contracting, fatigue-resistant fibers. Lysozyme and pepsinogen have been identified in Nerve cells of the autonomic ganglia (Auerbach’s some but not all of the secretory cells. Myoepithelial plexus) are present between the inner and outer cells are associated with the secretory units of human layers of smooth muscle. esophageal glands. Zones of intraluminal high pressure occur at the ends of the esophagus and are referred to as the upper esophageal sphincter (UES) and the lower esophageal sphincter (LES). Both are physiologic rather than

177 Stomach

The stomach forms an expanded portion of the tubular digestive tract between the esophagus and small intestine. In humans, the stomach is divided into the cardia, a short region where the esophagus joins the stomach; the fundus, a dome-shaped elevation of the stomach wall above the esophagogastric junction; the corpus, the large central part; and the pylorus, a narrow region just Fig. 14-2. A scanning electron micrograph of the luminal surface of a above the gastrointestinal junction. The digestive human fundic stomach illustrates numerous gastric pits. (X 200). functions of the stomach involve mechanical and chemical breakdown of ingested materials. The epithelial cells on the surface are replaced every Mucosa 4 to 5 days. New cells are derived from a small population of relatively undifferentiated cells

located in the bottoms of the gastric pits. Cells VOCABULARY: ruga, gastric lining from this region gradually migrate upward along the epithelium, simple columnar, gastric pit walls of the gastric pit to replace the cells of the (foveola) surface epithelium.

Three types of glands occur in the gastric mucosa: The mucosa of the empty stomach has numerous cardiac, gastric (oxyntic), and pyloric. folds or ridges called rugae that mostly disappear when the stomach is filled or stretched. The gastric lining epithelium is simple columnar and begins Cardiac Glands abruptly at the junction with the wet stratified squamous epithelium of the esophagus and ends VOCABULARY: simple branched tubular just as abruptly at its junction with intestinal gland, mucous cell, endocrine cell epithelium. The lining epithelium has a similar structure throughout the stomach and consists of The cardiac glands begin immediately around the secretory columnar cells that collectively form a esophageal orifice and extend about 3.4 cm along secretory sheet. The neutral mucin produced by the the proximal stomach. They attain their greatest surface epithelium is secreted continuously and depth near the esophageal-gastric junction. Here, forms a mucous film that helps protect the mucosa the tubular glands branch freely and appear to be from the acid/pepsin in the gastric lumen and aggregated into lobule-like complexes by the lubricates the surface. surrounding connective tissue of the lamina The apices of the columnar cells are held in close propria. Near the junction with the fundus, the apposition by tight junctions, and the lateral cell cardiac glands show less branching, the distinct membranes show numerous desmosomes. grouping disappears, and the thickness of the Scattered, stubby microvilli are present on the glandular area decreases. For the most part, cardiac apical surfaces, and numerous discrete mucin glands are simple branched tubular glands that granules fill the apical cytoplasm. Granular open into the overlying gastric pits. The depth of endoplasmic reticulum is present in the basal the gastric pit and the length of the cardiac gland cytoplasm, and well-developed Golgi complexes are approximately equal. The secretory units consist occupy the supranuclear region. mainly of mucous cells, but occasional parietal The gastric mucosa contains about 3 million cells are present and appear to be identical to those minute tubular infoldings of the surface epithelium of gastric glands. A number of endocrine cells of that form the gastric pits or foveolae, which are undetermined nature also are present. lined by the same simple columnar epithelium that covers the surface (Fig. 14-2).

178 Gastric (Oxyntic) Glands series of smooth cytoplasmic membranes forms small tubules and vesicles that make up a VOCABULARY: simple branched tubular tubulovesicular system. The tubulovesicular gland, mucous neck cell, parietal cell, membranes are rich in a unique hydrogen, potassium intracellular canaliculus, tubulovesicular ATPase, which acts as a proton pump in the system, hydrochloric acid, gastric intrinsic stimulated parietal cell. The membranes also factor, histamine, chief (zymogen) cell, contain potassium and chloride ion channel pepsinogen, pepsin, endocrine cell proteins. Secretion of hydrochloric acid occurs along the cell membrane that lines the canaliculi. The gastric (oxyntic) glands are the most abundant During stimulation of parietal cells, the glands in the stomach (about 15 million) and are tubulovesicular membranes diminish, canalicular found along the entire corpus of the stomach. They microvilli become more abundant, and canaliculi are mainly simple branched tubular glands. The elongate; during acid inhibition, the reverse occurs. glands run perpendicular to the surface of the Following parietal cell stimulation, actin mucosa, and one or several glands may open into microfilaments polymerize, and their interaction the bottom of each gastric pit. The gastric glands with myosin promotes the movement and fusion of are about four times as long as the pits into which tubulovesicular with canalicular membranes, they open. Each gland contains mucous neck cells, resulting in the increased canalicular membrane area parietal cells, chief (zymogen) cells, and endocrine containing hydrogen, potassium-ATPase and ion cells. channel proteins. Parietal cells contain abundant Mucous neck cells occur primarily in the upper carbonic anhydrase, which catalyzes the joining of regions of the gastric glands, where the glands open carbon dioxide to water, forming carbonic acid into the gastric pits. The cells often appear to be (H2CO3). Carbon dioxide and water diffuse into the - sandwiched between other cell types, and their cell from adjacent capillaries. Once formed, H2CO3 irregular shape is characteristic. Some have broad immediately dissociates into hydrogen ions (H+) - apices and narrow bases; others show narrow apices and bicarbonate ions (HCO3 ). Hydrogen ions are and wide bases. The nucleus is confined to the base actively pumped into the lumen of the canaliculus, of the cell, surrounded by basophilic cytoplasm. and bicarbonate ions pass back into the Secretory granules fill the apical cytoplasm. Unlike bloodstream of the gastric mucosa to cause an the mucous cells that line the gastric surface and increase in pH known as the alkaline tide. Ion gastric pits, mucous neck cells produce an acidic channels promote the movement of potassium and mucin. chloride ions into the canalicular lumen. The Although most numerous in the central area of the hydrogen, potassium-ATPase moves the potassium gastric glands, parietal cells also are scattered ion back into the parietal cell in exchange for among mucous neck cells in the upper parts of the hydrogen ion. The net result of this activity is the gland. They are large, spherical cells whose bases formation of hydrochloric acid in the canalicular often appear to bulge from the outer margin of the lumen. Water enters the canaliculi osmotically glands into the lamina propria. In routine sections, driven by the movement of ions. The increase in parietal cells are distinguished by their acidophilic canalicular membrane surface area during parietal cytoplasm. Each cell contains a large, centrally cell stimulation makes these events possible (Fig. placed nucleus and numerous mitochondria and is 14-3 & Fig. 14-4). characterized by intracellular canaliculi. Mitochondria make up nearly 40% of the parietal cell volume and account for the eosinophilia in hematoxylin and eosin stained preparations. The canaliculi are invaginations of the plasmalemma that form channels near the nucleus and open at the apex of the cell into the lumen of the gland. Numerous microvilli project into the lumina of the intracellular canaliculi, their number and length varying according to the secretory activity of the cell. Immediately adjacent to each canaliculus, a

179 acids, stimulates G-cells within the pyloric mucosa to secrete gastrin which also targets parietal and ECL cells. The histamine released by stimulated ECL cells acts synergistically with both gastrin and acetylcholine to enhance parietal cell secretory activity. These physiologic agonists (gastrin, histamine, acetylcholine) attach to individual transmembrane receptors in the parietal cell plasmalemma. Somatostatin from adjacent D cells in the pyloric mucosa acts on neighboring G cells to inhibit gastrin release and thereby inhibit parietal cell activity. Chief (zymogen) cells are present mainly in the Fig. 14-3. A transmission electron micrograph illustrating the basal half of the gastric glands and in routine ultrastructural features of a non-stimulated human parietal cell. (X 10,000). sections are distinguished by their basophilia. The cells contain abundant granular endoplasmic reticulum in the basal cytoplasm, well-developed Golgi complexes in the supranuclear cytoplasm, and apical zymogen granules, features that characterize cells involved in protein (enzyme) secretion. Chief cells secrete pepsinogen, a precursor of the enzyme pepsin that reaches its optimal activity at pH 2.0. The acid environment in the gastric lumen is created by the parietal cells. Pepsin is important in the gastric digestion of protein, hydrolyzing proteins into peptides. Also scattered within the bases of the gastric glands are endocrine cells, peptide/amine- producing cells that contain specific granules Fig. 14-4. A transmission electron micrograph illustrating the ultrastructural features of a stimulated human parietal cell enclosed by smooth membranes. The polarization sampled 10 minutes following gastrin administration. of these cells suggests that they secrete into the (X 10,000). bloodstream or intercellular space rather than into the lumina of gastric glands. The gastrointestinal In addition to hydrochloric acid, parietal cells of the hormones somatostatin, glucagon, pancreatic human stomach secrete a glycoprotein, gastric peptide, histamine, and the amine 5- intrinsic factor, which complexes with dietary hydroxytryptamine (5-HT, serotonin) have been vitamin B12; the complex is absorbed in the distal identified in the endocrine cells of the gastric small intestine. A deficiency of intrinsic factor glands. Gastric A cells produce glucagon which results in decreased absorption of vitamin Bl2, an stimulates hepatic glycogen degradation and essential vitamin for the maturation and production increases blood glucose levels. of red blood cells. Lack of this vitamin results in pernicious anemia and is a condition often associated Kinetics of the Gastric (Oxyntic) Glands with atrophic gastritis. Parietal cell stimulation is initiated and modulated Pluripotent stem cells located in the region where a by the vagus nerve during the cephalic phase of gastric gland joins the bottom of a gastric pit (the digestion in response to taste, smell, sight, chewing isthmus) divide to maintain themselves as well as or even the thought of food. This activity results in give rise to several committed cell types: pre-pit, the release of acetylcholine from postganglionic pre-mucous neck, pre-parietal, and pre-endocrine. cholinergic muscarinic neurons that act to stimulate Chief cells are derived from mucous neck cells both parietal and ECL (enterochromaffin-like) cells. through a pre-zymogenic stage. During the gastric phase of digestion, stomach distension, the presence of proteins and amino

180 All pit cells migrate in the direction of the gastric Lamina Propria lumen to become gastric lining epithelial cells and are eventually exfoliated into the lumen in about 5 The lamina propria of the stomach is obscured by days. Mucous neck cells, however, migrate inward the close approximation of the glands in the gastric toward the base of the gastric glands and in about 2 mucosa. It consists of a delicate network of weeks become pre-chief (zymogenic) cells. As fully collagenous and reticular fibers that surrounds and differentiated chief cells form, they migrate to extends between the glands and gastric pits. The occupy the bottom of the gastric glands and remain lamina propria contains numerous capillaries. active for up to 6 months. Thereafter, they are Numerous lymphocytes, some plasma cells, either lost by necrosis and shed into the lumen or, if eosinophils, and mast cells are found within the apoptosis occurs, phagocytosed by adjacent chief lamina propria. Lymphatic nodules also may occur. cells or macrophages entering the area. Endocrine cells also arise at the isthmus and migrate toward Muscularis Mucosae the base of the glands where they are abundant. Their turnover time is about 3 months. In contrast, The muscularis mucosae consists of an inner differentiating parietal cells migrate in both circular, an outer longitudinal and, in some places, directions with about equal numbers migrating an additional outer circular layer of smooth muscle. toward either the gastric lumen or the bottoms of Slips of smooth muscle extending into the lamina the gastric glands. They have a turnover rate of propria form a loose network around individual about 60 days and, if they become necrotic, are lost glands and extend to the gastric lining epithelium. by extrusion into the lumen of the gland. If the The contraction of these smooth muscle cells acts parietal cells become apoptotic, they are to compress the contained gastric glands. The phagocytosed by neighboring cells or macrophages. muscularis mucosae gives additional mobility to the gastric mucosa. Pyloric Glands Submucosa VOCABULARY: simple or branched tubular gland, mucous cell, endocrine cell, G cell, The submucosa consists of a coarse connective gastrin tissue rich in mast cells, eosinophils, and lymphatic cells. This layer contains lymph vessels and the The pyloric glands are confined to the distal 4 or 5 larger blood vessels. Parent vessels originating from cm of the stomach. They are simple or branched the celiac axis pierce the stomach wall at the lesser tubular glands composed of mucous cells similar and greater curvatures and supply smaller branches in appearance to mucous neck cells of the fundic that run about the circumference of the stomach region. An occasional parietal cell may be present. within the submucosa. Smaller branches of the The pyloric glands empty into gastric pits that are submucosal vessels enter the mucosa to provide its about the same length as the glands. Numerous vascular supply. endocrine cells, mainly G cells, are present. These unicellular endocrine glands produce gastrin, the Muscularis Externa peptide hormone that stimulates acid secretion by the parietal cells in the remainder of the stomach. The muscularis externa may consist of three layers of Endocrine cells that produce somatostatin (D cells) smooth muscle: outer longitudinal, middle circular, and and serotonin (5-HT) are present in large numbers in sometimes an inner oblique layer in the body and the pyloric glands also. Thus, in addition to the fundic regions. Strong contractions of the muscle exocrine function of mucus production, pyloric wall of the stomach create a churning action that glands have important endocrine functions as well. contributes mechanically to the breakdown of The serotonin produced increases gut motility ingested material. In the pylorus, the middle circular whereas somatostatin acts to inhibit the secretion of layer thickens to form the pyloric sphincter, which is adjacent endocrine cells. thought to aid in controlling the emptying of the stomach. Isotonic contents act to stimulate whereas fats inhibit gastric emptying.

181 High hydrogen ion concentrations in the proximal morphological features provide an absorptive duodenal lumen also inhibit gastric emptying. surface area of 200 M2. Plicae circulares are large, permanent folds that Serosa consist of the intestinal mucosa and a core of submucosa. These shelflike folds spiral about the A thin layer of loose connective tissue covers the lumen of the intestine for one-half to two-thirds of muscularis externa and is covered on its outer its circumference and may branch. Plicae begin in aspect by the mesothelial lining of the peritoneal the upper duodenum, reach their maximum cavity, the visceral peritoneum, forming a serosa. development in the proximal jejunum, and thereafter diminish in size and disappear in the Small Intestine distal half of the ileum. The intestinal mucosa also presents numerous The small intestine extends between the stomach finger-like evaginations, the villi, that cover the and colon and is divided into the duodenum, entire surface of the small intestine. They consist of jejunum, and ileum. Although there are minor a lamina propria core and a covering of intestinal microscopic differences among these subdivisions, epithelium. Villi are 0.5 to 1.5 mm long and are best all have the same basic organization as the rest of developed in the duodenum and jejunum, where the digestive tube - mucosa, submucosa, muscularis they form broad, spatulate structures. In the ileum externa, and serosa or adventitia. The transition they become shorter and more finger-like. from one segment to another is gradual. The The surface of the intestine is increased almost 30- proximal 12 inches of its length is generally fold by the large numbers of closely packed considered duodenum, the remaining proximal two- microvilli on the luminal surfaces of the intestinal fifths jejunum and the distal three-fifths ileum. The absorptive cells. Microvilli make up the striated small intestine moves chyme from the stomach to border seen by light microscopy. Each microvillus the colon and completes the digestive processes by represents a cylindrical extension of the plasma adding enzymes secreted by the intestinal mucosa membrane enclosing a small core of cytoplasm. and accessory glands (liver and pancreas). Its primary Each microvillus contains numerous thin actin function, however, is absorption. Approximately 8 to 9 filaments that extend into the apical cytoplasm of liters of fluid enters the small intestine on a daily the cell and contribute to the terminal web, a basis. Food and liquid intake represents 1-2 liters network of fine filaments that lies just beneath and of this volume the remainder coming from parallel to the microvillus border. The actin endogenous sources such as salivary, gastric, filaments within the microvillus core are linked intestinal, pancreatic, and biliary secretions. together to form a bundle by the proteins fimbrin Of this volume 6-7 liters is absorbed in the small and villin. A brush border myosin links the actin intestine with only 1-2 liters entering the colon the bundle to the surrounding plasmalemma. Near the majority of which is absorbed at this location. Only lateral surface of the cell, the filaments merge with as very small amount of fluid is evacuated in the those associated with junctional complexes. The stool. The majority of water is absorbed passively in filaments and terminal web contribute to the the gut and is largely dependent on an osmotic cytoskeleton of the cell and give stability to the gradient. microvillus border. Contraction of the terminal web region may act to spread the tips of microvilli and Specializations for Absorption increase absorptive efficiency.

VOCABULARY: plicae circulares, villus, Mucosa microvillus, striated border, terminal web VOCABULARY: simple columnar epithelium, Three specializations - plicae circulares, intestinal villus, intestinal gland (crypt of Lieberkühn) villi, and microvilli - markedly increase the surface area of the intestinal mucosa to enhance the The intestinal mucosa consists of a simple absorptive process. It is estimated that these columnar epithelium, a lamina propria, and a muscularis mucosae.

182 The numerous villi of the mucosal lining give a shows a prominent striated (microvillus) border velvety appearance to the interior of the small at its free surface. Prominent terminal bars unite intestine. Simple tubular intestinal glands (crypts the intestinal absorptive cells at their apices, and of Lieberkühn) empty between the bases of the desmosomes scattered along the lateral surfaces villi (Fig. 14-5). The glands measure 0.3 to 0.5 mm help hold the cells in close apposition. The in depth and extend through the mucosa to the microvilli that make up the striated border are level of the muscularis mucosae, separated from covered by a layer of glycoprotein (glycocalyx) that is one another by the connective tissue of the lamina elaborated by the intestinal epithelial cells. This propria. The intestinal glands contribute secretions glycoprotein layer has been called the surface coat that aid in the digestive process, but more and is resistant to mucolytic and proteolytic agents. important, they are the sites of renewal of intestinal It is believed to have a protective function and to epithelial cells as they are shed at the tips of the be involved in the digestive process. villi. Although considerable digestion occurs in the intestinal lumen due to the presence of pancreatic enzymes and bile salts, a significant amount of digestion also occurs near the microvillus surface prior to absorption. Enzymes that take part in the breakdown of disaccharides and polypeptides are present within the glycocalyx or along the plasma membrane of the microvilli. Numerous enzymes are present at this location and include enterokinase, peptidases, sucrase, maltase, and lactase. If the enzyme lactase is absent a condition known as lactose intolerance results. In addition to contributing to the digestive process, the intestinal absorptive cells are actively involved in the absorption of tripeptides, dipeptides, amino acids, monosugars (glucose, galactose, fructose), and lipids from the intestinal lumen. Absorbed materials passing through the epithelium must enter the cell through the apical plasmalemma and are prevented from passing between the cells by the zonulae occludens, which form a tight seal around the apices of the cells. The breakdown products of sugars and proteins pass through the basal plasmalemma to enter blood capillaries within the connective tissue cores of the villi. Fig. 14-5. A scanning electron micrograph illustrating isolated villi and intestinal glands of the duodenum. (X 200). Lipid absorption is peculiar in its pathway through Micrograph courtesy of J.E. Magney, University of the intestinal absorptive cells and in the fact that Minnesota. lipids enter lymphatic channels rather than the blood vascular system. Fats (primarily triacylglycerols) are Epithelial Lining emulsified by bile salts and broken down to fatty

acids and monoacylglycerols by pancreatic lipase. VOCABULARY: enterocyte, striated Long chain fatty acids, cholesterol, (microvillus) border, terminal bar, surface coat, monoacylglycerol, fat soluble vitamins (A, D, E, goblet cell, endocrine cell, caveolated cell and K) are then packaged into micelles and absorbed.

Resynthesis of triacylglycerol occurs in the smooth The epithelium that lines the intestinal lumen endoplasmic reticulum of the apical cytoplasm after contains a heterogeneous population of simple columnar they pass through the apical cell membrane. The cells. The principal cell type is the intestinal reformed lipid makes its way to Golgi complexes, absorptive cell, or enterocyte, a tall, cylindrical cell where it is complexed with apoproteins to form that rests on a delicate basement membrane and lipoprotein droplets called chylomicrons.

183 Chylomicrons have no limiting membranes and are Fructose is absorbed by a facilitated diffusion discharged through the lateral cell membranes of mechanism. Calcium absorption is dependent on the intestinal absorptive cells into the intercellular the presence of the active form of vitamin D (1, 25- space at the level of the nucleus, thus bypassing the dihydroxycholecalciferol) which promotes the basal regions. The chylomicrons pass down the synthesis of a Ca++ binding protein (calbindin) in intercellular spaces, through the basal lamina, and enterocytes. Small intestinal enterocytes absorb enter lymphatic channels (lacteals) after passing phosphate utilizing a sodium/phosphate between endothelial cells. In contrast, short- and transporter the activity of which is also stimulated medium-chain fatty acids enter the blood vascular by the activated form of vitamin D. Iron is system. The majority of fat absorption occurs in the absorbed primarily as ferroheme (bound to ileum. myoglobin or hemoglobin) or as free Fe++ and As in other transporting epithelia, the zonula transported in the blood plasma by a protein known occludens junction not only tightly unites the apices as transferrin. of enterocytes into a continuous epithelial layer but Some ion trapping also occurs in the intestinal tract also functions to clearly separate the enterocyte depending on the circumstance. Ammonia (NH3) is plasmalemma into apical and basolateral domains. lipid soluble and enters the intestinal lumen by Each contains specific transmembrane proteins passing through the intestinal lining epithelium. unique to its domain. The apical cell membrane When it combines with hydrogen ion to form + domain is rich in transmembrane proteins ammonium (NH4 ) the hydrogen ion is trapped as concerned with electrolyte transport and ion- ammonium which is not lipid soluble and is lost in coupled transport processes that bring materials the feces. into the cell from the intestinal lumen. Much of Goblet cells are scattered between the intestinal enterocyte absorption is linked to the transport of absorptive cells and increase in number distally in the sodium. Sodium can enter the cell via a intestinal tract. The mucus secreted by these transmembrane protein ion channel or by unicellular exocrine glands lubricates and protects transmembrane proteins that must bind to both the mucosal surface. The apical regions of the sodium and chloride before either ion enters the goblet cells often are expanded by the accumulation cell, a coupled cotransport process. In addition, of secretory granules; the base forms a slender sodium can be absorbed by a sodium-solute-coupled region that contains the nucleus, scattered profiles cotransport mechanism. Solutes such as galactose, of granular endoplasmic reticulum, and occasional glucose, dipeptides, tripeptides, amino acids, water- Golgi complexes. Goblet cells lie within the lining soluble vitamins, and some bile acids are epithelium and are united to intestinal absorptive cotransported with sodium through the apical cells by apical tight junctions and scattered plasmalemma and into the cell by this mechanism. desmosomes along the lateral surfaces. Goblet cells Once within the cytosol the solute follows a also migrate from the intestinal glands, eventually to concentration gradient and is discharged from the exfoliate at the tip of the villus. In humans, the life cell through the basolateral plasmalemma. Sodium- span of goblet cells is about 4 to 5 days. They may hydrogen ion and chloride-bicarbonate ion secrete continuously but are believed to pass exchange mechanisms also exist in the apical through only one secretory cycle. Mucin granules plasmalemma and are usually coupled to the are released by exocytosis and within a few seconds absorption of sodium chloride. expand several hundred-fold in volume due to the The absorptive sodium gradient established at the hydrophilic nature of the mucin granules. The apical plasmalemma is maintained by a sodium, resulting mucus is a viscid fluid gel consisting of potassium-ATPase transmembrane protein located in the glycoproteins. It is less viscid in the intestinal gland basolateral plasmalemma that functions as a sodium region. pump moving excess sodium into the extracellular Endocrine cells also are present in the epithelial space in exchange for potassium ions. The lining, scattered within the epithelium of the villi movement of water generally follows the movement and intestinal glands, but are far fewer in number of sodium, chloride and solutes (glucose, amino than goblet cells. Endocrine cells secrete acids) from the intestinal lumen. In general, sodium peptides/amines that influence gastric and intestinal ion is transported first creating a transepithelial secretion and motility, gallbladder contraction, and gradient followed by chloride ion and water. pancreatic and liver function (Table 15-2). They are

184 characterized by dense secretory granules, scattered duodenum and jejunum. Likewise, opioids such as profiles of granular endoplasmic reticulum, and an enkephalin occur in the intrinsic nerves of the electron-lucent cytoplasm. In addition to the stomach and pyloric sphincter whereas dynorphin is hormones of the enteroendocrine cells, numerous more restricted to the submucosal plexus of the peptide hormones (neurotensin, substance P, duodenum and ileum. The regulatory peptides vasoactive intestinal polypeptide, somatostatin, found within the gastrointestinal tract (both within galanin, calcitonin gene-related peptide, gastrin endocrine cells or enteric neurons) function as local releasing peptide, neuromedin U, enkephalins, regulatory molecules and neurotransmitters and/or dynorphin) occur in the neurons of the enteric plexus. neuromodulators. They effect all aspects of Some peptides such as VIP and galanin are found digestive function including secretion, motility, in neurons of the enteric system throughout the absorption, regulation of growth, and may even gastrointestinal tract. Others such as neuromedin U mediate inflammation occurring within the are confined to the neuronal population of the gastrointestinal tract.

Table 15-2. Hormones and related factors found in endocrine cells of the gastrointestinal epithelium.

Hormone Primary location of endocrine cell Primary action

Gastrin Mucosa of stomach (antrum) and duodenum Stimulates gastric acid secretion, trophic (G cell) factor for mucosa of stomach body

Secretin Mucosa of duodenum and jejunum (S cell) Stimulates pancreatic bicarbonate secretion in the ductal system

Cholecystokinin Mucosa of duodenum and jejunum (I cell) Stimulates pancreatic enzyme secretion by pancreatic acinar cells; gallbladder contraction

Enteroglucagon Mucosa of ileum and colon (L cell) Inhibits gastric acid secretion

Gastric inhibitory Mucosa of duodenum and jejunum (K cell) Inhibits gastric acid secretion; stimulates peptide insulin secretion

Motilin Mucosa of duodenum and jejunum (Mo cell) Acts to regulate the interdigestive myoelectric migrating complex (MMC)

Somatostatin Mucosa of stomach, small intestine, and colon Paracrine inhibitor (D cell)

Neurotensin Mucosa of ileum (N cell) Stimulates contractions of muscularis externa

Histamine Mucosa of body of stomach (enterochromaffin- Stimulates gastric acid secretion like (ECL) cell)

Peptide tyrosine Co-localized with enteroglucagon in the mucosae Inhibits gastric acid secretion tyrosine of ileum and colon (L cell)

Another cell type, the caveolated cell, may be seen between microvilli into the deep, apical cytoplasm. occasionally in the epithelial lining. It is a pear- This cell type occurs elsewhere in the shaped cell with a wide base and narrow apex that gastrointestinal tract and in other organs of protrudes slightly into the lumen. These cells are endodermal origin. They may be chemoreceptors in held in close apposition to neighboring epithelial some regions, as in the respiratory system, but their cells by tight junctions and desmosomes. function in the intestine is unknown. Caveolated cells have large microvilli that typically contain bundles of actin filaments that extend deep into the supranuclear region. Between the bundles of filaments, caveolae form small elongated channels that extend from the apical cell membrane

185 Intestinal Glands plasmalemma. The CFTR protein is an active transporter and requires energy (ATP) to transport VOCABULARY: undifferentiated cell, chloride ions. Transcellular chloride secretion is oligomucous cell, Paneth cell, intraepithelial thought to be the driving force for the movement lymphocyte of water and sodium through the paracellular route, the net result of which is secretion rich in sodium All epithelial cells that line the intestinal surface chloride and water entering the intestinal lumen. arise from cells in the intestinal glands. The upper During secretion of water by the small intestine it is regions of the glands are lined by simple columnar the chloride ion that is transported first creating a epithelium that is similar to and continuous with transepithelial gradient followed by sodium and that lining the intestinal lumen. It contains then water. The transmembrane conductance absorptive cells, goblet cells, scattered endocrine regulator protein that regulates the passage of cells, and a few caveolated cells. Undifferentiated chloride through the apical plasmalemma is thought cells line the basal half of the intestinal glands and to be controlled at least in part by two intestinal undergo frequent mitoses. Intestinal absorptive peptide hormones called guanylin and uroguanylin. cells (enterocytes) and oligomucous cells, which Certain pathogenic strains of enteric bacteria represent intermediate stages of goblet cell secrete heat-stable enterotoxins that mimic the differentiation, lie in the mid-gland region. New actions of these peptides, which results in "traveler's intestinal epithelial cells are formed continually in secretory diarrhea" and is illustrative of the the basal region of the glands, migrate by upward magnitude of fluid production through this system displacement of cells, and ultimately come to cover if unchecked. the villi and the entire intestinal surface. The oldest Small groups of pyramidal-shaped cells with intestinal epithelial cells are shed at the tips of the conspicuous granules are found at the bases of the villi. Complete turnover of the epithelial lining of intestinal glands. These are Paneth cells, which, the intestine occurs every 4 to 5 days. As absorptive unlike other cells of the intestinal epithelium, form intestinal cells migrate up the villus, they increase in a relatively stable population with a low rate of height and the striated border becomes more turnover. Paneth cells have been shown to contain prominent. The function of the enterocyte differs lysozyme and secrete substances called defensins, depending on its location along the villus-crypt axis. which protect against infection. Those enterocytes deep within the intestinal glands Intraepithelial lymphocytes often are seen are relatively undifferentiated, whereas enterocytes between and within intestinal epithelial cells that that line the upper one-third of the intestinal glands cover villi or line intestinal glands (crypts of and cover the lower one-third of villi are involved Lieberkühn). The intraepithelial lymphocytes are T- primarily in secretion. Enterocytes covering the upper lymphocytes (about 80% of these are T-suppressor two-thirds of villi are involved primarily in cells, the remainder being T-helper lymphocytes). absorption, although not exclusively, as these enterocytes also are involved in secretion but to a Lamina Propria lesser extent. Thus, enterocyte function changes during its lifespan as it migrates along the villus- VOCABULARY: lymphatic nodule, Peyer's crypt axis to eventually be shed at the villus tip. patch, M-cell, central lacteal Chloride ion is moved into enterocytes active in secretion via a sodium, potassium-2 chloride The connective tissue of the lamina propria forms cotransport mechanism. These transmembrane the cores of the villi and fills the areas between proteins are located in the basolateral plasmalemma. intestinal glands. The intestinal lamina propria is Potassium protein channels and sodium, potassium rich in reticular fibers that form a delicate pumps also localized within the basolateral meshwork containing numerous reticular cells, plasmalemma participate in a cooperative way so lymphocytes, plasma cells, macrophages, and that intracellular chloride levels are maintained eosinophils, giving the lamina propria a very cellular sufficiently high to allow for passive chloride appearance. The lamina propria of the intestinal outflow through chloride channel proteins and tract represents a special type of lymphatic tissue CFTR (cystic fibrosis transmembrane regulator and is part of the gut-associated lymphatic tissue protein) chloride channels in the apical (GALT).

186 The free cells of the lamina propria provide a a similar function in monitoring the external protective sleeve that encircles the intestinal lumen environment and relaying information to the immediately beneath the epithelium. Lymphocytes immune system. Both are considered to be antigen- are the most numerous cells present and provide a presenting cells. vast reserve of immunocompetent cells. The Lymphatic vessels occur in the lamina propria and majority of immunocompetent cells in the diffuse are important in fat absorption. Dilated lymphatic lymphatic tissue of the gut lamina propria are T- channels begin blindly in the cores of villi near their lymphocytes (about 70% are T-helper cells and the tips and form the central lacteals. Their walls remaining 30% are T-suppressor cells). However, consist of thin endothelial cells surrounded by basal the lymphatic nodules and Peyer’s patches do lamina and reticular fibers. At the bases of the villi, contain both B- and T-lymphocytes. Plasma cells the lymphatic capillaries anastomose with those within the lamina propria elaborate coursing between intestinal glands and form a immunoglobulin A (IgA), which is taken up by the plexus before passing through the muscularis intestinal absorptive cells and complexed to a mucosae. In the submucosa they join larger protein (secretory piece) that is synthesized by the lymphatic vessels. Vessels that pierce the muscularis enterocyte. The complex is then released from the mucosae may contain valves. cell and provides a protective surface for the epithelial cells - an immunologic border - against Muscularis Mucosae viruses and bacteria. The secretory piece acts as a carrier for the IgA and possibly protects it from VOCABULARY: inner circular layer, outer lysosomal digestion while it is within the cell and longitudinal layer from enzymatic digestion on the luminal surface. Lymphocytes often leave the lamina propria to The muscularis mucosae is well defined and migrate through the intestinal epithelium into the consists of an inner circular layer and an outer lumen of the intestine. longitudinal layer of smooth muscle. It is Lymphatic nodules of variable size lie in the intimately associated with thin elastic fibers. Slips of lamina propria, scattered along the entire intestinal smooth muscle leave the muscularis mucosae and tract, but become larger and more numerous enter the cores of villi to provide the means by distally. They are particularly numerous and well which villi contract. Contraction of villi developed in the ileum and often extend the full continuously provides the intestinal epithelium thickness of the mucosa. The lymphatic nodules covering the villi with a fresh environment for may be solitary or grouped together into aggregates maximum absorptive efficiency. The pump-like called Peyer's patches. These oval structures may action during contraction also aids in moving be quite large - 20 mm in length - and are visible to absorbed materials into the blood and lymphatic the naked eye. Peyer's patches may occupy the full capillaries, out of vessels in the lamina propria, and depths of the mucosa and submucosa. into the larger vessels of the submucosa and thus A peculiar type of cell, the M-cell, lies in the further aids the absorptive process. intestinal epithelium that covers the lymphatic Small arteries run on the inner surface of the nodules. These cells show characteristic microplicae muscularis mucosae and break up into capillary on the luminal surface, rather than microvilli. M- networks that form a dense network of fenestrated cells sequester intact macromolecules from the capillaries just beneath the intestinal epithelium. intestinal lumen and transport them in membrane- Near the tips of villi, the capillaries drain into small bound vesicles to intraepithelial lymphocytes. The veins that run downward to anastomose with a sensitized lymphocytes then migrate to various venous plexus surrounding the intestinal glands, aggregates of lymphatic tissue in the lamina propria before joining veins in the submucosa. or mesenteric lymph nodes, transporting information to sites of antibody production. M-cells provide a way for the immune system to maintain immunologic surveillance of the environment in the gut lumen and to respond appropriately to any changes. M-cell monitoring of the intestinal lumen is not unique. Langerhans' cells of the epidermis perform

187 Submucosa Nerve fibers and parasympathetic ganglia form a submucosal plexus (Meissner’s plexus) found VOCABULARY: duodenal (Brunner's) gland, throughout the submucosa of the esophagus, branched tubular gland, epidermal growth stomach, and small and large intestines. This plexus factor, submucosal plexus provides motor innervation for the muscularis mucosae and the slips of smooth muscle that enter The submucosa consists of coarse collagenous villi or other regions of the mucosa and provide fibers and numerous elastic fibers. It often contains motility. Large blood and lymphatic vessels also are lobules of adipose tissue as well as the cells found in the submucosa. The large mesenteric normally associated with loose connective tissue. In vessels pierce the muscularis externa at its the ileum it may contain large aggregates of attachment to the mesentery and then enter the lymphoid cells that from lymphatic nodules (Peyer’s submucosa, branch, and run around the patches). The submucosa of the duodenum houses circumference of the intestinal tube. The the duodenal (Brunner's) glands, the ducts of submucosal vessels supply and receive small which pierce the muscularis mucosae and drain into tributaries from the overlying mucosa. overlying intestinal glands. These branched tubular glands are first encountered immediately Muscularis Externa distal to the pyloric sphincter and extend as far as the proximal jejunum. They often occur in the VOCABULARY: inner circular layer, outer submucosal cores of the plicae circulares. The longitudinal layer, myenteric plexus, interstitial duodenal glands are the only submucosal glands cells of Cajal present in the gastrointestinal tract and consist of secretory cells with ultrastructural features of serous The muscularis externa consists of inner circular and mucous cells. In addition to mucin and outer longitudinal layers of smooth muscle. glycoproteins and perhaps a small amount of Both layers take a helical course around the bicarbonate, Brunner’s glands have been shown to intestinal tube. The fibers in the outer layer form a elaborate several other factors. Epidermal growth more gradual helix, giving the impression of a factor is thought to play an important role during longitudinal arrangement. Between the two layers of the development of the alimentary canal and is smooth muscle are the nerve fibers and believed to be important in wound healing parasympathetic ganglia of the myenteric plexus. (cytoprotective effects) and liver regeneration. This extensive plexus is found throughout Epidermal growth factor is a peptide that is a alimentary canal and supplies the motor innervation potent inhibitor of gastric acid secretion and a to smooth muscle forming the muscularis externa peptide that may control proliferation and of the entire gastrointestinal tract. The number of migration of intestinal epithelial cells in the multipolar neurons within the myenteric plexus is proximal small intestine. Bactericidal factors such as substantial (in excess of 100 million neurons) and lysozyme (muramidase) and proteinase inhibitors may exceed the number of neurons located within have been demonstrated in Brunner’s glands and the spinal cord. Specialized cells known as the are thought to represent a protective mechanism interstitial cells of Cajal also are found for the overlying duodenal mucosa against the throughout the muscularis externa of the action of microbial and pancreatic proteolytic gastrointestinal tract and are essential for normal enzymes in the luminal environment of this region. gastrointestinal motility. The interstitial cells of A sulfated glycoprotein (Apolipoprotein J) also Cajal are divided into several subgroups of functions to protect the mucosal barrier cells from mesenchymal derived cells that are found in close surface-active components such as bile, in the association with elements of the enteric nervous luminal environment. Thus, incorporated within the system as well as smooth muscle cells. A mucus layer are factors that guard against the subpopulation of these cells is regarded as the degradation of the mucus layer and underlying source of spontaneous slow peristaltic waves within mucosa by pancreatic enzymes and other surface the gastrointestinal musculature and function as active agents associated with this region, as well as pacemaker cells. Others of this family of cells are factors that promote cellular proliferation and believed to be involved in the relaxation of smooth differentiation. muscle triggered by nitric oxide.

188 The observed gastrointestinal movements such as: The large intestine is continuous with the rectum gastric slow waves (function to mix stomach and anal canal, the latter terminating as the anus. contents), segmentation waves of small intestine (function in a mixing action), short and long range Mucosa peristaltic waves (function to move contents over distance) and haustration (contraction rings of VOCABULARY: lacks villi, absorptive cell colon) appear to be regulated by the interaction of (enterocytes), goblet cell, fluid absorption, these interstitial cells and the enteric nervous mucus secretion system. In addition to these movements, powerful contractions of the muscularis externa (migrating The mucosa of the large intestine is similar to that myoelectric contractions) also occur that move of the small intestine but lacks villi and has a contents of the stomach and small intestine rapidly smooth interior surface (Fig. 14-6). The lumen is lined into the colon. A similar mass movement of by a simple columnar epithelium that consists of contents occurs in the colon. intestinal absorptive cells (enterocytes) and goblet cells. The latter increase in number toward Serosa the rectum so that in the distal region of the colon the epithelium consists mainly of goblet cells. The muscularis externa is invested by a thin layer of Intestinal glands of the colon are longer and more loose connective tissue covered by a layer of closely packed than those in the small intestine, and mesothelial cells of the visceral peritoneum. Where the they increase in length distally to reach their mesentery attaches to the intestinal wall, the serosa maximum depth (0.7 mm) in the rectum. The becomes continuous with both sides of the glands contain numerous goblet cells, endocrine mesentery and encloses connective tissue elements, cells and in the basal half of the glands, proliferating blood vessels, and nerves. An adventitia is and undifferentiated epithelial cells. Paneth cells associated with the duodenum usually are absent. The turnover rate for intestinal epithelial cells in the colon is about 6 days. Ileocecal Junction

The lumen of the ileum becomes continuous with that of the large intestine at the ileocecal junction. Here the lining is thrown into anterior and posterior folds called ileocecal valves, which consist of both the mucosa and submucosa surrounded by a thickening of the inner circular layer of the muscularis externa. The ileocecal valves act to prevent contents of the ileum from passing into the cecum prematurely and also prevent a backflow of fecal contents from the colon into the ileum. The presence of gastrin acts to relax (dilate) the ileocecal valve.

Large Intestine

The large intestine is approximately 180 cm long and is divided into several regions. The cecum is continuous with the ileum at the ileocecal junction and forms a blind pouch at the proximal end of the colon. It bears the appendix, a small, slender diverticulum. The remainder of the colon is divided Fig. 14-6. A scanning electron micrograph of a region of isolated into ascending, transverse, descending and sigmoid colonic mucosa illustrating the intestinal glands and their openings on parts that, histologically, show the same features. the mucosal surface. Note that villi are absent. (X 120). Photomicrograph courtesy of J. E. Magney, University of Minnesota.

189 The composition of the lamina propria and Muscularis Externa muscularis mucosae is the same as that of the small intestine. Scattered lymphatic nodules are present VOCABULARY: taeniae coli, haustrum, plicae and may protrude through the muscularis mucosae semilunares to lie in the submucosa. Ingested material enters at the cecum as a The outer, longitudinal coat of the muscularis semifluid and becomes semisolid in the colon. externa of the cecum and colon differs from that of Although digestive enzymes are not secreted by the the small intestine in that it forms three longitudinal colon, some digestion does occur as a result of bands called the taeniae coli. Between the taeniae enzymes present in the developing fecal mass and the muscle coat is thinner and may be incomplete. through the action of bacterial flora normally The inner circular layer is complete and appears associated with the large intestine. However, the similar to that of the small intestine. Components majority of digestion and absorption of nutrients is of the myenteric plexus lie just external to the completed before the ingesta enters the colon. The circular layer, as in the small intestine. Due to the primary functions of the large intestine are fluid tonus of the taeniae coli, the wall of the colon is absorption (sodium, chloride and water) and gathered into outwardly bulging pockets, the mucus secretion to protect the mucosa from haustra. Crescentic folds called plicae abrasion by the developing feces. With semilunares project into the lumen of the colon approximately 9 liters of fluid entering the intestinal between the haustra. tract daily only about 100 ml are lost in the feces. The cellular mechanisms involved with enterocyte Serosa absorption and secretion in the colon are thought to be similar to those occurring in enterocytes of VOCABULARY: adventitia, appendices the small intestine. The secretory activities of epiploicae enterocytes in both regions of the gut appear to be influenced by the peptides guanylin and The serosa is incomplete in the colon, as the uroguanylin. ascending and descending portions of the colon are The latter, which also occurs in relatively high retroperitoneal. Here the muscular wall of the colon concentrations in the blood, has been suggested to is attached to adjacent structures by an adventitia. act as an intestinal natriuretic factor. It is well Where a serosa is present, it may contain large, established that when large amounts of salt are pendulous lobules of fat called appendices taken orally, a large and rapid increase in urinary salt epiploicae. excretion occurs. The same amount of salt given intravenously causes only small changes in urinary Appendix salt excretion. Aldosterone and the atrial natriuretic peptides do not appear to be involved in this The structure of the appendix resembles that of the phenomenon, suggesting the presence of intestinal colon, but in miniature. The lumen is small and natriuretic factors such as uroguanylin. irregular in outline, and taeniae coli are absent. The

lamina propria is extensively infiltrated with Submucosa lymphocytes, and details of the mucosa often are obscured by the many lymphatic nodules that may The submucosa of the colon is similar to that of the fill the mucosa and submucosa. small intestine and contains the larger blood vessels and the submucosal nerve plexus. Glands are not present. As in the remainder of the digestive tube, blood vessels pierce the muscularis externa and course around the circumference of the intestinal wall. The large vessels supply tributaries that penetrate the muscularis mucosae and form capillary networks in the lamina propria, around intestinal glands, and beneath the lining epithelium. The veins follow the same course as the arteries.

190 Rectoanal Junction muscle fibers circumscribe the distal anal canal as it passes through the pelvic diaphragm and form the VOCABULARY: rectal valve, pectinate line, external anal sphincter, which is under voluntary circumanal gland, rectal column, anal valve, control. hemorrhoidal plexus, internal anal sphincter, external anal sphincter Gastrointestinal mucus

The rectum is a relatively straight continuation of The secretions of the gastric surface lining the colon about 12 cm in length. Three internal epithelium, Brunner’s glands and goblet cells transverse rectal valves (of Houston) occur in the contribute to a layer of mucus that forms a slippery, distal rectum. Infoldings of the submucosa and the viscoelastic gel that lubricates the mucosal lining of inner circular layer of the muscularis externa form the gastrointestinal tract. This layer consists of a these permanent sickle-shaped structures. The mixture of glycoproteins, phospholipids, sloughed valves function in the separation of flatus from the cells, various cellular and serum macromolecules, developing fecal mass. The mucosa of the first part electrolytes and water. The unique capacity of of the rectum is similar to that of the colon except gastrointestinal mucus to protect delicate underlying that the intestinal glands are slightly longer and the epithelial surfaces is due primarily to the gel- lining epithelium is composed primarily of goblet forming properties of its glycoprotein molecules cells. The distal 2 to 3 cm of the rectum forms the referred to as mucins. anal canal, which ends at the anus. Immediately Gastrointestinal mucus protects underlying proximal to the pectinate line, the intestinal glands mucosal epithelial cells by maintaining a favorable become shorter and then disappear. At the pH gradient and preventing autodigestion from the pectinate line, the simple columnar intestinal lumen. This adhesive gel-like material is replenished epithelium makes an abrupt transition to continuously by mucus secreting cells distributed noncornified stratified squamous epithelium. After along the length of the gastrointestinal tract. It is a short transition, the noncornified stratified the hydrophilic and viscoelastic properties of the squamous epithelium becomes continuous with the mucin glycoprotein molecules together with their keratinized stratified squamous epithelium of the ability to adhere to particulate matter (nondigested skin at the level of the external anal sphincter. substances, sloughed cells, inert particles) and Beneath the epithelium of this region are simple remove it from the gut lumen by fecal flow, but tubular apocrine sweat glands, the circumanal without damaging delicate mucosal surfaces, that glands. makes these molecules so unique. The mucus layer Proximal to the pectinate line, the mucosa of the is viscous enough to flow like a fluid under mild anal canal forms large longitudinal folds called shearing forces yet sufficiently elastic to recover rectal columns (of Morgagni). The distal ends of from deformation stress and maintain stability of the rectal columns are united by transverse mucosal structure. The mucus layer is porous enough to folds, the anal valves. The recess above each valve allow for the rapid diffusion of nutrients and forms a small anal sinus. It is at the level of the anal secretion, back and forth between underlying valves that the muscularis mucosae becomes epithelial cells and the lumen. Mucins, because of discontinuous and then disappears. their highly variable elongated branching The submucosa of the anal canal contains oligosaccharide side chains are thought to be numerous veins that form a large hemorrhoidal arranged in loose, random coils to produce an plexus. When distended (varicosed), these vessels amorphous network. The gaps between groups of protrude into the overlying mucosa and form these molecules are filled with water, ions, serum internal hemorrhoids (piles). The inner circular layer proteins, lipids, enzymes and other factors. The vast of the muscularis externa increases in thickness and diversity of carbohydrate units associated with these ends as the internal anal sphincter. In the distal large mucin molecules provides an extraordinary rectum the taeniae coli come together to invest the number of potential recognition sites for rectum as a complete outer longitudinal layer. This pathogenic and commensal organisms in the distal thin outer longitudinal muscular layer breaks up and gastrointestinal tract. ends by blending with the surrounding connective tissue and muscle of the pelvic diaphragm. Skeletal

191 Some mucin binding sites compete with those of Structural Organization microorganisms for binding sites of underlying epithelial cells, thus interfering with the access of VOCABULARY: capsule, hepatocyte, sinusoid, microorganisms to the mucosal surface and central vein, hepatic lobule, portal area, portal contribute to their removal. vein, hepatic artery, bile duct, lymphatic The ability of the mucin glycoproteins forming the channel, liver acinus, portal lobule, subcapsular mucus layer to resist luminal and bacterial enzymes limiting plate that destroy mucosal surfaces is thought to be due to the structural organization of the mucin The liver is invested by a delicate connective tissue molecules themselves. The protein core of the capsule that is continuous with the peritoneum. glycoprotein molecules is protected by a The capsule contains numerous elastic fibers and is surrounding sleeve of oligosaccharide chains. covered by a mesothelium except for a small bare area where the liver abuts the diaphragm. Liver The liver is composed of epithelial cells, the hepatocytes, arranged in branching and The liver is the largest gland in the body weighing anastomosing plates separated by blood sinusoids. about 1.4 kg in an average adult. It acts as both an Both form a radial pattern about a central vein that endocrine and exocrine gland, releasing several is the smallest tributary of the hepatic vein. The spokelike substances directly into the bloodstream and arrangement of hepatic plates about a central vein secreting bile into a duct system. An important constitutes the basis of the classic hepatic lobule, function of the liver is to conjugate lipid soluble which appears somewhat hexagonal in cross molecules (steroids, bilirubin, some drugs) with SO3 section, with a central vein at the center and portal or glucuronide which renders them water soluble areas at the corners. The liver consists of about 1 for release into the bile and ultimate elimination million such units. through the alimentary canal. A portal area contains a branch of the portal The liver is uniquely situated with respect to the vein, a branch of the hepatic artery, a bile duct, venous blood flow from the gastrointestinal tract. and a lymphatic channel. All are enclosed in a Blood from the portal vein, which drains the entire common investment of connective tissue. Blood gastrointestinal tract and spleen, passes through the passes from small branches of the hepatic artery substance of the liver before entering the systemic and portal vein into the sinusoids that lie between circulation. The smooth muscle in the wall of the plates of hepatocytes. Blood flows slowly through the portal vein is unique in comparison to other regions sinusoids toward the center of the lobule and exits of the vasculature in that it is phasic smooth muscle. through the central vein. Branches of the hepatic Phasic smooth muscle has the capacity to generate artery carry oxygenated blood and provide about action potentials and this wave of activity results in 20% of the blood flow within hepatic sinusoids. In rhythmic contractions that aid in moving blood to contrast, branches of the portal vein carry nutrient the liver. The liver therefore receives all of the rich blood from the gastrointestinal tract and materials absorbed by the gastrointestinal tract contribute the remaining 80% of the sinusoidal except some lipids, most of which are carried by blood flow. Hepatocytes nearest the branches of lymphatics to enter the general circulation. Blood the portal vein and hepatic artery - that is, at the from the spleen, the primary organ in which aged periphery of the lobule – receive blood with the erythrocytes are removed from the blood and highest nutrient and oxygen content. Both diminish broken down, carries the breakdown products of as blood flows toward the central vein. Due to this hemoglobin to the liver. Tributaries of the portal arrangement, three zones can be recognized in a vein empty into hepatic sinusoids that drain into hepatic lobule according to the metabolic activity: a hepatic veins. The arrangement in the liver of a set zone of permanent function (zone 1) at the periphery, a of sinusoids between veins is called the hepatic portal zone of intermittent activity (zone 2) near the center of system. The liver also has an arterial supply from the the lobule, and a zone of permanent repose (zone 3) near hepatic artery, and both the venous (portal) and the central vein. The boundaries of the hepatic arterial (hepatic) bloods percolate through the liver lobule can be estimated by observing the central sinusoids and exit by way of the hepatic vein. vein and noting the portal areas at the corners of the lobule (Fig. 14-7).

192 Also related to the blood supply is a smaller unit Hepatic Sinusoids of liver structure called the liver acinus or functional unit. The liver acinus is defined as the hepatic VOCABULARY: discontinuous endothelium, tissue supplied by a terminal branch of the hepatic fenestration, perisinusoidal space, hepatic artery and portal vein and drained by a terminal macrophage, Kupffer cell, lipocyte branch of the bile duct. It is represented by a diamond-shaped area with central veins at two of Hepatic sinusoids are larger and more irregular in the opposite comers (Fig. 14-7). Branches of the shape than ordinary capillaries. The sinusoidal lining blood vessels and a branch of the bile duct lie consists of a simple layer of squamous epithelium between the two portions of adjacent hepatic supported by very little connective tissue. Three lobules that they supply. types of cells are associated with the sinusoidal An additional lobule is related to the exocrine lining: endothelial cells, stellate cells (Kupffer cells secretion (of bile) in the liver. This unit is the portal or hepatic macrophages), and fat-storing cells lobule and has a portal canal at its center. It (lipocytes). consists of the hepatic tissue that is drained by the Endothelial cells constitute the major cellular bile duct of the portal area. A portal lobule is element of the sinusoidal lining and form a triangular in shape and contains parts of three discontinuous endothelium. There is no basal adjacent hepatic lobules (Fig. 14-7). A central vein lamina, and the cells are separated by gaps 0.2 to 0.5 is located at each corner of this unit. The bile µm wide. The endothelial cells also show numerous produced by hepatocytes flows in a direction opposite intracellular fenestrations or pores. The sinusoidal that of blood toward the periphery of the classic lining is separated from the liver cells by a narrow hepatic lobule. perisinusoidal space (of Disse) (Fig. 14-8).

Fig. 14-7. A diagrammatic sketch illustrating the lobules of the Fig. 14-8. A scanning electron micrograph demonstrating a portion liver. of a hepatic sinusoid. Note the surrounding hepatocytes and the perisinusoidal space of Disse. (X 6,000). Photomicrograph courtesy A single layer of small, dark hepatocytes limits the of G.H. Haggis, Ottawa, Canada. liver parenchyma beneath the capsule and is called the subcapsular limiting plate. A similar wall of Blood plasma flows freely through the endothelium hepatic cells surrounds the portal areas and forms and into the space, but the sinusoidal lining does the periportal limiting plate, which is pierced by hold back erythrocytes. Although occasional tributaries of the hepatic artery, portal vein, bundles of reticular fibers and fine collagenous lymphatic vessels, and bile ductules. The limiting fibers are present in the perisinusoidal space, there plates of hepatocytes prevent blood from escaping is no ground substance, and the flow of blood the classic hepatic lobules. plasma is unhindered. Because the plasma has direct access to the perisinusoidal space, the liver cells are constantly bathed on one surface by fluid that is rich in the nutrients absorbed by the intestinal tract.

193 Thus, the perisinusoidal space has considerable supported by a delicate stroma that consists significance in the exchange of materials between primarily of reticular fibers. the liver and plasma. The plasmalemma of the The nuclei of hepatocytes are large and round and hepatocytes that face the perisinusoidal space bear usually occupy the center of the cell. A single numerous well-developed microvilli that project nucleus usually is present, but as many as 25% of into the space and greatly increase the surface area hepatocytes are binucleate. There also is and facilitate absorption. considerable variation in the size of nuclei from cell Not all the formed tissue fluid is absorbed by the to cell, reflecting the polyploid nature of hepatocytes or passes back into the sinusoidal hepatocytes. lumen and as a result a considerable amount of The cytoplasm of hepatocytes is rich in organelles- lymph is produced by the liver. The liver produces large arrays of granular endoplasmic reticulum, a about half of the lymph found within the thoracic duct. Like moderate amount of smooth endoplasmic bile flow, lymph flows within the perisinusoidal space reticulum, mitochondria, Golgi complexes, from the center of the hepatic lobule toward the peroxisomes, and lysosomes. Inclusions such as periphery. Lymphatic vessels within the portal areas glycogen and lipid also are common. Glycogen course parallel to the branches of the portal vein often appears as dense rosettes (alpha particles) and ultimately empty into the thoracic duct. made up of smaller beta particles. The alpha Also present in the sinusoidal lining are actively particles measure 20 to 30 nm in diameter. The phagocytic cells, variously called hepatic cytoplasm is variable in appearance and changes macrophages or Kupffer cells. These form part with the nutritive state of the organ. of the sinusoidal lining and are irregularly shaped Tiny channels, the bile canaliculi, course through cells that expose the greater part of their cytoplasm the parenchyma between hepatocytes to end in the to the blood in the sinusoid and extend processes bile ducts of the portal areas. Bile canaliculi between the endothelial cells. represent an expansion of the intercellular space, and Unlike the neighboring endothelial cells, the their walls are formed by the adjacent plasmalemma cytoplasm of the hepatic phagocytes contains of two neighboring hepatocytes. Short microvilli vacuoles, lysosomes, Golgi bodies, and short extend from the cell membrane into the lumen. At profiles of granular endoplasmic reticulum. the margins of the canaliculus, the plasma Phagocytized material also may be present. These membranes are joined by occluding junctions similar to cells are part of the mononuclear system of the zonula occludens of other epithelia. They form macrophages and arise from monocytes of the bone an occluding seal to prevent bile from escaping into marrow. the intercellular spaces between hepatocytes. Golgi The third type of cell is located on the side of the complexes of hepatocytes often lie adjacent to the sinusoidal lining that faces the perisinusoidal space. canaliculi. The bile canaliculi show intermittent This cell accumulates lipid and is most numerous in contractions. the peripheral and intermediate zones of the hepatic lobule. They have been called lipocytes, fat-storing, Hepatocyte Function stellate, or interstitial cells, and function to store vitamin A. Hepatocytes are extraordinarily metabolically active cells that perform a wide variety of functions. Hepatocytes Hepatocytes are involved in the conversion of ammonium to urea, can perform gluconeogenesis VOCABULARY: polyploid, bile canaliculus (conversion of lipids and amino acids into glucose), synthesize cholesterol, as well as take up The parenchyma of the liver consists of large immunoglobulin A (IgA) and release it as a polyhedral hepatocytes arranged in plates that secretory from of IgA into the bile and ultimately radiate from the region of the central vein. The into the lumen of the small intestine. Hepatocytes surfaces of an individual hepatocyte either contact play a major role in the metabolism and storage of an adjacent liver cell or border on a perisinusoidal carbohydrates, fats, and proteins. space. This latter surface bears numerous well- A primary function of hepatocytes is to store developed microvilli. The plates of hepatocytes are carbohydrate in the form of glycogen.

194 Hepatic glycogen functions to maintain normal Amino acids are deaminated in the liver to blood glucose levels (70-100 mg/100 ml). During a produce urea which is excreted by the kidney. The high-carbohydrate meal as the blood glucose liver also has functions in protein metabolism; concentration begins to rise, a rapid pulse of insulin synthesis of fibrinogen, prothrombin, and albumin; secretion occurs that results in an increase in the and storage of several vitamins (primarily A, D, B2, insulin glucagon ratio. The rise in insulin B3, B4 and B12). concentration suppresses hepatic glucose Enzymes within hepatic peroxisomes have the production and increases its uptake by insulin- ability to metabolize alcohol. Enzymes associated dependent tissues. It is thought that a glucokinase, with the smooth endoplasmic reticulum, on the translocated from the nucleus to the cytosol during other hand, are involved in the deactivation of high glucose concentration catalyzes the some hormones, lipid soluble drugs, and toxins phosphorylation of glucose and promotes glucose which is another important function of hepatocytes. entry into hepatocytes. Insulin regulates glucokinase Detoxified materials are excreted by hepatocytes activity by both transcription and translocation in into the bile and conducted by the biliary duct the hepatocytes. The storage form of glucose system to the intestinal tract for elimination. (glycogen) consists of glucose molecules linked by α Bile is a complex exocrine alkaline secretion of the 1, 4 glycosidic bonds. Glycogen synthase catalyzes the liver and contains ions, water, bicarbonate, bile synthesis of glycogen. acids (taurocholic and glycocholic), bile pigment Glycogen breakdown occurs during fasting and (bilirubin glucuronide), phospholipids, cholesterol, exercise that results in a decrease in the insulin lecithin, and neutral fats. Bile acids/salts act as glucagon ratio, and secretion of epinephrin. The emulsifying agents and are important in the latter binds to α- and β-adrenergic receptors on the breakdown of fat to micelles in the intestinal lumen hepatocytes. Liver glycogen is degraded by a hepatic during digestion. Most bile acids (about 80%) are glycogen phosphorylase to glucose-6-phosphate reabsorbed in the ileum to be secreted once again which in turn is catalyzed to free glucose by the into the bile. This reuse of bile acids is referred to enzyme, glucose-6-phosphatase, localized within the as the enterohepatic circulatory system. cisternae of the endoplasmic reticulum. Glucose-6- Bilirubin is produced in the spleen and liver as a phosphate is transported into the lumen of result of the breakdown of the heme component of endoplasmic reticulum by a specific glucose-6- hemoglobin in damaged or old erythrocytes by phosphate transporter protein where it is macrophages at these locations. The bilirubin taken hydrolyzed to glucose and phosphate by glucose-6- up by hepatocytes is then conjugated with phosphatase. Glucose is transported out of the glucuronic acid to form bilirubin glucuronide which endoplasmic reticulum and across the plasmalemma is excreted into the bile. Bacterial action on this of the hepatocyte by glucose transport proteins. molecule after its entry into the intestinal tract Hepatocytes also play a vital role in maintaining converts it to urobilinogen. Of the urobilinogen blood lipid levels; by the uptake of fatty acids and formed, some is lost in the feces, some is absorbed the esterification of fatty acids to triglycerides which and returned to the liver, and some is excreted by occurs within the smooth endoplasmic reticulum. the kidneys. The triglycerides are then complexed to proteins within the Golgi complexes of hepatocytes to form Bile Ducts a variety of lipoproteins. Hepatocytes synthesize high-density lipoproteins (HDL) molecules which facilitate VOCABULARY: bile ductule, interlobular bile the flow of excess cholesterol and triacylglycerides duct, extrahepatic duct, common hepatic duct, in the plasma back to the liver for metabolism. common bile duct Hepatocytes also produce very-low-density lipoproteins (VLDL) that are rich in triacylglycerides that travel Bile canaliculi unite with bile ducts in the portal to skeletal muscle and fat cells where the canals via small, interconnecting channels called triacylglycerides are hydrolyzed by the enzyme bile ductules. They are small and have thin walls, lipoprotein lipase to fatty acids. Low-density lipoprotein and their small lumina are surrounded by a low (LDL) molecules on the other hand are rich in cuboidal epithelium that rests on a distinct basal cholesterol and carry cholesterol to cells throughout lamina. The terminal ductules pass through the the body that have LDL receptors. periportal limiting plate and empty into interlobular

195 bile ducts of the portal areas. The lumina of the Gallbladder bile ducts increase in diameter as they course toward the exterior, and the lining epithelium VOCABULARY: cystic duct, mucous increases in height. Interlobular ducts unite to form membrane, simple columnar epithelium, the extrahepatic ducts, in which the surrounding lamina propria, muscularis, adventitia, serosa layers of connective tissue become thicker and the lining epithelium becomes tall columnar. Two large The gallbladder is a saclike structure on the inferior extrahepatic ducts, the left and right hepatic ducts, surface of the liver, measuring about 4 cm wide and exit the lobes of the liver and unite to form the 8 cm long. It is joined to the common hepatic duct major excretory duct of the liver, the common by the cystic duct, whose mucous membrane hepatic duct. It is joined by the cystic duct from forms prominent spiraling folds that contain the gallbladder to form the common bile duct bundles of smooth muscle. These folds make up (ductus choledochus), which empties into the the spiral valve that prevents the collapse or duodenum. distention of the cystic duct during sudden changes The major extrahepatic ducts are lined by a tall in pressure. The wall of the gallbladder consists of a columnar, mucus-secreting epithelium. The mucous membrane, a muscularis, and a serosa or remainder of the wall consists of a thick layer of adventitia. connective tissue that is rich in elastic fibers and The mucous membrane of the gallbladder wall often contains numerous lymphocytes and consists of a simple columnar epithelium and an occasional migrating granulocytes. Bundles of underlying lamina propria. The oval nuclei are smooth muscle cells, running in longitudinal and located basally in the cells and the luminal surfaces oblique directions, are present in the common bile show numerous short microvilli. The apices of duct and form an incomplete layer that spirals adjacent cells are joined, near the lumen, by typical about the lumen. Near the wall of the duodenum, zonula occludens junctions. The epithelium rests on the smooth muscle forms a complete investment a thin basal lamina that separates it from the and thickens to form a small sphincter, the sphincter delicate connective tissue of the lamina propria, choledochus. Distal to this region, the common bile which contains numerous small blood vessels. duct and the major pancreatic duct merge as they Occasional glands are found in the lamina propria, pass through the intestinal wall and empty through especially where the gallbladder joins the cystic a common structure, the hepatopancreatic ampulla. As duct. These small, simple tubuloacinar glands are the ducts pierce the duodenal wall, they are thought to secrete mucus. The mucosa of the non- surrounded by a common sphincter of smooth distended gallbladder forms large irregular folds muscle. called rugae, which flatten out as the gallbladder fills Bile is produced continuously by the liver and with bile. ultimately leaves the organ through the extrahepatic There is no submucosa in the gallbladder. The duct system. Hepatocytes produce between 500-900 muscularis consists of interlacing bundles of ml of bile daily. Resistance at the sphincters forces smooth muscle that spiral around the lumen of the bile to enter the cystic duct and pass into the gallbladder. The smooth muscle cells contain gallbladder, where it is stored. Bile formation occurs numerous receptors for cholecystokinin. Gaps between primarily at two anatomic sites: bile canaliculi and the smooth muscle bundles are filled with bile ductules. Secretin increases secretion of collagenous, reticular, and elastic fibers. Because of ductular bile, which is rich in bicarbonate. the musculoelastic wall and the rugae, the Glucagon increases canalicular bile secretion. gallbladder has considerable capacity for distention. The surrounding fibroconnective tissue of the adventitia is fairly dense and is continuous with the connective tissue of the liver capsule. The free surface of the gallbladder (that exposed to the abdominal cavity) is covered by a mesothelium, forming a serosa. The gallbladder stores and concentrates bile, which is elaborated continuously by the liver.

196 In humans the gallbladder may contain 30 to 50 ml perinuclear cytoplasm is filled with granular of bile. On stimulation by cholecystokinin, the endoplasmic reticulum and mitochondria. gallbladder wall contracts and the sphincters of the Extensive Golgi complexes occupy the common bile duct and ampulla relax, allowing bile supranuclear region and are associated with forming to be released into the duodenum. zymogen granules. Mature zymogen granules are large, spherical, homogeneous structures that are Pancreas limited by a membrane and often fill the apical cytoplasm. The pancreatic acinar cell, as indicated The pancreas is the second largest gland associated by its morphology, is actively involved in the with the gastrointestinal tract. In humans it is a synthesis and release of proteins (enzymes). retroperitoneal organ, 20 to 25 cm long, extending Digestive enzymes of the pancreas are transversely from the duodenum across the synthesized in the granular endoplasmic reticulum posterior abdominal wall to the spleen. of the acinar cell and pass through the cisternae of Macroscopically it consists of a head that lies in the this organelle to reach the Golgi complex in small C-shaped curve of the duodenum, a slightly transport vesicles. Precursors of the enzymes are constricted neck, and a body that forms the bulk of packaged into zymogen granules by membranes of the pancreas. It lacks a definite capsule but is the Golgi complex. The granules then are liberated covered by a thin layer of areolar connective tissue into the apical cytoplasm, where they accumulate. that extends delicate septa into the substance of the When the enzymes are to be released from the cell, pancreas and subdivides it into numerous small the granules migrate to the apical cell membrane lobules. Blood vessels, nerves, lymphatics, and and discharge their contents into the acinar lumen excretory ducts course through the septa. The by exocytosis. The contents of the granules also major excretory duct runs the length of the may be discharged into fine secretory canaliculi pancreas, collecting side branches along its course. that lie between adjacent acinar cells and are The major pancreatic duct and its branches have continuous with the acinar lumen. Pancreatic acinar the general appearance of a fish backbone and gives cells secrete elastase, amylases, endopeptidases some internal structural support to this gland. (lipases, trypsin, and chymotrypsin) that cleave The pancreas consists of an exocrine portion, central peptide bonds, and exopeptidases which elaborates numerous digestive enzymes and (carboxypeptidases A and B) that cleave terminal bicarbonate, and an endocrine portion, whose peptide bonds. Together these proteolytic enzymes secretions are important in carbohydrate cleave proteins in the intestinal lumen into peptide metabolism. Unlike the liver, the exocrine and fragments that then are reduced to amino acids endocrine functions of the pancreas are performed prior to absorption. by different groups of cells. Deoxyribonuclease and ribonuclease also are produced by pancreatic acinar cells and digest Exocrine Pancreas deoxyribonucleoprotein and ribonucleoprotein, respectively. Pancreatic amylase breaks down starch VOCABULARY: compound tubuloacinar and glycogen to disaccharides, and pancreatic lipase gland, pyramidal cell, zymogen granule, cleaves fat micelles into fatty acid and glycerol. digestive enzyme, secretory canaliculus Duct System The exocrine pancreas is a large, lobulated, compound tubuloacinar gland in which the VOCABULARY: centroacinar cell, intercalated secretory units are tubular or flask-shaped. A duct, intralobular duct, interlobular duct, delicate network of reticular fibers surrounds each secretin, cholecystokinin, enterokinase secretory unit and forms the supporting stroma. Each acinus consists of a single layer of large An extensive duct system permeates the pancreas. pyramidal cells whose narrow apices border on a At their beginnings, the ducts extend into the acini lumen, while their broad bases lie on a thin and are interposed between the acinar cells and the basement membrane. A single, spherical nucleus is lumen. Ductal cells within the pancreatic acini are located near the base of the acinar cell, and one or called centroacinar cells and appear as flattened, more nucleoli may be present. The basal and light-staining cells with few organelles.

197 The wall of the short initial segment of the duct secreted as inactive precursors (zymogens) that, in system formed by centroacinar cells is continuous part, are converted to their active forms by outside the secretory unit with intercalated and enterokinase, an enzyme from the intestinal intralobular ducts. These ducts are tributaries of mucosa. Pancreatic amylase and lipase are said to be the interlobular ducts found in the loose secreted in their active forms. Cholecystokinin also connective tissue between lobules; the transition stimulates the gallbladder to contract, thereby adding between ducts is gradual. The epithelial lining bile to aid in neutralizing the intestinal contents and begins as simple squamous in the intercalated ducts, providing bile acids that act as emulsifying agents in increases in depth to cuboidal in the intralobular the breakdown of neutral fats. ducts, and is columnar in the interlobular ducts. Scattered goblet cells and endocrine cells also are Endocrine Pancreas found in the ductal system. The cells lining the ducts stain lightly, and organelles are not prominent VOCABULARY: pancreatic islet, alpha cell, in their cytoplasm. Desmosomes are scattered along glucagon, beta cell, insulin, delta cell, the lateral cell surfaces, and the apices of adjoining somatostatin, pancreatic polypeptide, PP cell, cells are united by tight junctions. The apical exocytosis, fenestration surfaces bear microvilli. A scanty connective tissue consisting mainly of Scattered among the pancreatic acini are irregular, reticular fibers supports the intercalated and elongate masses of pale-staining cells called the intralobular ducts. The interlobular and major pancreatic islets (of Langerhans), which make up secretory ducts are contained within the interlobular the endocrine portion of the pancreas. In humans septae and thus are surrounded by a considerable they number about 1 million. The islets are amount of fibroconnective tissue. The interlobular separated from the surrounding exocrine pancreas ducts drain into the primary and accessory by a delicate investment of reticular fibers, and as pancreatic ducts. with all endocrine glands, the islets have a rich The primary duct runs the length of the pancreas, vascular supply. In ordinary tissue sections, the increasing in size near the duodenum, where it runs islets appear to be composed of a homogeneous parallel to the common bile duct, with which it population of pale polygonal cells, but with special often shares a common opening at the greater stains and in electron micrographs, several distinct duodenal papilla. The openings of both ducts are cell types can be identified (Table 15-3). The controlled by the sphincter of the papilla. An accessory majority of cells are alpha and beta cells. duct may lie proximal to the main duct and opens Alpha cells make up about 20% of the islets and independently into the duodenal lumen. generally are located at the periphery of the islet. Secretion by the exocrine pancreas is under They contain large, dense, spherical granules that nervous and hormonal control. Two polypeptide are limited by a membrane. Mitochondria are long hormones, secretin and cholecystokinin, are and slender with a typical internal structure. A few released from cells of the intestinal mucosa and profiles of granular endoplasmic reticulum are influence exocrine pancreatic secretion. Secretin, scattered about the cytoplasm, and Golgi bodies elaborated by S cells in the mucosa of the usually are found near the nucleus. They contain duodenum and proximal jejunum, stimulates the secretory granules with diameters of about 250 nm. ductal system of the pancreas to secrete a large volume Alpha cells produce the peptide hormone of fluid that is rich in bicarbonate. In the intestinal glucagon. Glucagon is released in response to lumen, this alkaline secretion neutralizes the acid hypoglycemia and acts on the liver to convert chyme from the stomach and deactivates the gastric glycogen into glucose, stimulates hepatic enzyme pepsin, whose optimal activity occurs at a gluconeogenesis from amino acids, and stimulates low pH (2.0). It also establishes the neutral to adipose tissue to release fatty acids that are in turn alkaline conditions needed for the optimal activity metabolized by the liver to produce keto acids. The of pancreatic enzymes. net effect of glucagon secretion is that blood levels Cholecystokinin, elaborated by I cells in the of glucose, free fatty acids and ketones increase. duodenal and jejunal mucosa, stimulates secretory units Thus, glucagon functions as a major fuel mobilizing of the pancreas to synthesize and release pancreatic hormone. (digestive) enzymes. The proteolytic enzymes are

198 Beta cells form about 78% of the islet cells and Arterioles derived from these arteries give rise to tend to locate near the center of the islet. Their capillaries at the periphery of the islets, in the region secretory granules are smaller than those of alpha of the alpha, delta, and PP-cells. The capillaries then cells. Beta cell granules contain small, dense crystals run centrally within the islet, after which they that give them a distinctive appearance. The supply adjacent acinar cells of the exocrine granular endoplasmic reticulum is less extensive and pancreas. Thus, the pancreatic islet cells can interact the Golgi complexes are more distinctive than in with each other and influence the function of the alpha cells. Beta cells produce insulin, released in exocrine pancreas as well. response to hyperglycemia, which acts on the plasmalemma of various cell types, especially liver, Table 15-3. Endocrine cell types of the pancreas. muscle, and fat cells, to facilitate the entry of Cell type Hormonal Primary function glucose into the cell and thus lower blood glucose. substance Insulin also stimulates fat synthesis, inhibits A (alpha) cell Glucagon Increases blood sugar lipolysis, and stimulates the movement of amino acids into cells and their assembly into proteins. B (beta) cell Insulin Decreases blood sugar The net effect of insulin is opposite that of glucagon. Insulin acts as a fuel storage hormone and D cell Somatostatin Inhibitor of hormone secretion (paracrine decreases blood levels of glucose, free fatty acids, and function)* ketones. Several other types of endocrine cells are present PP cell Pancreatic Opposes action of in small numbers in the islets. Delta cells secrete polypeptide cholecystokinin the hormone somatostatin, which inhibits hormone secretion by adjacent alpha and beta G cell Ψ Gastrin Increases HCL secretion in stomach endocrine cells within the pancreatic islet. Pancreatic polypeptide, secreted by PP cells, inhibits the pancreatic acinar secretion of enzymes *Paracrine function. Secretion of somatostatin is and bicarbonate secretion by the ductal system. directly into the intercellular space from which it Endocrine cells of the kind found in the islets, diffuses to inhibit the secretion of adjacent endocrine including alpha and beta cells, also are scattered cells. ΨG cells are present only in some species or within the ducts and acini of the exocrine pancreas. restricted to fetal life in others. The number of different endocrine cells may vary according to their locations in the head or body of Organogenesis the pancreas and may relate to the different origins of these two parts. There is considerable species When the three germ layers have formed, the variation in the distribution of cells within the islets. embryo appears as flattened disk with the yolk sac In humans, beta cells tend to be located centrally, extending from its ventral surface. This surface of while alpha, delta, and PP cells are concentrated at the embryo and the interior of the yolk sac are the periphery of the islets. covered by endoderm that forms the lining for the Islet cells are intimately associated with entire digestive tract. With further development, the surrounding capillaries, and the secretory granules embryo and gastrointestinal endoderm fold to form often appear to be located near the cell membrane a cylinder. Folding begins at the ends of the that is adjacent to the vasculature. Secretory embryo, creating two blind endodermal tubes, both granules are released from islet cells by exocytosis of which are continuous with the lumen of the yolk into the surrounding blood vessels. The sac at an area called the intestinal portal. The endothelium of the islet capillaries contains proximal endodermal tube forms the foregut, which numerous fenestrations to facilitate entry of gives rise to the oral cavity, pharynx, stomach, and secretory products into the vasculature. In contrast, proximal duodenum. The midgut communicates the endothelium of capillaries of the exocrine with the yolk sac, and from it develops the jejunum, pancreas is not fenestrated. ileum, cecum, appendix, ascending colon, and part A significant proportion of the arterial blood of the transverse colon. The caudal tube becomes enters the pancreas via small interlobular and the hindgut, which differentiates into the rest of the intralobular arteries that first supply the islets. large intestine.

199 Oral Cavity Teeth have a dual origin: enamel arises from ectoderm; dentin, pulp, and cementum arise from The lips begin as a thickened band, the labial lamina, mesoderm. Tooth development begins with the that grows from the ectoderm over the primitive appearance of the dental lamina, a plate of epithelium jaw, into the underlying mesenchyme. The labial on which knoblike swellings (enamel organs) appear lamina thickens, and as the central cells degenerate, at intervals. These give rise to enamel and also act splits to form the lips and gums. The cheeks arise as molds for tooth development. The enamel by fusion of the upper and lower lips at their lateral organs grow into the mesenchyme to form inverted, angles, thus reducing the original broad mouth cuplike structures invaginated at the bases by dental opening. Skeletal muscle of the lip and cheeks arise papillae. The latter are condensations of from mesenchyme that migrates from the second mesenchyme that give rise to the dentin and pulp of branchial arch to lie between mucosal and teeth. The enamel organ forms a double-walled sac, of epidermal coverings. which the outer and inner walls differentiate into The tongue arises from the ventral ends of the outer and inner enamel layers, respectively. Loosely branchial arches. An ectoderm-covered oral part arranged cells lie between the two layers and form from the mandibular arches forms the body of the the enamel pulp. Cells of the inner layer become tongue: an endodermal pharyngeal part from the ameloblasts and produce enamel prisms, beginning at the second branchial arch gives rise to the rest of the interface with the dental papillae. Mesenchymal tongue. The junction between ectoderm and cells of the papillae next to the inner dental layer endoderm is just anterior to the sulcus terminalis. enlarge and differentiate into odontoblasts. These Thus, fungiform and filiform papillae are form a single layer of columnar cells whose apices ectodermal in origin, and the circumvallate papillae face the ameloblasts. Odontoblasts produce predentin are endodermal in origin. At first the tongue is that soon calcifies to become definitive dentin. covered by cuboidal epithelium, which later Mesenchyme central to the odontoblasts forms the becomes stratified squamous. dental pulp. All papillae initially form as elevations of the Dentin is the first of the hard tissues to appear in mucosa. A thickened epithelial ring surrounds each the tooth. Thereafter, both enamel and dentin are circumvallate papilla, grows into the mesenchyme, laid down, beginning at the apex and progressing and splits to form a trench around each papilla. toward the root. As the enamel and dentin layers Solid epithelial cords grow from the bottom of the thicken, the odontoblasts and ameloblasts gradually trench and give rise to von Ebner’s glands. Taste move away from each other. Ameloblasts migrate buds develop on the dome of each circumvallate peripherally, remaining at the outer surface of the papilla but are replaced by definitive buds along the enamel, and eventually are lost from the tooth at lateral sides of the papilla. Taste buds first appear as eruption. Before they are lost, ameloblasts lay down local thickenings of epithelium in which basal cells an acellular layer called the enamel cuticle that covers elongate, grow toward the surface, and give rise to the enamel adhering tightly to it. This cuticle may supporting and taste cells. persist for a considerable time after teeth erupt and The major and minor salivary glands are considered to the ameloblasts are lost. Odontoblasts progress arise from ectoderm. All begin from the epithelial centrally into the pulp cavity where they persist. As surface as buds that grow and branch, treelike, to they retreat, the odontoblasts trail fine cytoplasmic form a system of solid ducts. Terminal twigs, which processes that extend through the dentin to the eventually form intercalated ducts, round out at dentinoenamel junction. The entrapped processes their ends to form secretory tubules and acini. form the dentinal fibers contained in fine dentinal Canalization of the duct follows. The sublingual tubules. gland differs slightly in that several buds form from Mesenchyme around the developing enamel organ the oral epithelium and each remains a discrete differentiates into the loose connective tissue of the gland. The parotid appears first, followed by the dentinal sac. Near the root, the inner cells of the sac submandibular, sublingual, and, later still, minor become cementoblasts and cover the dentin with salivary glands. Mesenchyme surrounding the cementum. Cells at the exterior of the sac differentiate epithelial primordia provides the capsule and septae to produce the alveolar bone that surrounds each of the salivary glands. tooth.

200 The connective tissue between the external and a caudal growth indicates the initial development of internal layers of the dental sac gives rise to the the cecum from which the appendix arises as the periodontal membrane. Before the tooth erupts, a small result of extremely rapid growth at the blind end. In crater appears in the gingival epithelium and the duodenum, a rapid proliferation of cells in the connective tissue, immediately above the crown. epithelial lining temporarily occludes the lumen. The crater, which expands to accommodate the Vacuoles later appear in the epithelium, coalesce, tooth as it emerges, develops from focal pressure and restore patency to the lumen. To a lesser necrosis as the tooth grows upward, or by means of degree, similar events occur in the remainder of the lytic activity of enzymes produced by the dental small intestine and colon. The lining epithelium epithelium, or both. now is four to five cells thick, has smooth luminal and basal surfaces, and lies on a distinct basement Alimentary Canal membrane. The underlying mesenchyme has not differentiated into lamina propria and submucosa. The esophagus, stomach, small intestine, and colon Proliferation of the epithelium, together with initially consist of an endodermal tube surrounded envaginations of the mesenchyme, produces folds by a layer of splanchnic mesoderm. The endoderm or ridges in the mucosa, and as they increase in gives rise to the lining epithelium and all associated number, the luminal border becomes irregular. The glands of the digestive tube; mesoderm differentiates into first villi arise from the breakup of the mucosal the supporting layers of the gastrointestinal tube. ridges, and since some folds are covered by The esophagus begins as a short tube connecting stratified cuboidal and others by simple columnar the pharynx and stomach. As the neck region and epithelium, these villi vary in thickness. thorax develop, the tube elongates. Initially, the Subsequently, new generations of villi form as esophagus is lined by simple columnar epithelium, simple evaginations of epithelium and connective which gradually becomes stratified and thickened. tissue, without the prior development of mucosal Ciliated columnar and occasional goblet cells appear folds. Villi that develop in the colon, cecum, and among the squamous cells. Ciliated cells may persist appendix disappear in the second half of fetal life. until birth thereafter the lining remains a wet Intestinal glands develop as simple tubular stratified squamous epithelium. downgrowths of the epithelium between villi. The first glands to appear are the esophageal Duodenal glands arise from outgrowths of the cardiac glands, followed by the esophageal glands epithelium on the intestinal floor between villi proper, which continue to develop postnatally. and/or from intestinal glands. Outgrowths at the base of the epithelium give rise As villi form, myoblasts differentiate in the to the ductal system of these glands; the secretory mesenchyme to establish the muscularis mucosae. units arise from epithelial buds on the terminal At this time, the epithelium is simple columnar, and branches of the ducts. goblet and enteroendocrine cells develop in a The stomach begins as a fusiform expansion of cranial-caudal progression. Paneth cells are among the foregut, lined by a simple or pseudostratified the last cells to appear. The muscularis externa of columnar epithelium of endodermal origin. Gastric the small intestine arises before the muscularis pits arise as simple invaginations of the surface mucosae with the inner coat being the first to form. epithelium into underlying mesenchyme. The pits Its formation involves differentiation and form first along the lesser curvature, then gradually proliferation of smooth muscle cells from in other regions. About three weeks later, they mesenchyme, followed by hypertrophy, which appear in the pyloric and cardiac areas. Soon after accounts for most of the final thickness of the the pits form, oxyntic glands develop, arising from muscle coat. Plicae circulares form late, long after epithelial buds at the base of the pits. Pyloric and villi have developed. cardiac glands develop similarly, but later than the Postnatally, a well-developed endocytic complex is oxyntic glands. present in the epithelial cells that cover the villi in Development of the intestine and the appearance the distal ileum. The complex consists of numerous of villi, glands, and the various cell types usually vesicles and tubules formed by invaginations of the follow a proximal to distal progression. The intestinal cell membrane between microvilli. These join with tract begins as a simple endodermal tube extending inclusion-containing vacuoles of various sizes in the from the stomach to cloaca. As the tube elongates, supranuclear cytoplasm.

201 The complex is implicated in the absorption of separated by islands of hemopoietic cells, but there macromolecules and immunoglobulins during the is little organization into plates and sinusoids that first weeks of nursing. are clearly defined. As differentiation proceeds, The muscularis externa of the entire gut tube is irregular plates of cells form, extending from the derived from surrounding mesenchyme and central veins toward the periphery of each differentiates into the inner circular and outer developing lobule, and sinusoids become apparent. longitudinal layers of smooth muscle cells. The lobulation seen in the adult is not present in Initial development consists of an early period of the developing embryos and is thought to result intense proliferative activity and the differentiation of from the hemodynamics of blood flow through the myoblasts from mesenchymal cells. The inner liver. Early liver growth results mainly from circular layer generally is established first followed hyperplasia, but some hypertrophy also occurs. by the outer layer which initially consists only of a Lipid is lost from hepatocytes, and hemopoietic single layer of scattered, discontinuous myoblasts. A activity wanes and ceases before or shortly after low rate of mitotic activity continues throughout birth. development and into adulthood suggesting that the Bile canaliculi first appear at six weeks, before bile muscularis externa represents a slowly renewing secretion begins. During the next three or four tissue. The second major factor in establishing the weeks, the rest of the intrahepatic biliary system muscularis externa is the hypertrophy of the (bile ductules) forms from hepatocytes near the established muscle layers. Effective innervation of limiting plate of the lobules and then joins with the the muscularis externa is established early while the interlobular ducts of the portal areas. muscle tissue is differentiating (about 6 weeks) The caudal hepatic diverticulum soon becomes a solid during the period of the most active proliferation of cord of cells that gives rise to the cystic duct and smooth muscle cells (myoblasts). The period of gallbladder. Epithelial cords and blood vessels grow hypertrophy occurs after the myenteric plexus is into the connective tissue between lobules and established. The neurons of the myenteric plexus differentiate into hepatic ducts that unite with bile (and submucosal plexus) are derived from neural ducts. What factors control how the two join is crest. A congenital defect known as Hirschsprung unknown. disease (colonic aganglionosis) is a congenital defect that The pancreas develops from endodermal results from the failure of the neural crest cells to evaginations of the gastrointestinal tract wall in two migrate into the forming wall (muscularis externa) sites on the opposite sides of the duodenum. These of the sigmoid colon and rectum to form this form the ventral and dorsal pancreatic buds, which fuse plexus. after the ventral bud has migrated dorsally. The major excretory ducts develop from the ventral Liver and Pancreas bud; the accessory duct arises in the dorsal bud. The pancreatic parenchyma at first consists of a The liver primordium begins as a thickening of series of blind tubules of simple columnar endodermal epithelium in the anterior intestinal epithelium that branch and expands into the portal (future duodenum) of the developing surrounding mesenchyme. Acini form at the ends gastrointestinal tube. A hollow ventral outgrowth of the smallest ducts, but some also arise as appears, lined by simple columnar epithelium, that paratubular buds from larger ducts. As ducts and is continuous with the duodenal lining. This hepatic acini form, lobes and lobules gradually take shape diverticulum enlarges, grows into the vascular outlined by the surrounding connective tissue. mesenchyme of the septum transversum, and then Some acinar cells expand proximally along the divides. A large cranial portion differentiates into the ducts, and some of the terminal ductal cells become hepatic parenchyma and associated intrahepatic bile incorporated into the acini as centroacinar cells. ducts; a small caudal part forms the common bile Further increase in acinar cells occurs mainly by duct, cystic duct, gallbladder, and interhepatic bile division of differentiated cells and may extend into ducts. The surrounding mesenchyme forms the late postnatal life. capsule and stroma. Simultaneously, islet cells differentiate from At first the hepatic cells are small and irregular in endodermally derived progenitor cells within the shape and usually contain numerous lipid droplets. ductal system. Most islets develop at the ends of They may form an irregular network of cords ducts, but paratubular outgrowths also occur.

202 As ducts and acini continue to develop near the direct food to the teeth during mastication, direct it periphery of the lobules, the previously formed to the back of the throat for swallowing, and islets come to be more centrally placed in the provide the voluntary control for suckling and lobule; these are the secondary islets. Many islets retain speech. their connections with the ductal epithelium from The mixture of saliva and ground food is formed which they arose, but as the exocrine parenchyma into a semisolid bolus that, when of the right increases, continuity of islet and duct is obscured. consistency, is swallowed. Saliva is important for A few primary islets are found outside the lobule, this activity also; swallowing is impossible if the within the interlobular connective tissue. These mouth is parched. During the initial stage of represent the first endocrine cells to develop from swallowing, the bolus is directed to the oropharynx the first tubules, prior to formation of smaller ducts by several simultaneous, coordinated events. The and acini. Isolated endocrine cells or small groups anterior part of the tongue is pressed firmly against of endocrine cells remain scattered along the ductal the hard palate, and at the same time, the base of system or occur between acinar cells, even in the the tongue is retracted. Bone in the hard palate adult. provides a rigid platform against which the tongue can press and prevents collapse of the palate due to Summary the pressure exerted against it during swallowing. Formation of a mucoperiosteum prevents sliding The digestive system is designed for breaking down and tearing of the mucosa that covers the palate. As ingested materials to their basic constituents, which these events occur, the larynx and pharynx are then are absorbed and used by the individual. The elevated to receive the bolus. Skeletal muscles process of mechanical and chemical breakdown of associated with the soft palate contract, moving the food substances is called digestion. soft palate upward to seal off the oropharynx and The oral cavity is specialized to take in and prevent food from entering the nasopharynx and mechanically break down food through the cutting nose. As the pharynx is elevated, its lumen dilates to and grinding action of the teeth. The pulverized receive the bolus from the oral cavity. food is moistened and softened by secretions of the Simultaneously, the musculature of the larynx major and minor salivary glands. Digestion of contracts, closing the entrance to the respiratory carbohydrates is initiated in the oral cavity by the tree. Skeletal muscle fibers of the three pharyngeal enzymes amylase and maltase, which are secreted by constrictor muscles contract about the entering the major salivary glands. This chemical digestion bolus and quickly force it into the esophagus. of carbohydrates is short-lived, however, since both Skeletal muscle fibers in the muscularis externa of enzymes are destroyed by the acid environment of the upper esophagus are fast-acting, and their the stomach. Epidermal growth factor (EGF) and contraction carries the bolus of food into the nerve growth factors from the submandibular central region of the esophagus, where there is a glands influence the kinetics of epithelial tissues and gradual transition to slow-acting smooth muscle. other tissues lining the alimentary canal. Salivary Peristaltic waves are formed by the smooth muscle gland secretions also moisten and clean the mouth, in the lower half of the esophagus and move the provide a proper environment for taste, and act as bolus into the stomach. Because of peristalsis, vehicles to transport heavy metals out of the body. materials from the esophagus enter the stomach in When salivary gland secretion decreases during spurts. The lumen of the esophagus is lined by a dehydration, it initiates the sensation of thirst and wet stratified squamous epithelium that, as in the thus is a factor in maintaining fluid balance. The oral cavity, protects the surrounding structures stratified squamous epithelium that lines the oral from the abrasive action of materials as they pass cavity protects the underlying structures from through the lumen. The esophageal glands provide abrasion during eating. In regions subjected to a lubricant for passage of material. The main excessive abrasion (gingiva and palate), the function of the esophagus is to transport food from epithelium is cornified. the oral cavity to the stomach. It secondarily serves The tongue is important for chewing and to warm or cool ingested materials. swallowing. The interior of the tongue, cheek, and lips consists of skeletal muscle and is under voluntary control. The three structures serve to

203 In the stomach the arrangement of the muscle pancreas and liver elevate the pH to neutral or layers provides a churning action that promotes alkaline and provide an environment optimal for mixing with gastric secretions and mechanical the activities of digestive enzymes secreted by the breakdown of foodstuff. Protein is degraded by pancreas; the proteolytic enzyme of the stomach, pepsin, an endopeptidase that splits peptide pepsin, is deactivated. Bile salts also are added to linkages near the center of the molecule. In addition the intestinal contents and act as detergents or to secreting hydrochloric acid and pepsin, the emulsifying agents to aid in the digestion of fats by gastric mucosa secretes a considerable amount of pancreatic lipase. fluid (about 1000 ml of gastric juice after each Most of the digestive process takes place in the meal), most of which is reabsorbed in the intestinal intestinal lumen. The final degradation of materials tract. The parietal cells also secrete gastric intrinsic to their basic components (amino acids, factor, which binds to vitamin B12, important in monosugars, monoglycerides, and fatty acids) erythropoiesis. The complex is absorbed in the occurs on the luminal surface of the intestinal distal small intestine. If gastric intrinsic factor is not epithelial cells just prior to absorption. produced or is present in insufficient amount, much The intestinal wall is specialized for absorption of of the vitamin passes through the intestinal tract digested materials, and the surface area available is and is lost in the feces, resulting in development of greatly increased by the plicae circulares, villi, and pernicious anemia. The gastric mucosa of many microvilli. The efficiency of absorption is further suckling mammals contains rennin, an enzyme that increased by movement of the villi and by curdles milk, and a lipase that begins the digestion segmental movement of the luminal contents by of fat. contractions of the muscularis externa, which move The contents of the proximal stomach are the intestinal contents back and forth. Peristaltic semisolid, whereas those in the distal region form a waves of the muscularis externa move the contents pulplike, fluid mass called chyme. After reaching through the intestine and along the remainder of the proper consistency, chyme enters the the tract. As materials are absorbed, the luminal duodenum in small portions. The smooth muscle of contents become more concentrated. the pyloric sphincter helps to control the Correspondingly, there is an increase in the number evacuation of the stomach. of goblet cells distally in the small intestine to The simple columnar epithelium that lines the provide mucus for lubrication. stomach forms a glandular sheet that secretes a Goblet cells are among the most numerous cells neutral mucin. The epithelium and its secretions in the colon, and they increase in number toward protect the stomach from the erosive effect of acid the rectum. Villi are absent in the colon, but pepsin in the gastric lumen. Bicarbonate produced intestinal glands remain a prominent feature of the during hydrochloric acid formation by parietal cells mucosa. The primary function of the colon is to enters the mucosal microcirculation beneath the absorb fluid that remains in the luminal contents. surface lining and contributes to its protection also. As the fecal mass develops, secretions from the The mucus secreted by the cardiac and pyloric large population of goblet cells provide the glands helps to protect the mucosa where the lubrication necessary to prevent damage to the stomach joins the esophagus and duodenum, colonic mucosa. In the upper rectum, the lining respectively. Gastrin-producing endocrine cells are epithelium is made up primarily of goblet cells. present in large numbers in the pyloric glands; The anal canal is lined by a wet stratified gastrin stimulates acid secretion and gastric motility. squamous epithelium that protects this area from Because of the large population of G cells, the abrasion by the feces during defecation. Smooth pyloric region is not merely a mucus-secreting area muscle of the inner circular layer of the muscularis but also assumes the status of an endocrine organ. externa forms the internal anal sphincter. The As the acid chyme enters the duodenum it is surrounding skeletal muscle fibers of the pelvic subjected to a change of pH. Duodenal glands diaphragm adjacent to the anal orifice form the secrete an alkaline fluid that contains factors which external sphincter, which is under voluntary protects the proximal duodenal mucosa from the control. acid pepsin of the stomach. The duodenal glands, like the submandibular glands, are involved in the secretion of EGF. The alkaline secretions of the

204 Each individual is protected from a vast, ever- across cell membranes, and glyconeogenesis in the present population of microorganisms in the liver. Carbohydrate is stored by the liver as digestive system by immunoglobulins (mainly IgA) glycogen, which can be released into the that form a barrier against fungal, bacterial, and bloodstream as glucose. The liver contributes bile viral invasions. The immunologic barrier extends salts for the emulsification of fats during digestion, throughout the digestive tract and is the combined plays a vital role in detoxification, and supplies product of epithelial and lymphatic cells. The proteins that are important in blood coagulation. epithelial component, the secretory piece, is The liver functions in lipid transport and maintains thought to protect the immunoglobulin from lipid levels in blood. Lipid is complexed to protein digestion both within and outside the cell. In the in the hepatocytes, secreted into the perisinusoidal small intestine and colon, this barrier together with space, and transported in the blood as plasma phagocytes forms a protective sleeve around the lipoprotein. intestinal lumen. Large amounts of IgA also enter The endocrine cells of the gastrointestinal tract the intestinal lumen from the liver via the and pancreatic islets produce hormones that hepatobiliary pathway and play an important role in influence or control gastrointestinal secretions, keeping the population of microorganisms in check. gastrointestinal motility, gastric emptying, From its exocrine portion, the pancreas provides gallbladder contraction, and pancreatic and liver enzymes that digest carbohydrates, proteins, and secretion. How the specific hormones interact with lipids. The endocrine portion secretes insulin and one another or are influenced by other major glucagon, hormones that are instrumental in endocrine organs is under current investigation. regulating blood glucose levels, the flow of glucose

205 15 Urinary System Macroscopic Features

VOCABULARY: hilum, renal sinus, renal The total body water of an average person makes pelvis, major calyx, minor calyx, cortex, up about two-thirds of the total body weight. The medulla, medullary pyramid, renal column, total body water is subdivided into two major fluid renal papilla, medullary ray, renal lobe, renal compartments. An intracellular fluid compartment, the lobule, multilobular kidney water contained within the cells of the body, which accounts for about two-thirds of the total body Human kidneys are bean-shaped organs that lie in a water, and an extracellular fluid compartment, which retroperitoneal position against the posterior makes up the remaining third of the total body abdominal wall, one on either side of the upper water. The extracellular fluid is subdivided into the lumbar vertebrae. Each is contained within a thin interstitial fluids and the plasma within the blood but strong connective tissue capsule that contains vascular system. These are linked by vessels of the fat. The renal artery and nerves enter the kidney on lymphatic system. It is the homeostasis of the the medial border at the hilum, a concavity that extracellular fluid compartment that is maintained also serves as the point of exit for the renal vein, in part by the urinary system. lymphatics, and ureter. The hilum is continuous The urinary system consists of the kidneys, with the renal sinus, a large central cavity ureters, urinary bladder, and urethra. The kidneys surrounded by the parenchyma of the kidney and constitute the glandular component; the remainder filled with loose areolar connective tissue that of the urinary system forms the excretory passages. normally contains much fat. Nerves, lymphatics, The ureters conduct urine from the kidneys to the and branches of the renal artery and vein run bladder, where it is stored temporarily. In turn, the through the sinus. The renal pelvis is a funnel- bladder is drained by the urethra, through which the shaped expansion of the ureter where it joins the urine ultimately is voided from the body. kidney; it also passes through the sinus, dividing

into two or three short tubular structures called the Kidneys major calyces. These in turn divide into eight to twelve smaller units called minor calyces. Each The kidneys are a pair of compound tubular glands minor calyx forms a cylindrical attachment around a that clear the blood plasma of metabolic wastes, conical projection of renal tissue called a renal regulate fluid osmolality and volume, regulate papilla. electrolyte balance, eliminate foreign chemicals, and When a hemisection of the kidney is examined help maintain the acid-base balance of the body. macroscopically, the organization of the Excretion of metabolic products includes creatinine parenchyma into two distinct regions can be seen (from muscle creatine phosphate), uric acid (from readily. The darker, granular appearing outer region nucleic acids), urea (from amino acids), metabolites is the cortex, which forms a continuous layer of hormones and hemoglobin, as well as other beneath the capsule. The inner region, or medulla, substances. The kidneys also are instrumental in is paler and smoother in texture and consists of 8 to eliminating drugs, pesticides, and other 20 cone-shaped structures called medullary environmental factors from the blood plasma. In pyramids, which are separated from each other by addition to excretory functions, the kidneys have inward extensions of cortical tissue. The cortex that properties of endocrine organs and release two separates adjacent medullary pyramids makes up a substances, renin and erythropoietin, directly into renal column. The bases of the pyramids are the bloodstream. Renin is important in regulating directed toward the overlying cortex, while their blood pressure and sodium ion concentration; apices are oriented toward the renal sinus and form erythropoietin influences hemopoietic activity. The the renal papillae. From the bases of the pyramids, kidneys also are important in activating circulating groups of tubules extend into the cortex, giving it a vitamin D to an active form, 1, 25- striated appearance. These striations represent a dihydroxyvitamin D3, necessary for normal continuation of medullary tissue into the cortex and absorption of calcium ion in the gastrointestinal constitute the medullary rays. tract.

206 The arrangement of cortex and medulla allows Nephron subdivision of the kidney into smaller units, the lobes and lobules. A medullary pyramid, together VOCABULARY: renal corpuscle, proximal with its closely associated cortical tissue, forms a tubule, thin segment, distal tubule, loop of renal lobe, while a medullary ray, together with its Henle associated cortical tissue, forms a renal lobule. When the kidney consists of several lobes, it is There are 1 to 2 million nephrons in the human referred to as a multilobular kidney; the kidneys kidney, each representing a functional unit of the of humans are this type. kidney. A nephron is a blindly ending epithelial tubule that can be subdivided into several Microscopic Features components, each differing in structure, function, and position in the kidney. During urine formation, VOCABULARY: uriniferous tubule, nephron, the various segments of a nephron take part in collecting duct filtration, secretion, and resorption. A typical nephron consists of a renal corpuscle, a proximal Each renal lobule is made up of numerous epithelial tubule with convoluted and straight portions, a tubules called uriniferous tubules that collectively thin segment, and a distal tubule that also has form the parenchyma of the kidney (Fig. 15-1). straight and convoluted parts (Fig. 15-1). Those Each uriniferous tubule can be divided into a segments of the nephron between the proximal and nephron, and a collecting duct. Each of these distal convoluted tubules (i.e., the straight units has a different embryologic origin. descending part of the proximal tubule, the thin segment, and the straight ascending portion of the distal tubule) are collectively referred to as the loop of Henle. The renal corpuscle and the proximal and distal convoluted tubules occur only in the cortex, whereas Henle's loop generally is confined to the medulla or to a medullary ray. The size of the nephrons and the length of their various segments vary according to the position of the parent renal corpuscle in the cortex. Renal corpuscles near the medulla are larger and the tubules are longer than are those of nephrons whose renal corpuscles are located in the periphery of the cortex. Nephrons from the subcapsular region have small renal corpuscles and their thin segments of the loop of Henle extend for only a short distance into the medulla. Nephrons whose renal corpuscles occupy an intermediate position in the cortex show intermediate features.

Fig. 15-1. A diagrammatic sketch of a renal lobule illustrating uriniferous tubules (shown in black) and the various subdivisions of a nephron.

207 Renal Corpuscle

VOCABULARY: glomerulus, glomerular capsule, parietal layer, visceral layer, capsular epithelium, glomerular epithelium, capsular space, vascular pole, urinary pole, podocyte, primary process, secondary (foot) process (pedicle), slit pore (filtration slit), slit membrane, common basal lamina, fenestrated Fig. 15-3. A scanning electron micrograph of a fractured region glomerular endothelium, mesangial cell, of renal cortex illustrating the components of a renal corpuscle extraglomerular mesangium, intraglomerular as well as several proximal convoluted tubules. (X 250). Photomicrograph courtesy of P. Andrews, Georgetown mesangial cells, filtration barrier University.

The vascular pole is that area of the renal A renal corpuscle is roughly spherical and measures corpuscle at which the afferent and efferent 150 to 250 µm in diameter. It consists of a capillary arterioles enter and leave. As the afferent arteriole tuft, the glomerulus (Figs. 15-2 & 15-3), which enters the renal corpuscle, it immediately divides projects into a blind expansion of the uriniferous into several primary branches, each of which forms tubule called the glomerular capsule (Bowman’s a complex of capillaries (a capillary lobule) that then capsule). The outer layer of the capsule surrounds reunite to form the efferent arteriole (Fig. 15-2). the glomerulus as the parietal layer, which then Each glomerulus is made up of several such reflects onto the glomerulus, where it is intimately capillary lobules. On the side opposite the vascular applied to the glomerular capillaries to form a pole, the capsular space becomes continuous with complete epithelial covering called the visceral the lumen of the proximal convoluted tubule; this layer of the glomerular capsule. The parietal layer region is known as the urinary pole (Fig. 15-3). also is known as the capsular epithelium, while The parietal layer of the glomerular capsule the visceral layer frequently is referred to as the consists of a single layer of tightly adherent glomerular epithelium. The visceral and parietal squamous cells that contain few organelles. Near layers of the capsule are separated by a narrow the urinary pole there is an abrupt transition from capsular space. this simple squamous form to the large pyramidal

cells that line the proximal convoluted tubule. At

the vascular pole, the simple squamous capsular epithelium changes to a specialized cell type called the podocyte that forms the glomerular epithelium. Podocytes are large, stellate cells whose bodies lie some distance from the underlying capillaries, separated from them by several cytoplasmic extensions called the primary processes. These processes wrap around the glomerular capillaries and give rise to numerous secondary foot processes (pedicles) that interdigitate with similar processes from adjacent podocytes to completely invest the capillary loops of the glomerulus (Fig. 15-4). The plasmalemma of the foot processes exhibits a prominent glycocalyx rich in a sialoglycoprotein called podocalyxin. The narrow clefts between the Fig. 15-2. A scanning electron micrograph of a cast made of a renal glomerulus. (X 800). Photomicrograph courtesy of T. interdigitating processes form the slit pores Murakami, Okayama University. (filtration slits), which measure about 25 nm in width (Fig. 15-5). The gaps may be bridged by a thin slit membrane (diaphragm).

208

Fig. 15-6. A scanning electron micrograph of a portion of a fenestrated glomerular endothelial cell viewed from the capillary lumen. (X 35,000). Photomicrograph courtesy of P. Andrews, Georgetown University.

Fig. 15-4. A scanning electron micrograph of podocytes The renal corpuscle also shows stellate cells with comprising the glomerular epithelium which form a complete long cytoplasmic processes that contain numerous investment around the glomerular capillaries. (X 2,300). Photomicrograph courtesy of P. Andrews, Georgetown University. filaments. These are mesangial cells, which occupy the area between the afferent and efferent The foot processes of the podocytes and the arterioles at the vascular pole, where they lie in a endothelial cells of the glomerular capillaries share a matrix of amorphous material. These cells continuous common basal lamina that measures constitute the extraglomerular mesangium (Lacis 0.1 to 0.5 µm thick. Ultrastructurally, the basal cells) and are continuous with cells of similar lamina consists of a central electron-dense lamina appearance, the intraglomerular mesangial cells densa, a lamina rara externa adjacent to the that lie between the glomerular endothelium and podocyte foot processes, and a lamina rara interna the basal lamina. The latter cells are thought to clear adjacent to the glomerular endothelium. The lamina away large protein molecules that become lodged densa consists of type IV collagen, laminin and on the common basal lamina during filtration of heparan sulfate proteoglycan. The lamina rara blood plasma. Mesangial cells may participate in the externa and interna consist primarily of fibronectin, removal of older portions of the basal lamina from which is thought to firmly attach both epithelial and the endothelial side as it is added to by the endothelial cells to the lamina densa. Both cell podocytes. Mesangial cells are contractile and populations are essential to establish the common respond to angiotensin II and other basal lamina and thereafter continue to contribute vasoconstrictors. They also have receptors for the to its production and maintenance. atriopeptides. The contractile activity of the mesangial cells is thought to mediate blood flow through the glomerular capillaries. Mesangial cells are of clinical importance in some kidney diseases because of their tendency to proliferate. The renal corpuscle is the filter of the nephron. The fenestrated glomerular endothelium, the

Fig. 15-5. A transmission electron micrograph illustrating the common basal lamina, and the foot processes of details of the renal filtration barrier. (X 80,000). the glomerular epithelium form the filtration Photomicrograph courtesy of P. Andrews, Georgetown barrier of the renal corpuscle (Fig. 15-5). This University. barrier permits passage of water, ions, and small molecules from the capillaries into the capsular The glomerular capillaries are lined by a space, but larger structures such as the formed fenestrated glomerular endothelium. The elements of the blood and large, irregular molecules attenuated squamous cells show numerous large are retained. The capillary endothelial cells prevent pores (fenestrae) that measure 50 to 100 nm in passage of formed elements; the common basal diameter (Figs. 15-5 & 15-6). lamina restricts passage of molecules with a molecular weight greater than 70,000.

209 Substances of small molecular weight (40,000 or the blood enters the capsular space, the rise in less) cross the basal lamina and pass through the pressure forces the filtrate into the lumen of the filtration slits of the surrounding glomerular proximal convoluted tubule. The capsular epithelium to enter the capsular space. Material that epithelium forms a tight seal around each renal collects in the capsular space is not urine but a corpuscle, preventing leakage of filtrate into the filtrate of blood plasma. cortical tissue. Although materials with molecular weights larger than 45,000 or that have highly irregular shapes may Proximal Tubule pass through the endothelium and common basal lamina, they are unable to traverse the barrier VOCABULARY: proximal convoluted tubule, provided by the foot processes of the podocytes. microvillus (brush) border, straight portion, The filtration barrier limits passage of materials not apical canaliculus, endocytic complex, only on the basis of size and shape but also with aquaporins respect to their charge. Anionic molecules are more restricted in their passage through the filtration The proximal tubule begins at the urinary pole of barrier than are neutral molecules of similar size. the renal corpuscle and is the largest segment of the Heparan sulfate is a negatively charged human nephron. It is about 17 mm long and makes (polyanionic) molecule of the glomerular basal up most of the cortex. The proximal tubule is lamina. The sialoprotein (podocalyxin) coats the divided into a convoluted portion (pars convoluta) podocyte foot processes and together with heparan and a straight portion (pars recta). The convoluted sulfate gives the filtration barrier a net negative portion is the longer of the two parts and is the one charge. The negatively charged glomerular most frequently seen in sections of the cortex. After basement membrane prevents or restricts the a tortuous course through the cortex in the region filtration of molecules such as albumin and other of its parent renal corpuscle, the proximal tubule highly negatively charged molecules. Thus, the takes a more direct route through the cortex to glomerular epithelium and the common basal become the straight portion of the proximal tubule. lamina are important in limiting the kinds of It then enters the medulla or a medullary ray, where materials that pass from the blood into the capsular it turns toward a renal papilla as the first part of the space. loop of Henle. The energy for the filtration process is supplied by The proximal convoluted tubule consists of a the hydrostatic pressure of the blood in the glomerular single layer of large, pyramidal cells that have a well- capillaries. The pressure (about 70 mm Hg) developed microvillus (brush) border. The provides sufficient force to overcome the colloidal basolateral surfaces of these cells form an intricate osmotic pressure of substances in the blood system of interdigitating processes and ridges that (approximately 33 mm Hg) and the capsular often extend beneath neighboring cells to pressure of the filtration membrane (about 20 mm interdigitate with similar processes of adjacent cells. Hg). The resulting filtration pressure Tight junctions fuse the apices of the adjacent cells (approximately 18 mm Hg) is great enough to force together around the tubular lumen and clearly filtrable materials through all three layers of the delineate apical and basolateral cell membrane filtration barrier and into the capsular space. The domains. The resulting separation of hydrostatic pressure exerted within the glomerular transmembrane (transporter) proteins is vital to the capillaries results from the unusual vascular function of proximal tubular cells. Thus, a complex arrangement of the glomerulus. Most vascular areas labyrinth of cell membranes extends from the basal of the body are supplied by arterioles that form region of the epithelium to near the luminal surface capillaries that then reunite into venules; but the between individual epithelial cells. glomerular capillaries are interposed between an Compartmentalization of the lateral and basal afferent arteriole that conducts blood to the regions results in a greater surface area of cell glomerulus and an efferent arteriole that conducts membrane, which facilitates the transport of ions blood away from the glomerulus. This arrangement and other solutes. results in considerable pressure being exerted on the capillary walls and can be regulated by contraction of either arteriole. As more filtrate from

210 In typical tissue sections, profiles of proximal the proximal tubule these are aquaporin-I channels convoluted tubules are the most common which are not influenced by the presence of structures seen in the cortex and usually show a antidiuretic hormone (ADH). Aquaporin-I channels stellate lumen bounded by a distinct brush border. also are found in the descending thin limb of the Each cell of the proximal tubule has a large loop of Henle. A key factor in proximal tubular cell spherical nucleus, but in histologic sections not all reabsorption is a Na+, K+-ATPase enzyme pump cells show a nucleus because of the large size of the located within the basolateral cell membranes that cells. The cells contain a supranuclear Golgi limit the complex intercellular space. The complex and numerous rod-shaped mitochondria reabsorption of most substances including water is parallel to the long axis of the cell, closely linked to the function of this Na+, K+-ATPase associated with the lateral ridges and processes of system. Glucose, amino acids, lactate, and the basolateral plasmalemma. The cytoplasm usually phosphate are coupled to sodium entry across the is darkly staining and granular. apical plasmalemma of the cell by specific Structurally, the convoluted and straight parts of symporter (cotransport) and antiporter the proximal tubule are similar, but the cells of the (countertransport) transmembrane proteins. straight portion are shorter, and the brush border Sodium is then moved out of the cell and into the and basolateral infoldings are less well defined than intercellular space by the Na+, K+-ATPase in the convoluted part. Mitochondria are abundant mechanism, and glucose, amino acids, phosphate, and but are smaller and more randomly scattered. lactate follow by passive mechanisms. Glucose, One of the main functions of the proximal tubule amino acids, and lactate are usually completely is absorption of the glomerular filtrate. The brush removed from the tubular fluid in the first half of border, which consists of closely packed, elongated the proximal tubule by this mechanism. About 80% microvilli, increases the surface area available for of the filtered bicarbonate is reabsorbed in the proximal absorption by about 30-fold. The microvilli are tubule utilizing a Na+/H+- antiporter. Hydrogen embedded in a coat of extracellular glycoprotein ion is secreted into the lumen in exchange for (glycocalyx). The apical surfaces of these cells bicarbonate ions. The remainder of the proximal absorb sugars and amino acids from the luminal tubule is involved primarily in the reabsorption of contents in a manner similar to that of intestinal sodium chloride. Chloride ion concentration is high epithelial cells. Normally, all the glucose in the in this region because most bicarbonate, glucose, glomerular filtrate is absorbed in the proximal and organic anions have been preferentially convoluted tubule. If blood glucose levels exceed reabsorbed proximally, resulting in an enriched the absorptive capacity of the enzymes that control chloride ion solution in the distal half of the the absorption of glucose, the excess spills into the proximal tubule. Here sodium is reabsorbed with urine (glucosuria). chloride by parallel Na+-H+ and Cl--base- antiporter Protein is absorbed in the proximal tubule by a transmembrane proteins within the apical system of invaginations called apical canaliculi plasmalemma that also is dependent on the that give rise to a series of small vesicles containing Na+/K+-ATPase enzyme system of the basolateral protein sequestered from the lumen. The tubular membranes. Sodium and chloride ions in this invaginations, the vesicles, and the vacuoles portion of the proximal convoluted tubule also are together make up the endocytic complex, which is reabsorbed following a paracellular route between actively involved in protein absorption. The vesicles neighboring epithelial cells. Other inorganic ions coalesce to form large vacuoles that condense and (50% of potassium, 65% of ionized calcium, 15% ultimately fuse with lysosomes whose acid of magnesium, 85% phosphate), urea, uric acid and hydrolases reduce the protein to amino acids that water soluble vitamins (vitamin C and vitamin B are released back into the bloodstream. complexes) also are absorbed by the proximal Over 65% of the sodium chloride and water of the convoluted tubule. glomerular filtrate is absorbed in the proximal In addition to absorption, the proximal tubule tubule. Sodium ion is actively transported from the secretes organic cations (creatine, dopamine, lumen, and chloride ion and water follow passively acetylcholine, and epinephrine) and anions (bile with no expenditure of energy by the cell to salts, fatty acids, urate, and para-aminohippurate) maintain the osmotic balance. Water passes through that are destined to be eliminated in the urine. specialized protein channels called aquaporins. In

211 Hence, this region of the nephron serves as an Distal Tubule exocrine gland. The proximal tubule is a major site of ammonia production in the nephron as component VOCABULARY: straight portion, macula cells metabolize glutamine. Parathyroid hormone densa, pars maculata, juxtaglomerular cell, acts to decrease phosphate resorption in the convoluted portion, basal striation proximal convoluted tubule. Angiotensin II acts to stimulate the sodium-hydrogen antiporter which The distal tubule is divided into a straight part (pars increases hydrogen ion secretion and in addition recta), a portion that contains the macula densa promotes sodium, chloride, and water reabsorption. (pars maculata), and a convoluted portion (pars convoluta). Thin Segment The straight portion of the distal tubule begins as a gradual transition from the thin segment of the VOCABULARY: simple squamous epithelium nephron. The epithelium becomes cuboidal, and while a few microvilli are present, the cells lack the The simple cuboidal epithelium of the straight brush border and apical canaliculi of the proximal descending part of the proximal tubule (the initial tubule. Interdigitating processes are present along segment of the loop of Henle) changes abruptly in the basolateral surfaces of the cells, and these the thin segment to an attenuated simple contain numerous mitochondria. The straight part squamous epithelium. The luminal diameter of of the distal tubule constitutes the thick ascending limb the nephron also decreases markedly, from 65 µm of the loop of Henle and completes the looping course in the descending proximal tubule to about 20 µm of this structure as it ascends through the medulla. in the thin segment. The thin segment is confined It then re-enters the cortex to return to the parent largely to the medulla of the kidney and forms the renal corpuscle. thin limb of the loop of Henle. About 25% of the filtered load is reabsorbed by the The nuclei of the lining epithelial cells are centrally thick ascending limb. Reabsorption of sodium, placed and cause this area of the cell to bulge into chloride, and potassium at this location is driven by the lumen of the tubule. At the luminal surface, the a Na+-K+-2Cl- symporter located in the apical brush border of the proximal tubule is replaced by plasmalemma. Sodium chloride reabsorption by short, scattered microvilli. Ultrastructurally, the cells of the thick ascending limb relies on the cytoplasm shows fewer organelles; the lateral cell Na+/K+-ATPase enzyme system confined to the membranes have elaborate folds that interdigitate basolateral cell membrane as well as a kidney with those of adjacent cells. The cells are united at specific basolateral cell membrane chloride channel. their apices by typical junctional complexes. The Thus, movement of sodium across the apical cell lateral interdigitations are less well developed near membrane and into the cell is controlled by the the hairpin loops. The cells rest on a relatively transmembrane protein (symporter) that couples smooth basal lamina of moderate thickness. The the movement of sodium, chloride, and potassium length of the thin segment varies with each nephron ions. A specific ROMK channel (an inwardly- and depends on the position of the parent renal rectifying renal potassium channel) also located corpuscle in the cortex. Thin segments are short or within the luminal plasmalemma of cells forming even absent from nephrons whose renal corpuscle the thick ascending limb recycles potassium ion is located near the capsule and are longest when the back into the tubular fluid. This is necessary in renal corpuscle is located near the medulla. In these order to permit sustained Na+-K+-2Cl- cotransport nephrons, the thin segments may extend almost to activity in cells forming the thick ascending limb of the tip of a renal papilla. the loop of Henle. A positive membrane potential (luminal voltage) is generated and maintained by the Na+-K+-2Cl- cotransporter and the ROMK channel. This established positive transepithelial voltage plays an important role in the paracellular movement of cations (Na+, K+, Ca++, Mg++) from the lumen of this region of the nephron.

212 About 15% of the remaining bicarbonate is transporter is driven by low intracellular sodium reabsorbed at this location as is 25% of ionized and chloride concentrations created by Na+/K+ calcium, 15% of sodium, 30% of potassium, and ATPase and a basolateral cell membrane chloride about 70% of the magnesium from the remaining channel protein. Between 5 and 10% of the filtrate. remaining filtered magnesium also is absorbed in As it reaches the renal corpuscle, the distal tubule the distal convoluted tubule utilizing a luminal establishes a close relationship with the afferent magnesium channel and a basolateral cell arteriole, and where it contacts the arteriole, the membrane sodium-magnesium exchanger. tubule contains a group of specialized cells that Aldosterone acts on sodium channels to stimulate form the macula densa (dense spot). This segment sodium reabsorption and on potassium channels in of the distal tube is called the pars maculata. The cells of the distal tubule to stimulate potassium cells are taller and narrower than those in adjacent secretion. About 90% the bicarbonate in the areas so that their nuclei are closer together, more glomerular filtrate has already been absorbed in the prominent and lie just beneath the apical cell proximal tubule and loop of Henle. The distal membrane. Cells of the macula densa are polarized tubule also is the site where ammonia is converted toward the basal surface, that is, the Golgi to ammonium ion. Thus, this segment of the membranes and occasional granules are found basal nephron plays an important role in maintaining the to the nucleus, facing the basal cell membrane. The acid-base balance of the body. Parathyroid hormone macula densa is related to a modified region of the stimulates calcium ion reabsorption in cells forming afferent arteriole that contains the juxtaglomerular the initial portion of the distal tubule. The last cells and is considered to represent an endocrine portion of the distal tubule can become permeable portion of the kidney. This feature and the to water under the influence of antidiuretic polarization of the cell suggests that the macula hormone (ADH). densa monitors the luminal fluid in the distal tubule. Cells of the macula densa also are known to Loop of Henle produce the enzyme nitric oxide synthase. The nitric oxide released by the macula densa is thought to VOCABULARY: straight portion of proximal control vascular tone of the afferent and/or tubule, thin segment, straight portion of distal efferent arterioles, thereby influencing kidney tubule, counter-current multiplier system function. The macula densa marks the division between the The loop of Henle is interposed between the ascending and convoluted portion of the distal proximal and distal convoluted tubules and consists tubule. The convoluted portion runs a tortuous of the straight portion of the proximal tubule, course in the cortex near the renal corpuscle of its the thin segment, and the straight portion of the origin. The distal convoluted tubule is shorter than distal tubule. The loops descend in the medulla for the proximal convoluted tubule, so fewer sections variable distances, form hairpin loops, and then of the distal tubule are seen in histologic ascend through the medulla, parallel to the preparations. The lumen of the distal tubule descending limb, to the cortex. generally is wider than that of the proximal tubules, The loop of Henle is essential for the production the cells are shorter and lighter staining, and nuclear of hypertonic urine and is important in the profiles usually are seen in each cell, partly because conservation of body water. The loop consists of many are binucleate. A brush border is lacking. parallel limbs with tubular fluid flowing in opposite Electron micrographs show deep, elaborate directions that act as a countercurrent multiplier infoldings of the basolateral cell membrane system to aid in the concentration of urine. The associated with numerous elongated mitochondria descending thin segment is highly permeable to that lie parallel to the long axis of the cell, water but much less so to sodium and chloride ions producing the prominent basal striations of light and urea. In contrast, the ascending thin segment is microscopy. The cells of the distal tubules show a impermeable to water but permeable to sodium few luminal microvilli. chloride, which diffuses out of the tubule and The distal tubule is the site for further absorption contributes to the high osmolarity of the of sodium chloride utilizing an apical thiazide surrounding interstitium of the inner medulla in a sensitive sodium-chloride transporter. This passive fashion.

213 Some ions diffuse back into the lumen of the The lining epithelium of the collecting ducts adjacent descending limb. Cells comprising the consists of principal and intercalated cells. Similar- thick ascending limb (straight portion of the distal appearing cells also are found in the last segment of tubule) of the loop of Henle also are impermeable to the distal tubule. The principal (light) cells water but actively transport sodium and chloride generally are cuboidal with centrally placed, round ions out of the tubule. Such activity results in a nuclei and lightly staining cytoplasm with dilute tubular fluid and contributes to an increasing characteristic, distinct cell boundaries. hyperosmotic interstitium. Urea follows a route Ultrastructurally, the light cells show scattered, similar to that of these ions and makes a significant short microvilli on their apical surfaces, scattered contribution to the hyperosmolarity of the mitochondria, and some infolding of the basal interstitium. Thus, sodium and chloride ions, as plasmalemma. They also possess a single, centrally well as urea, are trapped in the interstitial substance located cilium (Fig. 15-7). of the medulla, resulting in an increase in the osmotic concentration around the tubules in the medulla. All three passively diffuse into the descending limb but either diffuse or are actively pumped out again by the ascending limb as the cycle repeats itself over and over. The sodium chloride-urea trap thus formed establishes an osmotic gradient that increases in strength toward the renal papillae. This gradient is important for the conservation of water and formation of hypertonic urine by the terminal portion of the distal tubule and the collecting ducts.

Collecting Ducts

VOCABULARY: initial segment, arched portion, medullary collecting duct, papillary duct, area cribrosa, principal (light) cell, intercalated (dark) cell, renal interstitium Fig. 15-7. A scanning electron micrograph of a portion of a human collecting tubule as viewed from the lumen The collecting ducts can be subdivided according to demonstrating the surface features of light (principal) and dark (intercalated) cells. (X 8,000). Photomicrograph courtesy of P. their location in the kidney. The initial segment is Andrews, Georgetown University. found in the cortex and includes short connecting portions that unite the distal tubules of cortical nephrons to collecting ducts and arched portions Principal cells reabsorb sodium and water and that are formed by the confluence of several secrete potassium ions. The activity of this cell type connecting portions from juxtamedullary nephrons. is dramatically influenced by antidiuretic hormone The arched portions originate deep in the cortex, (ADH) in fluid/volume homeostasis. In the absence of which they ascend through before arching to ADH the apical plasmalemma is impermeable to descend within a medullary ray. water and urea. In contrast, the basolateral Medullary collecting ducts occur primarily in plasmalemma is freely permeable to both. An ADH the medulla. As the ducts pass through the medulla receptor (an integral membrane glycoprotein), a they converge to form larger, straight collecting stimulatory G-protein and a membrane-bound ducts called papillary ducts, which end at the tip adenylate cyclase are present within the basolateral of a renal papilla. The numerous openings of the plasmalemma. In the presence of ADH, cytoplasmic papillary ducts give a sieve like appearance to the vesicles containing aquaporin-2 channels for water external surface of the papillae; this area has been and vesicles containing channels for urea are called the area cribrosa. The external surfaces of inserted into the luminal plasmalemma of these the papillae are covered by transitional epithelium. cells making the collecting tubule permeable to both water and urea.

214 Thus, urea reabsorbed proximally in the proximal secreted is dependent largely on the acid/base tubule or entering the descending limb of the thin status of the extracellular fluids. segment can be recycled back into the renal The collecting ducts function to conserve water interstitium from the collecting tubule by this and produce hypertonic urine. As the ducts traverse mechanism. the medulla to the tips of the pyramids, they pass Scattered between the light cells are the through the increasingly hypertonic environment intercalated (dark) cells, which have more established by the loop of Henle. The permeability mitochondria, stain more deeply, and show a large of the collecting ducts to water is controlled by number of vesicles in the apical cytoplasm. The ADH. In the presence of ADH, water is drawn cells lack central cilia, but the apical plasmalemma from the collecting ducts by osmosis because of the shows microplicae (Fig. 15-7). Some intercalated hypertonic environment maintained in the cells (α cells) secrete hydrogen ions in exchange for medullary interstitium. The loss of water from the potassium ions using an H+/K+-ATPase located tubular contents results in a concentrated, in the apical plasmalemma. For each hydrogen ion hypertonic urine. If ADH is lacking due to injury or secreted into the tubular lumen, a bicarbonate ion is disease, the kidney is unable to concentrate urine, released from the basolateral cell membrane using a and copious amounts of dilute urine are produced, - - Cl /HCO3 antiporter. Thus, intercalated cells resulting in severe dehydration. This condition is + - + secrete H and reabsorb HCO3 and K and known as diabetes insipidus. therefore play an important role in regulating acid- The renal interstitium fills the spaces between base balance. A small population of intercalated the tubular elements of the kidney. The cortical cells (β cells) have been observed that function interstitium is relatively scant except around blood opposite the α form and secrete potassium ion. As vessels and consists of fine bundles of collagen, the collecting tubules pass through the medulla, the fibroblasts, and scattered phagocytes. The cells increase in height to become tall columnar in medullary interstitium is more plentiful, and its cells the papillary ducts. lie parallel to the long axis of the tubules. These The mechanism of ammonia excretion is complex interstitial cells possess long, branching processes and follows a circuitous route to be excreted that encircle adjacent tubules and blood vessels. indirectly. Glutamine is metabolized to ammonia Pleomorphic interstitial cells filled with small lipid (NH3) and α-ketoglutarate by cells forming the droplets also occur in the medullary interstitium at proximal convoluted tubule. The NH3 is converted regular intervals between the epithelial tubules and + to ammonium (NH4 ) intracellularly and is vessels. The cells are suspected to be endocrine transported into the tubular lumen utilizing the cells and may secrete an antihypertension factor. + + + Na -H exchanger in which NH4 substitutes for The medullary interstitium contains an abundant H+. As the luminal fluid reaches the thick ascending intercellular ground substance and small groups of + + limb much of the NH4 is reabsorbed by the Na - collagen fibers. + - + K -2Cl cotransporter in which case NH4 is substituted for K+. Juxtaglomerular Apparatus Within the surrounding medullary interstitium + NH4 as well as NH3 contributes to the osmotic VOCABULARY: extraglomerular mesangium, gradient maintained by high concentrations of juxtaglomerular (JG) cell, renin, sodium, chloride, and urea. The collecting duct, angiotensinogen, angiotensin I, angiotensin II, which passes back through the interstitium, aldosterone, erythropoietin contains α-intercalated cells that contain two hydrogen transporters (H+-ATPase and H+-K+- The juxtaglomerular apparatus consists of the ATPase) located in the luminal plasmalemma. As macula densa of the distal tubule, the hydrogen ion is secreted into the tubular fluid the extraglomerular mesangium, and the lipid soluble NH3 passes through cells of the juxtaglomerular cells in the wall of the afferent collecting duct to enter the tubular fluid. Here the arteriole. The extraglomerular mesangium forms NH3 combines with secreted hydrogen ion to form a loose mass of cells between the afferent and + the water-soluble NH4 thereby effectively efferent arterioles. Component (Lacis) cells may “trapping” the hydrogen ion for excretion in the contain granules but their precise role in the urine. The amount of ammonia (hydrogen ion) function of the juxtaglomerular apparatus is

215 unclear. The juxtaglomerular (JG) cells appear to Vascular Supply be highly modified smooth muscle cells in the wall of the afferent arteriole as it enters the renal VOCABULARY: renal artery, segmental artery, corpuscle. The cells contain a number of secretory interlobar artery, arcuate artery, interlobular granules, well-developed Golgi complexes, and artery, intralobular artery, afferent arteriole, abundant granular endoplasmic reticulum. The efferent arteriole, peritubular capillary network, JG cells produce renin (a proteolytic enzyme), vasa recta, vascular countercurrent exchange which, when released into blood, converts a plasma protein angiotensinogen (a α2-globulin from the The vascular pattern of the kidneys is complex and liver) to angiotensin I. A converting enzyme in the shows regional specializations related to the blood (derived from endothelial cells lining organization and function of the various parts of capillaries of the lung) converts angiotensin I to the the nephron. About 20% of the cardiac output goes polypeptide angiotensin II, which acts on the zona to the kidneys. The renal arteries arise from the glomerulosa of the adrenal cortex, and stimulates abdominal aorta and before reaching the renal sinus the release aldosterone. Aldosterone secretion usually divide into branches that pass anterior and increases sodium ion absorption from the fluid posterior to the renal pelvis to enter the renal sinus. within the terminal portion of the distal tubule and Here they give rise to segmental arteries, which the cortical collecting ducts to the blood and also divide to form interlobar arteries that run increases movement of potassium ion from the between renal lobes. At the corticomedullary blood to the tubular fluid. Angiotensin II also is a junction, these arteries divide into several arcuate potent vasoconstrictor and elevates blood pressure. arteries that arch across the base of each medullary Within the kidney it acts a vasoconstrictor of the pyramid and give off interlobular arteries that afferent and efferent arterioles and causes the pass into the cortex between lobules. The contraction of mesangial cells. In addition, the interlobular arteries run peripherally in the cortex, juxtaglomerular apparatus has been implicated in giving rise to a system of intralobular arteries that the production of erythropoietin, an agent that enter renal lobules and provide afferent arterioles, stimulates erythropoiesis in the bone marrow. The which supply the glomeruli of renal corpuscles. As location of the macula densa, its basal polarization, the efferent arteriole leaves the renal corpuscle of and its orientation toward adjacent JG cells suggest a cortical nephron, it immediately breaks up into a that the three groups of cells may interact. The peritubular capillary network that supplies the macular densa may "sense" the ion (Na+, K+, Cl-) convoluted tubules. concentration in the distal tubule and thereby The main circulation of the renal cortex is unique influence the activity of JG cells. The in that the arterioles give rise to two distinct, tubuloglomerular feedback mechanism proposed sequential capillary beds: the glomerular and suggests a concentration-dependent uptake of Na+, peritubular capillaries. Efferent arterioles from K+, and Cl- by a Na+-K+-2Cl- cotransporter located juxtamedullary nephrons, on the other hand, form in the apical plasmalemma of macula densa cells. several long, straight vessels, the vasa recta, that The result of this activity is the generation of descend into the medullary pyramid and form adenosine that activates an adenosine A1 receptor in hairpin loops. Like the loop of Henle, the loops of the plasmalemma of extraglomerular mesangial the vasa recta are staggered throughout the medulla. cells. The activation of this receptor triggers an The walls of the vasa recta are thin, and the increase in cytoplasmic calcium the resulting action endothelium of the ascending (venous) limb is potential of which is transmitted to the afferent fenestrated. The vasa recta pass in close proximity arteriole by gap junctions resulting in to the loop of Henle, permitting passive vasoconstriction and inhibition of renin secretion. interchange between the two elements. The vasa Local angiotensin II, prostaglandins, and nitric recta form a vascular countercurrent exchange oxide modulate this activity. The juxtaglomerular system that removes excess water and ions. The cells also act as baroreceptors and can function medullary osmotic gradient is not disrupted due to independently in the release renin. Stimulation of the slower flow rate and the smaller volume (about renal sympathetic nerves results in the release of 8% of the total renal blood flow) in the vasa recta norepinephrine which also causes renin release. and because the vasa recta also form hairpin loops in which the blood flows down the descending side

216 of the loop and then back up the ascending side of It also is continuous with the epithelium of the the loop. In addition to making the collecting papillary ducts, thus providing a complete epithelial tubules more permeable to water, the presence of lining that prevents escape of urine into the ADH results in vasoconstriction of the vessels forming the neighboring tissues. Transitional epithelium forms a vasa recta thereby reducing the flow rate even further barrier to the diffusion of salts and water into and and protecting the established interstitial osmotic out of the urine. In the major and minor calyces, gradient from being disrupted. the epithelium is two to three cells thick, increasing The venous drainage of the kidney is similar to in the ureter to four or five layers and to six, eight, and follows the same course as the arterial supply. or more layers in the bladder. The surface cells are However, there is no venous equivalent of the large and rounded and in the relaxed bladder have glomerulus or the afferent and efferent arterioles. convex or dome-shaped borders that bulge into the The venous system of the medulla begins in the lumen. The superficial cells sometimes contain large ascending limb of the vasa recta, which drains into polyploid nuclei. The epithelium of the distended interlobular or arcuate veins. In the peripheral organ shows considerable changes in morphology. cortex, capillaries unite to form small veins that In the filled bladder, or as urine is propelled down assume a star-like pattern (stellate veins) as they the ureter, the epithelium is stretched and flattened drain into interlobular veins. The renal veins drain and temporarily assumes the appearance of a thin into the inferior vena cava. The left renal vein stratified squamous epithelium. When the differs in that it is much longer and receives venous intraluminal pressure is relieved, the epithelium drainage from the left gonad. again assumes its nondistended appearance. In the relaxed bladder, the apical cytoplasm of the Extrarenal Passages superficial cells contains fine filaments and fusiform vesicles that are limited by a membrane of the same The extrarenal passages consist of the minor and thickness as the cell membrane. These vesicles are major calyces, renal pelvis, ureter, urinary bladder, thought to represent reserve surface membrane for and urethra. They convey urine to the outside of use during distension. Transitional epithelium lies the body or, in the case of the bladder, store it on a very thin basement membrane that usually is temporarily. Except for the urethra, all have a not seen with the light microscope. similar basic structure with a mucosa, muscularis, and The epithelium lies on a lamina propria that adventitia. The layers are thinnest in the minor consists of a compact layer of fibroelastic calyces and increase in depth distally to reach their connective tissue. Some diffuse lymphatic tissue maximum development in the bladder. also may be present. The muscularis of the urinary passageways Calyces, Renal Pelvis, Ureters, and Bladder consists of bundles of smooth muscle separated by abundant fibroconnective tissue and begins in the minor calyces as two thin layers of smooth muscle. VOCABULARY: mucosa, transitional The inner, longitudinal layer begins at the attachment epithelium, lamina propria, muscularis, of the minor calyces to the renal papillae; the outer adventitia circular layer of muscle spirals around the renal

papilla forming a thin coat. The walls of the minor The thickness of the wall of the extrarenal passages calyces contract periodically around the renal gradually increases from the upper to lower parts; papillae and aid in moving urine from the papillary except for this, all parts of the extrarenal passages ducts to the calyces and into the renal pelvis. The show a similar histologic structure. The lumen is muscularis of the renal pelvis and upper two-thirds lined by a mucosa consisting of transitional of the ureter consist of the same two layers and epithelium that rests on a lamina propria. There is differ only in thickness. An additional outer, no submucosa, and the lamina propria blends with longitudinal layer of smooth muscle appears in the the connective tissue of the well-developed lower one-third of the ureter. muscular coat. The distal ends of the ureters, the intramural Transitional epithelium covers the external portions, pass obliquely through the wall of the surfaces of the renal papillae and reflects onto the bladder to empty into its lumen. internal surfaces of the surrounding minor calyces.

217 The inner circular layer of smooth muscle bladder. The membranous urethra (pars disappears, and the contractions of the longitudinal membranacea) is very short (1.5 cm) and extends layers help dilate the lumen of the distal ureter so from the apex of the prostate to the root of the that urine can enter the bladder. Peristaltic waves penis. The membranous urethra pierces the skeletal periodically pass along the ureter to convey urine to muscle of the urogenital diaphragm immediately before the bladder, into which it empties in small spurts. it enters the penis. Skeletal muscle surrounding this As the bladder fills, the pressure of its contents part of the urethra forms the external sphincter keeps the intramural portions of the ureters closed (sphincter urethrae) of the urethra and is under due to their oblique course in the bladder wall. The voluntary control during micturition. ureters open only when urine is forced through The third and longest segment, about 15 cm, is the them. Reflux of urine into the ureters also is penile urethra (pars cavernosa), which runs prevented by a valvelike flap of bladder mucosa longitudinally through the corpus cavernosa that lies over the ureteral openings. urethrae to end at the tip of the glans penis. The The muscularis of the urinary bladder is membranous and penile parts of the urethra are moderately thick and consists of inner longitudinal, lined by stratified or pseudostratified columnar middle circular, and outer longitudinal layers of smooth epithelium. Stratified squamous epithelium often muscle. The middle layer is the most prominent and occurs in patches of the penile portion and also spirals around each ureteral opening. Around the lines a distal enlargement, the fossa navicularis. internal urethral orifice, the muscle increases in The mucous membrane of the urethra shows small thickness to form the internal sphincter of the urethra. depressions or invaginations called the lacunae of A coat of fibroelastic connective tissue, the Morgagni, which are continuous with the branched adventitia, surrounds the muscularis and attaches tubular glands of Littré. The epithelium lining the extrarenal passages to surrounding structures. these glands is the same as that found in the In the renal pelvis it blends with the capsule of the intraepithelial nests of clear, mucus-secreting cells kidney. On the superior surface of the bladder the within the urethral epithelium. The lamina propria fibroelastic coat is covered by peritoneum. The beneath the urethral epithelium is a highly vascular, adventitia contains numerous blood vessels, loose connective tissue rich in elastic fibers. The lymphatics, and nerves. Blood vessels pierce the mucosa is bounded by inner longitudinal and outer muscularis, provide it with capillaries, and then circular layers of smooth muscle. form a plexus of small vessels in the lamina propria. A rich capillary layer lies immediately beneath the Female Urethra epithelium. Nerve fibers and small ganglia also occur in the adventitia and represent the The female urethra is shorter than the male (3 to 5 sympathetic and parasympathetic divisions of the cm long) and is lined by stratified squamous autonomic nervous system. Parasympathetic fibers epithelium, although patches of stratified or are important for control of micturition. pseudostratified columnar may be found. Glands of Littré are present throughout its length. As in the Male Urethra male, the lamina propria is a vascular, fibroelastic connective tissue that contains numerous venous VOCABULARY: prostatic urethra, sinuses. The surrounding muscularis consists of an membranous urethra, sphincter urethrae, inner longitudinal layer of smooth muscle bundles penile urethra, fossa navicularis, glands of and an outer circular layer. The female urethra is Littré, intraepithelial nests surrounded by skeletal muscle of the urogenital diaphragm that forms the external urethral sphincter In men, the urethra conveys urine from the bladder near its orifice. to the exterior and serves for the passage of seminal fluid during ejaculation. The male urethra is 18 to Organogenesis 20 cm long and has three segments. The first segment is 3 to 4 cm long and lies within the The urinary and reproductive systems arise in prostate, an accessory sex gland. This part forms common from mesoderm of the urogenital ridge. A the prostatic urethra (pars prostatica) and is lined nonfunctional pronephros, a mesonephros functional in by transitional epithelium similar to that of the the fetus, and a metanephros (the definitive kidney)

218 arise successively each caudal to the last with some maintaining cell-to-cell union as the glomerular overlapping. A functional pronephros occurs only epithelium changes from simple cuboidal to a layer in chordates such as Amphioxus and myxinoid of podocytes. The attachments are transitory and are fishes. Except for lacking loops of Henle, lost as the podocytes differentiate. With pronephric and mesonephric nephrons resemble development, podocytes become cuboidal and those of the metanephros. Although the nephron extend fingerlike processes from their basal surfaces and collecting tubules form continuous structures toward adjacent cells. The cells begin to separate in the definitive kidney, each has a separate origin, from one another, and their foot processes increase unlike other exocrine glands in which secretory and in size and mingle with others to cover the ductal units arise from the same primordium. expanding capillaries. Mesonephroi are drained by mesonephric ducts that The opposite end of the S-shaped tubules unites regress in the female but in the male are with nearby collecting tubules, and the lumina incorporated into the reproductive tract. By a series become continuous. The tubules elongate, become of subdivisions, an outgrowth of the mesonephric U-shaped, and give rise to proximal and distal tubules and duct, the uteric bud, gives rise to ureters, renal pelvis, loops of Henle. The latter are the last segments to major and minor calyces, papillary ducts, and form and are not numerous until late in collecting tubules. The ureteric bud extends into the development. metanephric blastema, a mass of mesoderm that gives Initially, development of the kidney is concerned rise to nephrons. The looser, more external layer of with formation of nephrons; this is followed by a mesoderm forms the interstitial tissue and capsule period of growth and differentiation of established of the kidney. nephrons. In humans, nephronogenesis is complete Proliferation of mesoderm in the subcapsular by the thirty-fifth week. Renin-immunoreactive cells nephrogenic zone results in solid ovoid masses in appear in the mesonephros by the sixth week of which the mesenchymal cells that lie adjacent to gestation and in the metanephros by the eighth collecting tubules form double-layered caps. The week, suggesting that the renin-angiotensin system rest of the cell mass forms a primitive renal vesicle may be active even during early fetal life. located between a main collecting tubule and one of Early in development, the digestive and urogenital its branches. Cells of the renal vesicle and collecting systems open into a common space, the cloaca. tubule do not make contact. Later, a wedge of mesenchyme, the urorectal Each renal vesicle gives rise to a nephron unit. The septum, divides the cloaca into a dorsal rectum and single cell-layered vesicle lengthens and two a ventral bladder and urogenital sinus. The indentations appear, transforming it into an S- endodermal lining of the newly formed bladder shaped tubule. The space between the middle and becomes stratified and the surrounding distal limbs of the S fills with mesodermal cells and mesenchyme differentiates into the muscular wall a capillary. Together, the vessel and the distal limb of the bladder. of the tubule evolve into a renal corpuscle. The capillaries lie immediately adjacent to the concave, Summary external surface of the lower limb and as they expand into the tubule, they acquire an epithelial The kidney serves as an organ of excretion, is both covering and form the glomerular capillaries. The an endocrine and exocrine gland, is a target organ parietal layer of the capsule, which originates from of other endocrine glands, and plays an important cells on the opposite side of the tubule, constricts role in acid-base balance as well as in maintaining around the point of entry of the capillary loops and fluid osmolality and volume homeostasis. establishes the vascular pole. Where the tubule The uriniferous tubules perform three separate surrounds the developing capillaries, the columnar functions in the formation of urine: filtration, cells and endothelial cells share a basal lamina. secretion, and selective absorption. The tubules do The provisional glomerular epithelial cells show not synthesize and release new material in numerous desmosomes and zonula adherens-like significant amounts but eliminate excess water and structures that maintain contact between the waste products of metabolism that are being epithelial cells as the capillary loops expand. The transported in the blood plasma. attachments occur mainly along the central, lateral regions of the cell, rather than at their apices, thus

219 After filtration of the plasma, metabolic wastes such of Henle and creates and maintains a gradient of as urea, uric acid, and creatinine are not absorbed osmotic pressure that increases from the base of but primarily remain in the tubular lumen and form the medullary pyramid to the papillary tip. Henle's constituents of the urine that will be eliminated. loops, with their hairpin turns, act as a countercurrent The renal corpuscles filter blood plasma, and in multiplier system to maintain the gradient. humans, the kidneys produce about 125 ml of The distal tubule is the principal site for glomerular filtrate each minute. About 124 ml of acidification of urine and is the site for further this is absorbed by the rest of the uriniferous absorption of bicarbonate in exchange for secretion tubule, resulting in the formation of approximately of hydrogen ions. The conversion of ammonia to 1 ml of urine. The total glomerular filtrate in ammonium ions also occurs in the distal tubule humans during a 24-hour period is 170 to 200 liters, trapping hydrogen ions for elimination in the urine. of which about 99% is absorbed. The filtration Therefore this region of the nephron plays an process is driven by the hydrostatic pressure of important role in acid-base balance. Absorption of blood, which is sufficient to overcome the colloidal sodium ions in the distal tubule is referred to as osmotic pressure of plasma and the capsular facultative absorption and is controlled by the pressure at the filtration membrane. The resulting steroid hormone aldosterone, which increases the filtrate contains ions, glucose, amino acids, small rate of absorption of sodium ion and excretion of proteins, water soluble vitamins, and the potassium ion. If aldosterone is absent, large nitrogenous wastes of metabolism. Blood cells, amounts of sodium are lost in the urine. proteins of large molecular weight, and large Parathyroid hormone also acts on the distal negatively charged molecules are prevented from convoluted tubule of the nephron to promote entering the capsular space by the filtration barrier. absorption of calcium ion and inhibit absorption of The glomerular filtrate is reduced to about 35% phosphate ion from the developing urine. of its original volume in the proximal convoluted Collecting ducts pass through the hypertonic tubule. In addition to the obligatory absorption of environment of the medulla. The permeability of sodium chloride and water, glucose, amino acids, the collecting ducts as well as the last portion of the proteins, and ascorbic acid are actively absorbed in distal tubule to water is controlled by ADH. In the the proximal convoluted tubules. The majority of presence of ADH, the collecting ducts and the last the bicarbonate also is absorbed at this location. segment of the distal tubule become permeable to Exogenous organic cations and anions are actively water, which, by osmosis, leaves the lumen to enter secreted into the lumen by the epithelium of the the surrounding interstitium. The flow of water out proximal convoluted tubule, thus fulfilling the of the tubular fluid thus produces hypertonic urine. requirements of an exocrine gland. Most of the If the concentration of ADH in the blood is low, a materials that have passed through the filtration larger volume of dilute urine results because the barrier are immediately resorbed by the epithelium permeability of the collecting ducts to water is of the uriniferous tubules and put back into the decreased. Aldosterone targets the distal convoluted circulation. Materials in excess of the body's needs tubule and collecting duct to increase reabsorption as well as any metabolic wastes (urea, uric acid, and of sodium, chloride, and water and increases creatinine) pass along the tubular lumen and are potassium secretion. excreted in the urine. Parathyroid hormone acts on Atrial natriuretic factor (ANF) belongs to a family the proximal convoluted tubule to decrease of atrial peptides stored in atrial-specific granules phosphate reabsorption and on the thick ascending within atrial cardiac myocytes. When released, ANF limb of the loop of Henle and distal tubule to acts on the kidneys causing vasoconstriction of the increase calcium reabsorption. Angiotensin II acts afferent arteriole which increases both glomerular on the proximal convoluted tubule to increase filtration pressure and filtration rate. ANF also acts sodium, chloride, and water reabsorption and on the distal and collecting tubules to decrease increases the secretion of hydrogen ion at this sodium chloride resorption and inhibits secretion of location. ADH, aldosterone and renin. These actions result The loop of Henle is essential for the in increased sodium excretion (natriuresis) in a large conservation of water and production of hypertonic volume of dilute urine. urine. An active sodium pump mechanism resides in the cells of the thick ascending limb of the loop

220 An endocrine part of the kidney, the A minor calyx is attached around each renal juxtaglomerular apparatus, elaborates renin, an papilla and represents the beginning of the enzyme that acts on angiotensinogen in the blood extrarenal passageways. Minor calyces undergo plasma, converting it to an inactive protein called periodic rhythmic contractions that aid in moving angiotensin I. A converting enzyme in the blood urine from the papillary ducts into the extrarenal (derived from endothelial cells in the lung) converts system. The walls gently contract around each renal angiotensin I to the angiotensin II that stimulates papillae and transport the urine to the renal pelvis. the zona glomerulosa of the adrenal gland to release Periodic contractions of the muscular walls propel aldosterone. Aldosterone stimulates the terminal small amounts of urine from the renal pelvis distal tubule and collecting tubule to absorb sodium through the ureter to the urinary bladder, where it is ions in exchange for potassium ions. Angiotensin II temporarily stored until a sufficient volume is is a potent vasoconstrictor also and raises blood obtained for urine to be evacuated through the pressure. The renin-angiotensin system is urethra. Contraction of the muscularis of the influenced by blood flow through the kidney and is bladder wall (the detrusor muscle) together with an important factor in hypertension. The voluntary relaxation of the skeletal muscle that juxtaglomerular apparatus also produces a blood- forms the sphincter urethrae accomplishes this borne factor called erythropoietin, which stimulates process during micturition. erythropoiesis in the bone marrow.

221 16 Meiosis Prophase I

VOCABULARY: leptotene, zygotene, synapsis The male and female reproductive organs produce (conjugation), bivalent, pachytene, tetrad, the gametes, provide a mechanism by which the diplotene, crossing over, chiasmata, gametes can be brought together to yield a new recombination, diakinesis individual, and in the female, house and nourish the zygote until birth of that new individual. The first meiotic prophase is long and complex and The generative organs for the production of male varies markedly from that of mitosis. It customarily gametes (sperm) are the testes, while in the female is divided into five stages: leptotene, zygotene, the ovary is the site of production of female pachytene, diplotene, and diakinesis. gametes, the ova. Gametes cannot contain the In leptotene the chromosomes begin to condense diploid (somatic) number of chromosomes, since and become visible as individual, slender threads their fusion would result in a doubling of the that resemble those of early prophase of mitosis. chromosomes with each new generation. Thus, the The ends of the chromosomes become oriented to gametes can contain only half the somatic number the side of the nucleus nearest the centrosome, with of chromosomes, and reduction to the haploid the bodies of the chromosomes extending in loops number occurs through a special form of cell division into the interior of the nucleus. called meiosis. During zygotene, the homologous chromosomes

come together in pairs in which the chromosomes VOCABULARY: two cell divisions, one DNA lie side by side, aligned point for point along their replication, first meiotic division, separation of lengths. Because of the close apposition of the homologous chromosomes, second meiotic homologous chromosomes at this stage, they division, separation of chromatids appear to be present in the haploid number. This

pairing is called synapsis (or conjugation), and Meiosis is characterized by two cell divisions but each pair of homologous chromosomes forms a only one replication of deoxyribonucleic acid bivalent. (DNA). Chromosomal DNA is duplicated during The chromosomes continue to contract and at interphase, prior to the first division, and results in pachytene are short and thick. Each chromosome a double or tetraploid amount of DNA within the of the bivalent begins to split lengthwise and then diploid number of chromosomes. This is no can be seen to consist of two chromatids; the different from what occurs during replication of bivalent therefore consists of four chromatids and DNA prior to ordinary mitosis. The first meiotic is frequently called a tetrad. division brings about separation of homologous In the diplotene stage, each chromosome splits chromosomes, and while this results in halving the into its constituent chromatids, which remain number of chromosomes, each chromosome attached only at their centromeres (kinetochores). contains a double amount of DNA so that the total The homologous chromatids move apart slightly, content of DNA within the resulting cells remains and the tetrad formation becomes more obvious. diploid. A second meiotic division then occurs At points along their lengths, the homologous without a period of DNA replication. Sister chromatids make contact with each other and chromatids are separated at this time to yield exchange segments; this exchange constitutes cells with the haploid number of chromosomes and crossing over. The physical regions where the the haploid amount of DNA. contacts are made are called chiasmata. The stages of meiotic division are the same as The exchange of segments between homologous those of mitosis, namely, prophase, anaphase, chromatids results in re-assortment or metaphase, and telophase. recombination of the genetic material. The

chromosomes continue to shorten and thicken, and

the nucleolus begins to fragment and disappear.

Diakinesis is marked by an even greater

contraction of the chromosomes that are scattered

throughout the cell.

222 The homologous chromosomes still are paired but Differences in Meiosis of Male and Female are joined only at the chiasmata. At the end of Germ Cells prophase I, the nuclear envelope and nucleoli have disappeared, and the tetrads have begun their The mechanics of meiosis are the same regardless migration to the equator of the cell. of whether sperm or ova are produced, but there are marked differences in the net yield of the two Metaphase I types of gametes and in the times at which meiosis is initiated. In the male, four viable, functioning Metaphase I is similar to the metaphase of mitotic sperm are produced from each germ cell that enters division except that pairs of chromosomes meiosis, whereas the same events in the female yield (bivalents), not single chromosomes, are arranged only a single functioning ovum. After the first and about the spindle. The homologous pairs are second meiotic divisions, the cytoplasm is aligned so that the members of each pair lie at distributed evenly among the developing sperm, but either side of the equatorial plate, with the in the formation of ova, the bulk of the cytoplasm centromeres of homologous chromosomes facing is passed to only one cell. The remaining ova, called opposite poles. polar bodies, receive so little cytoplasm that they are unable to survive. Anaphase I and Telophase I The time at which meiotic division is initiated in the male and female germ cells is remarkably different. In the Anaphase I and telophase I also are similar to those human female, the germ cells begin their meiotic of mitotic cell division. However, at anaphase I of divisions in the embryo, and by the fifth month of meiosis, the centromeres have not split so that intrauterine life, the developing ova are in the rather than chromatids separating and moving to diplotene stage of the first meiotic division, but the opposite poles, whole chromosomes, each division is not complete until just before ovulation. consisting of two chromatids, are separated. Hence, many of the ova remain suspended in the Reconstruction of the nuclei and cytoplasmic diplotene stage for several decades. The second separation occur at telophase I to yield two new meiotic division and the formation of the second cells. However, since the orientation of the polar body occur only after fertilization. In contrast, bivalents on the equatorial plate is random, there is meiotic division in the male germ cells is not a random assortment of maternal and paternal initiated until puberty, and once the cell has entered chromosomes in each of the telophase nuclei. meiosis, the process is carried to completion with no prolonged interruption. Meiotic Division II Summary VOCABULARY: no DNA synthesis, splitting of centromeres, separation of chromatids The primary function of the reproductive systems is to provide haploid germ cells for procreation, and The second meiotic division is more like that of one of the consequences of meiotic division is to mitosis and occurs after a brief interphase during reduce the number of chromosomes to half that which there is no synthesis of DNA. In some present in somatic cells. This ensures that on union species, the cells produced at the end of the first of gametes, the normal diploid number of division enter directly into metaphase II and bypass chromosomes will be maintained. A second a second prophase. The centromeres split at consequence of meiosis that is of great importance metaphase and the chromatids separate during is provision of genetic variation as the result of the anaphase. Unlike mitosis, the sister chromatids are exchange of genetic material between homologous not identical, due to the crossing over that occurred chromosomes and from the random distribution of during the first meiotic prophase and to the random homologous chromosomes between daughter alignment of the homologous chromosomes at the nuclei. The unequal division of cytoplasm among first division. Following telophase and cytokinesis, ova provides one cell with sufficient material to four haploid cells result. nourish the zygote until other forms of nourishment can be established.

223 17 Male Reproductive System compartments are directed toward the mediastinum, and each lobule contains one to four

convoluted seminiferous tubules that represent The male reproductive system consists of the the exocrine portion of the testis. Their product is gonads (testes), their excretory ducts, the accessory whole cells (spermatozoa). sex glands, and the external genitalia. The gonads The inner region of the tunica albuginea, the supply the male germ cells or gametes (sperm), tunica vasculosa, consists of loose connective which are conducted to the exterior by an excretory tissue and contains numerous small blood vessels duct system. The accessory sex glands contribute that supply the testis. The connective tissue extends secretions that, together with the sperm, form the into each lobule and fills the spaces between the semen. seminiferous tubules, forming the interstitial

tissue of the testis. The interstitial tissue is rich in Testes extracellular fluid and contains abundant small blood and lymphatic vessels that form a plexus VOCABULARY: tunica vaginalis, tunica around the seminiferous tubules. In addition to albuginea, mediastinum testis, septula testis, fibroblasts, the interstitial tissue contains lobuli testis, seminiferous tubules, tunica macrophages, mast cells, mesenchymal cells, and vasculosa, interstitial tissue, interstitial cells (of large polyhedral cells 15 to 20 µm in diameter. Leydig), testosterone These are the interstitial cells (of Leydig), which commonly occur in groups and make up the The testes are compound tubular glands that lie endocrine portion of the testis. They secrete the male within a scrotal sac, suspended from the body by a steroid hormone testosterone. Interstitial cells spermatic cord. The testes are dual organs that act usually contain single, large, spherical nuclei, but as exocrine glands producing a holocrine secretion, binucleate cells are not uncommon. Leydig cells are the sperm, and as endocrine organs that secrete the characterized by numerous LH receptors. In electron male sex hormone, testosterone. Each normal adult micrographs, the cytoplasm shows abundant testis weighs between 12-15 gm and the right testis smooth endoplasmic reticulum, well-developed is commonly slightly larger and heavier than the left Golgi complexes, and numerous mitochondria, testis. which contain tubular rather than lamellar cristae. Each testis is covered anteriorly and laterally by a Morphologically, interstitial cells are characterized simple squamous epithelium (mesothelium) called by large cytoplasmic crystals called the crystals of the visceral layer of tunica vaginalis. On the Reinke. These proteinaceous bodies are highly posterior aspect of the testis, this mesothelium variable in shape and size but are readily seen with reflects onto the scrotal sac and lines its interior as the light microscope. In electron micrographs they the parietal layer of the tunica vaginalis. The serous show a highly ordered structure. The crystals appear cavity between visceral and parietal layers allows the in the interstitial cells of most postpubertal testes to move freely and reduces the chance of individuals and vary considerably in number. Their injury from increased pressure on the exterior of significance is unknown. Most of the enzymes the scrotum. involved in the synthesis of testosterone are located A thick, fibrous capsule, the tunica albuginea, in the smooth endoplasmic reticulum and lies beneath the visceral layer of tunica vaginalis, mitochondria of interstitial cells. separated from it only by a basal lamina. Tunica Only about 1 to 2% of circulating testosterone is albuginea consists of a dense fibroelastic connective in free form with the remainder being bound to a tissue that contains scattered smooth muscle cells. sex steroid-binding globulin or albumin produced The muscle cells are concentrated in the posterior by the liver. Testosterone and its metabolites are region, where the tunica albuginea thickens and essential for the proliferation and the differentiation projects into the testis to form the mediastinum of excretory ducts and male accessory sex glands testis. Connective tissue partitions, the septula and for maintaining these structures in a functional testis, extend from the mediastinum into the state. Metabolites of testosterone include two interior of the testis and subdivide it into potent androgens. approximately 250 pyramid-shaped compartments called lobuli testis. The apices of the

224 Dihydrotestosterone (DHT) produced by the activity of Sertoli Cells the enzyme 5 α-reductase and 5 α-androstenediol due to the activity of the enzyme 3 α-reductase. VOCABULARY: tall columnar, basal Testosterone and its derivatives influence other compartment, adluminal compartment, blood- tissues and are responsible for beard growth, low testis barrier, androgen-binding protein, pitch of the voice, muscular build, and male inhibin distribution of body hair. Production of testosterone by the interstitial cells is controlled by The Sertoli cell is a tall columnar cell that spans a gonadotropic hormone called interstitial cell- the germinal epithelium from the basal lamina to stimulating hormone (ICSH) or LH, which is secreted the luminal surface. The cell has an elaborate shape by cells (gonadotrophs) located within the anterior with numerous lateral processes with recesses or pituitary. concavities that surround differentiating spermatogenic cells. The apical portion of the cells Seminiferous Tubules also envelops developing germ cells and releases them into the lumen of the seminiferous tubule. VOCABULARY: peritubular tissue, myoid The expanded portion of the cell contains an (peritubular) cell, germinal epithelium, irregularly shaped nucleus distinguished by a large, supporting (Sertoli) cell, spermatogenic cell prominent nucleolus that is readily seen with the light microscope. The basal cytoplasm contains abundant Each testicular lobule contains one to four highly smooth endoplasmic reticulum, and a large, well- convoluted seminiferous tubules that measure 150 developed Golgi complex occupies the to 250 µm in diameter and 30 to 70 mm long. supranuclear region. The cytoplasm contains many Added together the seminiferous tubules have a lipid droplets, lysosomes, thin elongated tubular length that exceeds 500 m. Each mitochondria, scattered profiles of granular seminiferous tubule is organized into a non- endoplasmic reticulum, and a sheath of fine branched closed loop, both ends of which open filaments that envelops the nucleus and separates it into the first portion of the excurrent duct system, from the organelles. Microtubules are present also, the rete testis. A seminiferous tubule consists of a their numbers depending on the state of activity of complex stratified germinal (seminiferous) the cell. Human Sertoli cells feature membrane- epithelium surrounded by a layer of peritubular bound inclusions called the crystalloids of Charcot- tissue. Böttcher; their function is unknown. Sertoli cells are The peritubular tissue is separated from the further characterized by FSH receptors. germinal epithelium by a basal lamina and consists Tight junctions occur between adjacent Sertoli cells of collagenous fibers and flattened cells that, near their bases and subdivide the germinal depending on the species, may contain numerous epithelium into basal and adluminal compartments, actin filaments. These are the myoid (peritubular) each of which has a separate, distinct population of cells. In humans, the myoid cells are said to be only spermatogenic cells. The basal compartment moderately contractile. The production of testicular extends from the basal lamina of the germinal fluid by the germinal epithelium and its flow epithelium to the tight junctions; the adluminal through the seminiferous tubules moves the compartment lies between the tight junctions and spermatozoa into the excretory duct system. the lumen of the tubule (Fig. 17-1). The tight The germinal epithelium of the adult is unique junctions between the Sertoli cells appear to form, among epithelia in that it consists of a fixed, stable at least in part, a blood-testis barrier. population of supporting (Sertoli) cells and a proliferating population of differentiating spermatogenic cells. Developing germ cells slowly migrate upward along the lateral surfaces of the supporting cells to be released at the free surface into the lumen of the seminiferous tubule.

225 movement of germ cells from the basal lamina through the epithelium to the lumen and are important in the release of germ cells into the lumen. The microtubules and actin filaments in the cytoplasmic processes of the Sertoli cells provide these processes with the mobility they need to carry out their functions. The numerous gap junctions that occur between adjacent Sertoli cells facilitate communication between cells along specific segments of a seminiferous tubule during migration and release of germ cells. Sertoli cells also secrete a Fig. 17-1. A diagrammatic sketch of the germinal epithelium. glycoprotein known as inhibin. Inhibin suppresses the secretion of FSH by gonadotrophs in the anterior Germ cells in the basal compartment are contained pituitary and via this feedback loop acts to control within an environment that has access to substances the rate of spermatogenesis. Likewise, the concentration of in the blood plasma, while the germ cells in the testosterone in the blood plasma is maintained at a adluminal compartment reside in a specialized milieu specific concentration by circulating levels of this that is maintained and controlled by the Sertoli hormone that influence the secretion of pituitary cells. A number of plasma proteins are present in gonadotrophs and gonadotrophin releasing the basal compartment that are not found in the hormone (GnRH) containing neurons located in luminal contents of the seminiferous tubule that is, the arcuate nucleus of the hypothalamus via a however, rich in other amino acids and ions. Sertoli negative feedback loop mechanism. cells are thought to provide all the nutrients for the avascular germinal epithelium. Spermatogenic Cells In addition to secreting testicular fluid, Sertoli cells release androgen-binding protein, whose VOCABULARY: spermatogenesis, type A synthesis is stimulated by a pituitary gonadotropin, spermatogonia, type B spermatogonia, primary follicle-stimulating hormone (FSH). The androgen- spermatocyte, secondary spermatocyte, binding protein binds testosterone, thus providing spermatid the adluminal compartment with the level of hormone needed for the normal differentiation and The spermatogenic cells of the germinal epithelium development of germ cells. The blood-testis barrier consist of spermatogonia, primary and secondary helps to confine the high concentration of spermatocytes, and spermatids. These are not testosterone to the adluminal compartment and separate cell types but represent stages in a thus allows a different environment to be continuous process of differentiation called established in the basal compartment. Many of the spermatogenesis. The term includes the entire developing germ cells in the adluminal sequence of events in the transformation of diploid compartment are haploid and might be regarded as spermatogonia at the base of the germinal foreign material by the body if released into epithelium into haploid spermatozoa that are surrounding tissues. The tight junctions between released into the lumen of the seminiferous tubule. Sertoli cells are thought to prevent haploid germ Spermatogonia lie in the basal compartment of the cells from contacting general body tissues and thus germinal epithelium, immediately adjacent to the prevent an autoimmune response to the individual's basal lamina. The cells are 10 to 20 µm in diameter own germ cells. Although tight junctions contribute and round or ellipsoidal in shape and the nucleus of to the blood testis barrier, other factors may be each cell contains the diploid number of involved. chromosomes. Two types of spermatogonia can be Sertoli cells phagocytize degenerating germ cells differentiated. Type A spermatogonia replicate by and take up the residual cytoplasm that normally is mitosis and provide a reservoir of stem cells for the shed during release of mature germ cells into the formation of future germ cells. lumen of the seminiferous tubule. In addition to providing mechanical and nutritional support for developing sperm, Sertoli cells also control the

226 During mitotic division, some type A only when the sperm are finally released by the spermatogonia give rise to intermediate forms that Sertoli cell. eventually produce type B spermatogonia; these are committed to the production of primary Spermiogenesis spermatocytes. The two types of spermatogonia can be differentiated by their nuclei. Type A VOCABULARY: acrosome, acrosomal vesicle, spermatogonia have spherical or elliptical nuclei head cap, axoneme, implantation fossa, with a fine chromatin distribution and one or two connecting piece, annulus, middle piece, head nucleoli near the nuclear envelope. Many nuclei of this cell type also exhibit a clear nuclear vacuole. The The process by which spermatids differentiate into lighter-stained, spherical nuclei of type B slender, motile sperm is called spermiogenesis. spermatogonia contain variably sized clumps of Newly formed spermatids are round cells with chromatin, most of which are arranged along the central, spherical nuclei, prominent Golgi nuclear envelope. Only a single, centrally placed complexes, numerous mitochondria, and a pair of nucleolus is found. centrioles. Each of these components undergoes Primary spermatocytes at first resemble type B changes during spermiogenesis. spermatogonia, but as they migrate from the basal At the onset of spermiogenesis, many small lamina of the germinal epithelium, they become granules appear in the Golgi membranes and larger and more spherical, and the nucleus enters eventually coalesce to form a single structure called the initial stages of division. Primary spermatocytes the acrosome. The developing acrosome is usually are found in the central zone of the germinal bounded by a membrane, the acrosomal vesicle, epithelium. How these large cells pass from the which also is derived from the Golgi complex and basal to the adluminal compartment is unknown. is closely associated with the outer layer of the The primary spermatocyte undergoes the first nuclear envelope. The acrosomal vesicle expands meiotic division to produce secondary and then collapses over the anterior half of the spermatocytes. The homologous chromosomes nucleus to form the head cap. have separated and the number is reduced by half, The acrosome, which contains hydrolytic enzymes, but the cells contain the diploid amount of DNA. remains within the acrosomal membrane. As these Secondary spermatocytes lie nearer the lumen than events occur, the two centrioles migrate to a the primary forms and are about half their size. position near the nucleus on the side opposite of Unlike the extended division of primary the forming acrosome. Nine peripheral doublets spermatocytes, secondary spermatocytes divide plus a central pair of microtubules develop from the quickly in the second meiotic division to produce distal centriole and begin to form the axoneme of spermatids, which contain the haploid number of the tail. The proximal centriole becomes closely chromosomes and the haploid amount of DNA. associated with a caudal region of the nucleus called Because they divide so quickly after being formed, the implantation fossa. secondary spermatocytes are seen only rarely in the As the axoneme continues to develop, nine germinal epithelium. Spermatids are about half the longitudinal coarse fibers extend around it and size of secondary spermatocytes. Numerous blend with nine short, segmented columns that spermatids in different stages of maturation border form the connecting piece, which unites the the lumen of the seminiferous tubule. Early nucleus (head) with the tail of the spermatozoon. spermatids appear as small spherical cells with round The annulus, a ringlike structure, forms near the darkly stained nuclei. Late spermatids appear as tailed centrioles and migrates down the developing spermatozoa within the recesses of the Sertoli cell. flagellum. Randomly distributed mitochondria The cell divisions that take place during formation migrate to the flagellum and become aligned in a of male germ cells are unique in that not only is the tight helix between the centrioles and the annulus. genetic material reduced by half; the division of the This spirally arranged mitochondrial sheath cytoplasm is incomplete. Thus, the cells resulting characterizes the middle piece of the tail of a from a single spermatogonium remain in mature spermatozoon. cytoplasmic continuity throughout the different Simultaneously, marked changes occur in the stages of differentiation. The continuity is broken nucleus: it becomes condensed, elongated, and slightly flattened.

227 Together with the acrosome, it forms the sperm The structural details of the different segments are head. The bulk of the cytoplasm now is associated best seen with the electron microscope. with the middle piece of the evolving spermatid, The neck is the region where the head and tail and as differentiation nears completion, the excess unite. It contains the connecting piece that joins the cytoplasm is shed as the residual body, leaving only a nine outer dense fibers of the tail to the thin layer of cytoplasm to cover the spermatozoon. implantation fossa of the nucleus. The region of the The residual cytoplasm is phagocytized by Sertoli connecting piece that joins the implantation fossa is cells as the spermatozoa are released into the lumen expanded slightly to form the capitulum. The of the seminiferous tubule. Although middle piece extends from the neck to the annulus morphologically the spermatozoa appear mature, and consists of the axoneme, the nine coarse fibers, they are nonmotile and largely incapable of and the helical sheath of mitochondria. The fertilization at this time. principal piece is the longest part of the tail and In most species, spermatids at specific stages of consists of the axoneme and the nine coarse fibers a differentiation always are associated with (2 + 9 + 9 arrangement) enclosed by a sheath of spermatocytes and spermatogonia, which also are at circumferential fibers. The circumferential fibers specific stages of development. A series of such join two longitudinal thickenings (columns) of this associations occurs along the length of the same sheath located on opposite sides. The end piece is seminiferous tubule, and the distance between two the shortest segment of the tail and consists only of identical germ cell associations is called a "wave" of the axoneme surrounded by the plasmalemma (Fig. seminiferous epithelium. Human germinal 17-2). epithelium exhibits a mosaic of such areas, and six different cell associations have been described. Such associations represent a fundamental pattern of cycling of the germinal epithelium during sperm production. The time taken for spermatogonia to become spermatozoa is relatively constant and species-specific: in humans it is about 64 days. If germ cells fail to develop at their normal rate, they degenerate and are phagocytosed by adjacent Sertoli cells.

Spermatozoa

VOCABULARY: head, tail, acrosomal cap, neck, capitulum, middle piece, principal piece, end piece, capacitation

Spermatozoa that lie free in the lumina of seminiferous tubules consist of a head containing Fig. 17-2. A diagrammatic sketch illustrating the structure of a the nucleus and a tail, which eventually gives human spermatozoon. motility to the free cell. The chromatin of the nucleus is very condensed and reduced in volume, The last step in the physiologic maturation of providing the functionally mature sperm with spermatozoa takes place in the female reproductive greater mobility. The condensed form of chromatin tract and is called capacitation. This final also protects the genome while the spermatozoon is activation occurs in the oviduct and is characterized in route to fertilize the ovum. The acrosomal cap by the removal of a glycoprotein coat and seminal covers the anterior two-thirds of the nucleus and proteins from the plasmalemma overlying the contains enzymes that are important for penetration acrosome, by changes in the acrosomal cap, and by of the ovum at fertilization. The size and shape of changes in the respiratory metabolism of the the nucleus vary tremendously in different species. spermatozoa. The sperm tail is about 55 µm long and consists of a neck, middle piece, principal piece, and end piece.

228 Capacitation substantially increases the number of the ductus epididymis. The efferent ductules follow spermatozoa capable of fertilization, but the a convoluted course and, with their supporting mechanism of this activation is not fully tissue, make up the initial segment of the head of understood. the epididymis. The luminal border of the efferent ductules shows a characteristic irregular contour Excretory Ducts due to the presence of alternating groups of tall and short columnar cells. Ciliated and The excretory ducts consist of a complex system of nonciliated cells are present. Each contains the tubules that link each testis to the urethra, through organelles normally associated with epithelia as well which the exocrine secretion, semen, is conducted as supranuclear granules that are lysosomal in to the exterior during ejaculation. The duct system nature. The nonciliated cells show numerous consists of the tubuli recti (straight tubules), rete microvilli on their apical surfaces and are thought to testis, ductus efferentes, ductus epididymis, ductus absorb much of the testicular fluid that is present in deferens, ejaculatory ducts, and prostatic, this part of the ductal system. Cilia on the other cell membranous, and penile urethra. type beat toward the epididymis and help move the nonmotile sperm and testicular fluid in that Tubuli Recti direction. The epithelium lies on a basal lamina and is VOCABULARY: simple columnar epithelium bounded by a layer of circularly arranged smooth muscle that thickens toward the duct of the Near the apex of each testicular lobule, the epididymis. Cilia are not found beyond the ductuli seminiferous tubules join to form short, straight efferentes, and the movement of sperm through the tubules called the tubuli recti. The lining epithelium remainder of the duct system depends on the has no germ cells and consists only of Sertoli cells. This contractions of the muscular wall and fluid flow. simple columnar epithelium lies on a thin basal lamina and is surrounded by loose connective Ductus Epididymidis tissue. The lumina of the tubuli recti are continuous with a network of anastomosing channels in the VOCABULARY: pseudostratified columnar, mediastinum, the rete testis. principal cell, stereocilia, basal cell, smooth muscle, three-layered coat Rete Testis The efferent ductules gradually unite to form a VOCABULARY: simple cuboidal epithelium single ductus epididymidis that measures 5 to 7m long. This highly coiled duct and its associated The rete testis is lined by simple cuboidal vascular connective tissue form the rest of the head, epithelium in which each of the component cells body, and tail of the epididymis. The epithelial bears short microvilli and a single cilium on the lining is pseudostratified columnar and consists apical surface. The epithelium lies on a delicate of principal and basal cells. basal lamina. The channels of the rete testis are Principal cells are the most numerous and are surrounded by a dense bed of vascular connective very tall (80 µm) in the proximal segment but tissue. gradually decrease in height distally to measure only 40 µm near the junction with the ductus deferens. Ductuli Efferentes Long, branched microvilli, inappropriately called stereocilia, extend from the apical surfaces of the

principal cells. Like microvilli found elsewhere, they VOCABULARY: initial segment, head of contain an axial bundle of actin microfilaments. The epididymis, tall and short columnar, ciliated actin microfilaments extend from the stereocilia and nonciliated cells into the terminal web region of the principal cells.

A very large Golgi complex takes up a supranuclear In men, 10 to 15 ductuli efferentes emerge from the position in the cell, and the basal region is filled by mediastinum on the posterosuperior surface of the an abundance of endoplasmic reticulum. testis and unite the channels of the rete testis with

229 Lysosomes, multivesicular bodies, and numerous Ductus Deferens coated vesicles also are present. One of the functions of the principal cell is absorption; together, VOCABULARY: mucosa, pseudostratified the proximal portion of the ductus epididymidis columnar, stereocilia, lamina propria, and the ductuli efferentes absorb over 90% of the muscularis, adventitia, ampulla fluid produced by the seminiferous epithelium. Principal cells also phagocytose degenerate The ductus deferens unites the ductus epididymidis spermatozoa and residual bodies. Ubiquitin, a highly and the prostatic urethra. It is characterized by a conserved protein molecule, is secreted by principal wall consisting of a mucosa, a thick muscularis, and cells of the epididymal epithelium. It forms an adventitia. It is often referred to as the most covalently linked polyubiquitin chains on structures muscular tube in the body. The mucosa is thrown into within the epididymal fluid, such as defective sperm longitudinal folds that project into the lumen and is and debris, during epididymal transit. These lined by a pseudostratified columnar epithelium identified targets are recognized for endocytosis and resting on a thin basal lamina. The principal cells of removed from the epididymal fluid by the principal the epithelium bear stereocilia on the luminal cells of the epididymal epithelium. The epithelium surface. The surrounding lamina propria is dense lining of the mid and distal regions of the ductus and contains numerous elastic fibers. The epididymis also secrete glycoproteins, sialic acid and muscularis is the dominant feature of the ductus other factors that are believed to play a role in deferens and consists of three layers of smooth sperm maturation. muscle cells arranged longitudinally in the inner and Basal cells are small, round cells that lie on the outer layers and circularly in the middle layer. The basal lamina, interposed between the principal cells. muscularis is surrounded by the loose connective They have a lightly staining cytoplasm with few tissue of the adventitia, which blends with organelles. Scattered intraepithelial lymphocytes neighboring structures and anchors the ductus often are present in the epithelium of the ductus deferens in place. Powerful contractions of the epididymidis. muscular wall propel sperm from the distal ductus The epididymal epithelium lies on a basal lamina epididymidis and rapidly transport them through and is surrounded by a thin lamina propria and a the ductus deferens during ejaculation. thin layer of circularly arranged smooth muscle Near its termination, the ductus deferens dilates to cells. Near the ductus deferens, the muscle layer form the ampulla. The lumen expands and the thickens and becomes three-layered (inner mucosa is folded, creating a labyrinth of pocket-like longitudinal, middle circular, outer longitudinal); it recesses. The lining epithelium is pseudostratified is continuous with that of the ductus deferens. The columnar. The muscular coat of the ampulla is regional differences reflect differences in the thinner and the layers are less distinct than in other mobility of the ductus epididymidis. Proximally, the parts of the ductus deferens. Nearly all the smooth duct shows spontaneous peristaltic contractions muscle in the muscularis of the ductus deferens is that slowly move spermatozoa through the phasic smooth muscle characterized by distinct action epididymis; distally, the peristaltic contractions are potentials and rapid contraction. Nearly every reduced, and this region serves to store sperm. smooth muscle cell is innervated (multiunit Spermatozoa become physiologically mature as they pass innervation) by the autonomic nervous system. This through the epididymis. Those entering the accounts for the rapid, forceful, pulse-like proximal part of the duct mostly are incapable of contractions of the ductus deferens during fertilization and swim in a weak, random fashion. ejaculation. Spermatozoa from the distal portion are capable of fertilizing ova and show strong, unidirectional Ejaculatory Duct motility. How the ductus epididymidis contributes to the maturation of spermatozoa is unknown. VOCABULARY: pseudostratified or stratified Transit time through the epididymis is about 3 columnar epithelium weeks for humans. A short, narrow region of the ductus deferens extends beyond the ampulla and joins the duct of the seminal vesicle.

230 This combined duct forms the ejaculatory duct, honeycombed appearance. All the compartments which empties into the prostatic urethra. It is about communicate with a central lumen, although in 1 cm long and is lined by a pseudostratified or histologic sections the impression is one of stratified columnar epithelium. The mucosa individual chambers. forms outpockets that are similar to, but not as well The mucosal folds are lined mainly by developed as, those in the ampulla. The remainder pseudostratified columnar epithelium consisting of the wall of the ejaculatory duct consists only of of rounded basal cells interposed between cuboidal fibrous connective tissue. or columnar cells. Regions of simple columnar epithelium may be present also. The mucosal cells Prostatic Urethra contain numerous granules, some lipid droplets, and lipochrome pigment, which first appear at VOCABULARY: transitional epithelium, sexual maturity and increases with age. The colliculus seminalis, prostatic utricle ultrastructural features indicate a cell that is active in protein synthesis. The cytoplasm contains The ejaculatory duct penetrates the prostate and abundant granular endoplasmic reticulum, a opens into the prostatic part of the urethra. The prominent supranuclear Golgi complex, and prostatic urethra is lined by a thin transitional conspicuous, dense secretory granules in the apical epithelium and bears a dome-shaped elevation, the cytoplasm. colliculus seminalis, on its posterior wall. A small The epithelium rests on a thin lamina propria of blind diverticulum called the prostatic utricle lies loose connective tissue with many elastic fibers. A on the summit of the colliculus and represents a muscular coat is present and consists of an inner remnant of the Müllerian duct in the male. The two layer of circularly arranged smooth muscle cells and ejaculatory ducts, one draining each testis, empty an outer layer in which the smooth muscle cells into the prostatic urethra on either side of the have a longitudinal orientation. Both layers of prostatic utricle. The numerous glands of the muscle are thinner than those in the ductus surrounding prostate also empty into this part of deferens. External to the muscle coat is a layer of the urethra. loose connective tissue (adventitia) rich in elastic fibers. Accessory Sex Glands Secretions from the seminal vesicles form a substantial part (65-70%) of the total ejaculate. It is Male accessory sex glands include the seminal a yellowish, viscid secretion that contains fructose, vesicles, prostate, and bulbourethral glands. The prostaglandins, factors that increase sperm motility, secretion from each of these glands is added to the factors that suppress the immune response in the testicular fluid and forms a substantial part of the female reproductive tract against semen, and factors semen. that clot and then liquify semen in the vagina. Fructose is a source of energy for sperm whereas Seminal Vesicles prostaglandins are thought to stimulate smooth muscle contractions within the walls of the tubular

structures of the female reproductive tract to aid in VOCABULARY: folds, honeycombed moving sperm to the site of fertilization. In appearance, pseudostratified columnar sections, the secretions appear as deeply stained, epithelium coagulated masses, often with a netlike structure.

The seminal vesicles depend on testosterone, and The seminal vesicles are elongated, saclike removal of the hormone, as by castration, results in structures that lie posterior to the prostate gland. their involution and loss of secretory function. The duct of each joins with the distal end of the ductus deferens to form an ejaculatory duct. The mucosa of the seminal vesicles is thrown into numerous complex primary folds that give rise to secondary and tertiary folds. These project into the lumen and subdivide the seminal vesicle into many small, irregular compartments that give the lumen a

231 Prostate on testosterone and its metabolites. Blood levels of prostate-specific antigen (PSA) produced by the prostate VOCABULARY: composite gland, compound is an important factor in screening for prostatic tubuloalveolar, capsule, septa, prostatic cancer. concretion, fibromuscular stroma Bulbourethral Glands The prostate is the largest of the accessory sex glands in men and surrounds the urethra at its VOCABULARY: compound tubuloalveolar, origin from the bladder. It is a composite gland, simple cuboidal to simple columnar made up of 35 to 60 small, compound tubuloalveolar glands from which 20 or more The bulbourethral glands (of Cowper) are a pair of ducts drain independently into the prostatic urethra. pea-sized structures located in the urogenital These small glands appear to form strata around the diaphragm, close to the bulb of the penis. They are urethra and consist of the periurethral mucosal glands, compound tubuloalveolar glands whose long submucosal glands, and the main or principal prostatic ducts drain into the proximal part of the penile glands, which lie peripherally and make up the bulk urethra. Each gland is limited by a connective tissue of the prostate. The prostate is contained within a capsule from which septa, containing elastic fibers vascular, fibroelastic capsule that contains many and smooth and skeletal muscle cells, extend into smooth muscle cells in its inner layers. Broad septa the glands, dividing them into lobules. extend into the prostate from the capsule and The ducts and secretory portions are irregular in become continuous with the dense fibroelastic shape and size, and at their terminations, the tissue that separates the individual glandular secretory parts may form cystlike enlargements. The elements. glandular epithelium varies from simple cuboidal The secretory units of the glands are irregular and to simple columnar depending on the functional vary greatly in size and shape. The glandular state, but in distended alveoli the epithelium may be epithelium differs from gland to gland and even flattened. Active cells show a lightly stained within a single alveolus. It usually is simple or cytoplasm filled with mucin granules that confine pseudostratified columnar but may be low cuboidal the nucleus to the base of the cell. Excretory ducts or squamous in some of the larger saccular cavities. are lined by simple columnar epithelium that The epithelium is limited by an indistinct basal becomes pseudostratified near the urethra. In lamina and rests on a layer of connective tissue that smaller ducts, the epithelium appears to be contains dense networks of elastic fibers and secretory. The surrounding connective tissue numerous capillaries. The cells contain abundant contains an incomplete layer of circularly arranged granular endoplasmic reticulum and many apical smooth muscle. secretory granules. The lumina of secretory units Bulbourethral glands produce a clear, viscid fluid may contain spherical bodies, the prostatic that is rich in amino sugars and contains concretions, that are thought to result from sialoprotein. The glands secrete in response to condensation of secretory material. The concretions erotic stimulation and the secretion serves as a appear to increase with age and may become lubricant for the penile urethra. calcified. The connective tissue surrounding the individual glandular units contains numerous Semen smooth muscle cells (a fibromuscular stroma), which aids in the rapid discharge of prostatic fluid The final product formed by the exocrine at ejaculation. secretions of the testes and accessory sex glands is a The prostatic secretion is a thin, milky fluid of pH whitish fluid called seminal fluid or semen. The 6.5 that is rich in zinc, citric acid, phosphatase, average ejaculate of men is about 3 ml of semen factors that enhance sperm motility, and proteolytic which, in addition to about 300 million sperm, enzymes, one of which, fibrinolysin, is important in contains degenerating cells exfoliated by the ductal the liquefaction of semen. Prostatic secretion system, occasional wandering cells from connective accounts for about one-third of the semen volume. tissues, pigment granules, and prostatic concretions. Like the seminal vesicles, the development and Hyaline bodies of unknown origin, lipid granules, functional maintenance of the prostate is dependent fat, and protein also are present.

232 In normal individuals about 60% of the drawing the testes closer to the abdominal wall by spermatozoa exhibit normal motility and about contraction of the layer of striated muscle (cremaster 60% show normal cytologic morphology. muscle) that invests the spermatic cord. In humans, failure of the testes to descend (cryptorchidism) results External Genitalia in sterility.

In men, the two structures that make up the Penis external genitalia are the scrotum and penis. VOCABULARY: erectile tissue, corpora Scrotum cavernosa penis, pectiniform septum, tunica albuginea, cavernous spaces, corpus VOCABULARY: skin, tunica dartos, spermatic cavernosum urethrae (corpus spongiosum), cord, pampiniform plexus glans penis, prepuce

The scrotum is a pendulous, cutaneous pouch The penile urethra serves as a common channel for situated at the base of the penis and below the conducting urine and seminal fluid to the exterior. symphysis pubis. It is divided into two It is contained within a cylinder of erectile tissue, compartments, each of which houses a testis, an the corpus cavernosum urethrae (corpus epididymis, and the lower part of the spermatic spongiosum) that lies ventral to a pair of similar cord. The scrotum consists only of skin and a erectile bodies called the corpora cavernosa penis. closely associated tunica dartos. Together the three structures make up the bulk of The scrotal skin is thin, pigmented, and the penis, the copulatory organ of the male. commonly thrown into folds. It contains many The corpora cavernosa penis begin as separate sweat glands, sebaceous glands that produce an bodies along the rami of the pubis on either side odorous secretion, and some coarse hairs, the and join at the pubic angle to form the shaft of the follicles of which are visible through the thin skin. penis. They are united by a common connective The tunica dartos underlies the skin and forms the tissue septum called the pectiniform septum, and septum that divides the scrotum into its two each corpus is surrounded by a thick, primarily compartments. It is firmly attached to the skin and collagenous sheath, the tunica albuginea. consists largely of smooth muscle cells and Trabeculae of collagenous and elastic fibers with collagenous connective tissue. The appearance of numerous smooth muscle cells extend into the the scrotum varies with the state of contraction of corpora from the tunica albuginea and divide the the smooth muscle. Under influence of cold, central regions of the corpora cavernosa into exercise, or sexual stimulation, the muscle contracts numerous cavernous spaces, those near the center and the scrotum becomes short and wrinkled. being the larger. These spaces are endothelial-lined The spermatic cord consists of several thin layers vascular spaces and are continuous with the arteries of connective tissue that are acquired from the that supply them and with draining veins. The anterior abdominal wall as the testes descend from the cavernous tissue of each corpus cavernosum penis abdominal cavity into the scrotal sac during communicates with the other through numerous development. It contains the ductus deferens, nerve slitlike openings in the pectiniform septum. fibers, lymphatic channels, testicular artery, and a The ventrally placed corpus cavernosum pampiniform plexus of testicular veins. As the urethrae (corpus spongiosum) ends in an testicular artery nears the testis, it becomes highly enlargement, the glans penis, which forms a cap convoluted and is surrounded by the venous plexus. over the ends of the corpora cavernosa penis. The proximity of the surrounding, cooler venous Structurally, corpus spongiosum is similar to the blood causes the arterial blood to lose heat and corpora cavernosa penis, but the tunica albuginea is provides a thermoregulatory (counter-current heat exchange) thinner and contains more elastic fibers and smooth mechanism for precooling incoming arterial blood. muscle cells, and the trabeculae are thinner and In this way the temperature of the testes is contain more elastic tissue. maintained a few degrees (2-3ºC) below body The three corpora are bound together by temperature, a condition necessary for production subcutaneous connective tissue that contains of sperm. The temperature can be elevated by numerous smooth muscle cells but is devoid of fat.

233 The shaft of the penis is covered by a thin, mobile penetrate the tunica albuginea to drain to the skin that shows a slight increase in pigmentation. exterior. The skin of the distal shaft, unlike that of the root, The structure and arrangement of the blood lacks hair but does contain scattered sweat glands. vessels in the cavernous tissues provide the The hairless skin of the glans penis is fused to the mechanism for erection. During erotic stimulation, the underlying connective tissue and is nonmobile. smooth muscle of the arterial and trabecular walls Unusual sebaceous glands not associated with hair relaxes, and blood pressure overcomes the elastic follicles occur in this region. resistance of the arteries. The helicine arteries dilate The glans penis is covered by a fold of skin called and straighten, and the vascular spaces of the the prepuce or foreskin, the inner surface of which cavernous tissues quickly fill with blood. The is moist and resembles a mucous membrane. lacunae, especially near the center of the cavernous Numerous free nerve endings are present in the bodies, become engorged with blood, thus epithelium of the glans penis, prepuce, and compressing the small peripheral spaces and veins subepithelial connective tissue of the urethra and against the non-yielding tunica albuginea, retarding skin. Meissner's corpuscles are associated with the the egress of blood; the erectile tissues of corpora epidermis of the genitals, and Vater-Pacini cavernosa penis become enlarged and rigid. During corpuscles are present in the deeper layers of the erection the intracavernous blood pressure may dermis. reach 1100 and 1200 mmHg in some individuals which is about 10 times normal blood pressure. Arterial Supply Corpus spongiosum does not become as rigid as the corpora cavernosa penis because there is less VOCABULARY: helicine arteries, intimal ridge compression of the venous drainage and the tunica albuginea is thinner and more yielding. The lesser The principal arterial supply to the penis is from rigidity of corpus spongiosum allows the urethra to two arteries that lie dorsal to the corpora cavernosa remain patent, which is essential for the passage of penis and deep arteries that run within the erectile semen during ejaculation. tissue of these structures. Branches from the dorsal After cessation of sexual activity, the smooth arteries penetrate the tunica albuginea and enter the muscle in the arteries and cavernous tissue regains cavernous tissue, where the arteries branch and its tone. The intimal ridges once again partially either form capillary plexuses or course distally in occlude the lumina of the arteries, thus reducing the the cavernous tissue. These helicine arteries are volume of incoming blood. Excess blood in the highly convoluted in the flaccid penis and take a vascular spaces of the erectile tissue is forced out by spiral course through the trabeculae of the contraction of smooth muscle cells in the cavernous tissue. The intima of most of these trabeculae and by recoil of surrounding elastic arteries, even before they enter the cavernous tissue. Gradually, the normal route of blood flow tissue, have long, ridgelike thickenings that project through the penis is restored, and the penis returns into and partially occlude the lumina. These intimal to the flaccid condition. ridges are most frequent where vessels branch. Sexual reflexes of men can be triggered by a variety of They consist of loose connective tissue that stimuli and involve intricate coordination of both contains many smooth muscle cells. Blood from the somatic and autonomic nerve pathways. During large central lacunae drains peripherally toward the arousal, tactile (cutaneous) stimuli are transmitted smaller vascular spaces and finally into a plexus of to the sacral region (S2-S4) of the spinal cord by veins at the periphery. The veins run along the afferent somatic fibers within the dorsal penile interior of the tunica albuginea, pierce the limiting nerve, a branch of the pudendal nerve. Impulses of tunica, and drain into the deep dorsal vein of the efferent parasympathetic fibers via the pelvic penis. splanchnic nerves result in vasodilatation of arteries The arterial blood supply to the corpus supplying the erectile tissues causing an erection. spongiosum is similar to that of the corpora Efferent sympathetic fibers stimulate smooth cavernosa penis, except for the venous drainage. muscle contraction within the following portions of Beginning at the lacunae, the veins of the corpus the male reproductive system: epididymis, ductus spongiosum have large openings and immediately deferens, prostate, and seminal vesicles.

234 These contractions are coordinated with rhythmic sequence of spermatogenic cells from contractions of skeletal muscle (primarily the spermatogonia to spermatids does not appear until bulbospongiosus) associated with the exterior of puberty. Thereafter, the single layer of columnar the penis that forcefully expel the ejaculate from the shaped Sertoli cells prominent in the seminiferous penile urethrae at emission. The muscle cells of these tubules of the prepubertal testis is hidden within the skeletal muscles are innervated by efferent somatic germinal epithelium by the large number of fibers of the pudendal nerve. spermatogenic cells. In old age, when the number of spermatogenic cells decreases, the Sertoli cell Organogenesis once again becomes a prominent feature of the seminiferous tubule. An indifferent gonad arises in the urogenital ridge, a The mesenchymal tissue in which the testis cords thickened area of mesoderm that contains the develop forms the septula testes, connective tissue primordia for the kidneys and gonads. Proliferation of the mediastinum testis, and tunica albuginea. of the peritoneal epithelium (mesothelium) on the Cells within the mesenchymal stroma differentiate ventromedial aspect of this ridge gives rise to a into interstitial cells (of Leydig). These are large and genital ridge, a longitudinal thickening several cells abundant from the forth to the sixth months of deep that runs parallel to the mesonephric ridge. development and produce large amounts of The indifferent gonad consists of a superficial testosterone. They then regress after birth only to epithelium and an internal gonadal blastema made increase again in number and size at puberty. up of ill-defined cords of cells derived from the Specific development of excurrent ducts does not superficial epithelium. Male sexual development is occur in the male. As the mesonephric kidney dependent on the expression of a sex-determining degenerates and is resorbed, its ducts are region (SRY gene) of the Y-chromosome or a female incorporated into the male reproductive system and phenotype will develop complete with ovary and transformed into genital ducts. The cranial oviduct. The SRY gene promotes the differentiation mesonephric tubules are modified and unite with and development of the Sertoli and Leydig cells. tubules of the rete testis. These efferent ductules In the XY genotypic male, the gonad rounds up, are continuous with mesonephric (Wolffian) ducts increases in size, and acquires a network of whose proximal region becomes highly convoluted branching (testis) cords and a tunica albuginea, to form the ductus epididymidis. Distally, the which identify it as a testis. With the appearance of mesonephric duct remains straight, forming the the tunica albuginea, the germinal epithelium ejaculatory duct and ductus deferens, which, near its distal reverts to a typical peritoneal mesothelium and no end, dilates to form the ampulla. Just beyond the longer plays a part in testicular development. ampulla, the epithelium of the ductus deferens Primordial germ cells migrate from the yolk sac during evaginates to provide the primordia for the seminal the fifth week of gestation and give rise to gonocytes; vesicles. Muscle coats are derived from the with cells from the gonadal blastema, they form the surrounding mesenchyme. The seminal vesicles testis cords. By the third month, fetal spermatogonia attain the adult pattern by the seventh fetal month are present. The testis cords extend radially toward and slowly grow until puberty, after which the adult the mesorchium (gonadal mesentery) and a dense proportions are reached. Testosterone is essential in region that will become the rete testis. The two the normal development of the epididymis, ductus become continuous and the testis cords elongate deferens, seminal vesicle and ejaculatory duct in the and differentiate into seminiferous tubules. Where they genotypic XY fetus. join the rete testis, the tubules remain straight and The prostate develops from multiple outgrowths of the form the tubuli recti. The seminiferous tubules only urethral epithelium, above and below the entrance contain only spermatogonia and Sertoli cells at this of the ejaculatory duct. Some 60 glands develop, stage of development. During development Sertoli organized into five groups that will become the cells elaborate Müllerian inhibitory factor (MIF), which lobes of the prostate. The mesenchyme into which inhibits further development of the the prostatic glands grow provides the paramesonephric (Müllerian) duct in the male fetus. fibromuscular stroma. The glands remain small The number of Sertoli cells remains constant from until puberty when, under the influence of birth, but the spermatogonia increase markedly testosterone, development is complete. between about 10 years of age and puberty. The full

235 The bulbourethral glands also arise as buds of as a consequence of rapid body growth and traction ectodermal epithelium, but in the region of the created by the gubernaculum. The testis descends urogenital sinus that forms the membranous beneath the peritoneum to reach the entrance of urethra. The outgrowths evaginate through the the presumptive inguinal canal by the third month investing mesenchyme to lie in the urogenital of fetal life. Shortly before birth the testis completes diaphragm. Mucous cells appear at about four its decent through the inguinal canal (trans-inguinal months. The bulbourethral glands, also are descent) and into the scrotum. A funnel-shaped influenced by testosterone, grow throughout outpocketing of the peritoneum called the processus puberty. The urethral glands (of Littré) develop vaginalis passes with the testis into the scrotal from outgrowths of the epithelium that lines the swelling along with other layers acquired from the urethra. abdominal wall. The processus vaginalis is Early in development of the external genitalia, the obliterated with the exception of a small space that cloacal membrane becomes surrounded by partially surrounds the testis. That region of the mesenchyme and forms two folds that unite processus vaginalis that remains to line the scrotal anteriorly to form the cloacal swelling. From the cavity forms the parietal layer of the tunica anterior part of the swelling, the genital (urethral) vaginalis, that which is associated with the testis folds develop, while the posterior part forms the anal forms the visceral layer of the tunica vaginalis. folds. The cloacal swelling increases in size, giving rise to a genital tubercle, at which time the genital Summary swellings become visible just lateral to the genital fold. In the male, the swellings are called scrotal Production of sperm occurs in the germinal swellings, which move caudally, fuse, and form the epithelium of the seminiferous tubules and scrotum. represents a holocrine (exocrine) secretion of the The genital tubercle elongates to form a phallus, testes. Spermatogenesis and spermiogenesis depend pulling the genital fold forward as it does. The on high concentrations of testosterone, the genital folds form the lateral walls of the urogenital endocrine secretion of the testes produced by groove, which extends along the caudal surface of the interstitial cells. phallus. The genital folds close over, fuse, and form Germinal epithelium essentially contains two the penile urethra, the tip of which lacks an orifice. major classes of cells: a mobile population of germ Ectodermal cells at the tip of the glans grow or spermatogenic cells and a stable population of inwards as a solid cord that later canalizes to supporting (Sertoli) cells. Seminiferous epithelium is establish continuity with the remainder of the unique in that it is divided into basal and adluminal urethra. Corpora cavernosa penis and urethrae compartments, each differing in their cellular develop from paired and single columns of content and environments. The basal compartment mesenchyme, respectively. The single column forms contains stem cells (spermatogonia) and newly around the penile urethra within the shaft and also formed primary spermatocytes that are exposed to gives rise to the glans. The external genitalia an environment similar to that of blood plasma. develop slowly after birth, and then grow rapidly The primary spermatocytes soon migrate into the during puberty. Dihydrotestosterone is the primary adluminal compartment, where, along with mediator in growth of the prostate, penis, and secondary spermatocytes, spermatids, and scrotum. spermatozoa, they are exposed to an environment By the end of the second month of prenatal life, rich in ions that contains amino acids not found in the differentiating testis lies on a common the basal compartment and has a high peritoneal fold called the urogenital fold which forms concentration of testosterone. The two different the gonadal ligaments. The cranial suspensory ligament environments are established and maintained by develops between the testis and the dorsal Sertoli cells, and the tight junctions between them abdominal wall near the 12th rib. The caudal contribute - at least in part – to a blood-testis ligament is known as the gubernaculum and links the barrier. Any substances that enter or leave the developing testis to the scrotal swelling. In humans, adluminal compartment must pass through the the gubernaculum proper consists only of cytoplasm of the Sertoli cells. mesenchymal cells. Both are important in the descent of the testis. A period of transabdominal descent occurs

236 Under the influence of gonadotropic hormone is absorbed in the ductuli efferentes and proximal (FSH), Sertoli cells release androgen-binding ductus epididymidis. protein that complexes with testosterone to Sperm entering the epididymis show weak, maintain a high concentration of hormone in the random movement, and for the most part they are adluminal compartment. The blood-testis barrier incapable of fertilization; spermatozoa from the also prevents escape of developing germ cells out of distal epididymis show strong, unidirectional the adluminal compartment and into general body motility and are capable of fertilization. Sperm are tissues. If this escape occurred, the haploid germ slowly propelled through the long ductus cells might be interpreted as foreign elements, epididymidis by peristaltic contractions of its resulting in immunologic responses directed against muscular walls. During their passage through the developing germ cells in the germinal epithelium epididymis, spermatozoa show continued and thus causing sterility. differentiation of the acrosome and sperm head, as Development of germ cells occurs in the well as progressive increase in the fertilizing adluminal compartment, in the embrace of the capacity. Sperm require the environment provided complex cytoplasmic processes of Sertoli cells. As by the epididymis to become physiologically mature spermatogenic cells develop and differentiate, the and viable. Although the maturation process is Sertoli cell is thought to move them actively along androgen-dependent, how the various segments of its lateral surfaces toward the lumen, where they are the epididymis control and regulate the physiologic released from the apical surface of the Sertoli cell. maturation of spermatozoa is unknown. The distal Gap junctions permit communication between ductus epididymidis stores sperm. Because of its Sertoli cells and allow for coordinated release of thick muscular wall, it can contract forcibly and, in spermatozoa from a given segment of a coordination with the wall of the ductus deferens, seminiferous tubule. Testosterone is present in the empty stored sperm into the prostatic urethra. adluminal compartment of the seminiferous tubules During ejaculation, the primary accessory sex and in the proximal duct system as far as the distal glands contribute successively to the seminal fluid. regions of the ductus epididymidis. Thus, there The bulbourethral and intraepithelial urethral glands exists the unusual circumstance of an endocrine secrete fluids that lubricate the urethra during secretion of the testis (testosterone and/or its sexual arousal and erection of the penis. At the metabolites) not only entering the bloodstream but onset of ejaculation, muscular tissue of the prostatic also passing through the lumen of the proximal stroma contracts, discharging the secretory product duct system to have an effect on the ductus into the prostatic urethra. The abundant, thin epididymidis. Systemic concentrations of secretion is slightly acidic and contains acid testosterone are essential for development and phosphatase. Sperm and their suspending fluids are functional maintenance of the rest of the excretory expelled from the distal ductus epididymidis and duct system and for the male secondary sex ductus deferens. The last component added to the characteristics. In many instances, testosterone ejaculate is the viscous secretion of the seminal serves primarily as a prohormone and its vesicles. This secretion is rich in fructose, an metabolites are the active factors. important energy source for the active, motile Dihydrotestosterone, for example, is the active spermatozoa. Seminal vesicles also secrete agent for the prostate and epididymis. prostaglandins in high concentration, that stimulate Much testicular fluid is produced by the germinal contractions within the female reproductive tract epithelium and serves as a vehicle for the transport that aid in moving spermatozoa to the site of of spermatozoa out of the seminiferous tubules and fertilization (ampulla of the oviduct). The final into the excretory duct system. Spermatozoa phase of physiologic maturation of spermatozoa is released into the seminiferous tubules are capacitation and occurs in the female reproductive nonmotile and are moved into the tubuli recti by tract, substantially increasing the number of the flow of testicular fluid and the shallow spermatozoa that are capable of fertilization. peristaltic movements of the seminiferous tubules. Testosterone and its metabolites are vital for the The ciliated epithelium and the increased smooth male reproductive system to develop properly and muscle in the walls of the ductuli efferentes to maintain normal structure and function. contribute to the movement of spermatozoa into Testosterone levels are high during in utero the proximal epididymis. Most of the testicular fluid development and produce the male phenotype.

237 These levels then fall, peak again at birth to fall which are under the influence of FSH, produce once again during childhood. Testosterone levels androgen-binding protein that functions to rise steeply at puberty, plateau in the adult and concentrate testosterone within the seminiferous slowly decline in old age. It is synthesized and tubules and the genital duct system. Secretion of released by the interstitial (Leydig) cells of the testis. FSH by cells (gonadotrophs) of the anterior Secretion is controlled by circulating levels of pituitary also is controlled by releasing factors from testosterone that are monitored by neurons within neurons within the hypothalamus. Inhibin released the arcuate nucleus of the hypothalamic region of by Sertoli cells suppresses FSH secretion by the brain that, through a releasing hormone anterior pituitary gonadotrophs and, through this (GnRH), influence the secretion of ICSH (LH) by negative feedback loop, functions to control the cells (gonadotrophs) of the anterior pituitary. rate of spermatogenesis. It is through this feedback system that normal testosterone levels are maintained. Sertoli cells,

238 compact feltwork of fine collagenous fibers and 18 Female Reproductive numerous fibroblasts. Elastic fibers are associated System mainly with blood vessels and otherwise are rare in the cortex. Scattered throughout the cortical tissue The female reproductive system consists of internal are the ovarian follicles, whose size varies with organs - the ovaries, oviducts, uterus, and vagina – their stage of development. Immediately beneath and external structures (external genitalia), which the surface epithelium, the connective tissue of the include the labia majora, labia minora, and clitoris. cortex is less cellular and more compact, forming a Although not genital organs, the mammary glands dense layer called the tunica albuginea. are important accessory organs of the female The stroma of the medulla consists of a loose reproductive system. For the first 10 or 11 years of connective tissue that is less cellular than that of the life, the reproductive organs remain immature and cortex and contains many elastic fibers and smooth growth parallels that of the body generally. During muscle cells. Numerous large, tortuous blood the 2 to 3 years prior to the first menstrual period, vessels, lymphatics, and nerves also are present in the generative organs increase in size, the breasts the medulla. enlarge, and pubic and axillary hair appears. Following the first menses and thereafter Ovarian Follicles throughout the reproductive period, the ovaries, oviducts, uterus, vagina, and mammary glands VOCABULARY: cortex, ovum, epithelial cells undergo cyclic changes in function and structure associated with the menstrual cycle and pregnancy. The follicles are located in the cortex of the ovary, During menopause, the cycles become irregular and deep to tunica albuginea, and consist of an eventually cease; in the postmenopausal period the immature ovum surrounded by one or more layers reproductive organs atrophy. of epithelial cells. At puberty, the human ovary contains 300,000 to 400,000 ova embedded in the Ovaries cortical stroma, but only a few reach maturity and are ovulated. Approximately 300 to 400 ova are

ovulated by a normal woman during her lifespan. The ovaries are paired oval bodies that lie on each During the menstrual cycle, several follicles (15-25) side of the uterus, suspended from the broad begin to grow and develop, but only one attains full ligament by a mesentery, the mesovarium. The maturity; the rest degenerate. The size of the follicle ovaries are homologous with the testes and are and the thickness of the epithelial envelope vary compound organs with exocrine and endocrine with the stage of development. During their functions. The exocrine secretion consists of whole growth, the follicles undergo a sequence of changes cells - the ova - and thus the ovary can be classified in which primordial, primary, secondary, and as a holocrine or cytogenic gland. In cyclic fashion mature follicles can be distinguished. it secretes female sex hormones directly into the bloodstream and therefore is an endocrine gland. Primordial Follicles Structure VOCABULARY: unilaminar, follicular cell,

primary oocyte, diplotene VOCABULARY: surface epithelium, cortex, ovarian follicle, tunica albuginea, medulla In the mature ovary, follicles are present in all

stages of development. Most, especially in young Each ovary is covered by a mesothelium that is women, are primordial or unilaminar follicles, continuous with that of the mesovarium. As this which are found in the peripheral cortex, just membrane extends over the surface of the ovary, beneath tunica albuginea. These follicles consist of the squamous cells become cuboidal and form the an immature ovum surrounded by a single layer of surface epithelium of the ovary. Sections of the flattened follicular cells that rest on a basement ovary show an outer cortex and an inner medulla, membrane. At this stage, the ovum is a primary but the boundary between the two regions is oocyte, suspended in the diplotene stage of the indistinct. The stroma of the cortex consists of a meiotic prophase.

239 The primary oocyte is a large cell with a prominent stratum granulosum by a distinct basement vesicular nucleus and nucleolus. The Golgi membrane. The cells differentiate into an inner apparatus is well developed, and spherical secretory layer, the theca interna, and an outer mitochondria tend to concentrate in the region of fibrous layer, the theca externa, but the boundary the centrosome. As the primordial follicle develops between the layers is somewhat indistinct. The into a primary follicle, changes occur in the ovum, theca externa merges imperceptibly into the follicular cells, and adjacent connective tissue of the surrounding stroma. Many small blood vessels cortex. penetrate the theca externa to provide a rich vascular network to the theca interna. The stratum Primary Follicles granulosum remains avascular until after ovulation. Follicle stimulating hormone (FSH) promotes the VOCABULARY: zona pellucida, granulosa cell, growth of a group of follicles resulting in an stratum granulosum, theca folliculi, theca increase in the production of estradiol and inhibin by interna, theca externa the granulosa cells. As estradiol and inhibin levels rise, secretion of FSH is decreased. By the end of During growth of a primary follicle, the oocyte the first week of the cycle falling FSH levels result increases in size, and the Golgi complex, originally a in the selection of a dominant follicle, the one that is single organelle next to the nucleus becomes a most sensitive to FSH. The dominant follicle multiple structure scattered throughout the continues to grow, produces large amounts of cytoplasm. Free ribosomes increase in number, estradiol, and is the one destined to ovulate while mitochondria become dispersed, and granular the others of the group are destined to become endoplasmic reticulum, although not prominent, atretic. As estradiol levels increase there is an becomes more extensive. A few lipid droplets and increase in the amount of LH produced by lipochrome pigments appear in the cytoplasm, and gonadotrophs in the anterior pituitary that is not yolk granules accumulate; these, however, are secreted. smaller and less numerous than in nonprimate species. As the oocyte grows, a clear refractile Secondary Follicles membrane, the zona pellucida, develops between the oocyte and adjacent follicular cells. The zona VOCABULARY: liquor folliculi, follicular pellucida consists of glycoproteins (ZP1, ZP2, ZP3) antrum, antral follicle, cumulus oophorus, secreted primarily by the oocytes, although a lesser corona radiata amount may be produced by surrounding granulosa cells. Microvilli extend from the oocyte into the As the follicular cells continue to proliferate, the zona pellucida. growing follicle assumes an ovoid shape and As the ovum grows, the flattened follicular cells of gradually sinks deeper into the cortex. When the the primordial follicles become cuboidal or stratum granulosum has become 8 to 12 cell layers columnar in shape and proliferate to form several thick, fluid-filled spaces appear between the layers. These cells now are referred to as granulosa granulosa cells. The fluid, the liquor folliculi, cells, and the layer of stratified epithelium that they increases in amount and the spaces fuse to form a form is called stratum granulosum. Mitochondria, single cavity called the follicular antrum. Liquor free ribosomes, and endoplasmic reticulum increase folliculi contains several growth factors, steroids, and the Golgi element become prominent. gonadotrophic hormone, and other substances Irregular, slender cytoplasmic processes extend many times their concentration in blood plasma. from the granulosa cells and penetrate the zona The follicle is now a secondary or antral follicle, and pellucida to make contact with processes from the the oocyte has reached its full size and undergoes oocyte. The processes of the oocyte and granulosa no further growth. The follicle, however, continues cells are united by gap junctions. to increase in size, due in part to continued While these changes occur in the oocyte and accumulation of liquor folliculi. The ovum is follicular cells, the adjacent stroma becomes eccentrically placed in the follicle, surrounded by a organized into a sheath or capsule, the theca mass of granulosa cells that projects into the fluid- folliculi, which surrounds the growing primary filled antrum, forming the cumulus oophorus. follicle. The theca folliculi is separated from the

240 The cells of the cumulus are continuous with those albuginea appears to be responsible for the granulosa cells lining the antral cavity. Granulosa breakdown of collagen fibers at the site of the cells that immediately surround the oocyte form the stigma. The stigma protrudes as a small blister and corona radiata and are anchored to the zona ruptures, and the ovum, with its surrounding zona pellucida by cytoplasmic processes. pellucida and adherent corona radiata, is extruded along with follicular fluid. Ovarian contractility aids in Mature Follicles and Ovulation ovulation. The released ovum measures about 120 µm in diameter. VOCABULARY: endocrine gland, aromatase enzyme complex, secondary oocyte, ovulation, Corpus Luteum stigma (macula pellucida) VOCABULARY: estrogen, progesterone, In women, a follicle requires 10 to 14 days to reach inhibin, granulosa lutein cell, theca lutein cell, maturity. The dominant follicle continues to make luteinization, corpus albicans more and more estradiol as the follicle continues to acquire an increase in the number of FSH and later The maturing follicle that acquires LH receptors is LH receptors. In addition to estradiol the growing the one of the group of developing follicles that goes follicle produces a small amount of progesterone. on to ovulate and is critical for subsequent corpus At maximum size, the follicle occupies the luteum formation after ovulation. Following thickness of the cortex and bulges from the surface ovulation, the follicle is transformed into a of the ovary. Fluid spaces appear between granulosa temporary endocrine structure, the corpus luteum, cells of the cumulus oophorus, and the connection which elaborates estrogens, progesterone and between the ovum and stratum granulosum is inhibin. The walls of the follicle collapse, and weakened. The theca folliculi has attained its stratum granulosum is thrown into folds. Bleeding greatest development, and the theca interna from capillaries in the theca interna may result in a assumes the cytologic characteristics of a steroid- blood clot in the center of the corpus luteum. At secreting endocrine gland. Androstenedione and this time it is sometimes referred to as a corpus testosterone are important androgens secreted by the hemorrhagicum. Granulosa cells increase greatly in theca interna that serve as substrates for estrogen size, take on a polyhedral shape, and transform into biosynthesis by the granulosa cells. The theca is large, pale-staining granulosa lutein cells. Lipid thought to be influenced primarily by luteinizing accumulates in the cytoplasm of the cells, smooth hormone. An aromatase enzyme complex within endoplasmic reticulum becomes abundant, and the granulosa cells converts the substrates to mitochondria show tubular cristae. Cells of the estradiol, a form of estrogen. Estradiol then diffuses theca interna also enlarge and become epithelioid in back across the limiting basement membrane of the character to form theca lutein cells. stratum granulosum to enter the circulation via The lutein cells derived from the theca interna are capillaries within the theca interna. Just before somewhat smaller than granulosa lutein cells. The ovulation, the oocyte completes the first meiotic division and process of formation of granulosa and theca lutein gives off the first polar body. Thus, at ovulation a cells is called luteinization. secondary oocyte is liberated. With the depolymerization of the basement Rupture of the mature follicle and liberation of the membrane between the theca interna and the ovum constitute ovulation, which normally occurs granulosa cells, capillaries from the theca interna at the middle (day 14) of the menstrual cycle. invade the lutein tissue to form a complex vascular Immediately before ovulation, further expansion of network throughout the corpus luteum. Connective the follicle occurs due to increased secretion of tissue from the theca interna also penetrates the liquor folliculi. Where the follicle bulges from the mass of lutein cells and forms a delicate network ovary, its wall becomes thinner, and a small, about them. The fully formed corpus luteum avascular, translucent area appears. This is the secretes estrogens, inhibin, and progesterone; stigma, or macula pellucida. The tunica albuginea secretion of progesterone increases rapidly during thins out, and the surface epithelium of the ovary luteinization, and high levels are maintained until becomes discontinuous in this area. A collagenase the corpus luteum involutes. produced by granulosa cells adjacent to the tunica

241 If the ovum is not fertilized, the corpus luteum interna frequently becomes thicker and forms a persists for about 14 days and then undergoes hyalinized, corrugated layer, the glassy membrane. involution. The cells decrease in size, accumulate This structure is characteristic of growing follicles much lipid, and degenerate. Hyaline material that have undergone atresia and aids in accumulates between the lutein cells and the corpus distinguishing a large atretic follicle from a corpus luteum gradually is replaced by an irregular white luteum. Other differences include degenerative scar of dense connective tissue, the corpus changes in the granulosa cells and the presence of albicans. Over the following months, the corpus fragments of zona pellucida at the center of a albicans itself disappears. If fertilization occurs, the follicle without an associated oocyte. Ultimately, the corpus luteum enlarges further and persists for degenerated remains of the follicle are removed and about the first 6 months of pregnancy and then a scar resembling a small corpus albicans is left. gradually declines. Following delivery, involution of This, too, eventually disappears into the stroma of the corpus luteum is accelerated, resulting in the the ovary. formation of a corpus albicans. In some mammals, especially rodents, clusters of Estradiol, progesterone and inhibin initially epithelioid cells are scattered in the stroma of the suppress FSH production by gonadotrophs in the cortex. These interstitial cells contain small lipid anterior pituitary, but if pregnancy does not occur and droplets and bear a marked resemblance to luteal the corpus luteum declines, FSH levels rise and a cells. It is thought that these interstitial cells arise new group of ovarian follicles is stimulated to from the theca interna of follicles that are develop and the cycle repeats. undergoing atresia. In humans, interstitial cells are most abundant during the first year of life, the Atresia of Follicles period during which atretic follicles are most numerous. In women, they are present only in VOCABULARY: atresia, glassy membrane, widely scattered, small groups. Their exact role in hilus cells ovarian physiology is unknown; in humans they elaborate androgens. During the early part of each menstrual cycle, Other large epithelioid cells, the hilus cells, are several primordial follicles begin to grow, but found associated with vascular spaces and usually only one attains full development and is unmyelinated nerves in the hilus of the ovary. The ovulated. The remainder undergo a degenerative cells appear similar to the interstitial cells of the process called atresia, which may occur at any stage testis and contain lipid, lipochrome pigments, and in the development of a follicle. In atresia of a cholesterol esters. Hilus cells are most commonly primary follicle, the ovum shrinks, degenerates, and found during pregnancy and at menopause, but undergoes cytolysis; the follicular cells show similar their function is unknown. Tumors arising from degenerative changes. The follicle is resorbed, and these cells have a masculinizing effect. the small space that is left is rapidly filled by connective tissue from the stroma. Similar The Ovarian Cycle degenerative changes occur in larger follicles, but the zona pellucida may persist for a time after VOCABULARY: follicle-stimulating hormone, dissolution of the oocyte and follicular cells. luteinizing hormone, inhibin Macrophages invade the atretic follicle and engulf the degenerating material, including fragments of During the ovarian cycle gonadotrophin releasing the zona pellucida. hormone (GnRH) is synthesized by neurons in the Cells of the theca interna persist longer than those arcuate nucleus of the hypothalamus and released in of the stratum granulosum but also show pulsatile fashion to stimulate follicle-stimulating hormone degenerative changes. The theca cells increase in (FSH) and luteinizing hormone (LH) synthesis and size, lipid droplets appear in the cytoplasm, and the release by gonadotrophs within the anterior cells assume an epithelioid character, similar to that pituitary. The receptor for GnRH on the of lutein cells. The cells assume a cordlike gonadotrophs is a G-protein. The gonadotrophic arrangement, the cords being separated by hormones also are released in pulsatile fashion connective tissue fibers and capillaries. The basal corresponding to the GnRH pulses. lamina between granulosa cells and the theca

242 Both the FSH and LH glycoprotein molecules bind Oviducts to a G-protein linked cell-surface receptor which increases the production of cAMP in the The oviducts (uterine tubes) are a pair of muscular appropriate cell. Maturation of ovarian follicles, tubes that extend from the ovary to the uterus their endocrine functions, and the phenomenon of along the upper margin of the broad ligament. One ovulation are regulated by the follicle stimulating end of the tube is closely related to the ovary and, and luteinizing hormones. at this end, is open to the peritoneal cavity. The Follicle-stimulating hormone is responsible for other end communicates with the lumen of the the growth of follicles and stimulates the aromatase uterus. The oviduct delivers the ovum, released at enzyme complex within granulosa cells to produce ovulation, to the uterine cavity and provides an estrogens and inhibin. Together with FSH, environment for fertilization and initial segmentation of the luteinizing hormone induces ripening of the fertilized ovum. mature follicle and ovulation. Alone, LH converts the ovulated follicle into a corpus luteum and Structure induces it to secrete progesterone, estrogen and inhibin. The cyclic nature of follicle formation and VOCABULARY: infundibulum, fimbria, ovulation is the result of reciprocal interaction ampulla, isthmus, intramural (interstitial) part between the hypothalamic-pituitary axis and ovarian hormones. The oviduct is divided into several segments. The As production of estrogens and inhibin by the ovarian end, the infundibulum, is funnel-shaped, granulosa cells increases, release of FSH from the and its margin is drawn out in numerous tapering pituitary is inhibited, and the level of FSH falls below processes called fimbria. The infundibulum opens that needed for maturation of new follicles. into the tortuous, thin-walled ampulla of the tube, However, the rising level of estrogen does which makes up slightly more than half the length stimulates a surge in the release of LH from the of the oviduct. The ampulla is continuous with the pituitary, resulting in ovulation, formation of a isthmus, a narrower, cordlike portion that makes corpus luteum, and secretion of progesterone, up about the medial one-third. The intramural estrogen and inhibin by this structure. Increasing (interstitial) part is the continuation of the tube levels of progesterone from the corpus luteum now where it passes through the uterine wall. As in other inhibit release of LH from the pituitary, and as the hollow viscera, the wall of the oviduct consists of level of LH declines, the corpus luteum can no several layers: an external serosa, an intermediate longer be maintained. With decline of the corpus muscularis, and an internal mucosa. luteum, progesterone and estrogen levels diminish, the pituitary no longer is inhibited from secreting Mucosa FSH, and a new cycle of follicle formation is initiated. The ovarian steroid hormones have only VOCABULARY: plica, columnar epithelial cell, minor effects on the pituitary directly; their primary lamina propria, ciliated cell, nonciliated cell, action is mediated through neurons in the cyclic change hypothalamus. Both FSH and ovarian androgens stimulate granulosa cells to synthesize and release a The mucosa of the oviduct presents a series of glycoprotein called inhibin. Inhibin preferentially longitudinal folds called plicae. In the ampulla the suppresses the synthesis and secretion of FSH by plicae have secondary and even tertiary folds to gonadotrophs in the anterior pituitary. create a complex labyrinth of epithelial-lined spaces.

In the isthmus the plicae are shorter with little Age and the Ovary branching, while in the intramural part the plicae form only low ridges. Throughout the tube, the The ovary looses its ability to produce viable ova plicae consist of a single layer of columnar with age. In women at 42 years of age the ability of epithelial cells resting on an incomplete basement the ovary to produce viable ova is reduced to about membrane and a lamina propria of richly cellular 50%. By 45 years of age ovarian failure in women is connective tissue that contains a network of nearly 100%. reticular fibers and fibroblasts.

243 The epithelium decreases in height from ampulla to 5 cm at its widest (upper) part, and 2.5 to 3.0 cm uterus and consists of ciliated and nonciliated cells. thick. It is slightly flattened dorsoventrally, and the The ciliated cells are most numerous on the luminal cavity corresponds to the overall shape of surface of the fimbria and progressively decrease in the organ. The uterus receives the fertilized ovum number through the ampulla, isthmus, and and nourishes the embryo and fetus throughout its intramural portions. The nonciliated cells appear development until birth. to be secretory and help establish an environment that is suitable for the survival and fertilization of Structure the ovum and for maintenance of the zygote. A third cell type, an undifferentiated cell with a darkly VOCABULARY: body, cervix, cervical canal, staining nucleus, may represent a precursor of the endometrium, myometrium, perimetrium secretory cells or an exhausted secretory cell. The epithelium shows cyclic changes associated Several regions of the uterus can be distinguished. with ovarian cycles. During the follicular phase, The bulk of the organ consists of the body, which ciliated cells increase in height, reaching their comprises the upper expanded portion. The dome- maximum at about the time of ovulation, and there shaped part of the body between the junctions with is evidence of increased activity in the secretory the oviduct constitutes the fundus. Below, the uterus cells. In the luteal phase, ciliated cells decrease in narrows and becomes more cylindrical in shape: height and lose their cilia, and there is augmented this region forms the cervix, part of which secretory activity by the secretory cells. Loss of cilia protrudes into the vagina. The cervical canal is greatest in the fimbria and least in the isthmus. passes through the cervix from the uterine cavity The cilia are responsive to steroid hormones: estrogen and communicates with the vaginal lumen at the appears to be responsible for the appearance and external os. maintenance of cilia, and progesterone increases the The wall of the uterus is made up of several layers rate at which they beat. Other ciliated cells located that have specific names: the internal lining or elsewhere in the body show no such hormone mucosa is called the endometrium; the middle responsiveness. muscular layer forms the myometrium; and the external layer is referred to as the perimetrium. Muscular Coat and Serosa The perimetrium is the serosal or peritoneal layer that covers the body of the uterus and supravaginal VOCABULARY: inner circular muscle, outer part of the cervix posteriorly and the body of the longitudinal muscle uterus anteriorly.

The mucous membrane rests directly on the muscle Myometrium coat, and there is no submucosa. The muscle coat consists of two layers of smooth muscle cells, but VOCABULARY: smooth muscle, internal layer, the layers are not sharply defined. The inner layer middle layer, stratum vasculare, outer layer is circular or closely spiraled; the outer layer of longitudinal muscle is the thinner. The muscularis The bulk of the uterine wall consists of the increases in thickness toward the uterus due to the myometrium, which forms a thick coat about 15 to increased depth of the inner layer. Externally, the 20 mm in depth. The myometrium consists of oviduct is covered by a serosa that represents the bundles of smooth muscle cells separated by thin peritoneal covering of the organ. Fertilization usually strands of connective tissue that contain fibroblasts, occurs in the ampulla of the oviduct. collagenous and reticular fibers, mast cells, and macrophages. The muscle forms several layers that Uterus are not sharply defined because of the intermingling of smooth muscle cells from one layer to another. The human uterus is a single, hollow, pear-shaped Generally, however, internal, middle, and outer organ with a thick muscular wall; it lies in the pelvic layers of smooth muscle can be distinguished. The cavity between the bladder and rectum. The internal layer is thin and consists of longitudinal nonpregnant uterus varies in size depending on the and circularly arranged smooth muscle cells. individual but generally is about 7 cm in length, 3 to

244 The middle layer is the thickest and shows no cycle. The cyclic activity begins at puberty and regularity in the arrangement of the smooth muscle continues until menopause. In the body of the cells, which run longitudinally, obliquely, circularly, uterus, the endometrium consists of a thick lamina and transversely. This layer also contains many large propria (endometrial stroma) and a covering blood vessels and has been called the stratum epithelium. The stroma resembles mesenchymal vasculare. The outer layer of smooth muscle tissue and consists of loosely arranged stellate cells consists mainly of longitudinally oriented cells, with large, round or ovoid nuclei supported by a some of which extend into the broad ligament, network of fine connective tissue in which oviducts, and ovarian ligaments. Elastic fibers are lymphocytes, granular leukocytes, and macrophages prominent in the outer layer but are not present in are scattered. There is no submucosa, and the the inner layer of the myometrium except around stroma lies directly on the myometrium, to which it blood vessels. is firmly attached. In the nonpregnant uterus, the smooth muscle The stroma is covered by a simple columnar cells are 30 to 50 µm long, but during pregnancy, epithelium that contains ciliated cells and they hypertrophy to reach lengths of 500 to 600 µm nonciliated secretory cells. The epithelium dips into or greater. New smooth muscle cells are produced the stroma to form numerous uterine glands that in the pregnant uterus from undifferentiated cells extend deeply into the stroma, occasionally and possibly from division of mature cells also. The penetrating into the myometrium. Most are simple connective tissue of the myometrium also increases tubular glands, but some may branch near the in amount during pregnancy. In spite of a total muscle. There are fewer ciliated cells in the glands increase in muscle mass, the layers are thinned than in the covering epithelium. A basement during pregnancy as the uterus becomes distended. membrane underlies the glandular and surface After delivery, the muscle cells rapidly decrease in epithelia. size, but the uterus does not regain its original, The endometrium can be divided into a stratum nonpregnant dimensions. basale (basal layer) and a stratum functionale The myometrium normally undergoes intermittent (functional layer), which differ in their structure, contractions that, however, are not intense enough function, and blood supply. The stratum basale is to be perceived. The intensity may increase during the narrower, more cellular, and more fibrous layer menstruation to result in cramps. The contractions and lies directly on the myometrium. Occasionally, are diminished during pregnancy, possibly in small pockets of stratum basale may extend into the response to the hormone relaxin. At parturition, myometrium, between muscle cells. This layer strong contractions of the uterine musculature undergoes few changes during the menstrual cycle occur, causing the fetus to be expelled. Uterine and is not shed at menstruation but serves as the source contractions increase in response to or after from which the functional layer is restored. administration of oxytocin. This hormone is The stratum functionale extends to the lumen of synthesized by neurons forming the supraoptic and the uterus and is the part of the endometrium in paraventricular nuclei of the hypothalamus and which cyclic changes occur and which is shed released at the neurohypophysis. Uterine during menstruation. The stratum functionale contractions also increase in response to sometimes is subdivided into the compacta, a narrow prostaglandins. A rise in the level of prostaglandins superficial zone, and the spongiosa, a broader zone occurs just prior to delivery. that forms the bulk of the functionalis. The blood supply of the endometrium is unique Endometrium and plays an important role in the events of menstruation. Branches of the uterine arteries VOCABULARY: endometrial stroma, simple penetrate the myometrium to its middle layer, columnar epithelium, uterine gland, stratum where they furnish arcuate arteries that run basale, stratum functionale, radial artery, circumferentially in the myometrium. One set of straight artery, spiral artery, lacuna branches from these arteries supplies the superficial layers of the myometrium, while other branches, The endometrium is a complex mucous membrane the radial arteries, pass inward to supply the that, in women, undergoes cyclic changes in endometrium. structure and function in response to the ovarian

245 At the junction of myometrium and endometrium, Cyclic Changes in the Endometrium the radial branches provide a dual circulation to the endometrium. VOCABULARY: proliferative phase, Straight arteries supply the stratum basale, while maturation of follicle, secretory phase, corpus the stratum functionale is supplied by highly coiled luteum, ischemic (premenstrual) phase, spiral arteries (Fig. 18-1). As the latter pass menstrual phase through the functional layer, they provide terminal arterioles, which unite with a complex network of During a normal menstrual cycle, the endometrium capillaries and thin-walled, dilated vascular undergoes a continuous sequence of changes in structures, the lacunae. The venous system forms which four phases can be described. The phases an irregular network of venules and veins with correlate with the functional activities of the ovaries irregular sinusoidal enlargements and then drains and constitute the proliferative, secretory, ischemic into a plexus at the junction of myometrium and (premenstrual), and menstrual phases. endometrium. During menstrual cycles, the spiral The proliferative phase begins at the end of the arteries constrict periodically, subjecting the functional menstrual flow and extends to about the middle of layer to intermittent periods of anoxia. The distal the cycle. This stage is characterized by rapid portions of the arterial supply in the functionalis regeneration and repair of the endometrium. Epithelial undergo degeneration and regeneration with each cells in the remnants of the uterine glands in the menstrual cycle, whereas the straight arteries of the stratum basale proliferate and migrate over the raw basal layer show no such changes. surface of the mucosa; stromal cells also proliferate. In the early part of this period, the endometrium is of limited thickness, and its glands are sparse and fairly straight and have small lumina. The epithelium of the glands and surface is simple cuboidal to low columnar, and mitoses are present in the glandular lining. As the proliferative phase advances, the endometrial glands increase in number and length and become more tortuous and more closely spaced, and the lumina widen. Toward the end of the proliferative phase (days 14-16), glycogen accumulates in the basal region of the glandular epithelium. The surface and glandular epithelia now are tall columnar with fewer ciliated and more secretory cells. The secretory cells have large numbers of small mitochondria, but the endoplasmic reticulum and Golgi complex are poorly developed. Microvilli are present on the free borders of the cells. The nuclei are round or oval and contain finely granular chromatin with one or more nucleoli. The spiral arteries lengthen but are lightly coiled and do not extend into the superficial third of the endometrium. The proliferative phase corresponds to the maturation of the ovarian follicle up to the time of ovulation. Estrogen, secreted by the developing follicles, stimulates growth of the endometrium; some growth may continue for a day or two after

ovulation. Fig. 18-1. A diagrammatic sketch illustrating the arrangement of blood vessels in the endometrium. During the secretory phase, the endometrium

continues to increase in thickness as a result of

hypertrophy of stromal and glandular cells, stromal

edema, and increased vascularity.

246 The glands lengthen and become irregularly coiled Cervix and convoluted and show wide, irregular lumina. The epithelial cells become larger, and their free VOCABULARY: dense connective tissue, surfaces show many long microvilli. Mitochondria endocervix, exocervix, nonkeratinized stratified are large, there is a rich endoplasmic reticulum, and squamous epithelium, secretory change the Golgi apparatus is prominent. Nuclei are enlarged and contain distinct nucleoli. The wall of the cervix differs considerably from Glycogen and mucoid materials rapidly increase, that of the body of the uterus. Little smooth muscle first in the bases of the glandular cells, then in the is present, and the cervical wall consists mainly of apical portions of the cells, and then into the lumina dense connective tissue and elastic fibers. In the of the glands. Elongation and coiling of the spiral part that protrudes into the vaginal canal, smooth arteries continue, and the vessels extend into the muscle is lacking. The cervical canal is lined by a superficial part of the endometrium. mucosa, the endocervix, which forms complex, The secretory phase is associated with branching folds. The epithelial lining consists of development of a corpus luteum and is maintained tall, mucus-secreting columnar cells along with a as long as the corpus luteum remains functional. few ciliated columnar cells. Numerous large, Progesterone secreted by the corpus luteum is the branched cervical glands are present that are lined by steroid primarily responsible for the secretory mucus-secreting columnar cells similar to those of changes in the endometrium. the lining epithelium. The cervical canal usually is The ischemic (premenstrual) phase is filled with mucus. Occasionally the glands become characterized by intermittent constriction of the occluded and filled with secretion, forming spiral arteries, resulting in vascular stasis and Nabothian cysts. reduced blood flow to the functional layer. The The portion of the cervix that protrudes into the outer zone of the endometrium is subjected to vaginal canal, the exocervix, is covered by a anoxia for hours at a time, resulting in breakdown nonkeratinized stratified squamous epithelium of the stratum functionale. The stroma becomes whose cells contain much glycogen. The transition increasingly edematous and infiltrated by from the simple columnar epithelium of the cervical leukocytes. canal is abrupt and usually occurs near the external In the menstrual phase, the functional layer os. The cervical mucosa does not take part in the becomes necrotic and is shed. The spiral arteries dramatic cyclic changes that occur in the also become necrotic, and blood is lost from the endometrial lining of the body of the uterus and is arteries and veins. Small lakes of blood form and not shed. The glandular elements do show changes coalesce, and overlying patches of mucosa are in secretory activity, however. At midcycle there is detached, leaving a denuded stromal surface. a copious secretion of a thin alkaline fluid, probably Sloughing of the endometrium continues until the the result of increased stimulation by estrogen. entire functional layer has been discarded. Blood After ovulation and establishment of a corpus oozes from veins torn during the shedding of the luteum, the amount of secretion decreases and the endometrial tissue. The menstrual discharge mucus becomes thicker and more viscous. consists of arterial and venous blood, autolyzed and The lumen of the cervix is usually less than 1 cm degenerated epithelial cells, and glandular in diameter but will expand to 10 cm or more at the secretions. The straight arteries of stratum basale do time of birth. The dilatation is the result of not constrict during menstruation, thus preserving increased elasticity and a softening of the cervical the basal layer to provide for restoration of the stroma during labor. The changes that occur within endometrium during the following new proliferative the cervical stroma are complex and not completely stage. The onset of the menstrual cycle coincides understood. However, changes are known to occur with the beginning involution of a corpus luteum. with regard to the making up of glycosaminoglycans in the ground substance during labor. Hyaluronic acid levels increase, binding more water, whereas concentrations of dermatan sulfate decrease, reducing the number of cross linkages between extracellular fibers within the stroma.

247 The net result of these changes is a softening of the into the surrounding loose connective tissue around matrix with a separation of connective tissue fibers. other organs and acts to hold the vagina in place. In addition, the collagen and elastic fibers within the stroma are shortened and rearranged in such a Cytology of the Cervical Canal way as to decreased tensile strength and resistance allowing the dilatation to occur. VOCABULARY: basal cell, parabasal cell, intermediate squamous cell, superficial Vagina squamous cell

The vagina is the lower-most portion of the female The nonkeratinized stratified squamous epithelium reproductive tract and is a muscular tube that joins that lines the vagina and covers the exocervix is the uterus to the exterior of the body. arranged in layers similar to those of the epidermis. It represents the copulatory organ of the female. Deepest is the basal layer (stratum germinativum); Ordinarily the lumen is collapsed and the anterior followed by an intermediate (spinous) layer and a and posterior walls make contact. superficial layer (stratum corneum) from which cells are shed. Desquamated cervical cells can be Structure recovered from vaginal secretions, or cells can be obtained more directly by gentle abrasion of the VOCABULARY: mucosa, rugae, exocervix. The cells can be studied in smear nonkeratinized stratified squamous epithelium, preparations, and examination of Papanicolaou's lamina propria, muscularis, adventitia ("Pap") smears has become a standard procedure for early detection of cervical cancer. The vaginal wall consists of a mucosa, a muscularis, In normal women, four types of cells usually are and an adventitia. The mucosa is thrown into folds recognized cytologically, loosely corresponding to (rugae) and consists of a thick surface layer of the layers of the cervicovaginal epithelium. These nonkeratinized stratified squamous epithelium include basal (lower basal) cells, parabasal (outer overlying a lamina propria. The epithelial cells basal) cells, intermediate (precornified) squamous contain abundant glycogen, especially at midcycle. cells, and superficial (cornified) squamous cells. The glycogen of sloughed cells is broken down by Germinal cells from the basal layer are exceedingly commensal lactobacilli within the vaginal lumen and rare in smears from normal adults, and the basal results in lactic acid being formed. The acid pH and parabasal cells recognized cytologically are creates an environment favorable to the commensal derived from different levels of the intermediate bacterial flora and deters growth of fungi (Candida layer. They do represent less mature cells, and their albicans) and bacterial pathogens. The lamina presence indicates that immature cells are at higher propria consists of a fairly dense connective tissue than normal levels in the epithelium. These cells are that becomes more loosely arranged near the associated with marked deficiency of estrogens and muscle coat. Immediately beneath the epithelial are common before puberty and during menopause. lining, elastic fibers form a dense network. Diffuse Basal cells arise from the lower levels of the and nodular lymphatic tissues are found transitional zone and are rounded or oval cells, occasionally, and many lymphocytes, along with about 4 to 5 times the size of a granular leukocyte. granular leukocytes, invade the epithelium. The The central nucleus is deeply stained, but a pattern vagina lacks glands, and the epithelium is kept moist of fine chromatin granules and dense patches can by secretions from the cervix. be made out. In some cells a Barr body is distinctly The muscularis consists of bundles of smooth visible. The moderate amount of basophilic muscle cells that are arranged circularly in the inner cytoplasm stains deeply and evenly. Parabasal cells layer and longitudinally in the outer layer. The arise higher up in the transitional zone and also are longitudinally oriented smooth muscle cells become round or oval cells, but they are larger than basal continuous with similarly oriented cells in the cells with a more abundant cytoplasm that is less myometrium. basophilic and often shows a somewhat "blotchy" The adventitia is a thin outer layer of connective pattern. The central nucleus remains about the tissue rich in elastic fibers. It merges imperceptibly same size but may be more dense than that of a basal cell.

248 Intermediate squamous cells vary in size, but all tissue papillae. The deeper layers of epithelium are appear as thin, polygonal plates with abundant pigmented. Both surfaces of the labia minora are transparent cytoplasm. The nucleus is smaller, devoid of hair, but large sebaceous glands are vesicular, and centrally placed. The cytoplasm stains present. somewhat variably and may be lightly basophilic or show some degree of eosinophilia. Superficial Clitoris squamous cells represent dead surface cells. They are large, with voluminous eosinophilic cytoplasm VOCABULARY: erectile, corresponds to dorsal that is thin and transparent with sharply defined penis, corpora cavernosa, glans clitoridis, no borders. Because of the thinness, the cytoplasm corpus spongiosum often is curled, wrinkled, and folded. The nucleus is very small - about one-half to one-third that of an The clitoris is an erectile body that corresponds to intermediate squamous cell - and is densely stained the dorsal penis. It consists of two corpora and pyknotic. cavernosa enclosed in a layer of fibrous connective The superficial and intermediate squamous cells are tissue and separated by an incomplete septum. The the largest cells seen in a routine preparation and free end of the clitoris terminates in a small, range from 40 to 50 µm in diameter. Occasionally, rounded tubercle, the glans clitoridis, which anucleate squamous cells may be found. They have consists of spongy erectile tissue. The clitoris is a somewhat shriveled appearance, and the site of covered by a thin layer of nonkeratinized stratified the nucleus is suggested by a pale central zone. squamous epithelium with high papillae associated They represent cells that are more completely with many specialized nerve endings. The clitoris keratinized. has no corpus spongiosum and therefore does Cells that originate from the endocervix also may not contain the urethra. be present and often occur in small sheets or strips; their appearance depends on the orientation. From Vestibular Glands end on, the cells appear as groups or nests of small polyhedral or round cells with sharp cell boundaries VOCABULARY: stratified squamous and relatively large central nuclei. In profile, the epithelium, mucous cell, major vestibular cells show their columnar shape with the nuclei gland, compound tubuloalveolar gland close to one pole. Strips of these cells give a "picket fence" appearance. The vestibule is the cleft between the labia minora and in it are the vaginal and urethral openings. It is External Genitalia lined by stratified squamous epithelium and contains numerous small vestibular glands The external genitalia of the female consist of the concentrated about the openings of the vagina and labia, clitoris, and vestibular glands. urethra. The glands contain mucous cells and resemble the urethral glands of the male. A pair of Labia larger glands, the greater or major vestibular glands, are present in the lateral walls of the VOCABULARY: labia majora, homologue of vestibule. They are compound tubuloalveolar the scrotum, labia minora glands that secrete a clear, mucoid lubricating fluid. The major glands correspond to the bulbourethral The labia majora consist of folds of skin covering glands of the male. an abundance of adipose tissue. In the adult, the outer surface is covered by coarse hair with many Pregnancy sweat and sebaceous glands. The inner surface is smooth and hairless and also contains sweat and Pregnancy involves implantation of a blastocyst sebaceous glands. The labia majora are the into a prepared uterine endometrium and homologue of the scrotum in the male. The labia subsequent formation of a placenta to nourish and minora consist of a core of highly vascular, loose maintain the developing embryo. connective tissue covered by stratified squamous epithelium that is deeply indented by connective

249 Prior to implantation, fertilization of the ovum and spermatozoon pushes through the hole in the zona cleavage of the resulting zygote usually occurs in pellucida to enter the ovum, a period of hyperactivity the ampulla of the oviduct. of flagellar beat occurs, propelling the spermatozoon into the ovum. The entire spermatozoon is Fertilization engulfed by the cytoplasm of the ovum. Thus, flagellar beat appears to be more important at the VOCABULARY: secondary oocyte, female moment of fertilization rather than getting to the pronucleus, male pronucleus site of fertilization. Electron micrographs suggest that the plasma membranes of the sperm and ovum Before an ovum can be fertilized, it must undergo fuse, that of the spermatozoon being left at the maturational changes, chief of which is reduction of surface of the ovum. Penetration is followed by the chromosome complement to the haploid release of electron-dense cortical granules that underlie number. The oocyte passes through the early stages the plasmalemma of the ovum (cortical granule of the first meiotic division during fetal life, and it is reaction) and by immediate changes in the only just before ovulation that the division is completed and permeability of the zona pellucida, which thereafter the first polar body is given off. The resulting secondary excludes entry by any additional competing sperm oocyte immediately enters the second meiotic (polyspermy). division, which, however, proceeds only to The ovum now completes the second maturation division metaphase and is not completed until fertilization occurs. and extrudes the second polar body. The remaining At the time of ovulation, the oviduct shows active chromosomes (23) reconstitute and form the movements that bring the infundibulum and female pronucleus. The nucleus (head) of the fimbria close to the ovary. Cilia on the surface of spermatozoon swells and forms the male the fimbria sweep the ovum into the ampulla of the pronucleus, and the sperm body and tail are oviduct where fertilization, if it is to occur, takes resorbed. The two pronuclei move to the center of place. The human ovum remains fertile for between the cell, and two centrioles, supplied by the anterior 24 and 48 hours, after which it degenerates if centriole of the spermatozoon, appear. The fertilization does not occur. chromatin of each pronucleus resolves into a set of Approximately 300 million sperm are released into chromosomes that align themselves on a spindle to the vaginal lumen during coitus but it is estimated undergo a normal mitotic division of the first that only 300-500 spermatozoa reach the site of cleavage. Each cell resulting from this division fertilization. These remain viable for about 72 receives a full diploid set of chromosomes. hours. Muscular contractions within the walls of the uterus and oviduct propel the spermatozoa to the Cleavage proximal region (ampulla) of the oviduct where fertilization takes place. The smooth muscle cells VOCABULARY: blastomere, morula, are thought to contract in response to blastocele, blastocyst, inner cell mass, prostaglandins and/or oxytocin released during trophoblast sexual intercourse. Of the millions of sperm initially deposited in the The zygote undergoes a series of rapid divisions female tract, only one penetrates the ovum. There is no called cleavage that result in a large number of cells, evidence for chemotactic attraction, and random the blastomeres. The cells are bounded by the movement brings sperm and ovum together. The zona pellucida, and a mulberry-like body, the zona pellucida is important in fertilization as it provides morula, is formed. Cleavage is a fractionating sperm recognition sites for sperm binding and is the process: no new cytoplasm is formed, and at each most efficient trigger of the sperm acrosome reaction. Both division the cells become smaller until a normal, sperm binding and induction of the acrosome predetermined, cytoplasmic-nuclear ratio is reached. reaction are ligand-receptor interactions, the ligands Thus, the total size of the morula is not increased. of which are located in the ZP3 glycoprotein of the Cleavage occurs as the morula slowly is moved zona pellucida. The acrosome reaction results in the along the oviduct by the waves of peristaltic release of acrosomal enzymes (acrosin and trypsin- contractions in the muscle coat. When the morula like enzymes) needed to digest a hole in the zona reaches the uterine cavity, it is at the 12- to 16-cell pellucida. At the time of fertilization, when a stage.

250 At about this time, fluid penetrates the zona the two layers become closely locked by numerous pellucida and diffuses between the cells of the tight junctions that develop between them. The morula. The fluid increases in amount, the uterine epithelial cells degenerate and are engulfed intercellular spaces become confluent, and a single by the trophoblast, the cellular debris appearing in cavity, the blastocele, is formed. The morula has phagosomes within the trophoblast cytoplasm. now become a blastocyst that forms a hollow Where it is fixed to the endothelium, the sphere containing, at one pole, a mass of cells called trophoblast proliferates to form a cellular mass the inner cell mass that will form the embryo between the blastocyst and maternal tissues. No cell proper. The capsule-like wall of the blastocyst boundaries can be made out in this cell mass, which consists of a single layer of cells, the trophoblast. is called the syncytial trophoblast. The syncytium After reaching a critical mass, the blastocyst breaks continues to erode the endometrium at the point of through the surrounding zona pellucida and remains contact, creating a ragged cavity into which the free within the uterine cavity for about a day; then it blastocyst sinks, gradually becoming more deeply attaches to the endometrium, which is in the embedded until it lies entirely within the secretory phase. endometrial stroma. The surface defect in the Encasement within the zona pellucida protects the endometrium is closed temporarily by a fibrin plug. forming blastocyst from the possible damaging Later, proliferation of surrounding cells restores the effects of oviductal movements, possible adverse surface continuity of the endometrial lining. effects of oviductal and uterine secretions, and/or As the blastocyst sinks into the endometrium, the destruction by maternal tissues until it reaches a syncytial trophoblast rapidly increases in thickness critical mass for survival. During cleavage, the zona at the original site of attachment and progressively pellucida progressively thins and coupled with the extends to cover the remainder of the blastocyst. expansion of the blastocoele, the blastocyst hatches and When completely embedded, the entire wall of the crawls through and out of the surrounding zona blastocyst consists of a thick outer syncytial pellucida. The hatched blastocyst makes contact with trophoblast and an inner cytotrophoblast, which is the maternal endometrial surface through composed of a single layer of cells with well-defined apposition of its trophectoderm to the uterine boundaries. The cytotrophoblast shows active mitosis lining epithelial cells. The initial contact is mediated and contributes cells to the syncytial trophoblast, by cell surface oligosaccharides that play an where they fuse with and become part of that layer. important role in recognition, adhesion and The syncytial trophoblast continues to erode the attachment to the uterine epithelium. Following uterine tissues, opening up the walls of maternal these events, trophoblast cells penetrate between blood vessels. Spaces appear in the syncytial surface uterine epithelial cells and establish direct trophoblast and these lacunae expand, become contact with underlying decidual cells. confluent, and form a labyrinth of spaces. Many of the spaces contain blood from eroded maternal Implantation blood vessels; this blood supplies nourishment for the embryo and represents the first step in the VOCABULARY: secretory phase, syncytial development of uteroplacental circulation. trophoblast, cytotrophoblast, lacunae Placenta At the time of implantation, the endometrium is in the secretory phase and, having been under the VOCABULARY: primary villus, chorion, influence of progesterone from the corpus luteum secondary villus, chorionic plate, chorion for several days, has reached its greatest thickness frondosum, chorion laeve, tertiary placental and development. Implantation is initiated by close villus, decidua, human chorionic gonadotropin approximation of the trophoblast to the microvilli (hCG), estrogen, progesterone, human and surface projections of the uterine epithelial chorionic somatomammotropin (hCS) cells. At the points of contact, the cytoplasm of the trophoblast contains clusters of coated vesicles and As the lacunae enlarge, intervening strands of numerous lysosomes. The microvilli shorten and trophoblast form the primary villi that cover the disappear, and the trophoblast extends finger-like entire periphery of the blastocyst. processes between the uterine epithelial cells, and

251 Each villus consists of a core of cytotrophoblast containing considerable glycogen. External to the covered by a layer of syncytial trophoblast. cytotrophoblast is a layer of syncytial trophoblast of Trophoblastic cells at the tips of the villi apply variable thickness (Fig. 18-2). The cells of the themselves to the endometrium and form a lining cytotrophoblast decrease in number in the latter for the cavity in which the blastocyst lies. When the half of pregnancy, and only a very few are present embryonic germ layers have been established, at term. The syncytial trophoblast also thins out to mesoderm grows out from the embryo as the form a narrow layer. chorion and forms a lining for the trophoblast that surrounds the blastocyst. Mesoderm extends into the primary villi to form a core of connective tissue and convert the primary villi to secondary villi. The deeply embedded portion of the chorion constitutes the chorionic plate from which numerous villi project to form the chorion frondosum. Villi on that part of the chorion facing the uterine cavity grow more slowly and are less numerous; ultimately, these villi disappear; leaving a smooth surface that forms the chorion laeve. Blood vessels develop in the mesenchymal cores of the secondary villi and soon make connection with the fetal circulation. With vascularization, the secondary villi become the definitive or tertiary placental villi. At parturition, all but the deepest layers of the endometrium are shed; thus the superficial part of the endometrium is called the decidua. A feature of the stroma is the alteration of its cells to form enlarged, decidual cells that contain much glycogen.

According to the relationship with the implantation Fig. 18-2. A diagrammatic sketch illustrating the structure of site, three areas of the decidua are recognized. The the placenta. decidua capsularis is the part that lies over the surface The placenta transfers oxygen and nutrients from of the blastocyst, while the decidua basalis underlies the maternal to the fetal circulation and waste the implantation site and forms the maternal component products from the fetal to the maternal circulation. of the placenta. The endometrium lining the Although maternal and fetal circulations are closely remainder of the pregnant uterus is the decidua apposed, they remain separated by the syncytial parietalis. As the embryo grows, the decidua trophoblast (and early in pregnancy by the capsularis becomes increasingly attenuated. cytotrophoblast also), a basement membrane, the Eventually, the decidua capsularis makes contact connective tissue of the villi, and the wall of the with decidua parietalis on the opposite surface of fetal blood vessels. Transport of material between the uterus, and the uterine cavity is obliterated. fetal and maternal blood appears to be regulated by The mature placenta consists of maternal and fetal the syncytial trophoblast. Being without cell components. The maternal part is decidua basalis; the boundaries and intercellular spaces, any materials fetal portion consists of the chorionic plate and the villi passing into or out of the fetal blood must pass arising from it. Maternal blood circulates through the through the cell membranes and cytoplasm of the intervillous spaces and bathes the villi, of which syncytium. Electrolytes, steroids, fatty acids, there are two types. Some pass from the chorionic oxygen, and carbon dioxide traverse the syncytial plate to decidua basalis as anchoring villi from which trophoblast by passive diffusion. Other substances, secondary and tertiary branches float in the such as glucose, cross this barrier by carrier intervillous spaces as the floating villi. Both types of molecules. villi consist of a core of loose connective tissue in which lie fetal capillaries. Covering each villus is an inner layer of cytotrophoblast cells, which have large nuclei and lightly basophilic cytoplasm

252 In addition, the syncytial trophoblast plasmalemma glands and are of cutaneous origin, representing contains receptors for macromolecules such as modified apocrine sweat glands. transferrin, insulin, and immunoglobulins that are taken in by receptor-mediated endocytosis and Structure of the Resting Gland cross the epithelial barrier in transport vesicles. The cytotrophoblast continues to serve as the source of VOCABULARY: duct, simple cuboidal cells for the syncytial trophoblast. epithelium, lactiferous sinus, stratified cuboidal The placenta also is a multipotential endocrine organ epithelium, stratified squamous epithelium, essential for maintaining pregnancy. The alveolus, myoepithelial cell trophoblast secretes a glycoprotein, human chorionic gonadotrophin (hCG) that can be Each breast consists of 15 to 20 individual pyramid- detected by the end of the first week after shaped lobes that radiate from a nipple. The lobes fertilization. It increases in amount to the fifth week are separated by septa of dense connective tissue. of pregnancy and maintains and stimulates the Each lobe represents an individual mammary gland corpus luteum to secrete estrogens and and contains glandular tissue drained by its own progesterone during early pregnancy. The placenta ductal components, embedded in connective tissue. also secretes estrogens and progesterone that aid The proportions of glandular and nonglandular in maintaining the uterine environment for tissue vary with functional status. continued fetal development. At about 9 weeks’ In the resting gland, the principal glandular gestation the placenta takes over the production of elements are the ducts, which are grouped together progesterone. A variety of other factors, similar to to form lobules. The smallest branches of the ductal releasing hormones of the hypothalamus as well as system are lined by simple cuboidal epithelium, pituitary-like hormones, are synthesized and but the lining increases in height as the ducts unite released by the placenta into the maternal blood. and pass toward the nipple. Just beneath the areola, One of these, human chorionic the ducts expand to form the lactiferous sinuses, somatomammotropin or hCS (also referred to as which are lined by a two-layered stratified human placental lactogen) is important not only in cuboidal epithelium. As the duct ascends through breast development and lactation but also in the nipple, it becomes lined by stratified making glucose available from the maternal squamous epithelium. circulation for both the placenta and fetus. Human Secretory units consist of clusters of alveoli chorionic somatomammotropin imparts a condition surrounding a small duct. The alveolar wall consists of insulin resistance to the mother so that glucose of simple cuboidal epithelium resting on a rather than being rapidly removed from the basement membrane. Between the epithelial cells circulation by maternal tissues is available for the and the basement membrane are highly branched developing fetus. myoepithelial cells whose long, slender processes embrace the alveolus in a basket-like network. Breasts (Mammary Glands) Alveoli are not prominent in the resting gland, and there is some question as to whether they are Mammary glands are present in both sexes but in present at all. When present, they usually occur as men remain rudimentary throughout life. In small, budlike extensions of the terminal ducts. women, the size, shape, and structure vary with age The intralobular connective tissue around the and functional status. Prior to puberty, the female ductules and alveoli is loosely arranged and cellular, breasts are undeveloped but enlarge rapidly at whereas that surrounding the larger ducts and lobes puberty due mainly to accumulation of adipose (interlobular connective tissue) is variably dense and tissue. They reach their greatest development at late contains much adipose tissue. pregnancy and lactation. In women, the breasts are variably hemispherical and conical in shape, each surmounted by a cylindrical projection, the nipple. Surrounding the nipple is a slightly raised, circular area of hairless pigmented skin, the areola. The mammary glands are compound tubuloalveolar

253 Nipple and Areola True milk secretion begins a few days after parturition, but not all the breast tissue is VOCABULARY: keratinizing stratified functioning at the same time. In some areas, alveoli squamous epithelium, lactiferous duct, smooth are distended with milk, the epithelial lining is muscle, areolar gland flattened, and the lumen is distended; in other areas, the alveoli are resting and are lined by tall columnar The nipple is covered by keratinizing stratified epithelial cells. Secreting cells have abundant squamous epithelium continuous with that of the granular endoplasmic reticulum, moderate numbers skin overlying the breast. The dermis projects of relatively large mitochondria, and supranuclear deeply into the epithelium, forming unusually tall Golgi complexes. Milk proteins are elaborated by dermal papillae. The skin of the nipple is pigmented the granular endoplasmic reticulum and in and contains many sebaceous glands but is devoid association with the Golgi body form membrane- of hair and sweat glands. The nipple is traversed by bound vesicles. These are carried to the apex of the many lactiferous ducts, each of which drains a cell, where the contents are released by exocytosis. lobe and empties onto the tip of the nipple. The Lipid arises as cytoplasmic droplets that coalesce to outer parts of the ducts also are lined by form large spherical globules. The droplets and keratinizing stratified squamous epithelium. The globules reach the apex of the cell and protrude dense collagenous connective tissue of the nipple into the lumen. Ultimately, they pinch off, contains bundles of elastic fibers, and much surrounded by a thin film of cytoplasm and the smooth muscle is present. The smooth muscle detached portion of the plasmalemma. This method cells are arranged circularly and radially and, on of release is a form of apocrine secretion, but only contraction, produce erection of the nipple. minute amounts of cytoplasm are lost. The areola is the pigmented area of skin that Immunoglobulins in milk are synthesized by plasma encircles the base of the nipple. During pregnancy, cells in the connective tissue surrounding the alveoli the areola becomes larger and more deeply of the mammary glands. The secreted pigmented. It contains sweat, sebaceous, and immunoglobulin is taken up by receptor-mediated areolar glands. The latter appear to be endocytosis along the basolateral plasmalemma of intermediate in structure between apocrine sweat mammary gland epithelial cells and transported in glands and true mammary glands. small vesicles to the cell apex, where it is discharged into the lumen by exocytosis. Passage of milk from Pregnancy and Lactation alveoli into and through the initial ductal segments is due to the contraction of myoepithelial cells. VOCABULARY: proliferation of glandular Average milk production by a woman breast- tissue, colostrum, apocrine secretion feeding a single infant is about 1200 ml/day. Milk consists of about 88% water, 6.5% carbohydrate, During pregnancy, mammary glands undergo 3.3% lipid, and 1.5% protein. Lactose is the primary extensive development in preparation for their role carbohydrate and casein the primary protein. In in lactation. During the first half of pregnancy, the addition, immunoglobulins (IgE and IgA), terminal portions of the ductal system grow rapidly, electrolytes (Na+, K+, Cl-), minerals (Ca++, Fe++, branch, and develop terminal buds that expand to Mg++), and other substances occur in milk. Women become alveoli. Proliferation of glandular tissue in the fourth and fifth months of lactation may takes place at the expense of the fat and stromal secrete as much as 0.5 g of immunoglobulin per day connective tissue, which decrease in amount. The in her milk. Such immunoglobulins are important in connective tissue becomes infiltrated with the resistance of enteric infections and provide the lymphocytes, plasma cells, and granular leukocytes. infant with considerable passive immunity. During late pregnancy, proliferation of glandular With weaning, lactation soon ceases, and the tissue subsides, but the alveoli expand and there is glandular tissue returns to its resting state. some formation of secretory materials. The first However, regression is never complete, and not all secretion, colostrum, is thin and watery. It is poor the alveoli completely disappear. in lipid but contains a considerable amount of immunoglobulin that provides passive immunity to the newborn.

254 Hormonal Control present in the gonadal ridges until the sixth week of development. The epithelium of the gonadal ridge VOCABULARY: estrogen, progesterone, proliferates, and in places, clumps of cells become somatotropin, prolactin, adrenal corticoid, thickened to form a number of cellular gonadal cords human chorionic somatomammotropin, that penetrate the underlying mesenchyme. The neurohormonal reflex, oxytocin cords at the periphery form a primary ovarian cortex, while centrally; proliferation of mesenchyme Before puberty, growth of the mammary gland establishes a primary medulla. The gonadal cords parallels general body growth, but as the ovaries break up into irregular clusters separated by strands become functional, the mammary tissue comes of mesenchyme, an extension of which cuts off the under the influence of estrogen and progesterone. cell clusters from the surface epithelium and While some structural changes can be observed, the eventually forms the tunica albuginea. A second cyclic response of the breast is minor. During proliferation of the surface epithelium gives rise to pregnancy, the glands are continuously stimulated new cellular cords that invade the primary cortex, by estrogen and progesterone from the corpus dissociates into isolated clusters, and form the luteum and placenta. Generally, growth of the definitive cortex. The cell clusters surround one or ductal system depends on estrogen, but for alveolar more primitive germ cells (primary oocytes) that development, both progesterone and estrogen are enter prophase of the first meiotic division. The required. To attain the full development in primary medulla is replaced by a fibroelastic stroma pregnancy, other hormones -somatotropin, to establish a permanent medulla. prolactin, adrenal corticoids, and human Primordial germ cells, which give rise to the definitive chorionic somatomammotropin - appear to be oogonia, are segregated early in development. They necessary. appear first in the yolk sac, then migrate to the dorsal At the end of pregnancy (birth), the levels of mesentery and then to the genital ridges. It is circulating estrogen and progesterone fall abruptly. uncertain whether the primordial cells arise from a In the absence of their inhibition, there is an group of special blastomeres during cleavage or as increased output of prolactin by cells of the clones of a single blastomere. Their early adenohypophysis. Prolactin is a powerful lactogenic association with endoderm has suggested this as the stimulus, and full lactation is established in a few tissue of origin. The forming ovary contains days after birth. Maintenance of lactation requires between 6-7 million oogonia at 20 weeks gestation. continuous secretion of prolactin, which results The number of oogonia drops to between 1-2 from a neurohormonal reflex established by suckling. million by the time of birth. Periodic suckling also causes release of oxytocin By six weeks’ gestation, male and female embryos from the neurohypophysis, which stimulates have paired mesonephric and paramesonephric contraction of myoepithelial cells, resulting in release of ducts. In the female embryo, paramesonephric milk from alveoli and into the ducts (milk letdown). ducts become predominant, giving rise to oviducts, uterus, and possibly vagina. Unlike the male, the Organogenesis mesonephric ducts do not contribute to the female reproductive system. Mesonephric ducts regress, Early development of the female reproductive tract except for a few remnants. is related closely to that of the urinary system and Paramesonephric ducts begin to form at about 35 or of the male reproductive tract. All arise in the 36 days gestation. Each duct begins as an mesoderm of the urogenital ridges within epithelial invagination on the lateral side of the mesonephric thickenings on each side of the midline. Initially, the ridge. The ingrowing mesothelium forms a short ovaries and testes are indistinguishable and form a groove that sinks beneath the surface and seals off primitive, indifferent gonad that does not acquire male to form a tube. The anterior end develops first and or female morphology until the seventh week of opens into the coelomic cavity as a funnel-shaped development. The first indication of gonads structure that gives rise to the fimbriated and appears at four weeks, when a pair of longitudinal ampullary parts of the oviduct. Caudally, the ridges, the gonadal ridges, appear. These are divided paramesonephric ducts from each side lengthen and into lateral and medial parts and only the medial descend, then turn medially to meet each other and portion gives rise to the ovary. No germ cells are form a single structure, the utero-vaginal canal.

255 At first the two lumina are separate, but the and on cells (gonadotrophs) within the anterior intervening walls degenerate and a single cavity is pituitary. The hypothalamic neurons release a single formed. The descending part forms the body and factor (gonadotropin-releasing hormone) that cervix of the uterus. Mesenchyme surrounding the regulates the release of both FSH and LH from utero-vaginal canal supplies the endometrial stroma cells (gonadotrophs) of the anterior pituitary. and myometrium. Follicle-stimulating hormone influences the growth The vagina arises from an epithelial proliferation, of late primary and secondary follicles and the sinovaginal bulb, on the posterior wall of the promotes the formation of estrogens. Luteinizing utero-genital sinus. It is not certain whether this hormone stimulates ovulation and promotes corpus epithelium is true sinus epithelium or epithelium of luteum formation. Estrogen suppresses FSH the mesonephric duct that descended into this secretion by acting on both cells of the anterior region. The proliferating epithelium extends pituitary and neurons within the hypothalamus. cranially as a solid bar that forms the vaginal plate. Inhibin secreted by follicular granulosa cells Beginning at the caudal end, cells in the center of suppresses secretion of FSH directly by acting on the plate degenerate and a vaginal lumen is formed. cells of the anterior pituitary. Progesterone inhibits Canalization is complete by the fifth month. The LH release; therefore, the corpus luteum lacks the fetal vaginal epithelium is under the influence of stimulus essential for its maintenance and soon maternal hormones and in late fetal life becomes degenerates. markedly hypertrophied. It regresses after birth and The oviduct is the site of fertilization of an ovum assumes its inactive childhood form. and also transports the zygote to the uterus as the The external genitalia also pass through an result of muscular and ciliary actions. Conditions indifferent stage before they develop their definitive within the oviduct sustain the zygote as it sexual characteristics. The caudal end of the undergoes cleavage during passage through this primitive hindgut is closed by a cloacal membrane tube. Nutrition for the zygote is provided by around which the urogenital folds develop. The material stored in the cytoplasm of the ovum. folds unite at their cranial ends to form the genital On entering the uterine cavity, the blastocyst lies tubercle, which, in the female, elongates only slightly in secretions produced by the endometrium. The to give rise to the clitoris. A pair of genital swellings secretion is rich in glycogen, polysaccharides, and develops along each side of the genital folds; in the lipids, providing an excellent "culture medium" for female these remain separate and form the labia the dividing cells of the blastocyst. The uterine majora. Similarly, the urogenital folds do not fuse in endometrium, to which the blastocyst attaches, the female, and these give rise to the labia minora. provides for the sustenance of the embryo throughout its development. A rich food supply for Summary the implanting blastocyst comes from the secretions of the uterine glands and from products of the The ovary is a cytogenic gland releasing ova and uterine stroma as the blastocyst burrows into the also acts as a cyclic endocrine gland. During its endometrium. These products are absorbed by the growth in the follicles, the oocyte is nourished by syncytial trophoblast and diffuse to the developing blood vessels of the ovary via the theca interna. embryo. As the trophoblast continues to erode into When a corpus luteum is formed after ovulation, the endometrium, blood-filled spaces form, and the estrogens and progesterone are produced and are blastocyst is bathed by pools of maternal blood that responsible for development of the uterine mucosa supply the embryo with nourishment. When a prepared for reception of the blastocyst. The placenta has been established, the nutritional, periodic nature of hormone production by the respiratory, and excretory needs of the embryo are ovary establishes the menstrual cycle during which, met by this fetal-maternal membrane. The placenta in the absence of pregnancy, the uterine mucosa is also has protective functions, preventing passage of shed. If pregnancy occurs, the corpus luteum particulate matter to the embryo. It also has persists and its hormonal activity maintains the endocrine functions, elaborating a variety of factors endometrium in a prepared state. including gonadotropin (hCG), Female reproductive function is regulated somatomammotropin (hCS), estrogens, and primarily by positive and negative feedback loops progesterone. on neurons in the hypothalamic region of the brain

256 Human chorionic gonadotropin (hCG) is produced viscid mucus elaborated at midcycle appears to very early and maintains the corpus luteum during favor the movement of sperm into the proximal early pregnancy. regions of the female reproductive tract.. The basal layer of the endometrium is not shed at The mammary glands provide nourishment for parturition or at menstruation and provides for the the newborn, which is delivered in an immature and restoration of the uterine mucosa after these events dependent state. The first secretion, colostrum, has have occurred. The myometrium, by its muscular a high content of immunoglobulins and give passive contractions, expels the fetus at birth. immunity to the suckling young. Passage of milk Changes in the secretory activities of cervical from the alveoli into the initial segments of the glands during the menstrual cycle may have some ducts is accomplished by contraction of significance in fertility. During most of the cycle, myoepithelial cells under the influence of the the glands produce a thick, viscous mucus that hormone oxytocin. appears to inhibit passage of sperm. The thin, less

257 pregnant women. Portions of the brain and heart 19 Endocrine Glands also can function as endocrine organs. What are termed classic endocrine glands consist of Endocrine glands lack ducts, and their secretory discrete masses of cells with a relatively simple products (hormones) generally pass into the blood organization into clumps, cords, plates or follicles or lymphatic circulation. This type of secretion is supported by a delicate vascular connective tissue. referred to as endocrine secretion. However, hormones Endocrine glands have a rich vascular supply, and also may be secreted directly into the intercellular the secreting cells have direct access to capillaries. space (paracrine secretion) to elicit a local effect on Historically, the classic endocrine system consists of adjacent cells. In some instances cells may secrete a the pineal, parathyroid, thyroid, adrenal, and chemical messenger that acts on its own receptors pituitary glands. and is self-regulatory (autocrine secretion). Hormones A vast system of unicellular endocrine cells that are secreted into the vascular system eventually (glands) also exists, and collectively these cells form enter the tissue fluids and, depending on the a diffuse endocrine system. It is represented by scattered specificity of the hormone, can alter the activities of unicellular glands that often lie at some distance just one organ or influence several. The organs that from one another, separated by cells of a different respond to a specific hormone are the target organs type. These cells are present throughout much of of that hormone. Hormones may be classified into the gastrointestinal tract, in the ducts of the major three general categories according to their chemical digestive glands, and in the conducting airways of structure: amino acid polymers (polypeptides, the lungs. When the cells of this system are proteins, or glycoproteins), cholesterol derivatives considered collectively, they form a mass larger (sterols and steroids), and tyrosine derivatives than many of the individual organs of the classic (thyroid hormones and catecholamines). Many of endocrine system. the peptide and protein hormones exist in the circulation in a free state unbound to other elements in the plasma. In contrast, thyroid and Pineal Gland steroid hormones circulate in the blood bound to specific plasma carrier proteins. When the hormone The human pineal gland is a somewhat flattened molecules reach their destination they become body 5 to 8 mm long and 3 to 5 mm wide. In the involved in cell-to-cell signaling and interact with adult it weighs about 0.1 gm. It is attached to the their target cells through specific hormone-receptor brain (roof of the diencephalon) by a short stalk interactions. The targeted receptor may reside that contains small blood vessels, some nerve fibers within the plasmalemma, the cytoplasm, or the and their supportive elements. The pineal gland is nucleus. The resulting hormone-receptor included in a group of circumventricular organs that interaction may generate secondary messenger lack a blood-brain barrier. molecules or influence gene expression. In some glands, such as the liver, the epithelial Structure cells have both exocrine and endocrine functions. Hepatocytes fulfill the definition of endocrine cells VOCABULARY: pinealocyte, glial cell, corpora because they release glucose, proteins, and other arenacea (brain sand), melatonin, serotonin substances directly into the blood of the hepatic sinusoids, but the same hepatocytes also secrete bile The pineal is covered by a thin connective tissue into adjacent canaliculi (the beginnings of the liver capsule that is continuous with the surrounding duct system) and thus are exocrine cells also. meningeal (pial) tissue. Richly vascularized and Organs such as the pancreas, kidney, testis, and innervated septa extend into the parenchyma of the ovary are mixtures of endocrine and exocrine gland and subdivide it into poorly defined lobules. components in which separate or isolated groups of Two types of cells, pinealocytes and glial cells, are cells that secrete directly into the vasculature form present organized into clumps and cords. the endocrine portion. The rest of the gland makes Pinealocytes have relatively large lobulated up the exocrine portion that is drained by ducts. nuclei. Long cytoplasmic processes often radiate The placenta is a unique, temporary endocrine from the cell body to form club-shaped endings organ that persists only for about 9 months in near adjacent perivascular spaces or adjacent pinealocytes.

258 Small, membrane-bound vesicles, some with is thought to inhibit gonadotropin activity before electron-dense cores, are present in the swollen puberty and therefore acts to prevent the onset of ends of the processes. Synaptic ribbons may be puberty before the appropriate age. Pineal-specific present also. Nearer the cell body, the cytoplasm peptides produced by the pineal also are thought to contains abundant, fairly large mitochondria, influence or mediate hypophyseal function, but numerous free ribosomes, profiles of smooth their exact role is uncertain. The human pineal endoplasmic reticulum, lipid droplets, lipochrome begins to produce melatonin at about three months pigment, lysosomes, and large numbers of of age. Production of melatonin falls steadily after microtubules and intermediate filaments. puberty. Numerous gap junctions link groups of pinealocytes electrically and metabolically. Although Parathyroid Glands light and dark forms have been described, it is unknown whether each forms a distinct cell type or The parathyroid glands are small, brownish, oval whether they represent differences in the activity of bodies 4 to 8 mm long and 2 to 5 mm wide. There a single cell type. usually are four in humans, but as many as six or Glial cells form an interwoven network within more may lie within the capsule of the thyroid gland and around the parenchymal cords and clumps of or may be embedded in the substance of the middle the pineal gland. They are fewer in number than third of the thyroid. In about 8% of the human pinealocytes and their nuclei are smaller and stain population, additional parathyroid tissue is present more deeply. Glial cells show long cytoplasmic in the thymus, the association being due to the processes and often are regarded as a form of astrocyte. common developmental origin of these structures The cytoplasm contains many fine filaments, 5 to 6 from the third pharyngeal pouch. Each parathyroid nm in diameter, which may form large bundles. weighs about 130 mg; those in women are slightly Corpora arenacea (brain sand) are concretions heavier than in men. found in the pineal gland. These irregularly shaped structures occur in the capsule and substance of the Structure gland. They consist mainly of calcium carbonates and phosphates within an organic matrix. In VOCABULARY: chief (principal) cell, oxyphil humans, they increase with age beginning after cell, parathyroid hormone puberty, but their significance is unknown. The pineal gland has an extensive blood supply Each parathyroid gland is surrounded by a thin and is provided with many postganglionic sympathetic connective tissue capsule and is subdivided by nerve fibers from neurons in the superior cervical ganglia. delicate trabeculae that extend from the capsule. The pineal gland produces melatonin, an Blood vessels, lymphatics, and nerves course indolamine. In mammals such as the rat, through the trabeculae to enter the substance of the pinealocytes synthesize serotonin, a precursor of parathyroid. In older individuals fat cells often are melatonin that varies cyclicly (circadian and abundant and may form about 50% of the gland. seasonal) in amount and changes in relation to the The parenchyma consists of closely packed groups amount of light. Serotonin levels fall and melatonin or cords of epithelial cells supported by a delicate levels rise during darkness. These circadian rhythms framework of reticular fibers that contains nerve are controlled by the release of norepinephrine fibers and a rich capillary network. from the sympathetic fibers that enter the pineal The parenchyma contains two types of cells. from the superior cervical ganglia, which in turn is Chief (principal) cells are the more numerous and controlled by the light perceived by the retina. In measure 8 to 10 µm in diameter. They have round, some rodents and birds the pineal, through centrally placed, vesicular nuclei, and the cytoplasm secretion of melatonin, plays an active role in the contains the usual organelles as well as large reproductive cycles, which are influenced by the accumulations of glycogen and lipid droplets. In length of day light (photoperiod). The pineal in addition to small, dense, membrane-bound these species acts as a photoneuroendocrine organ, granules, lipofuscin granules often are present also. influencing the gonads in response to light. Oxyphil cells form only a small part of the cell The function of the pineal and melatonin in population and may occur singly or in small groups. humans, however, is poorly understood. Melatonin

259 They are not present in significant numbers until Thyroid Gland puberty and then increase with age. Oxyphil cells are larger than chief cells, and their cytoplasm stains The thyroid gland normally weighs between 20 and intensely with eosin and is packed with large, 30 g, being slightly heavier in men, and is located in elongated mitochondria that show numerous the anterior region of the neck, below the cricoid cristae. Between mitochondria are small cartilage. It consists of two lateral lobes and an accumulations of glycogen and occasional profiles isthmus that forms a narrow bridge across the of granular endoplasmic reticulum. The small nuclei trachea between the two lobes. A small pyramidal stain deeply and often appear pyknotic. lobe extends cranially in about 15% of the Numerous cells that are intermediate in population. appearance between chief and oxyphil cells also have been described. Chief cells are the primary Structure parenchymal element of the parathyroid gland, and other cell types (such as the oxyphil and VOCABULARY: follicle, colloid, principal intermediate cells) may represent a modification or (follicular) cell, colloid resorption droplet, a stage of development of the chief cell. thyroglobulin, triiodothyronine, The parathyroid secretes parathyroid hormone tetraiodothyronine (thyroxin), thyroxine (PTH), a polypeptide hormone that regulates the binding globulin (TBG), parafollicular cell calcium level of the blood. Nearly half the blood (light cell, C cell), calcitonin (thyrocalcitonin) calcium is bound to albumin; the remainder is present as free ions. It is the concentration of free The parenchyma of the thyroid gland is enclosed in calcium ion that governs the secretion of parathyroid a connective tissue capsule and organized into hormone. If the concentration of ionized calcium hollow spherical structures, the follicles, which drops below normal (8.6-10.6 mg/dl), parathyroid vary considerably in diameter and contain a hormone is secreted and acts directly on osteoclasts gelatinous material called colloid. The walls of the and osteocytes to mobilize calcium ion from bone follicles consist of a simple epithelium that rests on and on the renal tubules to promote absorption of a thin basal lamina, about 50 nm thick. Each follicle calcium ion and inhibit absorption of phosphate ion is supported by a delicate reticular network that from the glomerular filtrate. Parathyroid hormone contains a vast capillary plexus, numerous nerve acts on the proximal tubule within the kidney to fibers, and blindly ending lymphatic vessels. inhibit phosphate reabsorption (about 95% of The follicular epithelium consists mainly of phosphate is normally reabsorbed at this location) principal (follicular) cells, which usually are and on the thick ascending limb of the loop of squamous to cuboidal in shape; depending on the Henle and distal tubule to increase the reabsorption functional status of the thyroid, the cells may of calcium ion. Parathyroid hormone also acts on become columnar. The nuclei are spherical, contain vitamin D metabolism. Vitamin D is a sterol one or more nucleoli, and have a central position. produced by the skin or absorbed in the diet. The lateral cell membranes are united at the apex by Parathyroid hormone acts on the renal proximal junctional complexes, and the luminal surfaces bear convoluted tubule to promote the conversion of short microvilli. The basal plasmalemma is smooth, 25(OH)-vitamin D to the active form, 1, 25-(OH)2- with no infoldings. Mitochondria are evenly vitamin D. This active form of vitamin D increases distributed throughout the cytoplasm and vary in calcium absorption in the small intestine. Increase number according to the activity of the cell. Active in the concentration of calcium ion in the blood cells take on a cuboidal shape and show many results in a decrease in the amount of PTH released. profiles of granular endoplasmic reticulum. Inactive Blood levels of calcium are kept from exceeding the cells are squamous, and granular endoplasmic optimum by a second calcium-regulating hormone, reticulum is sparse. Golgi complexes usually have a calcitonin, produced by the parafollicular cells of the supranuclear location in active cells, and the apical thyroid gland. cytoplasm contains numerous small vesicles, Parathyroid hormone is essential for life. lysosomes, and multivesicular bodies. Complete removal of the parathyroid glands results Colloid resorption droplets also are found in the in a precipitous drop in blood calcium followed by apical cytoplasm. Follicular cells contain thyroid- tetany and death. stimulating hormone (TSH) receptors.

260 The thyroid is a unique endocrine gland in that the follicular cell. Thyroxin (tetraiodothyronine) secretory product, colloid, is stored extracellularly in molecules constitute what is known as thyroid the lumen of the follicle. Colloid consists of hormone and are released with triiodothyronine at the mucoproteins, proteolytic enzymes, and a base of the cell into blood and lymphatic capillaries glycoprotein called thyroglobulin, the primary (Fig. 19-1). Thyroxin is transported in the blood storage form of thyroid hormone. Synthesis of plasma complexed to a binding protein called thyroglobulin occurs in the principal cells along the thyroxine binding globulin. Triiodothyronine, which same basic intracellular pathway as glycoprotein in hormonally is the more potent of the two, but is cells elsewhere in the body. Amino acids are not as abundant, is not as firmly bound to the synthesized into polypeptides in the granular binding protein. endoplasmic reticulum and then carried in transport vesicles to the Golgi complex, where the carbohydrate moiety is conjugated to the protein. From the Golgi complex, the glycoprotein (noniodinated thyroglobulin) is transported to the apical surface in small vesicles, from which it is discharged by exocytosis into the follicular lumen and stored as part of the colloid. While thyroglobulin is being synthesized, thyroperoxidase is assembled in the granular endoplasmic reticulum and then passes through the Golgi complex and is released by small vesicles at the apical surface of the cells. Follicular cells have a unique ability to take up iodide from the blood using a Na+-I- cotransporter and concentrate it. The iodide subsequently is oxidized to iodine by this intracellular peroxidase and used in the iodination of tyrosine groups in thyroglobulin. Formation of monoiodotyrosine and diiodotyrosine is thought to occur in the follicle, immediately adjacent to the microvillus border of the follicular cells. When one molecule of monoiodotyrosine is linked to one of diiodotyrosine, a molecule of triiodothyronine is formed. Coupling of two molecules of diiodotyrosine results in the formation of tetraiodothyronine (thyroxin). The thyronines make up a small part of the thyroglobulin complex but represent the only constituents with hormonal activity. Thyroglobulin and the thyronines are stored in the lumen of the follicle as colloid until needed. Fig. 19-1. A diagrammatic sketch illustrating the synthesis of When follicular cells are stimulated by TSH, thyroid hormone. droplets of colloid are sequestered from the follicular lumen by endocytosis and the formed Thyroxine binding globulin (TBG) is colloid resorption droplets enter the apical important in regulating the amount of free cytoplasm of the follicular cells. Lysosomes coalesce hormone that circulates in the blood plasma. with the resorption droplets and hydrolyze the contained Thyroxin and triiodothyronine are released from thyroglobulin, liberating monoiodotyrosine, TBG only in response to metabolic needs, thus diiodotyrosine, triiodothyronine, and preventing rapid fluctuations in the levels of these tetraiodothyronine into the cytoplasmic matrix. The hormones. mono- and diiodotyrosines are deiodinated by the enzyme deiodinase, and the iodine is reused by the

261 As they are released and elicit their effects, the Adrenal Gland hormones are replaced by other thyroxin and triiodothyronine molecules from the thyroid. The The adrenal glands in humans are a pair of activity of the thyroid is regulated by a thyroid- flattened, triangular structures with a combined stimulating hormone (TSH) secreted by thyrotrophs weight of 14 to 16 gm. One adrenal gland is in the anterior lobe of the pituitary. situated at the upper pole of each kidney. The Most of the secreted thyroid hormone (90%) is adrenal gland is a complex organ consisting of a thyroxine but is converted to the more active form, cortex and medulla, each differing in structure, triiodothyronine, by peripheral target tissues. The function, and embryonic origin. kidney and liver are important deiodinators of thyroxin and convert it to the functionally more Adrenal Cortex potent triiodothyronine. Triiodothyronine then bids to a nuclear receptor in cells of the target organ the net VOCABULARY: zona glomerulosa, zona result of which is an increase in oxygen fasciculata, zona reticularis, mineralocorticoid, consumption and metabolic rate. Thyroid hormone glucocorticoid, adrenal androgens has general effects on the metabolic rate of most tissues, and among its functions are increased Each adrenal gland is surrounded by a thick capsule carbohydrate metabolism, increased rate of of dense irregular connective tissue that contains intestinal absorption, increased kidney function, scattered elastic fibers. The capsule contains a rich increased heart rate, increased ventilation, normal plexus of blood vessels - mainly small arteries - and body growth and development, and increased numerous nerve fibers. Some blood vessels and mental activities. nerves enter the substance of the gland in the The thyroid also contains a smaller number of trabeculae that extend inward from the capsule and cells variously called parafollicular, light, or C then leave the trabeculae to enter the cortex. cells, which are present adjacent to the follicular The parenchyma of the adrenal cortex consists of epithelium and in the delicate connective tissue continuous cords of secretory cells that extend from the between follicles. The C cells adjacent to the capsule to the medulla, separated by blood sinusoids. follicular epithelium appear to be sandwiched The cortex is subdivided into three layers according between the bases of follicular cells and lie to the arrangement of the cells within the cords. immediately adjacent to the basal lamina; These cortical layers consist of an outer zona parafollicular cells never directly border on the glomerulosa (10%), a middle zona fasciculata lumen of the follicle. Between follicles, the cells (75%), and an inner zona reticularis (15%). The may occur singly or in small groups. In electron cytologic changes from one zone to the next are micrographs, the parafollicular cells show numerous gradual. moderately dense, membrane-bound secretory Zona glomerulosa forms a narrow band just granules that measure 10 to 50 nm in diameter. The beneath the capsule. The columnar cells are cytoplasm also contains occasional profiles of arranged into ovoid groups or arcades and have granular endoplasmic reticulum, scattered centrally placed spherical nuclei. In electron mitochondria, and poorly developed Golgi micrographs, the cells show a well-developed complexes. smooth endoplasmic reticulum and numerous Parafollicular cells secrete calcitonin mitochondria that are evenly distributed throughout (thyrocalcitonin), another polypeptide hormone the cytoplasm. Occasional lipid droplets and that regulates blood calcium levels. Calcitonin lowers scattered profiles of granular endoplasmic reticulum blood calcium by acting on osteocytes and osteoclasts also are present. to suppress resorption of calcium from bone and its Zona fasciculata forms the widest zone of the release into the blood. Thus, calcitonin has an effect cortex and consists of long cords that usually are opposite that of parathyroid hormone, serves to one or two cells thick. The cords run parallel to control the action of parathyroid hormone, and one another, separated by sinusoids that are lined helps regulate the upper levels of calcium by an attenuated endothelium. concentration in the blood.

262 The cells of zona fasciculata are larger than those of kidney receives signals created by decreased arterial the other two zones and have a polyhedral shape, renal blood pressure (which reduces the degree of and their spherical nuclei are centrally placed; stretch on the juxtaglomerular cells) and/or by a binucleate cells are common. The cytoplasm decrease in the amount of sodium chloride and contains numerous rounded mitochondria with potassium detected by the macula densa. In tubular cristae, abundant smooth endoplasmic response to this signal, juxtaglomerular cells release reticulum, and well-developed granular endoplasmic renin, which acts enzymatically on a circulating reticulum. Many lipid droplets also are present and protein called angiotensinogen changing it to contain neutral fat, fatty acids, and fatty acyl esters angiotensin I. A converting enzyme produced of cholesterol; these represent stored precursors for primarily by endothelial cells within the lung the synthesis of the steroid hormones secreted by converts angiotensin I to angiotensin II. The latter the zona fasciculata. Short microvilli often are stimulates the zona glomerulosa to secrete present on the plasmalemma adjacent to sinusoids. aldosterone. Angiotensin II also is a vasoconstrictor Zona reticularis forms the innermost zone and is and acts directly to increases blood pressure. In made up of a network of irregular, anastomosing addition, angiotensin II acts on the renal proximal cords that also are separated by sinusoids. In tubule to promote sodium chloride and water general, the cells of this zone resemble those of reabsorption and proton (H+) secretion. zona fasciculata except that they are smaller, the Adrenocorticotrophic hormone (ACTH) cytoplasm contains fewer fat droplets, the nuclei elaborated by basophils (corticotrophs) in the stain more deeply, and lipofuscin granules are anterior pituitary has only a limited role in the prominent. Light and dark forms of the cells have control of secretion by the zona glomerulosa. been described, but their significance is not known. However, it has a permissive effect and small The adrenal cortex synthesizes and secretes in amounts of ACTH are needed for aldosterone excess of twenty-four steroid hormones, which secretion to occur in response to renin-angiotensin generally can be placed in three broad categories: system activity. Aldosterone is metabolized by the mineralocorticoids that control water and electrolyte liver and has a half life of about 20 minutes. balance, glucocorticoids that affect carbohydrate Glucocorticoids (cortisol, cortisone, and metabolism, and sex steroids (androgens). All are corticosterone) are secreted by cells of the zona derived from cholesterol. Enzymes for the synthesis fasciculata and zona reticularis. These steroid of these steroid hormones are located mainly in the hormones act mainly on the metabolism of fats, smooth endoplasmic reticulum and mitochondria proteins, and carbohydrates, resulting in an increase of the adrenal cortical cells. The pathway involves in blood glucose and amino acid levels and in the transfer of precursor molecules and intermediate movement of lipid into and out of fat cells. Cortisol products back and forth between these two also suppresses the inflammatory response and organelles. Adjacent lipid droplets provide the some allergic reactions by inhibiting the enzyme substrates needed for the biosynthetic processes. phospholipase A2. Cortisol is metabolized by the The mineralocorticoids, primarily aldosterone, are liver and has a half life of about 70 minutes. secreted by zona glomerulosa. Aldosterone plays a Adrenal androgens, dehydroepiandrosterone and major role in the regulation of extracellular fluid and androstenedione, also are secreted by cells of the blood volumes as well as maintaining potassium balance. zona fasciculata and zona reticularis, although cells It acts on the distal and collecting tubules of the of the reticularis are said to be the more active in kidney to increase the rate of sodium chloride and androgen secretion. Androgens are anabolic water reabsorption from the glomerular filtrate and, steroids leading to increased muscle mass and bone at the same time, to increase potassium secretion. It growth. In women adrenal androgens contribute to also lowers the concentration of sodium chloride in the development of secondary sexual characteristics the secretions of sweat and salivary glands and such as growth of axillary and pubic hair, and intestinal mucosa. Aldosterone secretion is controlled sebaceous gland hypertrophy at puberty. primarily by the renin-angiotensin system, which is Abnormally high levels of adrenal androgens may sensitive to changes in blood pressure and to the have masculinizing effects on women and in boys concentration of sodium chloride and potassium in induce precocious puberty. the blood plasma and extracellular fluid. It has been suggested that the juxtaglomerular apparatus of the

263 The adrenal cortex is the primary source of Secretion of both hormones is controlled by the sympathetic circulating androgens in women whereas in men nervous system. Epinephrine and norepinephrine are testicular androgens are of greater importance and metabolized by the liver and have a half life of are produced in much greater amounts. about 2 to 3 minutes. Secretion by zona fasciculata and zona reticularis is controlled by adrenocorticotrophic hormone (ACTH), a Blood Supply polypeptide hormone secreted by corticotrophs in the anterior pituitary. Adrenocorticotrophic The rich plexus of small arteries in the capsule of hormone stimulates the synthesis and release of the adrenal provides cortical arterioles that enter the steroids, increases blood flow through the cortex, parenchyma of the adrenal to empty into the vast and promotes growth of the two inner zones of the network of cortical sinusoids that separates the adrenal cortex. The adrenal cortex is essential for cords of epithelial cells making up the cortex. The life. attenuated endothelium of the sinusoids has numerous fenestrations and is supported only by a Adrenal Medulla thin basal lamina and a delicate network of reticular fibers. The sinusoids pass through all three layers of VOCABULARY: chromaffin cell, the cortex and near the corticomedullary junction catecholamine, epinephrine, norepinephrine begin to merge to form large collecting veins. These drain toward the central medulla and finally collect The adrenal medulla is composed of large round or into a single, large suprarenal vein that drains the polyhedral cells arranged in clumps or short cords. entire adrenal gland. The cortex has no separate These are the chromaffin cells, so named because venous drainage. Some small arteries enter the they show numerous brown granules when treated cortex in the trabeculae and provide a direct arterial with chromium salts (the chromaffin reaction). The supply to the medulla. Hence, the medulla receives parenchyma is supported by a framework of blood from cortical sinusoids and medullary reticular fibers, and this stroma contains numerous arteries. The capillaries that surround the medullary capillaries, veins, and nerve fibers. Sympathetic cords also are lined by a thin, fenestrated ganglion cells are present also and may occur singly endothelium. or in small groups. Chromaffin cells secrete In addition to their systemic effects, catecholamines. By special histochemical means, adrenocorticosteroids (especially glucocorticoids) two types of chromaffin cells have been identified, affect the adrenal medulla, and the steroid-rich one containing epinephrine and the other blood from the cortex influences the secretion of norepinephrine. epinephrine or norepinephrine by the chromaffin Ultrastructurally, chromaffin cells are cells. Cortisol induces an enzyme characterized by numerous electron-dense granules, (phenylethanolamine-N-methyl transferase) in the 100 to 300 nm in diameter, that are limited by a adrenal medullary cells that converts membrane. The granules from cells that secrete norepinephrine to epinephrine. Thus, epinephrine is norepinephrine have intensely stained electron- the major catecholamine secreted and accounts for dense cores, whereas cells that elaborate between 70%-80% of the medullary secretion. epinephrine have homogeneous, less dense secretory granules. Both cell types show profiles of Paraganglia granular endoplasmic reticulum, scattered mitochondria, and well-developed Golgi complexes VOCABULARY: chief cell, norepinephrine, that lie close to the nucleus. supporting cell Although not essential for life, hormones of the adrenal medulla help individuals meet stressful Clusters of chromaffin cells enclosed in thick situations. Epinephrine increases cardiac output, capsules of dense irregular connective tissue causes bronchial dilation, elevates blood glucose, normally occur outside the adrenal medulla and increases the basal metabolic rate. Norepinephrine associated with the sympathetic nervous system. acts primarily to elevate and maintain blood pressure by causing vasoconstriction in the peripheral segments of the arterial system.

264 These form the paraganglia, which are highly The entire hypophysis is surrounded by a thick vascularized structures that contain two types of connective tissue capsule. cells: chief and supporting cells. Chief cells contain numerous membrane-bound, electron-dense granules that are similar to those of chromaffin cells of the adrenal medulla. They are believed to secrete norepinephrine. The chief cells are surrounded, in part or completely, by elongated supporting cells that lack secretory granules. Some paraganglia, such as aortic chromaffin bodies, are paired and quite large. Paraganglia can be important clinically if they secrete abnormally high amounts of catecholamine.

Hypophysis

The hypophysis or pituitary is a complex endocrine gland located at the base of the brain, lying in the sella turcica, a small depression in the sphenoid bone. It is attached to the hypothalamic region of the brain by a narrow stalk and has vascular and neural connections with the brain. The pituitary weighs about 0.5 gm in adults but is slightly heavier in women and in multiparous women may exceed 1 gm in weight. Despite its small size, the pituitary Fig. 19-2. A diagrammatic sketch illustrating the subdivisions gland produces several hormones that directly of the hypophysis and their relationships. affect other endocrine glands and tissues. Vascular Supply Organization VOCABULARY: superior, middle, and inferior VOCABULARY: adenohypophysis, hypophyseal arteries; hypophyseal portal veins neurohypophysis, pars distalis (anterior lobe), pars intermedia, The pituitary is supplied by three pairs of arteries: pars tuberalis, pars nervosa, infundibular stalk, the superior, middle, and inferior hypophyseal median eminence arteries, all of which supply directly to the neurohypophysis. The middle hypophyseal artery The hypophysis consists of two major parts, an previously was believed to traverse the parenchyma epithelial component called the adenohypophysis of the adenohypophysis, but recent evidence has and a nervous component that forms the shown that it too passes directly to the neurohypophysis. The adenohypophysis is further neurohypophysis. Thus, the adenohypophysis has subdivided into the pars distalis (anterior lobe) no direct arterial blood supply but is linked to the that lies anterior to the residual lumen of Rathke's common capillary bed of the neurohypophysis by pouch; pars intermedia, which forms a thin large-bore, thin-walled veins, the hypophyseal partition behind the residual lumen; and pars portal veins. The hypophyseal portal veins form an tuberalis, which is an extension of pars distalis important link between the primary capillary plexus surrounding the neural stalk. associated with the median eminence and The neurohypophysis also contains three portions. infundibular stalk and the secondary capillary plexus of The major part is pars nervosa, which lies just the portal system associated with the posterior to pars intermedia and is continuous with adenohypophysis. the infundibular stalk and the median eminence Venous drainage occurs through confluent (Fig. 19-2). Pars intermedia of the adenohypophysis pituitary veins that carry blood from the and pars nervosa of the neurohypophysis often are adenohypophysis, pars intermedia, and regarded collectively as forming a posterior lobe. neurohypophysis through a common trunk to the

265 venous systemic circulation. Only a few lateral action of a family of protein molecules known as hypophyseal veins extend directly from the somatomedins produced in the liver. The principal adenohypophysis to the cavernous sinus. The only somatomedins have amino acid sequence homology direct drainage of the neurohypophysis is by the with proinsulin. This homology and their insulin- neurohypophyseal limbs of the confluent veins like biological activities have lead to the term located at the lower end of the neurohypophysis. insulin-like growth factors, a synonym for Pars intermedia is relatively avascular and few somatomedins. Somatomedins circulate bound to capillaries traverse it. The most prominent vessels carrier proteins and have half lives considerably of pars intermedia are confluent pituitary veins. longer than growth hormone. The second type of acidophil, the mammotroph, tends to be more Pars Distalis (Anterior Lobe) scattered within the cellular cords. Active cells contain granular endoplasmic reticulum, Golgi VOCABULARY: chromophilic cell, acidophilic complexes, and scattered lysosomes. The cytoplasm cell, somatotroph, somatotropin (growth also contains irregular granules that measure 550 to hormone), mammotroph, prolactin, basophilic 615 nm in diameter. Mammotrophs secrete cell, corticotrophs, adrenocorticotrophic prolactin, which promotes mammary gland development hormone, thyrotroph, thyrotropin (thyroid- and lactation. Prolactin together with growth stimulating hormone), gonadotroph, follicle- hormone, adrenocorticoids, estrogen and stimulating hormone, luteinizing hormone, progesterone are necessary for normal breast interstitial cell-stimulating hormone, development to occur. The concentration of chromophobe prolactin rises progressively during pregnancy but is kept from showing its lactogenic effect by the high Pars distalis makes up about 75% of the hypophysis concentration of progesterone and estrogen. The and consists of epithelial cells arranged in clumps or effect of these three hormones is to continue to irregular cords. The parenchyma and surrounding stimulate tubuloalveolar growth of the mammary sinusoids are supported by a delicate network of glands within the breast tissue. The decrease of estrogen reticular fibers. The parenchymal elements mostly and progesterone levels resulting from the loss of the consist of chromophobic and chromophilic cells, placenta after birth allows the lactogenic effects of distinguished by whether or not they take up stain. prolactin to be expressed and results in milk Chromophilic cells are further subdivided into production. acidophilic and basophilic cells according to the Basophilic cells of pars distalis consist of three staining properties of their secretory granules. All types of cells: corticotrophs, thyrotrophs, and three cell types -chromophobes, acidophils, and gonadotrophs. Corticotrophs generally are round basophils - are present in pars distalis. or ovoid and larger than most acidophilic cells. Acidophilic cells are large, round or ovoid cells, They are most numerous in the central anterior 14 to 19 µm in diameter, with well-developed portion of pars distalis. In humans, the cells contain juxtanuclear Golgi complexes. The secretory numerous secretory granules, 350 to 400 nm in granules stain with eosin in standard preparations. diameter that are similar in appearance to the Two types of acidophils are present: somatotrophs granules of somatotrophs. The cytoplasm often and mammotrophs. The cytoplasm of contains lipid droplets and small filaments ranging somatotrophs contains a well-developed granular from 6 to 8 nm in diameter. Corticotrophs endoplasmic reticulum and numerous spherical, synthesize a glycoprotein prohormone, protropin, electron-dense secretory granules that measure 350 that is then cleaved to form adrenocorticotrophic to 400 nm in diameter. Somatotrophs produce hormone (ACTH), beta-lipotropic hormone (β- somatotropin (growth hormone), a peptide LPH), alpha-melanocyte-stimulating hormone (α- hormone that is important in controlling body MSH), and beta-endorphin. The different growth and development in children and hormones produced by acidophils and basophils in adolescents. It acts primarily on epiphyseal cartilages to the adenohypophysis also are produced as increase linear growth of bone, increases organ size prohormones, but how these are cleaved to form and lean body mass and plays a role in regulating body the active hormone is not well understood. metabolism. Many of the effects attributed to growth hormone are through the production and

266 Adrenocorticotrophic hormone stimulates the zona and inhibin act together on the fasciculata and zona reticularis of the adrenal cortex to hypothalamic/pituitary axis to suppress FSH and produce and release glucocorticoids. LH secretion and thereby temporarily prevent Adrenocorticotrophic hormone also is important in another ovarian cycle from taking place. stimulating the production of adrenal androgens. The male the equivalent to luteinizing hormone, The thyrotroph type of basophil often is angular interstitial cell-stimulating hormone (ICSH), is in shape and may form small groups deep in the secreted by this type of basophil in the male. ICSH parenchymal cords, at some distance from the stimulates production of testosterone by the interstitial cells sinusoids. At 130 to 150 nm in diameter, their of the testis. Testosterone is essential for sperm secretory granules are the smallest of any granules maturation and for development and the in the parenchymal cells of the hypophysis. maintenance of secondary sex organs and Thyrotrophs secrete a glycoprotein, thyrotropin characteristics in men. In men FSH targets the (thyroid- stimulating hormone), which stimulates Sertoli cell of seminiferous tubules to produce the thyroid to produce and release thyroid hormone. androgen binding protein (ABP) and inhibin. Gonadotrophs are the third type of basophil in Androgen binding protein is essential for the normal the pars distalis. They are generally smaller than spermatogenesis and spermiogenesis to occur. corticotropic basophils, often show only a Inhibin stops the secretion of FSH by the scattering of granules, and usually are round in gonadotrophs and plays an important role in the shape. They often lie immediately adjacent to feedback loop controlling the rate of spermatogenesis. sinusoids. The cytoplasm shows profiles of granular The remaining general cell type, the endoplasmic reticulum, prominent Golgi chromophobe, usually is small and confined to the complexes, and scattered lysosomes. The few interior of the parenchymal cords. Lacking granules, spherical secretory granules present are about 150 they fail to color with routine stains, hence their nm in diameter. This type of basophil secretes two name. Their existence as a separate cell type is glycoprotein hormones. Follicle-stimulating under debate. They may represent a reservoir of hormone (FSH) stimulates growth of ovarian follicles in cells from which chromophilic cells originate. This women and, in men, stimulates synthesis of androgen- accords with the hypothesis that cells of the pars binding protein by Sertoli cells in the seminiferous distalis cycle, first accumulating secretory granules epithelium, thereby activating and promoting and then releasing them. spermatogenesis. In women, FSH acts on granulosa Occasionally, cystlike follicles, lined by cells to produce estrogens from androgens coming nonsecretory (follicular) cells and filled with a from surrounding thecal cells. Luteinizing colloid type of material, may be seen in the pars hormone (LH) is essential for ovulation, for distalis. Microvilli and occasional tufts of cilia stimulating secretion of estrogen by ovarian extend from the apices of the cells into the follicles, and for promoting luteinization of follicles follicular lumen. The follicular cells are unusual in following development induced by FSH. FSH that they also have long basal processes that extend stimulates granulosa cells of selected ovarian between adjacent secretory cells. Similar cells called follicles to proliferate and produce estradiol. folliculostellate cells are not organized into follicles and Simultaneously, LH stimulates thecal cells to extend long branching processes between secretory produce androgens which diffuse into the stratum epithelial cells. The processes of folliculostellate granulosum and are converted into estradiol. The cells, which form a network, are united by gap result is increased estradiol production. The rising junctions. Their cytoplasm contains intermediate levels of estradiol stimulate a surge in gonadotropin filaments (gliofibrillary acid protein) and suggests a releasing hormone (GnRH) release from function similar to that of glial cells in the central hypothalamic neurons. The resulting increase in LH nervous system, ie, providing a supporting and FSH secretion induces ovulation. Following framework and monitoring a microenvironment for ovulation the remaining cells of the ovulated follicle the secretory epithelial cells. are transformed into a corpus luteum in response to The various epithelial cell types forming the LH. The cells of the corpus luteum produce large anterior pituitary (pars distalis) have a regional amounts of progesterone and estradiol throughout distribution and show a variation in number. its life span. Granulosa lutein cells also produce a glycoprotein called inhibin. Estradiol, progesterone

267 The lateral regions contain the greatest number of Control of the Adenohypophysis somatotrophs while corticotrophs are concentrated in the medial posterior region adjacent to the pars VOCABULARY: hypothalamus, releasing nervosa of the posterior pituitary. Thyrotrophs are hormone, inhibitory hormone, somatostatin concentrated in the medial anterior region. Gonadotrophs and mammotrophs are scattered Secretions from target endocrine glands that are throughout the pars distalis. Somatotrophs make up under control of the adenohypophysis influence about 40-50%, mammotrophs 20-25%, cells in the adenohypophysis through regulatory corticotrophs 15-20%, thyrotrophs about 10%, and neurons in the hypothalamus. This important gonadotrophs about 10% of the epithelial cells region of the brain consists only of about 4 grams forming the pars distalis. of tissue. The perikarya of the hypothalamic neurons are located primarily in the paraventricular Pars Tuberalis nucleus, arcuate nucleus, and medial preoptic nucleus of the hypothalamus. Axons from these Pars tuberalis forms a sleeve of epithelial cells, up neurons project into the median eminence where to 60 µm thick, around the infundibular stalk. It is the axon terminals secrete releasing and inhibiting thickest anteriorly and may be incomplete hormones. These small peptides, released in a posteriorly. Pars tuberalis is characterized by pulsatile fashion, enter the primary capillaries longitudinally arranged cords of parenchymal cells surrounding the infundibular stalk and then are separated by sinusoids. The parenchyma of pars transported to the secondary capillaries of pars tuberalis is continuous with that of pars distalis and distalis via the hypophyseal portal veins where they consists of acidophils, basophils, and stimulate specific target cells to release hormones. undifferentiated cells. The latter usually are Hypothalamic releasing factors are known for columnar, and their cytoplasm shows numerous somatotropin (growth hormone releasing hormone small granules and large amounts of glycogen. (GHRH)), primarily from neurons of arcuate nuclei; Nests of squamous cells may be present. Pars thyrotropin (thyrotropin releasing hormone tuberalis has no known distinct hormonal function. (TRH)), primarily from neurons of dorsomedial nuclei; corticotropin (corticotropin releasing Pars Intermedia hormone (CRH)), primarily from neurons in anterior paraventricular nuclei; and gonadotropin VOCABULARY: chromophobe, basophil, (gonadotropin releasing hormone (GnRH)), melanocyte-stimulating hormone primarily from neurons in arcuate nuclei and preoptic area; and their amino acid sequences have Pars intermedia is rudimentary in humans, in whom been determined. Inhibitory hormones such as it forms only about 2% of the hypophysis. It somatostatin (a peptide of 14 amino acids) also are consists of chromophobe and basophil cells, the secreted by hypothalamic neurons. Somatostatin latter often encroaching into pars nervosa. Colloid- inhibits the secretion of growth hormone, thyroid filled cysts lined by chromophobes or basophilic stimulating hormone and prolactin. Dopamine also cells also may be present, and remnants of Rathke's inhibits prolactin secretion and is thought to be the cleft may persist as fluid-filled cysts lined by ciliated most important factor in the control of prolactin columnar epithelium. Pars intermedia in some release. Other factors having this action are species is responsible for the secretion of oxytocin and thyrotropin releasing hormone. melanocyte-stimulating hormone (MSH), a Somatostatin is released by neurons within the polypeptide hormone. Its role in mammals is paraventricular nuclei and dopamine by neurons unknown, but it may be involved in melanocyte within the arcuate nuclei. stimulation. Only the precursor molecule for MSH occurs in humans. MSH contains amino acid sequences that are identical to portions of the ACTH molecule, and the occurrence of these structurally similar regions is believed to account for the melanocyte-stimulating effect of ACTH.

268 Neurohypophysis express milk into the ductal system during nursing. Oxytocin also is released in response to vaginal VOCABULARY: hypothalamohypophysial stimulation during sexual intercourse resulting in tract, Herring bodies, pituicytes, neurophysin, smooth muscle contractions within the wall of the oxytocin, antidiuretic hormone uterus as well as the oviduct. This action may facilitate sperm transport within the female Macroscopically, the neurohypophysis consists of reproductive tract. Oxytocin levels rise and the median eminence, infundibular stalk, and increases uterine smooth muscle contractions infundibular process (pars nervosa). The greater during labor. Oxytocin also is believed to play a part of the neurohypophysis consists of major role in certain behaviors such as pair bonding unmyelinated nerve fibers of the and affection between adults and bonding between hypothalamohypophysial tract, which originates mother and child. mainly from neurons in the supraoptic and Antidiuretic hormone (ADH) acts primarily on the paraventricular nuclei. The tract receives additional collecting tubules of the kidney to increase their nerve fibers from other hypothalamic regions. The permeability to water and urea, thus promoting the fibers end blindly in the pars nervosa, close to a rich resorption of water from the glomerular filtrate. capillary plexus. The axons comprising the Antidiuretic hormone acts primarily on the principal hypothalamohypophysial tract commonly contain cells of the collecting duct. In the presence of this spherical masses of secretory material that vary hormone, intracellular vesicles containing proteins considerably in size. These are the Herring bodies, (aquaporin-2 and urea channels) are delivered to which consist of large accumulations of dense and inserted into the apical plasmalemma of this secretory granules that vary from 120 to 200 nm in cell type making them permeable to water. The net diameter. Hormones within the secretory granules effect of this action is the concentration of urine are synthesized in the perikarya of neurons forming and the reabsorption of water by the kidney. the supraoptic and paraventricular nuclei. They then Afferent nerve fibers traveling within the vagus pass down the axons to be stored in the nerve and glossopharyngeal nerves from the carotid and terminals that form the pars nervosa. When aortic bodies carry baroreceptor input to a released, the hormones cross a thin, fenestrated vasomotor center located in the medulla of the endothelium to enter the capillaries in the pars brain stem. Perikarya of neurons in the supraoptic nervosa and thence into hypophyseal veins to and paraventricular nuclei of the hypothalamus finally enter the systemic circulation. receive input from these peripheral baroreceptors as Scattered among the nerve fibers are cells called well as input from osmoreceptors and secrete in pituicytes, which vary in size and shape and may response to an increase in the osmolarity of the contain pigment granules. Pituicytes are considered body fluids as well as other factors. Decreased to be the equivalent of neuroglial cells of the central blood volume and pressure, increased body fluid nervous system, but whether they have only a osmolality and increased angiotensin II levels supportive function or actively participate in the stimulate ADH secretion resulting in a small volume secretory processes of adjacent nerve terminals is of concentrated urine being produced by the not known. kidney. Increased blood volume and pressure, Two cyclic polypeptide hormones, oxytocin and decreased body fluid osmolality, and atrial antidiuretic hormone (vasopressin), are stored and natriuretic peptides inhibit ADH secretion resulting released from axons forming the pars nervosa. in a large volume of dilute urine produced by the During axonal transport, each hormone is bound to kidney. a carrier protein called neurophysin, and it is the Pharmacologic doses of ADH cause contraction neurophysin-hormone complex that forms the of vascular smooth muscle and elevate blood major part of the Herring bodies. pressure. As a result of this action, this hormone is Oxytocin is released from neurons in lactating sometimes referred to as vasopressin. women by means of a neuronal reflex that is initiated during suckling and transmitted to the hypothalamus through the cerebral cortex. Oxytocin causes contraction of myoepithelial cells around the alveoli of the mammary gland and helps to

269 Organogenesis fuses with smaller caudal-lateral outpockets (the lateral thyroid) of the pharynx. The latter contribute Classic endocrine glands develop as a result of to the ultimobranchial bodies of the thyroid. The invaginations from an epithelial surface (ectoderm, median thyroid enlarges and forms an epithelial- endoderm, and mesoderm (mesothelium)) into the lined vesicle that separates from the pharyngeal underlying mesoderm. Connection with the floor. As the vesicle migrates caudally, it expands epithelium of origin is lost. laterally to form two lobes that fuse with tissues of The pineal gland arises from the roof of the the developing ultimobranchial bodies. The lumen developing brain by fusion of a solid anterior of the thyroid vesicle is lost, epithelial cells in the growth near the habenular commissure with a wall of the vesicle proliferate, and numerous hollow, saccular evagination from the roof of the anastomosing cords expand into the surrounding diencephalon. Cell proliferation first begins and is mesenchyme. A minute lumen lies in the center of more extensive in the anterior region, where the each cord. These features characterize the differentiating pineal neuroepithelial cells form a prefollicular stage. As a result of the secretory activities network of anastomosing cords and follicles within of the epithelial cells, the lumen expands and the mesenchyme. Together with contributions from segmentation of the epithelial cords results in neural crest, the mesenchyme forms the connective formation of follicles. Secondary follicles arise by tissue capsule, trabeculae, and stroma of the gland. budding and/or further subdivision and growth of Neuroepithelial cells differentiate into pinealocytes and the primary follicles. The capsule and stroma neuroglial cells. As the gland matures, lobules develop from the surrounding mesenchyme. The become more distinct and are composed mainly of connection or tract with the site of origin of the pinealocytes. thyroid is known as the thyroglossal duct. If the The parathyroids develop in the ectodermal epithelium thyroglossal duct fails to atrophy during of the third and fourth branchial pouches. Dorsolateral development it will persist as a cyst (thyroglossal thickenings of each pouch form solid masses of duct cyst). cells, each of which represents a primordium of a The origin of parafollicular (C) cells is obscure. parathyroid gland. As they invaginate, each cell They may be derived from neural crest or mass forms a vesicle that loses its connection to the endoderm associated with the ultimobranchial epithelium of the pharyngeal floor. The paired bodies. primordia from the third pouches migrate with the The adrenal cortex is a derivative of mesoderm, and thymic rudiment to the caudal border of the thyroid the medulla arises from neural ectoderm. A to become the inferior parathyroids. The pair associated provisional fetal cortex arises first from the mesothelial with the fourth pouches remains at the cranial border cells over the dorsal mesentery. The cells proliferate of the thyroid and forms the superior pair. and invade adjacent vascular mesenchyme, which The cells of the primordia mainly differentiate into subsequently forms the capsule and stoma of the chief cells, which form anastomosing cords and gland. Differentiation of the epithelial cells gives sheets. Growth of the glands occurs as the cords of rise to a thick zone of large, acidophilic cells that cells expand by addition of new cells derived by form the provisional cortex. This soon is enveloped by mitosis. The thin capsule and vascular stroma of an outer, compact layer of smaller, densely staining each parathyroid originate from the surrounding cells that will form the permanent cortex. The mesenchyme. Initially, the stroma is relatively provisional cortex continues to grow and at birth inconspicuous and free of fat, which accumulates makes up almost 80% of the cortex. Immediately with age. Oxyphils appear at about six years of age after birth, it begins to involute and disappears but are not present in appreciable numbers until within the first few weeks of postnatal life. As the after puberty and continue to increase with age. provisional cortex regresses, the cells of the definitive Generally, thyroid development can be divided into cortex differentiate and advance centrally. At birth, three main stages: formation and separation of the the zona glomerulosa is present and the zona thyroid anlagen, early differentiation (prefollicular fasciculata is in the initial stage of development. stage), and follicle formation. The thyroid is derived Zona fasciculata and zona reticularis become well from ventral outpockets of the endodermal epithelium over defined and assume adult form during the first few the floor of the pharynx near the base of the tongue. months of postnatal life. These outgrowths form the median thyroid, which

270 The adrenal medulla arises during the seventh week referred to as neurocrine secretion. Antidiuretic by migration of cells from developing ganglia in the hormone and oxytocin are synthesized by neurons region of the celiac plexus. The cells invade the in the supraoptic and paraventricular nuclei of the medial side of the cortical primordium, which hypothalamus, travel down their respective axons, subsequently envelops them. As a result, the and are released into the vasculature of the pars presumptive chromaffin cells take a central position nervosa. From here they enter the hypophyseal in the adrenal late in fetal life. Ultimately, the veins and make their way into the general chromaffin cells become arranged into cords and circulation. Antidiuretic hormone promotes plates separated by sinusoids. absorption of water from the glomerular filtrate, The neurohypophysis develops as an outgrowth of thus conserving body water and concentrating the the floor of the diencephalon while the urine. The stimulus for secretion of ADH is an adenohypophysis arises from an ectodermal epithelial increase in the osmolality of the blood plasma. The invagination of the developing oral cavity (Rathke’s cell bodies of neurons in the paraventricular and pouch). This vesicle loses its connection with the supraoptic nuclei are stimulated to secrete ADH in epithelium and comes to abut the rostral surface of response to osmoreceptors and other stimuli the infundibulum. It then differentiates into the coming from pressure-sensitive regions within the three primary epithelial subdivisions of the vascular system. Oxytocin, which is released in adenohypophysis. The rostral wall thickens to response to the suckling reflex of nursing women, become the pars distalis, the caudal wall thins to stimulates contraction of myoepithelial cells and form pars intermedia, and bilateral outgrowths of helps to express milk from the secretory units into the dorsolateral wall wrap around the infundibular the ductal system of the mammary glands (milk stem to become pars tuberalis. letdown). Oxytocin released during sexual The neurons destined to form the intercourse stimulates contractions of the uterus releasing/inhibiting hormone producing neurons and oviduct that may act to facilitate sperm develop in that region of the brain that will become transport in the female system. Oxytocin increases the hypothalamus with the exception of GnRH uterine contractions during labor. neurons. GnRH neurons initially differentiate in the Other hypothalamic neurons act on mediator olfactory placode and then migrate into the forming cells, the gonadotrophs of the adenohypophysis, hypothalamic region of the brain. and through them control follicular growth, As these events occur, the lumen of Rathke’s ovulation, and hormone production in women and pouch is obliterated or may form a residual lumen spermatogenesis and hormone production in men. that appears as cysts in the adult. Thickenings of Several hypothalamic neurons, acting through the rostral wall occur at a time when the releasing hormones, influence somatotrophs and parenchyma of the developing pars distalis thyrotrophs of the pars distalis to control the levels differentiates into cords or plates of cells that are of growth hormone and thyroid hormone, which separated by sinusoids. The latter arise from control body growth and metabolism, respectively. surrounding mesenchyme that is incorporated into Adrenocorticotrophic hormone elaborated by the parenchyma as the two major subdivisions corticotrophs of the pars distalis stimulates the blend into one. The capsule and septae arise from zona fasciculata and zona reticularis of the adrenal the surrounding mesenchyme. By the tenth week, cortex to secrete and release glucocorticoids. Like the major cell types have differentiated. other epithelial cells in the pars distalis, corticotrophs are controlled by inhibitory and Summary releasing hormones secreted by hypothalamic neurons. The parenchymal cells of the endocrine glands Unlike most of the classic endocrine glands, the synthesize hormones that may regulate specific chromaffin cells of the adrenal medulla are under tissues or have more general, systemic effects. the direct control of the sympathetic nervous Endocrine glands control and coordinate many of system. When stimulated to secrete, the chromaffin the physiologic activities of the body and often act cells release two active catecholamines - in concert with the nervous system. Neurons often epinephrine and norepinephrine - which, although perform endocrine functions by secreting peptides not essential for life, are important in meeting directly into the bloodstream. This phenomenon is stressful "fight or flight" situations by increasing

271 cardiac output, metabolic rate, and blood pressure. activities of osteoclasts and osteocytes. Thus, Likewise the zona glomerulosa responds primarily regulation of the concentration of calcium ion in to changes in blood volume and/or extracellular the blood is controlled by two endocrine glands fluid volume. For example, during episodes of that act in opposition to one another. hemorrhage or diarrhea, renin is releases from the Parathyroid hormone not only is the major JG cells in the kidney and via the renin-angiotensin hormone regulating the excretion of phosphate and circuit aldosterone released by the zona retention of calcium ion by the kidney, but also glomerulosa acts to increase extracellular fluid and controls the synthesis of 1, 25-dihydroxyvitamin D3 blood volumes. In contrast, if extracellular fluid and by cells in the proximal tubules of the kidney. Like blood volumes are high, as in congestive heart vitamin D3, 1, 25-dihydroxyvitamin D3 is lipid- failure, atrial natriuretic factor acts on the zona soluble, and its mechanism of action is similar to glomerulosa to inhibit aldosterone secretion. that of steroid hormones. In the proximal small Other classic endocrine glands are not under intestine, it induces an increase in mucosal weight direct nervous stimulation or control by neural and the length of villi and microvilli and accelerates hormones. The activity of the parathyroid gland, for the rate of mucosal turnover. It also is involved in example, is influenced primarily by the regulating calcium stores in the skeleton. Receptors concentration of calcium ion in the circulating for the hormone occur in a variety of tissues, blood. If the concentration falls below a specific including differentiating cells of the immune level, PTH is secreted and acts on the following: system. osteocytes and osteoclasts to promote release of The hypothalamus is the major neuroendocrine calcium ion from bone, kidney tubules (thick regulatory region of the brain and is restricted by ascending limb and distal tubule) to increase the blood-brain barrier, whereas the calcium absorption from the glomerular filtrate, and neurohypophysis is not. This may explain why proximal convoluted tubules to release 1, 25- oxytocin, ADH, and the releasing and inhibitory vitamin D3, which increases calcium absorption in hormones released by neurons in the hypothalamic the small intestine. Blood calcium is kept from region must travel considerable distances along rising above optimum levels by the hormone axons before being released into capillaries in the calcitonin, which is released by the parafollicular region of the median eminence, infundibular stalk, cells of the thyroid gland. Calcitonin counters the and infundibular process, finally to enter the action of PTH by suppressing the resorptive hypophyseal or systemic circulations.

272 The interior of the eyeball is subdivided into an 20 The Eye anterior chamber, a posterior chamber, and a vitreal cavity. Each is filled with a transparent medium that Eyes are photoreceptors containing systems of helps maintain the shape and turgor of the eye. The convex surfaces that transmit and focus light on a anterior and posterior chambers are continuous surface sensitive to the intensity and wavelength through the pupil and contain a watery aqueous (color) of light. Images formed there are humor, which provides nutrients for the anterior transmitted to the brain, where they are interpreted, structures of the eye. The large vitreal cavity is filled correlated, and translated into the sensation of with a viscous, transparent gel called the vitreous sight. Eyes can regulate the amount of light humor or vitreous body. The general structure of the admitted and change their focal lengths. eye is shown in Figure 20-1. The eyes are almost spherical and are located in and protected by boney sockets provided with a padding of fat. Because the eyes are offset from each other, the images formed are correspondingly offset and provide for binocular vision. Integration of the binocular images results in a three- dimensional quality of sight through which depth and spacial orientation are recognized.

General Structure

The wall of the eyeball consists of an outer corneoscleral coat, a central coat called the uvea, and an inner coat called the retina. The corneoscleral coat is subdivided into a smaller, transparent, anterior cornea that allows light to enter the eye and the sclera, a larger white segment. The sclera is a tough fibrous layer that protects the Fig. 20-1. A diagrammatic sketch of general eye structure. delicate internal structures of the eye and helps maintain the shape of the eyeball. Corneoscleral Coat The uveal coat consists of the choroid, ciliary body, and iris. The choroid is the vascular part of the The corneoscleral coat forms the outermost layer of uvea, immediately beneath the retina. It is the eyeball and consists of the cornea and sclera. continuous with the ciliary body, which forms a belt-like structure around the interior of the eyeball, Sclera just forward of the anterior margin of the retina. The ciliary body controls the diameter and shape of VOCABULARY: fibrous outer tunic, lamina the lens, which focuses light on the retina. The iris, cribrosa the last portion of the uvea, is continuous with the ciliary body and has a central opening, the pupil, The sclera forms the tough, opaque, fibrous outer whose diameter can be changed to regulate the tunic of the posterior five-sixths of the eyeball. It amount of light passing into the eyeball. varies in thickness in different regions but averages The retina consists of pigment-containing about 0.5 mm. It is composed of flat bundles of photoreceptors that, on exposure to light, produce collagen that run in various directions parallel to the chemical energy that is converted into the electric scleral surface. Delicate networks of elastic fibers energy of nerve impulses. A chain of conducting and elongated, flattened fibroblasts extend between neurons in the retina transmits the impulses to the the collagen bundles. Melanocytes occur in the optic nerve. The optic disc is the point where nerve deeper layers. The sclera thins and forms a fibers from the retina gather to form the optic fenestrated membrane, the lamina cribrosa, at the nerve and leave the eyeball. point where fibers of the optic nerve penetrate the sclera to exit the eye.

273 The sclera protects the interior of the eye, aids in proteoglycans are not present in the sclera. maintaining its shape, and serves as the site of Flattened fibroblast-like cells, the keratocytes, are attachment for the extrinsic muscles of the eye. present between the bundles of collagen fibers. Blood vessels and lymphatics normally are absent in Cornea the substantia propria, although occasional lymphoid wandering cells are seen. VOCABULARY: corneal epithelium, Descemet's membrane is a homogeneous Bowman's membrane, substantia propria, membrane, 6 to 8 µm thick, lying between the keratocyte, Descemet's membrane, corneal posterior surface of the substantia propria and the endothelium corneal endothelium. It corresponds to a thick basal lamina secreted by the corneal endothelium. It is The transparent cornea is the most anterior part of resilient and elastic, although elastic fibers are not the eye. It is about 11 mm in diameter, present, and appears to consist mainly of the type approximately 0.8 mm thick near the center, and VIII form of collagen. 1.00 mm at the periphery. Its curvature is The corneal endothelium lines the inner surface considerably greater than that of the posterior of the cornea and consists of a layer of large, sclera. The cornea is uniform in structure hexagonal squamous cells. The apices of the cells throughout and consists of corneal epithelium, are joined by tight junctions, and the cytoplasm Bowman's membrane, substantia propria, contains numerous mitochondria and vesicles. The Descemet's membrane, and corneal endothelium. cells are involved in transporting materials from the The corneal epithelium is stratified squamous anterior chamber. with a smooth outer surface and averages about 50 The transparency of the cornea is due to the µm in depth. It usually consists of five layers of uniform diameter and orderly arrangement of large squamous cells that have few organelles but collagen fibers and to the properties of the ground often contain glycogen. The superficial cells retain substance. The cornea is avascular and depends on their nuclei, and their external surfaces form the aqueous humor and blood vessels of the numerous fine ridges (microplicae) that help retain surrounding limbus to supply its nutrients. Ion moisture on the corneal surface. The epithelium lies pumps of the corneal endothelial cells maintain a on a distinct basal lamina. The lateral membranes of critical fluid level within the substantia propria. If adjacent cells are extensively interdigitated and excess fluid accumulates, the cornea becomes joined by numerous desmosomes. The corneal opaque. Corneal endothelial cells do not divide after epithelium contains many free nerve endings and is birth and therefore are not replaced. Each person is very sensitive to a number of stimuli, especially born with a complete complement of corneal pain. endothelial cells. The basal lamina of the corneal epithelium lies on the outer layer of the substantia propria and is Limbus called Bowman's membrane; it ends abruptly at the peripheral margin of the cornea. Bowman's VOCABULARY: external scleral sulcus, membrane has a homogeneous appearance and internal scleral sulcus, scleral spur, trabecular consists of a feltwork of small collagen fibrils (type meshwork, Schlemm's canal I) and lacks elastin. The substantia propria, or stroma, forms the The limbus represents a zone of transition 1.5 to bulk of the cornea. It consists of numerous bundles 2.0 mm wide between the transparent cornea and of collagen fibers (type I and type V) arranged into the opaque sclera. The outer surface bears a shallow 50-70 broad sheets called corneal lamellae that run groove, the external scleral sulcus, where the parallel to the corneal surface. The collagen bundles more convex cornea joins the sclera. A similar in each successive lamina run in different directions structure, the internal scleral sulcus, lies on the and cross at various angles. Adjacent lamellae are inner surface of the limbus. The scleral spur is a knit tightly together by interlacing collagenous small ridge of tissue projecting from the posterior fibers. The fibers, bundles, and lamellae of collagen lip of the internal scleral sulcus and attached are embedded in a matrix of sulfated proteoglycans anteriorly to tissue that comprises the outflow rich in chondroitin sulfate and keratosulfate. These system for fluid in the anterior chamber.

274 The limbus is further characterized by the space, between the sclera and choroid to enter the terminations of Bowman's and Descemet's choroid proper. membranes. The collagen fibers and bundles of the The choroid proper consists of three regions. The cornea become larger here and their arrangement outer vessel layer is made up of loose connective more irregular as they blend with those of the tissue with numerous melanocytes and contains the sclera. Numerous small blood vessels form arcades larger branches of ciliary arteries and veins. The in this region and nourish the peripheral portions of central choriocapillary layer consists of a net of the avascular cornea. capillaries lined by a fenestrated endothelium; these Descemet's membrane is replaced by the spongy vessels provide for the nutritional needs of the trabecular meshwork attached to the anterior part outer layers of the retina. The choriocapillary layer of the scleral spur. It contains numerous flattened, ends near the ora serrata. The innermost layer of anastomosing trabeculae that consist of collagenous the choroid is the glassy (Bruch's) membrane fibers and ground substance covered by an that lies between the remainder of the choroid and attenuated endothelium that is continuous with the the pigment epithelium of the retina. With the light corneal endothelium. The trabeculae form a microscope it appears as a homogeneous layer, 1 to labyrinth of spaces that communicate with the 4 µm thick. Ultrastructurally it shows five separate anterior chamber. Between the bulk of the limbal strata made up of basal laminae of capillaries in the stroma and the trabecular meshwork, a flattened, choriocapillary layer, the pigment epithelium of the endothelial-lined canal, Schlemm's canal, courses retina, and between them, two thin layers of around the circumference of the cornea. Aqueous collagen separated by a delicate elastic network. humor enters the trabecular spaces from the anterior chamber and crosses the endothelial lining Ciliary Body of the trabeculae, the juxtacanalicular connective tissue, and finally, the endothelium of the canal to VOCABULARY: ciliary epithelium, inner enter its lumen. Several channels arise from the nonpigmented cell, outer pigmented cell, peripheral wall of the canal to join veins in the blood-aqueous barrier, aqueous humor, ciliary limbus and eventually drain to episcleral veins. process, zonule fiber, ciliaris muscle Obstruction to the drainage of aqueous humor causes a rise in intraocular pressure, a characteristic The ciliary body is located between the ora serrata of the condition called glaucoma. of the neural retina and the outer edge of the iris, where it attaches to the posterior aspect of the Uveal Layer scleral spur at the corneoscleral junction. It forms a thin triangle when seen in section with the light The uvea, the middle vascular coat of the eye, is microscope and consists of an inner vascular tunic divided into choroid, ciliary body, and iris. and a mass of smooth muscle immediately adjacent to the sclera. The internal surface is covered by Choroid ciliary epithelium, a continuation of the pigment epithelium of the retina that lacks photosensitive VOCABULARY: suprachoroid lamina, cells. perichoroidal space, outer vessel layer, Ciliary epithelium consists of an inner layer of choriocapillary layer, glassy (Bruch's) nonpigmented cells and an outer layer of membrane pigmented cells, each resting on a separate basal lamina; it is unusual in that the cell apices of both The choroid is a thin, brown, highly vascular layers are closely apposed, apex-to-apex. membrane that lines the inner surface of the The outer pigmented cell layer is separated from posterior sclera. Its outer surface is connected to the stroma of the ciliary body by a thin basal lamina the sclera by thin avascular lamellae that form a continuous with that underlying the pigment delicate, pigmented layer called the suprachoroid epithelium in the remainder of the retina. The basal lamina. The lamellae mainly consist of fine elastic lamina of the nonpigmented layer lies adjacent to fibers between which are numerous large, flat the posterior chamber of the eye and is continuous melanocytes and scattered macrophages. The with the inner limiting membrane of the retina. lamellae cross a potential cleft, the perichoroidal

275 The basal plasmalemma of the nonpigmented cells Iris shows numerous infoldings and is involved in ion/fluid transport. The cells contain numerous VOCABULARY: pupil, anterior chamber, mitochondria and a well-developed, supranuclear posterior chamber, myoepithelial cell, sphincter Golgi complex. The adjacent pigmented cells of the muscle outer layer also show prominent basal infoldings, and the cytoplasm is filled with melanin granules. The iris is a thin disc suspended in the aqueous The apices of cells of the inner, nonpigmented humor between the cornea and lens and has a epithelium are united by well-formed tight junctions central, circular aperture called the pupil. The iris is that form the anatomic portion of the blood- continuous with the ciliary body at the periphery aqueous barrier, which selectively limits passage of and divides the space between the cornea and lens materials between the blood and the interior of the into anterior and posterior chambers. The anterior eye. chamber is bounded anteriorly by the cornea and The ciliary epithelium elaborates aqueous humor, posteriorly by the iris and central part of the lens. which differs from blood plasma in its electrolyte The posterior chamber is a narrow space between composition and lower protein content. Aqueous the peripheral part of the iris in front and the humor fills the posterior chamber, provides peripheral portion of the lens, ciliary zonule, and nutrients for the lens, and posteriorly, enters the ciliary processes. The two chambers communicate vitreous humor. Anteriorly, it flows from the through the pupil. The anterior surface of the iris is posterior chamber through the pupil into the irregular due to numerous fissures or crypts. The anterior chamber and aids in nourishing the cornea. margin attached to the ciliary body forms the ciliary It leaves the anterior chamber via the trabecular margin; that surrounding the pupil is the pupillary meshwork and Schlemm's canal to the episcleral margin. veins. The stroma of the iris consists primarily of loose, In its anterior part, the inner surface of the ciliary vascular connective tissue with scattered body is formed by 60 to 80 radially arranged, collagenous fibers, melanocytes, and fibroblasts elongated ridges called ciliary processes, which are embedded in a homogeneous ground substance. lined by ciliary epithelium and contain a highly The anterior surface lacks a definite endothelial or vascularized stroma and scattered melanocytes. mesothelial covering but is lined in part by a Zonule fibers, which hold the lens in place, are discontinuous layer of melanocytes and fibroblasts. produced mainly by the nonpigmented cell layer of Tissue spaces of the stroma often appear to the ciliary epithelium and are attached to its basal communicate with the anterior chamber. The lamina. They extend from the ciliary processes to posterior surface is covered by two rows of the equator of the lens. cuboidal, pigmented cells that are continuous with Most of the ciliary body consists of smooth the ciliary epithelium. Where it passes onto the muscle, the ciliaris muscle, which controls the posterior surface of the iris, the inner shape and therefore the focal power of the lens. nonpigmented layer of the ciliary epithelium The muscle cells are organized into regions with becomes heavily pigmented. Cells of the outer layer circular, radial, and meridional orientations. have less pigment and are modified into Numerous elastic fibers and melanocytes form a myoepithelial cells to form the dilator of the iris. sparse connective tissue between the muscle The dilator of the pupil consists only of a single bundles. The ciliaris muscle is important in eye layer of radially arranged myoepithelial cells whose accommodation, and when it contracts, it draws the contraction increases the diameter of the pupil. ciliary processes forward, thus relaxing the The sphincter muscle consists of a circularly suspensory ligament (zonule fibers) of the lens, arranged, compact bundle of smooth muscle cells allowing the lens to become more convex and to near the pupillary margin. Contraction of the focus on objects near the retina. sphincter muscle reduces the diameter of the pupil. The iris acts as a diaphragm, modifying the amount of light that enters the eye, thus permitting a range of vision under a variety of lighting conditions.

276 The color of the iris is determined by the amount and exit of the optic nerve. Lacking photoreceptors, and distribution of pigment. In the various shades this area is insensitive to light and forms the "blind of blue eyes, melanin is restricted to the posterior spot" of the retina. surface of the iris, whereas in gray and brown eyes melanin is found in increasing concentration within Neural Retina melanocytes present throughout the stroma of the iris. In albinos, melanin pigment is absent and the VOCABULARY: three-neuron conducting iris takes on a pink color due to the vasculature of chain, photoreceptors, bipolar neuron, ganglion the iridial stroma. cell, association neuron, glial cell

Retina Except for the extreme periphery and the fovea centralis, the neural retina is made up of the VOCABULARY: pigment epithelium, neural following layers, listed in order from the choroid, as retina (retina proper), ora serrata, macula lutea, seen with the light microscope: (1) layer of rods and fovea centralis, optic disc cones, (2) external limiting membrane, (3) outer nuclear layer, (4) outer plexiform layer, (5) inner The retina, the innermost of the three tunics, is a nuclear layer, (6) inner plexiform layer, (7) ganglion delicate sheet of nervous tissue that forms the cell layer, (8) layer of nerve fibers, and (9) internal photoreceptor of the eye. The outer surface is in limiting membrane. contact with the choroid; the inner surface is The layer of rods and cones represents the inner adjacent to the vitreous body. The posterior retina and outer segments of rods and cones. The rod consists of an outer pigment epithelium and an outer segments appear as darker-staining rods and inner neural retina (retina proper). The retina are more numerous than the cone outer segments. decreases in thickness anteriorly, and the nervous The latter are larger and exhibit a conical shape. component ends at a ragged margin called the ora Lying between this layer and immediately adjacent serrata. The thin prolongation of the retina extends to the outer nuclear layer is the external (outer) anteriorly to cover the ciliary processes as the ciliary limiting membrane. It appears as a fine epithelium and covers the posterior aspect of the discontinuous line and is formed by junctional iris, where it forms the iridia retinae. The forward complexes between the scleral tips of Müller's cells extension of the retina consists only of the and adjacent photoreceptor cells. The outer pigmented layer and an inner layer of columnar nuclear layer consists of the cell bodies and nuclei epithelial cells; the nervous component is lacking. of the rods and cones. The outer plexiform layer The neural retina is anchored only at the optic disc, consists primarily of rod spherules, cone pedicles where nerve fibers congregate before passing and dendrites of bipolar neurons. Nuclei and cell through the sclera to form the optic nerve, and at bodies of bipolar neurons, horizontal, amacrine and the ora serrata. Although cells of the pigment Müller's cells constitute the inner nuclear layer. epithelium interdigitate with photoreceptor cells of The inner plexiform layer is made up of axons of the neural retina, there is no anatomic connection bipolar neurons, dendrites of ganglion cells, and between the two components of the retina, and processes of amacrine cells. Ganglion cells and after trauma or disease, the neural retina may detach scattered neuroglial cells form the ganglion cell from the pigment epithelium. layer. The nerve fiber layer is made up primarily The exact center of the posterior retina of unmyelinated axons from ganglion cells and corresponds to the axis of the eye, and at this point, processes of Müller's cells. The very thin internal vision is most perfect. The region appears as a limiting membrane is formed by the vitreal small, yellow, oval area called the macula lutea. In processes of Müller's cells and their basement its center is a depression about 1.5 mm in diameter membrane. called the fovea centralis, where the sensory The structurally complex neural retina consists elements are most numerous and most precisely essentially of a three-neuron conducting chain organized. Nearer the periphery of the retina, neural that ultimately forms the nerve fibers in the optic elements are larger, fewer, and less evenly nerve. The neural elements of the basic conducting distributed. About 4.0 mm to the nasal side of the chain are photoreceptors (rod and cone cells), macula lutea is the optic disc, the site of formation bipolar neurons, and ganglion cells (Fig. 20-2).

277 adjacent to the vitreal body and forms the inner limiting membrane of the retina. The apices of Müller's cells are joined to adjacent photoreceptor cells by junctional complexes and form the outer limiting membrane. The cytoplasm is rich in smooth endoplasmic reticulum and contains abundant glycogen. In addition to providing structural support, Müller's cells are thought to aid in the nutritional maintenance of other retinal elements and have been equated with astrocytes of the central nervous system. A small number of spindle-shaped glial cells also are present around ganglion cells and between axons that form the nerve fiber layer of the retina.

Photoreceptor Cells

VOCABULARY: rod cell, rod proper, outer segment, rhodopsin, inner segment, ellipsoid, vitreal portion, myoid, outer fiber, cell body, inner fiber, spherule, synaptic ribbon, cone cell, iodopsin, cone pedicle

The two types of photoreceptors are rod cells and cone cells. Rod cells are long, slender cells that lie

Fig. 20-2. A diagrammatic sketch illustrating the basic three- perpendicular to the layers of the retina. They are

neuron chain system of the retina. 40 to 60 µm long and 1.5 to 3.0 µm wide,

depending on their location in the retina, and

number between 75 million and 170 million in each When stimulated by light, photoreceptor cells retina. The scleral (outer) third of each rod, called transmit an action potential to the bipolar neurons, the rod proper, lies between the pigment epithelium which in turn synapse with ganglion cells. and the outer limiting membrane. The scleral end of Unmyelinated axons from the ganglion cells enter the rods is surrounded by processes that extend the layer of retinal nerve fibers and unite at the from the underlying pigment epithelial cells; the optic disc to form the optic nerve, which transmits inner or vitreal end extends into the outer plexiform the impulse to the brain. Other intraretinal cells are layer. either association neurons (horizontal cells, Each rod proper consists of outer and inner amacrine cells) or glial cells. These elements are segments connected by a slender stalk containing organized into the various layers of the retina. nine peripheral doublets of microtubules. The

doublets originate from a basal body in the vitreal Glial Cells end of the inner segment, but the connecting stalk differs from a typical cilium in that it lacks a central VOCABULARY: Müller's cells, inner limiting pair of microtubules. membrane, outer limiting membrane The outer segment contains hundreds of flattened membranous sacs or discs of uniform The largest and most prominent glial elements in diameter. These sacs contain rhodopsin, the the neural retina are Müller's cells, which extend substance responsible for absorption of light. between the outer and inner limiting membranes of After exposure to light, rhodopsin changes its the retina. Their cytoplasmic processes run between molecular configuration from cis to trans and breaks the cell bodies and processes of neurons in the down, resulting in hyperpolarization of the retina and provide physical support for the neural plasmalemma of the rod cell, with formation of an elements. The basal lamina of Müller's cells lies electrical potential that is transferred to dendrites of

278 associated bipolar cells. After stimulation, approach the tip of the cone. The visual pigment of rhodopsin is rapidly reconstituted. cone cells, iodopsin, is associated with the outer The inner segment consists of an outer scleral segments. Absorption of light and generation of an region called the ellipsoid (which contains electrical impulse are similar to that occurring in numerous mitochondria) and a vitreal portion that rods. Cones function in color perception and visual houses the Golgi complex, free ribosomes, and acuity, responding to light of relatively high myoid. Microtubules are numerous in both regions. intensity. Detection of color is believed to depend The myoid region contains elements of granular on the presence of several pigments in the cones, and smooth endoplasmic reticulum and synthesizes whereas rods are thought to contain only one form and packages proteins that are transported down of pigment. the connecting stalk to the scleral end of the outer rod segment to be used in the assembly of new membranous discs. Older sacs are shed from the tips of the rods during the morning hours and are replaced by new discs. The discarded discs are phagocytized and destroyed by cells of the pigment epithelium. The remainder of the rod cell consists of an outer fiber, a cell body, and an inner fiber (Fig. 20-3). The outer fiber is a thin process that extends from the inner segment of the rod proper to the cell body, which contains the nucleus. The inner fiber joins the cell body to the spherule, a pear-shaped synaptic ending that contains numerous synaptic vesicles and a synaptic ribbon. The latter consists of a dense proteinaceous plaque that lies perpendicular to the presynaptic surface, often bounded by numerous vesicles. The cell body and nucleus of the rods are located in the outer nuclear layer of the retina; the spherule lies within the outer plexiform layer. Cone cells are 75 µm or more in length at the Fig. 20-3. A diagrammatic sketch illustrating rod and cone cells. fovea, decreasing to 40 µm at the edge of the retina. Each retina contains between 6 million and 7 Three different categories of cones have been million cones. In general, cone cells resemble the described. Long wavelength cones that respond best to rods but are flask-shaped with short, conical outer the red range, middle wavelength cones that respond to segments and relatively broad inner segments the green range, and short wavelength cones that united by a modified stalk similar to that of rod respond best in the blue range. cells. The membranous sacs of the outer cone Except for those in the outer fovea, cones lack an segments differ in that they may remain attached to outer fiber, and the inner cone segment blends with the surrounding cell membrane and decrease in the cell body. The nuclei of cone cells are larger and diameter as they approach the tip of the cone. paler than those of the rods and form a single row The inner segment, also called the ellipsoid portion, in the outer nuclear layer, adjacent to the outer shows a region with numerous mitochondria and a limiting membrane. Each cone has a thick inner myoid part that contains the Golgi complex and fiber that runs to the outer plexiform layer, where it elements of smooth and granular endoplasmic forms a club-shaped synaptic ending, the cone reticulum. As in rods, the cones synthesize proteins pedicle, which synapses with processes from that pass to the outer segments, where they are used bipolar and horizontal neurons. in the formation of new membranous sacs. Older Thus, photoreception results when light is sacs appear to be shed in the evening and are absorbed by visual pigments in the rods and cones. phagocytized in the pigment epithelial cells. Unlike those in rod cells, the sacs decrease in size as they

279 The photoactivated pigment activates a G protein, Association Neurons transducin, which then stimulates a phosphodiesterase to hydrolyze intracellular cyclic GMP. This action VOCABULARY: horizontal cell, amacrine cell, closes sodium channels, hyperpolarizes the inter plexiform cell photoreceptor, and slows glutamate release at the synaptic terminal. In this way, the photoreceptor The cell bodies and nuclei of horizontal cells lie in cells respond to light and pass on the generated the inner nuclear layer. Their dendrites enter the action potential to neurons of the inner retina and outer plexiform layer and synapse with a single cone ultimately to the brain. pedicle. The axons also enter the outer plexiform layer and run parallel to the adjacent retinal layers to Bipolar Cells end on terminal twigs that synapse with several rod spherules. Although the functional significance of VOCABULARY: rod bipolar cell, flat cone horizontal cells is not clear, because they synapse bipolar cell, invaginating midget bipolar cell, with cones of one area, with rods of another area, flat midget bipolar cell and with bipolar cells, it has been suggested that they may raise or lower the functional threshold of Like photoreceptor cells, bipolar cells lie these cells. perpendicular to the retinal layers with their cell Amacrine cells are pear-shaped neurons that lack bodies and nuclei in the inner nuclear layer of the axons but have several dendrites. The cell bodies lie retina. They give rise to one or more dendrites that in the inner nuclear layer, and their dendrites extend extend into the outer plexiform layer, where they into the inner plexiform layer. synapse with terminals of photoreceptor cells. A Perikarya of interplexiform cells also lie in the single axon extends into the inner plexiform layer inner nuclear layer and send processes to both and synapses with processes from ganglion cells. plexiform layers. They receive input from amacrine Several types of bipolar neurons have been cells, and their output is with both horizontal and described: Rod bipolar cells make contact with bipolar neurons. several rod cells; flat cone bipolar cells form All three types of association neurons are thought synapses with several cone pedicles; invaginating to act to modulate the passage of impulses from the midget and flat midget bipolar cells synapse with photoreceptors to ganglion cells. These association a single cone pedicle. Bipolar cells relay impulses neurons allow for the integration of signals between from the rod and cone cells to the ganglion cells of adjacent groups of photoreceptors. the next layer. Fovea Centralis Ganglion Cells The fovea centralis is a funnel-like depression on VOCABULARY: diffuse ganglion cell, midget the posterior surface of the retina, in direct line ganglion cell with the visual axis. At this point, those vitreal layers of the retina which are beyond the outer Dendrites of ganglion cells synapse with axons of nuclear layer are displaced laterally, giving light an bipolar neurons and represent the third and almost free pathway to the photoreceptors. The terminal link in the neuron chain. Axons of the central region of the fovea, about 1.5 mm in various ganglion cells pass along the vitreal surface diameter, consists only of cones that are longer and in the nerve fiber layer of the retina and join other thinner than cones elsewhere in the retina. They are axons at the optic disc to form the optic nerve. closely packed and number about 25,000 to 35,000. Although several morphologic varieties of ganglion Vision is most acute in this part of the retina. cells have been described, two forms have been identified by their synaptic relations with bipolar neurons. Diffuse ganglion cells synapse with several types of bipolar cells, and midget ganglion cells, a monosynaptic type, synapse with a single midget bipolar cell.

280 Pigment Epithelium Lens

VOCABULARY: cylindrical sheath, elongated VOCABULARY: capsule, anterior lens cell, microvillus, lipofuscin granule, melanin lens fiber, lens substance, cortex, nucleus, granule, residual body, vitamin A zonule fiber

The pigment epithelium of the retina consists of a The lens is a transparent, biconvex epithelial body simple layer of hexagonal cells that tend to increase placed immediately behind the pupil between the in diameter near the ora serrata. Their basal cell iris and vitreous body. It is about 10 mm diameter membranes show numerous infoldings with and 3.7 to 4.5 mm thick. The posterior surface is associated mitochondria and are thought to be more convex than the anterior surface. The lens active in transport. The basal lamina contributes to consists of a capsule, anterior lens cells, and lens the glassy (Bruch's) membrane of the choroid as substance. seen by the light microscope. The lateral cell On its anterior and posterior surfaces, the lens is membranes of adjacent cells show some covered by a homogeneous, proteoglycan-rich interdigitations and, near the apex, are united by capsule 10 to 18 µm thick. A single layer of tight junctions. Two types of cytoplasmic processes cuboidal cells, the anterior lens cells, lies arise from the apices of the cells: cylindrical immediately beneath the capsule and forms the sheaths invest the tips of the rod and cone outer epithelium of the lens, which is restricted to the anterior segments, and elongated microvilli extend toward surface. The posterior surface has no epithelium. The photoreceptor cells. The cytoplasm is characterized apices of the anterior lens cells face inward, toward by abundant mitochondria near the base of the cell the lens; the basal surfaces rest on a basal lamina. and by many lipofuscin and melanin granules. Anterior lens cells contain ion pumps that maintain Pigment epithelial cells may show numerous the proper hydration of the lens. Near the equator residual bodies that contain remnants of of the lens, the cells increase in height and gradually phagocytized membrane material shed by rod outer differentiate into lens fibers that make up the bulk segments. of the lens, referred to as the lens substance. The Pigment epithelium absorbs light after it has lens grows throughout life by addition of new fibers passed through the neural retina, thereby to the periphery of the lens substance. The outer preventing reflection within the eye, and the apical layers of the lens substance form the cortex. Nearer tight junctions prevent undesirable substances from the center, the lens substance consists of entering the intercellular spaces of the neural retina. condensed, concentrically arranged fibers that give The pigment epithelium also stores vitamin A, a it a more homogeneous appearance; this area is precursor of rhodopsin, needed by the outer rod called the nucleus of the lens. membranes. At the equator of the lens, the anterior lens cells

elongate and push into the lens to lie beneath the Vascular Supply of the Retina epithelium anteriorly and the capsule posteriorly. As the cells increase in length, they lose their nuclei The outer nuclear and plexiform layers and the layer and their basal attachment to the capsule and of rod and cone inner segments lack blood vessels. become lens fibers that show a homogeneous, These parts of the retina are nourished by capillaries finely granular cytoplasm with few organelles. They of the choriocapillary layer of the choroid. appear hexagonal in cross section and measure 7 to Nutrients cross the pigmented epithelium to enter 10 µm in length. The lateral membranes of the lens the intercellular spaces of the outer neural retina. fibers (cells) in the cortex show large protrusions The inner layers of the retina are supplied by retinal that interdigitate with concavities in adjacent fibers, vessels arising from the central retinal artery, which in a ball and socket manner. Nexus junctions are enters the eye in the optic nerve. Capillary networks common at these points. The ball and socket joints from this source lie in the nerve fiber layer and in maintain the relative positions of the fibers as the the inner plexiform layer. lens changes shape during focusing.

281 The lens is avascular and derives nourishment Accessory Structures totally from diffusion of material from the aqueous humor and vitreous body. Eyelids The lens is held in place by a system of fibers called zonule fibers that make up the suspensory VOCABULARY: conjunctiva, tarsal plate, ligament of the lens. Zonule fibers arise from the tarsal (Meibomian) gland, eyelash, gland of ciliary epithelium that covers the ciliary processes Zeis, Moll's glands and attach to the lens capsule just anterior and posterior to the equator of the lens. Zonule fibers The eyelids protect the anterior aspects of the eyes. consist of small bundles of fine filaments, They consist of a connective tissue core covered approximately 12 nm in diameter, which may externally by thin skin. The interior surface of each correspond to the microfibril (glycoprotein) lid and the anterior surface of each eye (except for component of elastic fibers. the cornea) are covered by a mucous membrane The thickness and convexity of the lens are called the conjunctiva. The palpebral conjunctiva lines controlled by the ciliary muscle. If the ciliary muscle the interior of the lid; the bulbar conjunctiva covers contracts, the ciliary body and choroid are pulled the anterior surface of the eye. Both consist of a forward and centrally, releasing tension on the stratified columnar epithelium interspersed with zonule fibers, and the lens "sags" to become thicker goblet cells, resting on a dense connective tissue and more convex. This permits focusing on near that is rich in elastic fibers and may contain objects. When the ciliary muscle relaxes, the zonule numerous lymphocytes. The goblet cells provide fibers are placed under tension, and the lens mucus, which lubricates and prevents abrasion of becomes thinner and less convex to focus on far the corneal epithelium. At the margin of the lid, the objects. palpebral conjunctiva is continuous with the

keratinized stratified epithelium of the skin. Vitreous Body The external surface of the eyelid consists of skin: an external layer of keratinized stratified squamous VOCABULARY: hyaluronic acid, hyalocyte, epithelium and an inner layer of loose connective hyaloid canal tissue with many elastic fibers. Fine hairs are present, associated with many small sebaceous and The vitreous body is a colorless, transparent, sweat glands. Skeletal muscle from the orbicularis gelatinous mass that fills the vitreal cavity. It is 99% oculi and superior levator palpebrae muscles is water and contains hyaluronic acid and other present in the substance of the lid. The curved hydrophilic glycosaminoglycans, as well as thin tarsal plate, composed of dense fibrous connective fibrils of collagen (type II and type XI) arranged tissue, is a major structure in the lid. It conforms to randomly in a network. The fibrils are most the shape of the eyeball and maintains the shape of prominent near the periphery. A few cells, the the lid. Large sebaceous glands, the tarsal hyalocytes, and occasional macrophages are (Meibomian) glands, are embedded in the tarsal present in the outermost parts of the vitreous body. plate, arranged in a single row with their ducts Hyalocytes may be responsible for the formation opening at the margin of the lid. Each gland and maintenance of the vitreous body. A thin, consists of a long central duct surrounded by cylindrical network of fibrils, the hyaloid canal, numerous secretory alveoli. They elaborate a lipid extends from the optic disc to the posterior surface secretion that lubricates the lid margins. of the lens. It represents the site of the embryonic The eyelashes are thick, short, curved hairs hyaloid artery. The vitreous body provides nutrients arranged in two or three irregular rows along the to the lens and adjacent structures and maintains margin of the lid. Their follicles extend into the the correct consistency and shape of the eye. tarsal plate. Sebaceous glands associated with eyelashes are called the glands of Zeis; the modified apocrine sweat glands located between the follicles are called Moll's glands. Moll’s glands secrete a variety of antimicrobial proteins (IgA,

282 lysozyme, lactoferrin, and β-defensin-2) which play mesenchyme that lies between or around these a role in the defense of the eye against infection. components. A summary of the contributions made by these embryonic tissues to adult structures of the Lacrimal Glands eye are shown in Table 20-1. Eye development begins with the formation of VOCABULARY: compound tubuloalveolar, shallow optic grooves on each side of the developing serous, myoepithelial cell, lacrimal duct, forebrain. As the neural tube closes, the optic lacrimal sac, nasolacrimal duct grooves expand to form optic vesicles that remain attached to the brain by a hollow optic stalk. Lacrimal glands are well-developed, compound Thickenings of the surface ectoderm over the optic tubuloalveolar glands of the serous type, located vesicles give rise to the lens placodes, which expand in the superior temporal region of the orbit. Each along their lateral walls to form lens pits from gland consists of several separate glandular units which lens vesicles eventually form. The walls of the that empty into the conjunctival sac via 6 to 12 lens vesicles consist of surface ectodermal cells. ducts. The secretory units consist of tall columnar With loss of the superficial (epitrichial) cells of the cells with large, pale secretory granules. Numerous surface ectoderm and of the connecting stalk to the myoepithelial cells lie between the bases of the surface, the underlying cuboidal cells of the lens secretory cells and their basal lamina. The ducts are vesicles come to form the definitive cells of the lined at first by simple cuboidal epithelium, which lens. becomes stratified columnar in the larger ducts. Simultaneously, the distal wall of the optic vesicle Small accessory lacrimal glands can occur in the collapses into itself to form a double-walled optic cup in inner surface of the eyelid. Accessory lacrimals which the apices of the cuboidal cells in the two occur in the fornix of the conjunctiva (glands of layers face each other. As the optic cup forms, the Krause) and just above the tarsal plate (glands of lens vesicle descends into the cavity of the cup, Wolfring). where it continues to develop. The optic cup Secretions of the lacrimal gland moisten, lubricate, remains attached to the brain by the optic stalk: the and flush the anterior surface of the eye and the optic cup forms the retina, the optic stalk becomes interior of the eyelids. Excess tears collect at a the optic nerve. medial expansion of the conjunctival sac, which is Meanwhile, ingrowth of mesenchyme from the paraxial drained by the lacrimal duct to a lacrimal sac at the area helps separate the lens vesicle from the surface medial corner of each eye. The lacrimal duct is lined ectoderm. Part of the mesenchymal ingrowth by stratified squamous epithelium. The lacrimal differentiates into the corneal endothelium; the rest sac and the nasolacrimal duct, which drains the contributes to the anterior chamber. A second sac into the inferior meatus of the nasal cavity, are ingrowth of mesenchyme extends between the surface lined by pseudostratified columnar epithelium. The ectoderm and the corneal endothelium to form the remainder of their walls consists of dense substantia propria of the cornea. The surface ectoderm, connective tissue. previously a simple epithelium, is now two cells The tear film ensures an optimal refractive surface thick and forms the corneal epithelium. A third and provides a mechanical and antimicrobial barrier ingrowth of mesenchyme extends into the optic cup on the corneal surface. The lipid component from the lateral side near the lens and forms a originates from Meibomian glands and forms the loose, acellular matrix between the lens and corneal superficial layer of the tear film. The aqueous endothelium. The matrix behind the central cornea component contains water, electrolytes, a large liquefies and an early anterior chamber is formed variety of proteins, peptides and glycoproteins that, as it widens, separates the corneoscleral region (mucins) secreted by the lacrimal glands. In from the developing iris. Bowman’s and addition, the tear film receives antimicrobial Descemet’s membranes appear during the fourth proteins produced by the glands of Moll. month, and collagen fibrils in the substantia propria reach their full thickness. As the corneal epithelium Organogenesis and a substantia propria develop, the cornea thickens to reach its full dimensions at the end of The eyeball arises from neuroectoderm of the the first postnatal year. embryonic forebrain, surface ectoderm of the head, and

283 The outer layer of the double-walled optic cup which the cells are arranged apex-to-apex. Zonule becomes highly pigmented to form the pigment fibers develop within the vitreous and extend from epithelium of the retina; endothelial cells in the the inner surface of the ciliary process to the lens adjacent, undifferentiated mesenchyme form the capsule. They appear during the third month and choriocapillaris. The collagenous portion of the form the suspensory ligament of the lens. glassy (Bruch’s) membrane develops first, followed The iris also originates from neuroectoderm and about six weeks later by the elastic part. mesenchyme. The double layer of epithelium Melanocytes have migrated from the neural crest into covering the iris develops from the double cell layer the outer layers of the choroid by the fifth month of the optic cup. The smooth muscle of the and later appear in its inner layer. sphincter and dilator muscles of the iris is unusual The sclera arises as a condensation of mesenchyme in that it develops from neuroectoderm. An ingrowth around the optic cup, just external to the of mesenchyme at the edge of the optic cup provides developing choroid. The condensation begins at the the stroma and vasculature of the iris. equator of the sclera and by the fifth month forms The optic nerve develops from axons of retinal a complete layer. ganglion cells, neuroectodermal cells, and mesoderm The posterior four-fifths of the inner layer of the associated with the optic stalk. The mesoderm gives optic cup thickens due to cell proliferation and rise to the vascular components and connective migration and forms the neural retina. The inner tissue of the nerve, including the meninges. At first, layer of the optic cup differentiates into inner and the optic stalk contains an open ventral groove, the outer neuroblastic layers. Cells of the inner choroid fissure, through which mesenchyme enters neuroblastic layer develop into the neurons and glial the optic cup. Hyaloid vessels that supply the lens and elements of the adult retina; rods and cones arise inner surface of the retina during their development from ciliated cells in the outer neuroblastic layer. A and axons from the brain to the retina also pass marginal layer in the forming retina receives axons within the groove. As the choroid fissure closes, the that extend posteriorly toward the optic stalk. optic stalk is transformed into the optic nerve with Differentiation of retinal cells begins in the the central retina (the previous hyaloid) artery at its posterior region of the optic cup and gradually center. That part of the hyaloid artery that supplied extends forward to the area of the (adult) ora the lens degenerates and is resorbed. The mesenchyme serrata. Neuroblastic layers of the macular region filling the posterior aspect of the optic cup becomes develop first so that the more peripheral, later the gelatinous, transparent vitreous. developing axons must course around this region to The lamina cribrosa develops from neuroectoderm as reach the optic disc. Some retinal differentiation, a condensation of glial cells. It is strengthened by the especially of the macular region, continues after mesenchymal component that gives rise to fibroblasts birth, and the density of the retinal pigment also and collagenous septae that pass among the increases postnatally. neuroglial cells. Cells in the anterior one-fifth of the inner layer of the optic cup, beyond the region of the ora serrata Accessory Structures of the adult eye, remain a single layer. The cells lie apex-to-apex with cells of the pigment layer that Folds of integument adjacent to the eyeball extend to the margin of the optic cup and cover the differentiate to form the eyelid. Ectoderm on the developing ciliary region and iris. exterior of the developing eyelid becomes the outer The ciliary body arises from neuroectoderm of the layer of stratified squamous epithelium (epidermis): anterior optic cup and its associated mesoderm. that covering the cornea and anterior part of the Neuroectoderm forms the outer pigmented and inner sclera forms the conjunctiva. As development nonpigmented epithelia that cover the ciliary body proceeds, the folds meet and fuse temporarily. The and ciliary processes; mesenchyme forms the ciliary epidermal union breaks down prior to the opening muscle, stroma, and blood vessels. As the ciliary of the eyes. Eyelashes and sebaceous, sweat, and muscle differentiates, folds appear in the outer layer tarsal glands develop along the edge of each eyelid of neuroectoderm, external to the margin of the while they still are fused. The follicles of the lashes optic cup. Each fold gives rise to a ciliary process, and their associated glands arise as epidermal complete with a core of mesenchyme and blood ingrowths that then develop as hairs elsewhere in vessels and covered by two epithelial layers in the body.

284 Lacrimal glands develop as a series of solid cords of epithelium from the nasolacrimal groove; a of cells from the conjunctival epithelium. The cords second growth from the epithelium of each eyelid grow, branch, and acquire lumina, thus establishing joins it. The distal end of the cord grows toward the the ductal system. Secretory alveoli are the last nasal cavity and fuses with the nasal epithelium components to form. The lacrimal sac and prior to acquiring a lumen. nasolacrimal duct first appear as a solid outgrowth

Table 20-1. Contributions to the eye.

Embryonic Layer Adult structures

Neuroectoderm Neural retina, pigment epithelium, epithelium of iris, dilator and sphincter pupillary muscles, nervous and neuroglial elements of optic nerve

Surface ectoderm Epithelium of cornea, lens

Mesenchyme Substantia propria, endothelium of cornea, sclera, choroid, stroma and vessels of iris, ciliary body, ciliary processes, ciliary muscle, sheaths of optic nerve; anterior, posterior, and vitreous chambers

Summary that extend from surrounding ciliary processes and focuses an inverted, real image on the retina. The The corneoscleral coat, together with the convexity and thickness of the lens are controlled intraocular pressure of the fluid contents within the by the ciliary muscle acting through the ciliary eye, maintains the proper shape and size of the processes and zonule fibers. If the ciliary muscle eyeball. Light entering the eye must cross several contracts, the ciliary body and choroid are pulled transparent media (cornea, aqueous humor, lens, centrally and forward, releasing tension on the and vitreous body) before reaching receptors in the zonule fibers; this allows the lens to become thicker retina. There are no blood vessels in the transparent and more convex, enabling the eye to focus on near elements, which rely on diffusion of materials for objects. When the ciliary muscle relaxes, the ciliary their nutrition. Peripheral regions of the cornea body slides posteriorly and peripherally, increasing receive nutrients from adjacent vessels in the the tension on the zonule fibers and making the limbus; the remainder of the cornea depends on lens thinner and less convex to focus on far objects. diffusion of nutrients from the aqueous humor. The iris is continuous with the ciliary body at the The lens receives all its nutrition from the aqueous periphery and divides the space between the cornea humor which is secreted continuously into the and lens into anterior and posterior chambers. The posterior chamber by the ciliary epithelium. The chambers communicate via the pupil, an aperture in aqueous humor enters the anterior chamber the iris through which light passes into the lens and through the pupil and diffuses posteriorly into the vitreous chamber. The dilator of the pupil consists vitreous chamber of the eye. From the anterior of a single layer of radially arranged myoepithelial chamber the aqueous humor passes through the cells along the posterior surface of the iris. trabecular network into Schlemm's canal and then Contraction of these cells increases the diameter of into adjacent episcleral veins. The aqueous humor the pupil. The sphincter of the iris consists of supplies nutrients to the transparent media of the smooth muscle arranged around the margin of the eye and is responsible for maintaining the correct iris that acts as a diaphragm to modify the amount intraocular pressure. of light entering the eye and permits vision under a Stationary refraction occurs through the variety of light conditions. transparent cornea, in contrast to variable refraction The rods and cones of the retina are the that occurs in the lens as it changes shape to focus photoreceptors that collect visual impressions (light near or far objects on the retina during eye patterns) and translate them into nerve impulses. accommodation. The other media have negligible refraction. The lens is held in place by zonule fibers

285 The membranous sacs of the outer rod segments membranes are carried back to the photoreceptor contain rhodopsin, which, on exposure to light, cells to be reused. The pigment epithelium is a changes from cis to trans and breaks down. The storage site for vitamin A, a precursor of rhodopsin, photoactivated pigment activates transducin which which is recycled to the membranes of the outer in turn stimulates a phosphodiesterase to hydrolyze rod segments. Apical tight junctions between cells intracellular cyclic GMP. This action closes cation of the pigment epithelium form a barrier to prevent channels, hyperpolarizes the photoreceptor, and unwanted materials from entering the neural retina. slows glutamate release at the synaptic terminal. In The neurons are nourished by diffusion from this way, the photoreceptor cells detects light and capillaries in the choriocapillary layer of the passes on a generated electrical potential that is choroid. transferred to dendrites of associated bipolar cells. Basically, the retina represents a three-neuron Rods have a lower threshold to light intensity than chain of receptors (rods and cones), bipolar cones and are important in dark and light neurons, and ganglion cells equivalent to the three- discrimination and in night vision. neuron sensory chains of the peripheral nervous The visual pigment associated with cone outer system. Rod and cones can be equated with any segments is iodopsin. Light absorption and other sensory receptors, bipolar neurons with generation of an electrical impulse in cones follow a craniospinal ganglia, and retinal ganglion cells with sequence similar to that in rods. Cones serve for internuncial neurons in the spinal cord and color perception and visual acuity, responding to brainstem. Thus, the arrangement of nervous light of relatively high intensity. Detection of color elements in the sensory chain of the retina is similar depends on the presence of different pigments in to that of other sensory pathways. The retina the cones. The central region of the fovea centralis represents an extension of the brain, modified to consists only of cones and is in direct line with the form a special receptor. Other neurons, horizontal visual axis of the eye. Here the inner layers of the cells, amacrine cells, and interplexiform cells in the retina that are beyond the outer nuclear layer are retina serve as association neurons. displaced laterally, allowing light almost a free The eyelids are mobile folds of skin that protect pathway to the photoreceptors; in this portion of the anterior of the eye from injury, desiccation, and the retina vision is most acute. excessive light. Lacrimal glands secrete tears that The pigment epithelium of the retina absorbs light moisten and lubricate the anterior surface of the after it has passed through the neural retina and eye. Tears prevent the cornea from drying and flush prevents reflection within the eye; melanocytes in the exposed surface of the eye and inner surface of the choroid, iris, and other regions of the eye serve the eyelid, washing away potentially dangerous a similar purpose. The cells of the pigment agents. The tear film provides an optimal refractive epithelium phagocytize the membranous sacs as surface and ensures a mechanical and antimicrobial they are shed from the tips of the outer cone and barrier on the corneal surface. rod segments. Some components of the digested

286 temporal bone. The external auditory meatus is 21 The Ear lined by keratinized stratified squamous epithelium that is continuous with the epidermis of the auricle. The ear contains receptors specialized for hearing The epithelium is firmly anchored to the and for awareness of head position and movement. surrounding perichondrium or periosteum by dense The ear is subdivided into the external ear, which collagenous connective tissue. Numerous small receives and directs sound waves from the external hairs are present in the epithelium of the outer part environment; the middle ear, which transforms of the meatus and are associated with large sound waves into mechanical vibrations; and the sebaceous glands in the underlying connective inner ear, where these mechanical vibrations are tissue. A special form of simple coiled tubular converted to nerve impulses and relayed to the apocrine sweat gland, the ceruminous gland, brain to be interpreted as sound. The inner ear also occurs in the skin that lines the meatus. Depending contains the vestibular organs that function in on the state of activity, the secretory cells may vary balance. The general structure of the ear is shown in shape from cuboidal to columnar. Each secretory in Figure 21-1. tubule is surrounded by a network of myoepithelial cells that lies between the basal lamina and the bases of the epithelial cells. The ducts may open directly on the surface or, together with adjacent sebaceous glands, into hair follicles. Their secretory product is cerumen, a waxy material that prevents drying of the skin that lines the external auditory meatus. Hairs and glands occur primarily along the roof of the inner (boney) part of the external auditory meatus.

Middle Ear

Fig. 21-1. A diagrammatic sketch illustrating the general VOCABULARY: tympanic cavity, auditory structure of the ear. ossicle, tensor tympani, stapedius, auditory tube, tympanic membrane

External Ear The tympanic cavity is an irregularly shaped space in the petrous portion of the temporal bone. It is VOCABULARY: auricle (pinna), elastic continuous anteriorly with the auditory (eustachian) cartilage, external auditory meatus, ceruminous tube and posteriorly with the mastoid air cells gland, cerumen (cavities) of the temporal bone. The lateral wall consists chiefly of the tympanic membrane The auricle (pinna) of the external ear consists of (eardrum), which forms a partition separating the an irregular plate of elastic cartilage surrounded tympanic cavity from the external auditory meatus. by a thick perichondrium rich in elastic fibers. The The inner, boney wall of the middle ear makes covering skin contains a few small hairs, their contact with the inner ear via two small, membrane- associated sebaceous glands, and an occasional covered openings called the oval and round windows. eccrine sweat gland. The skin adheres tightly to the The membrane of the oval window contains the perichondrium except on the posterior surface, base of an auditory ossicle, the stapes. The where a subcutaneous layer is present. The auricle is membrane that covers the round window often is associated with sheets of skeletal muscle. referred to as the secondary tympanic membrane. The external auditory meatus is about 2.5 cm The tympanic cavity contains the auditory long and follows an open S-shaped course. It ossicles, a chain of three bones (malleus, incus, and consists of an outer part whose cartilaginous walls stapes) that unites the tympanic membrane of the are continuous with the auricular cartilage and an middle ear with the oval window of the inner ear. inner portion whose walls are formed by the The ossicles consist of compact bone and are united by small synovial joints.

287 The bones are suspended in the air-filled tympanic encircling ring of fibrocartilage that unites the cavity by thin strands of connective tissue called eardrum to the surrounding bone. The inner ligaments. Small skeletal muscles, the tensor surface of the tympanic membrane attaches to the tympani and stapedius, are associated with two of malleus. the ossicles; the tendon of the tensor tympani Sound waves received by the tympanic membrane attaches to the malleus, and the tendon from the cause it to vibrate slightly. The vibrations then are stapedius attaches to the stapes. The bulk of the transmitted to the fluid-filled chambers of the inner muscles themselves are contained within small ear via the ossicles. The tympanic membrane is canals in the temporal bone. about 18 times as large as the oval window of the The auditory ossicles, their suspending ligaments, inner ear, which contains the foot plate of the and the inner walls of the tympanic cavity are stapes. Because of this arrangement, the eardrum covered by a thin mucous membrane that consists and auditory ossicles act as a piston that exerts of a simple squamous epithelium lying on a thin pressure on the confined fluid of the inner ear. layer of connective tissue. The mucous membrane Thus, the tympanic membrane and auditory ossicles is firmly attached to the periosteum of the temporal not only transmit vibrations but also are able to bone, lines the interior of the mastoid air cells, and amplify weak forces of sound waves without covers the inner surface of the tympanic expending energy. membrane. Where the tympanic cavity joins the The tensor tympani and stapedius muscles protect auditory tube, the lining epithelium becomes delicate structures in the inner ear by dampening ciliated columnar, interspersed with secretory cells. ossicle movement resulting from loud or sudden The auditory tube (eustachian tube) is about 4 noises. They also are thought to regulate the degree cm long and connects the anterior part of the of tension in the tympanic membrane so that tympanic cavity with the nasopharynx. The auditory sounds of moderate intensity can be picked up in a tube acts as a passageway to ventilate the tympanic noisy environment. cavity and allows equalization of pressure between the middle ear and pharynx. Near the tympanic cavity Inner Ear the supporting wall consists of compact bone, whereas in its medial two-thirds, the auditory tube VOCABULARY: boney labyrinth, perilymph, is supported by a J-shaped elastic cartilage. The membranous labyrinth, endolymph boney portion is lined by a simple columnar epithelium that becomes ciliated pseudostratified The inner ear consists of a hearing receptor organ, columnar epithelium in the cartilaginous part. Cilia the cochlea, and five vestibular sense organs beat toward the pharynx. Goblet cells are present in devoted to balance and head position. It is made up the lining epithelium near the pharynx, and mixed, of a system of canals and cavities within the petrous compound, tubuloalveolar glands often are found portion of the temporal bone. The compact bone in the underlying connective tissue. A mass of immediately surrounding the canals and cavities lymphoid tissue, the tubal tonsil, fills the connective forms the boney labyrinth and is filled with a fluid tissue and infiltrates the lining epithelium near the called perilymph. A series of fluid-filled pharyngeal opening of the eustachian tube. membranous structures, collectively called the The tympanic membrane forms most of the membranous labyrinth, lie suspended in the lateral wall of the tympanic cavity and is made up of perilymph. The membranous labyrinth is filled with three layers. The outer layer consists of stratified endolymph, a fluid with an ionic composition squamous epithelium, which reflects onto the similar to that of intracellular fluid, being rich in tympanic membrane from the external auditory potassium ions and low in sodium ions. Perilymph meatus. Two layers of collagenous fibers and resembles extracellular fluid, having a high content fibroblasts form the middle layer. In the external of sodium ions and a low concentration of layer, the fibers are arranged radially, whereas those potassium ions. Endolymph is produced by an area of the inner layer run a circular pattern. A simple of the cochlear duct called the stria vascularis; the squamous epithelium and its supporting connective exact site of perilymph formation is unknown. tissue form the third layer and are continuous with the mucosa lining the tympanic cavity. The fibrous layers of the tympanic membrane enter an

288 The boney and membranous labyrinths consist of cover an area approximately 2 mm square along the two major components: the vestibular labyrinth, superoanterior wall. It lies on a plane perpendicular which contains sensory elements for equilibrium, to the macula sacculi, which occupies an area and the cochlea, which contains sensory structures measuring 2 x 3 mm on the anterior wall of the for hearing. saccule (Fig. 21-2). The macula of the utricle lies on a horizontal plane positioned at a right angle to the Vestibular Labyrinth macula of the utricle. The macula of the saccule lies on a vertical plane at a right angle to the macula of VOCABULARY: semicircular canal, utricle, the utricle. The sensory epithelium of the cristae saccule, crista ampullaris, macula utriculi, and maculae consists of type I and type II sensory macula sacculi, type I hair cell, kinocilium, hair cells and supporting (sustentacular) cells. type II hair cell, supporting cell, planum semilunatum, cupula cristae ampullaris, otolithic membrane, otoliths (otoconia)

The vestibular labyrinth consists of three semicircular canals that are continuous with an elliptical structure called the utricle. The utricle in turn unites with an anteromedial spherical part of the vestibular system called the saccule by a thin duct that joins with a similar duct from the saccule. The two ducts unite to form the slender endolymphatic duct, which terminates as a small Fig. 21-2. A diagrammatic sketch illustrating the locations of the expansion called the endolymphatic sac. sensory regions in the membranous labyrinth (stippled area). Most of the membranous labyrinth that forms the vestibular portion of the inner ear is lined by a The type I hair cell is flask-shaped with a narrow simple squamous epithelium. The remainder of this apical region and a rounded base that contains the wall consists of fine connective tissue fibers and nucleus. A large part of the cell is enveloped by a stellate fibroblasts. Thin trabeculae of connective cup-like afferent nerve ending (calyx) (Fig. 21-3). tissue extend from the wall of the membranous Nearby efferent nerve endings may synapse with labyrinth and cross the perilymphatic space to blend the nerve calyx but do not directly contact the type with the periosteum of the surrounding bone. The I hair cell; a narrow intercellular cleft, 30 nm wide, trabeculae suspend the membranous component of separates the hair cell from the nerve ending. The the semicircular canals, utricle, and saccule in the space narrows to approximately 5 nm at gap perilymph contained within the osseous labyrinth. junctions that are scattered between the two The perilymphatic connective tissue is rich in blood compartments. Synaptic ribbons often are found in capillaries that supply the various segments of the the cytoplasm of type I hair cells, immediately membranous labyrinth. adjacent to the plasmalemma. Mitochondria are In specific regions of each subdivision of the concentrated around the nucleus and at the cell membranous labyrinth, the epithelium assumes a apex, and numerous microtubules are present, stratified appearance and serves in sensory especially in the apical region. The cytoplasm of the reception. The sensory epithelium of each nerve calyx shows scattered mitochondria and many semicircular canal is restricted to the dilated vesicles that range from 50 to 200 nm in diameter. ampullary portion and, together with an underlying The apical cell membrane of the type I hair cell core of connective tissue, forms a transverse ridge bears 50 to 100 large, specialized microvilli, known that projects into the lumen of the ampulla. The as "hairs". These nonmotile elements are limited by connective tissue contains many nerve fibers. These a plasmalemma, have cytoplasmic cores that sensory neuroepithelial regions of the semicircular contain numerous fine filaments, and are canals form the cristae ampullaris. Similar raised constricted at their base just before they join the regions of neuroepithelium are found in the utricle rest of the cell. and saccule and form the macula utriculi and the macula sacculi, respectively. The macula utriculi

289 The longitudinal filaments leave the microvilli and from the basal cytoplasm to the terminal web. The enter into a thick mat of filaments that forms a microtubules form an integral part of the terminal web in the apical cytoplasm of the cell. cytoskeleton and provide rigidity to the supporting The microvilli progressively increase in height from cells. Although the function of the supporting cells about 1 µm on one side to about 100 µm on the is uncertain, it has been suggested that they are opposite side of the cell. Fine extracellular filaments concerned with the metabolism of endolymph or link individual microvilli in each row. A single, that they contribute to the nutrition of hair cells. eccentrically placed cilium is present on the apical The supporting cells at the periphery of the sensory surface and is peculiar in that the two central epithelium form a simple columnar layer, the microtubules end shortly after originating from the planum semilunatum, which lacks hair cells. basal body. The cilium is believed to be non-motile The microvilli of the hair cells in the cristae are and often is called a kinocilium. embedded in a gelatinous structure called the Type II hair cells are simple columnar cells cupula cristae ampullaris, which is composed of surrounded by numerous separate afferent and viscous proteoglycans that project from the surface efferent nerve endings, rather than by a single of the crista into the ampullary lumen of each afferent nerve ending as seen surrounding type I semicircular canal. Supporting cells in the sensory hair cells (Fig. 21-3). The type II hair cell also bears epithelium also may contribute to the cupula. The a single, eccentrically placed, nonmotile cilium and sulfated proteoglycans may be secreted by the 50 to 100 large microvilli that are arranged planum semilunatum. identically to those of type I cells. The cytoplasm The tallest row of microvilli of the hair cells from contains scattered profiles of granular endoplasmic the maculae of the utricle and saccule also are reticulum, abundant mitochondria, smooth- embedded in a viscous proteoglycan that forms the surfaced tubules, and numerous vesicles 20 nm in otolithic membrane. Additionally, numerous diameter. A well-developed Golgi complex crystalline bodies called otoliths (otoconia) are occupies a supranuclear position. Synaptic ribbons suspended in this layer. These bodies consist of are present in the cytoplasm of type II hair cells, protein and calcium carbonate. immediately opposite the surrounding nerve The gelatinous cupula of the cristae projects terminals. across the lumen of the ampullary region of the semicircular canal like a swinging door. During angular movement of the head, the cupula is displaced by the motion of endolymph contained within this part of the membranous labyrinth. Displacement of the cupula excites the sensory hair cells, which, in turn, generate an action potential that is received by surrounding nerve terminals. Similarly, gravitational forces on the otolithic membrane and the otoconia embedded within it cause a shearing action on the microvilli of the underlying hair cells in the macula. Linear acceleration also results in stimulation of hair cells in the macula.

Although the exact mechanism is unknown, the Fig. 21-3. A diagrammatic sketch illustrating type I and type II hair sensory epithelium in the vestibular organs cells. transforms the mechanical energy of endolymph Adjacent columnar-shaped supporting cells movement into the electrical energy of a nerve (sustentacular cells) extend from the basal lamina to impulse. The bending or displacement of microvilli the free surface of the sensory epithelium. They is thought open mechanoreceptors which results in follow a very irregular course throughout the the depolarization of hair cells, the action potential epithelium and in electron micrographs show a being transferred to surrounding nerve endings to well-developed terminal web at the cell apex, a result in the generation of a nerve impulse. Efferent prominent Golgi complex, numerous secretory nerve endings probably have inhibitory functions to granules, and bundles of microtubules that extend control the threshold of activity of hair cells. It is

290 thought that movement of microvilli towards the The cochlear duct is part of the endolymphatic tallest row of microvilli depolarizes (excites) the hair system and is connected to the saccule of the cell whereas movement of microvilli towards the membranous labyrinth by the small ductus shortest row hyperpolarizes (inhibits) hair cell activity. reuniens. The opposite end of the cochlear duct terminates at the apex of the cochlea as the blindly Cochlea ending cecum cupulare. The scala vestibuli and the scala tympani contain perilymph and VOCABULARY: modiolus, spiral ganglion, communicate at the apex of the cochlea through a spiral lamina, basilar membrane, spiral small opening called the helicotrema. At the base ligament, vestibular membrane, scala vestibuli, of the cochlea, the scala tympani is closed by the cochlear duct, scala tympani, ductus reuniens, secondary tympanic membrane, which fills the cecum cupulare, helicotrema round window. This membrane separates the perilymph in the scala tympani from the middle ear. Like the vestibular portion of the inner ear, the The scala vestibuli extends through the cochlea consists of an outer portion of compact perilymphatic channels of the vestibule to end at bone and a central membranous part contained in the oval window, which is closed by the foot of the perilymph. The osseous part of the cochlea spirals stapes. Movement of the stapes in the oval window for two and three-fourths turns around a cone- exerts pressure on the perilymph in the scala shaped axis of spongy bone called the modiolus. vestibuli. Because fluid cannot be compressed, Blood vessels, nerve fibers, and the perikarya of waves of pressure either pass through the cochlear afferent bipolar neurons, called the spiral duct, displacing it to enter the scala tympani, or ganglion, lie within the boney substance of the enter the scala tympani directly through the modiolus. A projection of bone, the spiral lamina, helicotrema. The pressure is released from the extends from the modiolus into the lumen of the confined perilymphatic spaces of the cochlea by the cochlear canal along its entire course. A fibrous elasticity of the secondary tympanic membrane, structure, the basilar membrane, extends from the which bulges into the tympanic cavity of the middle spiral lamina to the spiral ligament, a thickening ear (Fig. 21-5). of periosteum on the outer boney wall of the cochlear canal. The thin vestibular membrane extends obliquely across the cochlear canal from the spiral lamina to the outer wall of the cochlea. The basilar and vestibular membranes divide the cochlear canal into an upper scala vestibuli, an intermediate cochlear duct, and a lower scala tympani. A cross section of the cochlear canal is shown in Figure 21-4.

Fig. 21-5. A diagrammatic sketch illustrating the mechanics of hearing.

Cochlear Duct

VOCABULARY: basilar membrane, spiral crest, vestibular membrane, stria vascularis, marginal cell, basal cell, intraepithelial capillary, spiral prominence

The cochlear duct is a triangular space that follows the spiral course of the cochlea. Its floor is formed by the basilar membrane and its roof by the

vestibular membrane. Fig.. 21-4. A diagrammatic sketch illustrating a cross sectional view of the cochlear canal.

291 The basilar membrane consists of a layer of phalangeal cells, border cells, cells of Hensen, and collagen-like fibers embedded in an amorphous cells of Claudius. matrix and extends from the osseous spiral lamina Inner and outer pillar cells form the boundaries of the modiolus to the spiral crest, a well- of the inner tunnel, a space that lies within and vascularized periosteal region along the outer wall extends the length of the organ of Corti. In both of the cochlea. The vestibular membrane consists types of pillar cells, the nucleus is located in the of two layers of simple squamous cells separated broad base. The elongated cell bodies contain mainly by their basal laminae. numerous microtubules that form a cytoskeleton. The outer wall of the cochlear duct is formed by a Inner pillar cells are slightly expanded at the apex vascular area called the stria vascularis. It occurs and extend over the outer pillar cells. The apices of along the entire length of the cochlear duct and the outer pillar cells also expand slightly to fit into consists of a pseudostratified columnar epithelium the concave apical undersurface of the inner pillar that rests on a vascular connective tissue and cells. In addition to forming the boundaries of the contains basal and marginal cells. The epithelium is inner tunnel, the pillar cells provide structural continuous with the simple squamous epithelium support for adjacent cells. that lines the interior of the vestibular membrane. The phalangeal cells serve as supporting elements Marginal cells show deep infoldings of the basal for the sensory hair cells. The inner phalangeal and lateral cell membranes and are associated with cells form a single row immediately adjacent to the numerous mitochondria, suggesting that these cells inner pillar cells and surround the sensory inner hair are involved in fluid transport. They may participate cells except at their apical regions. In contrast, the in the elaboration of endolymph. Basal cells show columnar outer phalangeal cells form three or few mitochondria or basal infoldings. The four rows and support outer hair cells, which also epithelium of the stria vascularis differs from that are arranged in rows (Fig. 21-6).The apex of each found elsewhere in the body in that it contains outer phalangeal cell forms a cup-like structure that intraepithelial capillaries. surrounds the basal one-third of an outer hair cell. The epithelium of the stria vascularis is Afferent and efferent nerves are located at the base. continuous with a simple layer of attenuated cells Each outer phalangeal cell gives rise to a slender that overlies the spiral prominence, a highly cytoplasmic process filled with microtubules. vascularized thickening of the periosteum. The The process extends to the surface, where it spiral prominence lies beneath the stria vascularis expands into a flat plate that attaches to the apical and extends the length of the cochlear duct. The edges of the outer hair cell and is supported laterally cells become cuboidal where the epithelium reflects by outer phalangeal cells and the outer hair cells in onto the basilar membrane from the outer wall of the adjacent row. The apical plates of the outer the cochlear duct. phalangeal cells provide additional support for the outer hair cells, the upper two-thirds of which are Organ of Corti not supported by adjacent cells and are surrounded by large, fluid-filled intercellular spaces. The fluid VOCABULARY: supporting cell, inner pillar contained in these spaces is believed to be similar to cell, outer pillar cell, inner phalangeal cell, that within the inner tunnel. outer phalangeal cell, border cell, inner spiral tunnel, spiral limbus, interdental cell, tectorial membrane, cells of Hensen, cells of Claudius, inner hair cell, outer hair cell

The cochlear duct contains a region of specialized cells, the organ of Corti, that transforms vibrations of the basilar membrane into nerve impulses. The avascular organ of Corti extends along the length of the cochlear duct and lies on the basilar membrane. It consists mainly of supporting and hair cells. Supporting cells are tall columnar and consist of inner and outer pillar cells, inner and outer Fig. 21-6. A diagrammatic sketch of the organ of Corti.

292 Border cells are supporting cells associated with narrow necks. Well-developed microvilli extend the inner edge of the organ of Corti. These slender from the apical surface of the inner hair cells, but cells undergo a transition to the squamous cells that unlike type I hair cells, they lack a kinocilium. The line the inner spiral tunnel. This space is formed microvilli contain numerous filaments that extend by the spiral limbus, which consists of periosteal into a dense terminal web in the apical cytoplasm. connective tissue that extends from the osseous Mitochondria aggregate just beneath the terminal spiral lamina and bulges into the cochlear duct. web and also are scattered throughout the Vertically arranged collagenous fibers, often called remainder of the cytoplasm, which contains auditory teeth, are contained within the substance of scattered ribosomes and profiles of smooth the limbus. Interdental cells are specialized cells endoplasmic reticulum. Numerous nerve endings along the upper surface of the spiral limbus. The synapse with the bases of the inner hair cells on a cells extend between collagen fibers to reach the plane below the level of the nucleus. lumen of the cochlear duct. On the upper surface The columnar outer hair cells usually form three of the limbus, they form a continuous sheet and are or four rows and are supported by the apices of the united by tight junctions. outer phalangeal cells. Microvilli are arranged in the The interdental cells secrete a sheet of material shape of a V (Fig. 21-8). There is no cilium. that forms the tectorial membrane, which consists Mitochondria aggregate at the base of the outer hair of a gelatinous matrix rich in proteoglycans and a cell, which makes contact with afferent and efferent filamentous protein thought to be similar to nerve fibers. In all other respects, the outer hair epidermal keratin. The tectorial membrane extends cells are similar to the inner hair cells, and both are over the interdental cells and beyond the substance receptors of sound in the organ of Corti. of the spiral limbus to overlie the hair cells of the organ of Corti. The tips of microvilli extending from the outer and inner hair cells contact the undersurface of the tectorial membrane (Figs. 21-6 & 21-7). Near the outer edge of the organ of Corti, immediately adjacent to the outer phalangeal cells, is another group of columnar supporting cells called cells of Hensen. They decrease in height and transform into the adjacent cells of Claudius, Fig. 21-8. A scanning electron micrograph of the surface of the organ of which form the outer edge of the organ of Corti. Corti showing the arrangement of microvilli extending from inner and outer hair cells. The hair cells are separated by inner pillar cells. (X 1,350). Micrograph courtesy of M. Lenoir, University of Montpellier, France.

Organogenesis

External Ear

Mesenchymal proliferation around the edges of the first and second pharyngeal arches, together with

Fig. 21-7. A scanning electron micrograph showing the external surface ectoderm, gives rise to the auricle. The border of the tectorial membrane and its relationship to the cells of Hensen. Note the exposed outer hair cells where the tectorial proliferations occur around the openings of the

membrane is lifted up. (X 380). Micrograph courtesy of M. developing external auditory meati and gradually Lenoir, University of Montpellier, France. fuse to form the definitive auricles.

The external auditory meatus develops from the first The sensory hair cells are divided into inner and pharyngeal groove and the bounding arches. As the outer hair cells. Inner hair cells form a single row pharyngeal groove grows centrally, it forms a along the inner aspect of the organ of Corti and funnel-shaped pit lined by ectoderm and surrounded extend the length of the cochlea. Like type I hair by mesoderm (cartilage). cells of the vestibular labyrinth, the inner hair cells of the cochlea are short, flask-shaped cells with

293 A thick epithelial plate forms in the ectoderm, sac. The otocyst elongates and a tubular outgrowth growing from the base of the pit towards the at the ventral pole grows spirally for two and three- developing tympanic cavity. Connective tissue fourths turns into the surrounding mesenchyme. between the two layers of epithelium develops from This is the cochlear duct, which remains connected to mesenchyme. Initially the eardrum lies horizontally, the saccule by a narrow ductus reuniens. The dorsal almost parallel to the floor of the meatus, but as the part of the otocysts expands and develops into the meatus continues to grow, the eardrum gradually semicircular canals; the intermediate region forms the becomes more erect. utricle and saccule. The epithelial lining of the various segments of the membranous labyrinth is, at first, Middle Ear simple low columnar. As nerve fibers grow among the cells of the cristae ampullaris, maculae of the The tympanic cavity is derived from the first utricle and saccule, and organ of Corti, the pharyngeal pouch and is lined by epithelium of epithelium thickens and differentiates into the endodermal origin. The distal part of the pouch, the special sensory and supporting cells. The tubotympanic recess, widens to form a provisional supporting cells are thought to secrete and maintain tympanic cavity. The narrow proximal connection the cupula cristae ampullaris and otolithic to the region of the developing nasopharynx membranes in the maculae. Differentiation of the becomes the auditory tube. Mesenchyme from the organ of Corti progresses slowly, beginning as a first and second pharyngeal arches that lie above thickening of the epithelium at the base of the the provisional cavity becomes cartilaginous and cochlea and progressing to the apex. Large inner forms the first models of the ossicles. These remain and small outer ridges of epithelium form both embedded in a spongy mesenchyme until the associated with a covering tectorial membrane. The tympanic cavity widens and the mesenchyme small outer ridge gives rise to the supporting cells degenerates. As this occurs, the epithelium expands and inner and outer hair cells of the organ of Corti. to line the newly formed cavity and inner surface of The large inner ridge involutes to form the lining of the eardrum and to cover the ossicles in a the spiral sulcus. mesentery-like fashion. The ossicles, thus are At first, the structures of the developing suspended in the tympanic cavity but actually lie membranous labyrinth are embedded in outside the epithelium of the cavity. Each ossicle mesenchyme that later undergoes chondrification undergoes endochondral ossification. and in turn is replaced by bone to form the boney The tensor tympani and stapedius muscles arise from labyrinth. Cartilage adjacent to the membranous mesenchyme of the first and second pharyngeal labyrinth degenerates to form a syncytial reticulum arches, respectively, and lie beyond the lining of the that becomes the perilymphatic spaces filled with tympanic cavity. Ligaments supporting the ossicles perilymph. In the region of the developing cochlear develop from the mesenchyme that lies between the duct, the cartilage is resorbed so that the two mesentery-like folds of the covering epithelium. perilymphatic spaces, the scala vestibuli and scala The lining mucosa of the tympanic cavity continues tympani, are formed on either side of the cochlear to expand and eventually lines the mastoid air cells duct. The modiolus of the cochlea develops directly of the temporal bone. from the mesenchyme as membrane bone.

Inner Ear Summary

The epithelium of the inner ear is ectodermal in The pinna of the external ear collects sound and origin. Thickenings of the ectoderm, the auditory directs it into the external auditory meatus. Sound placodes, occur midway along both sides of the waves cause the tympanic membrane to vibrate midbrain and invaginate to form cup-shaped slightly at the same frequency as the sound waves. auditory pits. These expand, lose their connections Auditory ossicles suspended in the tympanic cavity with the surface ectoderm, and become detached of the middle ear conduct the vibrations to the ovoid sacs called auditory vesicles or otocysts. A narrow, perilymph channels of the inner ear. The force of tubular recess, the endolymphatic duct, develops the vibrations created by the sound waves is from the region last in contact with ectoderm, amplified with no expenditure of energy, because eventually expanding into a blind endolymphatic the tympanic membrane is much larger than the

294 foot plate of the stapes. Together, the intervening Nerve impulses from the organ of Corti are ossicles act as a piston. With each movement, the transmitted via the bipolar neurons of the spiral foot plate of the stapes exerts pressure on the ganglion to the cochlear division of the eighth perilymph, which is confined to the scala vestibuli cranial nerve. From here, impulses are relayed to and scala tympani of the inner ear. The pressure appropriate regions of the brain to be interpreted as waves pass through the perilymph from the scala sounds. vestibuli to the scala tympani and are released from The vestibular portion of the inner ear is the perilymphatic space by bulging of the secondary important for coordinating and regulating tympanic membrane into the air-filled tympanic locomotion and equilibrium. The three semicircular cavity. As the pressure waves move from scala canals are located in posterior, superior and vestibuli to scala tympani, they must first pass horizontal positions. Type I and II hair cells of the through the intervening cochlear duct or travel to cristae in the ampullary region of each semicircular the helicotrema at the apex of the cochlea. When a canal are stimulated during angular movement of pressure wave passes through the cochlear duct, the the head. Sensory stimulation of the receptors vestibular and basilar membranes are displaced results from movement of endolymph and slightly, creating strong shearing forces between displacement of the cupula that overlies the cristae. microvilli at the apex of inner and outer hair cells Displacement of the cupula, in which microvilli of and in the overlying tectorial membrane. The underlying hair cells are embedded, causes a shearing action stimulates the hair cells, which in shearing force at the apices of the hair cells. The turn elicit a nerve impulse in the surrounding cristae are positioned to detect acceleration and afferent nerve endings. Supporting cells, especially rotary motion and thus provide information on the the pillar and phalangeal cells, provide strong direction and rate of acceleration of head structural support to the cell bodies of their hair movement. Similarly, forces created during head cells so that the latter are not displaced by the movement act on the otolithic membrane and its shearing action on the microvilli. contained otoconia, again resulting in shearing Large areas of the basilar membrane vibrate at forces being applied to the apices of hair cells. Head several frequencies, but sound waves of a given movement forward and backward is perceived frequency produce a maximum displacement along primarily by the macula of the utricle; head specific regions of the basilar membrane. At low movement going side to side is perceived by macula frequencies, maximum displacement occurs farther of the saccule. Both also perceive the direction of from the oval window. Because both efferent and gravity and the static position. Hair cells of the afferent nerve endings terminate on hair cells in the vestibular organs, as well as those of the cochlea, organ of Corti, both sensory reception and transform mechanical movements of the inhibition are said to occur. The inhibitory endolymph (whether produced by displacement of mechanism stems from the central nervous system the cupula, otolithic membrane, or basilar and may aid in discrimination of loudness and membrane) into the electrical energy of a nerve pitch. The two skeletal muscles of the middle ear, impulse. Bending or displacement of microvilli on the tensor tympani and stapedius, dampen the the various types of hair cells is thought to result in movement of the ossicles and protect the delicate depolarization of the hair cell, with the stimulus structures of the inner ear from loud and sudden then being transferred to surrounding afferent noises. Contraction of these muscles may play a nerve endings to generate a nerve impulse. role in regulating the degree of tension placed on Associated efferent nerve endings are thought to the tympanic membrane so that sounds of have an inhibitory action and may elevate the moderate intensity can be transmitted in a noisy threshold of activity of hair cells. environment.

295 INDEX Amylases, 197 Adrenal cortex Anagen,

cells of, 262-264 phase of hair growth, 156 A bands, Adrenal corticoids, 263-264 Anal canal, 191 of skeletal muscle 90 Adrenal glands, Anal valves, 191 A-cells, cortex of, 262-264 Anaphase, 30 pancreatic, 198-199 features of, 262-264 meiotic, 223 A-cells, medulla of, 264 Anaphylaxis, 48 of synovial membrane, 58 vascularization of, 264 Androgen-binding hormone, AB-cells, Adrenal medulla, 264 226 of synovial membrane, 58 Adrenocorticotrophic hormone Anemia ABO blood grouping, 68 (ACTH), 266-267, 268 in children, 77 Absorption, function of, 263-264, 267 white cell anomalies in, 77 defined, 5 Adsorptive micropinocytosis, 9 Angiotensin I, 216, 221, 263 Acellular cementum, 175 Adventitia, 177, 189, 190 Angiotensin II, 209, 216, 221, Acetylcholine, 96, 112-113, 130 of digestive tract, 170 263 Acid hydrolase, 13 Adventitial reticular cells, Angiotensinogen, 216, 221, 263 Acidophilic cells, 266 of bone marrow, 81 Anisocytosis, 69 Acidophilic erythroblasts, 83 Afferent lymphatics, 138 Ankyrin, 68 Acinar cells, Agranular leukocytes, 71, 73-76 Annulate lamellae, 17, 26 pancreatic, 197 Albinism, 152 Annuli fibrosi, 57, 123 Acini, Albumin, Annulospiral endings, 96 glandular, 40 normal blood values of, 67 Annulus, Acrocentric, Alder's anomaly, 76 of sperm, 227 defined, 22 Aldosterone, 213, 216, 263 Anterior chamber, Acrosomal cap, 227-228 Alimentary tract. See Digestive of eye, 273, 276 Acrosomal vesicle, 227-228 system 170-205 Anterior lens cells, 281 Acrosome, 227-228 Alpha cells, Anterior lingual salivary glands, ACTH (adrenocorticotrophic pancreatic, 198 172 hormone), 266, 268 Alpha helix, 44 Anterior lobe, function of, 263-264, 267 Alpha particles, of hypophysis, 265-268 Actin, 18 of glycogen, 21 Antibody, in cardiac muscle, 98 Alpha units, 44 secretion of, 48, 74 in platelets, 70 Alveolar ducts, 164-165 Antidiuretic hormone (ADH), in skeletal muscle, 93-94 Alveolar macrophages, 166 213-214 in smooth muscle, 100 Alveolar pores, 165 Antigen-presenting cells, 49, Actin-binding proteins, 18-19 Alveolar sacs, 165 76, 136, 139, 141, 144-145, 152, α-actinin, 19 Alveoli 163, 176, 187 Action potentials, 106, 109 defined, 40 Antigens, Active zones, 96 respiratory, 165 stimulation of lymphocytes Adenohypophysis, of salivary glands, 173-174 by, 75 control of, 268 of tooth, 174 Antilymphocyte serum, features of, 265-268 Amacrine cells, as mitogen, 75 ADH (Antidiuretic hormone), of retina, 280 Antiport mechanism, 8 269 Ameloblasts, 175, 200 Antral follicle, 240-241 Adhesion glycoproteins, 46 Amino acids, Anucleate, Adipose synovial membranes, absorption of, 184 defined, 68 59 Amphiarthrosis, 56 Anus, 189, 191 Adipose tissue, 43, 48-50 Ampulla Aorta, Adluminal compartment, of ductus deferens, 230 elastic laminae of, 124 of Sertoli cells 225-226 of oviduct, 243 features of, 124

Aortic bodies, 130

296 Apical canaliculi, 211 Auricle, 287 Basket endings, Apical foramen, Autocrine secretion 40, 258 of axon, 111 of tooth, 174-176 Autolysis, 13 Basophilic cells, Apical surface, Autonomic ganglia, 108 of hypophysis, 266, 268 of epithelium, 33 Autonomic nervous system, Basophilic erythroblasts, 83 Apocrine secretion, 41, 254 117-118 Basophilic stippling, 69 Apocrine sweat glands, 157 Autophagic vacuole, 14 Basophils, 71 Apoptosis, 31 Autophagy, 14 characteristics of, 73 Appendices epiploicae, 190 Autosomes, 22 formation of, 84-86 Appendix, Axoaxonic synapse, 111 Beta cells, features of, 190 Axodendritic synapse, 111 pancreatic, 199 Appositional growth, Axon hillock, 105-106, 112 Beta particles, of cartilage, 59 Axoneme, of glycogen, 21 Aquaporins 9, 211 of sperm, 227-228 Beta-endorphin, 266 Aqueous humor, 273, 276 Axons, 105-106 Beta-lipotropic hormone Arachnoid, 117 Axoplasm, 106 (LPH), 266 Area cribrosa, Axosomatic synapse, 111 Betz cells, 116 renal, 214 Azurophil granules, 72, 75, 84, Biconcave disk, Areola, 254 87 defined, 68 Areolar connective tissue, 43, B-cells, Bile, 195 49 pancreatic, 199 Bile canaliculi, 194 Areolar glands, 254 B-cells, Bile ducts, 192, 196 Areolar synovial membranes, of synovial membrane, 58 Bile ductules, 195-196 58 B-lymphocytes, 74, 135 Bipolar cells, Aromatase enzyme complex, Band form, retinal, 277, 280 241 of neutrophils, 71 Bipolar neurons, 107, 161 Arteries, 124-126 Banding, Bivalent, 222 conducting, 124 of muscle, 90 Bladder (urinary), 218 distributing, 125 Barrett's esophagus, 177 Blastocele, 251 elastic, 124 Basal body, 38 Blastocyst, 251 muscular, 125 of centriole, 18 Blastogenic transformation, 75 small, 125 Basal cells Blastomere, 250 Arterioles, 125-126 of cervical canal, 248 Blebs, Arteriovenous anastomoses, of cochlear duct, 292 in smooth muscle, 100 129 Basal compartment, Blood Articular cartilages, 52, 57-58 of Sertoli cells, 225-226 abnormalities of, 69 Articular disc, 57 Basal lamina, cells of, 68-77 Ascorbic acid, epithelial, 32 in children, 77 normal blood values of, 67 Basal striations, 38, 213 components of, 67 Association neurons, 107, 280 of salivary glands, 173 as connective tissue, 43 Asthma, 48 Basal surface, hemopoiesis, 79-88 Astrocytes, 113 of epithelium, 33 normal lab values for, 67 Atretic follicle, 242 Basal-lateral infolding, 38 Blood islands, 79 Atrial natriuretic factor (ANF), Basal-lateral plasmalemma, 38 Blood types, 68 121 Basement membranes, Blood vessels, 123-130 Atrioventricular bundle, 122 epithelial, 32 arteries, 124-126 Atrioventricular node, 122 Basilar membrane capillaries, 126-127 Attachment plaques, 35, 100 of cochlea, 291-292 veins, 128-129 Auditory meatus, 287 of organ of Corti, 292 Blood-air barrier, 165 Auditory ossicles, 287 Basket cells, Blood-aqueous barrier, 276 Auditory teeth 293 of cerebellar cortex, 116 Blood-brain barrier, 115 Auditory tube, 287-288 Blood-thymic barrier, 144

297 Bone of synovial joint, 57 Carotene, 151 cells of, 55 Calcitonin, 260 Carotid bodies, 130 as connective tissue, 43 Calcium Carrier proteins, 258 decalcified, 56 absorption of, 159, 184, 260, Cartilage development of, 59-63 272 development of, 59 endosteum of, 55 normal blood values of, 67 elastic, 43, 53 ground substance of, 56 Calyces, fibrous, 43, 53 matrix of, 54, 56 of axon, 111 hyaline, 43, 52 periosteum of, 55 Cambium, 60 repair of, 63 remodeling of, 62 Canal of Schlemm, 275-276 types of, 43, 52-53 repair of, 63-64 Canaliculi, Cartilaginous joints, 56 structure of, 53-56 of bone, 54 formation of, 63 Bone marrow Cancellous bone, Catagen, embryonic hemopoiesis in, structure of, 53 phase of hair growth, 156 79-80 Candidate stem cells, 82 Catalase, 15 hemopoiesis in, 81-88 Cap, Catecholamines, 264 lymphocyte conditioning in, of lymphatic tissue, 136 Caveolated cells, 185 87 Capacitation, Cecum, 189-190 structure of, 80 of sperm, 228-229 Cecum cupulare, 291 types of, 80 Capillaries, 126-127 Cell, vasculature of, 80-81 function of, 126-127 cytoskeleton of, 18-20, 27 Bony labyrinth, 288-289 lymph, 131 division of, 29-31 Border cells, Capillary sphincter areas, 126 inclusions in, 20-21, 28 of organ of Corti, 292-293 Capitulum, nucleus of, 21-23, 28 Bound water, 46 of sperm, 228 organelles of, 6-18, 26-27 Boutons en passage, 111 Capping protein, 19 protoplasm of, 5-6 Bowman's capsule, 208 Capsular epithelium, Cell adhesion molecules, 7 Bowman's glands, 161 of glomerulus, 208 Cell attachments, 34-37 Bowman's membrane, 274 Capsular space, Cell-mediated immunity, 74 Brain, of glomerulus, 208 Cell membrane melanin in, 20 Capsule See Plasmalemma 6-10 Brain sand, 259 of joint, 57 Cells of Claudius, 293 Bronchi of lens, 281 Cells of Hensen, 293 extrapulmonary, 162-163 Carbohydrate, 5 Cellular cementum, 175 features of, 162-163 absorption of, 184 Cement line, intrapulmonary, 163-164 storage of, 194-195 of bone, 54 Bronchioles, 163 Carbon dioxide, 68, 70, 77 Cementocytes, 175, 200 respiratory, 164 normal blood values of, 67 Cementum, Brown fat, 50 Carboxypeptidases, of tooth, 174-175, 200 Bruch's membrane, 275 pancreatic secretion of, 197 Central canal, Brunner's glands, 188 Cardia, spinal cord, 115 Brush border, 37, 210 of stomach, 178 Central depression, Brush cells, Cardiac muscle, 97-99 of erythrocyte, 68 of trachea, 162 fibers of, 97-98 Central lacteals, 187 Buccal salivary glands, 172 intercalated disks of, 98 Central longitudinal vein, Buffy coat, 67 organization of, 97-99 of marrow, 81 Bulbourethral glands, 232 Cardiac skeleton, 123 Central nutrient branches, C-cells, Cardiovascular system of marrow, 80 of thyroid, 262 arteries, 124-126 Central pair, Calcification and cell death, capillaries, 126-127 of microtubules, 38 zone of, 61 heart, 120-123 Centriolar satellite, 17 Calcified zone, veins, 128-129

298 Centrioles, 17, 27 Chromatin, 21-22 Collagenous connective tissue, replication of, 17, 30 nucleolus-associated, 23 43-45 Centroacinar cells, 197 Chromomere, 70 Collaterals, 106 Centromere, 29-30 Chromophilic cells, 266 Collecting ducts, 207, 214-215 in meiosis, 223 Chromophilic substance, 105 Collecting lymph vessels, 131 splitting of, 30, 223 Chromophobes, 267-268 Colliculus seminalis, 231 Centrosome, 17 Chromosomal fibers, 30 Colloid, 260-261 Ceramide, 7, 11, 15 Chromosomes, 21, 29, 30 Colloid resorption droplets, intracellular synthesis of, 7 daughter, 29, 30 260-261 Cerebellar cortex, 116-117 separation of, 30, 223 Colon, gray matter of, 116 Chylomicrons, 183-184 features of, 189-190 Cerebral cortex, 116 Chymotrypsin, 197 Colony-forming units (CFUs), gray matter of, 116-117 Cilia, 37, 38 82 Cerumen, 287 Ciliaris muscle, 276 Color, Ceruminous glands, 157, 287 Ciliary body, 275-276 discrimination of, 279, 286 Cervical canal, 244, 247 Ciliary epithelium, 275-276 Colostrum, 254 cytology of, 248-249 Ciliary processes, 276 Columnar, Cervix, 244, 247-248 Ciliated pseudostratified defined, 32 junction with vagina, 247 columnar epithelium, 38 Common bile duct, 196 Cheek, 170-171 Ciliated simple columnar Common hepatic duct, 196 Chemoreceptors, 164, 167 epithelium, 38 Communication junctions, 36- aortic bodies, 130, 167, 269 Ciliogenesis, 18 37 carotid bodies as, 130, 167 Circadian rhythms, 259 Compact bone, Chiasmata, 222 Circumanal glands, 191 structure of, 53, 54 Chief cells Circumferential lamellae, Composite glands, of paraganglia, 265 of bone, 54 defined, 174, 232 of parathyroid, 259-260 Circumvallate papillae, 171 Compound acinar glands, 40 of stomach, 180-181 Cisternae, 10-13 Compound glands, Children, Cisternal lumen, 11 defined, 40 blood in, 77 Clara cells, 163 Compound saccular glands, 39- Chloride Classic endocrine glands, 258 40 normal blood values of, 67 Clathrin, 9 Compound tubular glands, 39- Cholecystokinin, 185, 198 Clear cells, 40 Cholesterol of sweat glands, 157 Compound tubuloacinar in cell membrane, 7 Cleavage, 250-251 glands, 39-40, 173, 197 normal blood values of, 67 Climbing fibers, Compound tubuloalveolar Chondroblasts, 59 cerebellar, 117 glands, 39-40 Chondrocytes, 52-53, 59 Clitoris, 249 Condensing vacuoles, 12 Chondroitin, 46 Clotting, 67, 70, 77-78, 128, Conducting arteries, 124 Chondroitin sulfates, 46 133 Conduction, Chordae tendineae, 123 Coated pits, 9 heart, 121-122 Choriocapillary layer, 275 Coated vesicles, 9 Conductivity, Chorion, 252 Cochlea, 291-293 defined, 5 Chorion frondosum, 252 Cochlear duct, 291-292 Cone pedicle, 279 Chorion laeve, 252 Codon, 10 Cones, Chorionic plate, 252 Collagen, 44, 45, 47 structure of, 279-280 Choroid, 275 in bone, 55, 56 Conjugation, 222 Choroid plexus, 114-115 in cartilage, 52-53 Conjunctiva, 282 Chromaffin cells, 264 fibers of, 44 Connecting piece, Chromatids, 30 of tendon, 97 of sperm, 227 in meiosis, 222-223 synthesis of, 47 Connective tissue, types of, 45 cells of, 47-50

299 classification of, 43, 49-50 Crescents, Dendraxons, 107 dense, 43, 50 of salivary glands, 174 Dendrites, 105, 106 features of, 43-51 Cristae, Dendritic cells, 163 fibers of, 44-45, 47 of mitochondria, 16 Dendritic interdigitating cells, ground substance of, 45-46 Cristae ampullaris, 289-291 141 loose, 44-46 Cross-striations, Dendritic spines, 106 organization of, 43, 49-50 of muscle, 90-91 Dendrodendritic synapse, 111 special, 50, 52-66, 67-78, 79- Crossing over, 222 Dense bone, 88 Crown, structure of, 53-54 Connexin, 36 of tooth, 174, 175 Dense connective tissue, Connexons, 36 Crypts, types of, 43, 50 Constitutive exocytosis, 10 of tonsils, 137, 138 Dense irregular connective Continuous capillaries, 126 Crystalloid, tissue, 43, 50 Continuous fibers, 30 in eosinophils, 72 Dense regular connective Contractility, Crystalloids of Charcot- tissue, 43, 50 97 defined, 5 Böttcher, 225 Dentin, Corium, 149 Crystals, of tooth, 174-175 Cornea, 273-274 as cytoplasmic inclusions, 21, Dentinal fibers, 174 Corneal endothelium, 274 28 Dentinal tubules, 174 Corneal epithelium, 274 Crystals of Reinke, 224 Deoxyribonuclease, 197 Corneoscleral coat, 273-275 Cuboidal, Dermal papillae, 149, 153 Corona radiata, 241 defined, 32 Dermatan sulfate, 46 Corpora arenacea, 259 Cumulus oophorus, 240 Dermatoglyphics, 153 Corpora cavernosa clitoridis, Cupula cristae ampullaris, 290 Dermis, 50, 149, 153 249 Cuticle, 155 Descemet's membrane, 274 Corpora cavernosa penis, 233 Cystic duct, 196 Desmin, 19, 27, 91, 99 Corpora cavernosa urethrae, Cytokinesis, 30-31 Desmosomes, 35-36 233 Cytolysosome, 14 of cardiac muscle, 98 Corpus albicans, 242 Cytoplasm, Diads, Corpus luteum, 241-242 inclusions in, 20-21, 28 of cardiac muscle, 98 development of, 241-242 Cytoplasmic matrix, 6 Diakinesis, 222 Corpus spongiosum, 233 Cytoskeleton, 6, 18-20, 27 Diapedesis, 72 Cortex Cytosol, 6 Diaphysis, 53 adrenal, 262-264 Cytotoxic T-cells, 74 Diarthroses, 57 cerebellar, 116-117 Cytotrophoblast, 251-252 Diffuse ganglion cells, cerebral, 116 D-cells, retinal, 280 of hair, 155 pancreatic, 199 Diffuse lymphatic tissue, 134- of lens, 281 Dark cells, 136, 138 of lymph node, 138-139 of sweat glands, 157 Digestive enzymes, 197 renal, 206 Dark pole, Digestive system 170-205 of thymus, 143 of lymphatic tissue, 136 liver, 192-196 Cortical sinus, Daughter chromosomes, 30 oral cavity, 170-176 of lymph node, 139 Decalcified bone, 56 pancreas, 197-199 Corticosterone, 263 Decidua basalis, 252 pharynx, 176 Corticotrophs, 266 Decidua capsularis, 252 tubular digestive tract, 176- Cortisol, 263 Decidua parietalis, 252 191 Cortisone, 263 Definitive erythroblasts, 79 Diploë, 53 Cotransport mechanism, 8 Definitive osteons, 62 Diploid cells, 22 Countertransport mechanism, 8 Dehydroepiandrosterone, 263 Diplosome, 17 Cranial ganglia, 107 Delta cells, Diplotene, 222, 239 Crenated cells, 69 pancreatic, 199 Distal convoluted tubule, 207, Demarcation membranes, 86 212-213

300 Distal tubule, 207, 212-213 implantation of, 251 Ependyma, 114-115 Distributing arteries, 125 Enamel, Epicardium, 121 DNA (deoxyribonucleic acid) of tooth, 174, 175 Epidermal growth factor, 173, meiosis, 222 Enamel prisms, 175, 200 188 mitochondrial, 17 Enamelins, 175 Epidermal ridges, 149 nuclear, 5, 21-23 Encapsulated nerve endings, Epidermal-melanin unit, 152 replication of, 29-31 110-111 Epidermis, Döhle's bodies, 76 End feet, pigmentation of, 151-152 Doublets, of astrocytes, 113 stratum basale of, 149-150 of microtubules, 37, 38 End piece, stratum corneum of, 151 Drumsticks, of sperm, 228 stratum granulosum of, 150 in neutrophils, 71-72 Endocardium, 120 stratum lucidum of, 150 Ductuli efferentes, 229 Endocervix, 247 stratum spinosum of, 150 Ductus choledochus, 196 Endochondral ossification, 59- structure of, 149-153 Ductus deferens, 63 Epididymis, 229-230 features of, 230 Endocrine cells Epidural space, 117 Ductus epididymidis, 229-230 of intestine, 184-185 Epimysium, 89-90 Ductus reuniens, 291 of stomach, 178, 180-181, Epinephrine, 264 Duodenal glands, 188 185 Epineurium, 110 Duodenum, 182, 188 Endocrine glands, 39-41, 258- Epiphyseal ossification center, Dura mater, 117 272 61 Dust cells, 166 formation of, 41-42, 270-271 Epiphyseal plate, 56, 62 Dynein, 20, 38, 106 Endocytic complex, 211 Epiphyses, 53 Dystrophin, 94 Endocytosis, 9-10 Epitendineum, 97 Ear, Endolymph, 288 Epithelial attachment cuff, 176 external, 287 Endolymphatic sac, 289 Epithelial cells, 32-34, 39-42 inner, 288-293 Endometrial stroma, 245-247 Epithelium middle, 287-288 Endometrium, 245-247 avascularity of, 32 structure of, 287-293 cyclic changes in, 246-247 cell attachments in, 34-37 Eardrum, 287, 288 vascularization of, 245-246 classification of, 32-33 Eccrine sweat glands, 156-157 Endomitosis, 86 glandular, 39-41 Echinocytes, 69 Endomysium, 90, 98 specializations of, 37-38 Efferent lymphatics, 138 Endoneurium, 110 structure of, 32 Efferent nerves, 110 Endopeptidases, Eponychium, 154 Ejaculatory duct, 230-231 pancreatic secretion of, 197 Equatorial plate, 30 Elastic arteries, 124 Endoplasmic reticulum, 10-11 Erectile tissue, 233-234 Elastic cartilage, 53, 287 Endosomes, 10 Erythroblasts, 83-84 Elastic connective tissue, 43 Endosteum, 53, 55, 60 Erythrocytes, 67-68 Elastic fibers, Endotendineum, 97 abnormalities of, 69 of connective tissue, 44-45 Endothelial cells, 33, 128 anuclearity of, 68 Elastic laminae, 45, 124 Endothelin I, characteristics of, 68-70 Elastin, 44 secretion of, 128 formation of, 83-84 Electron transfer system, Endothelium, 33, 128 gas transport by, 69-70 in mitochondria, 16-17 Enterocytes, 183 morphology of, 68 Elementary particles, Enteroglucagon, 185 number of, 68 in mitochondria, 16 Enterokinase, 198 structure of, 68 Ellipsoid Eosinophilia, 76 Erythroplastid, 68 of cone cells, 279 Eosinophils, 49, 71 Erythropoiesis, 83-84 of rod cells, 279 characteristics of, 49, 72-73 Erythropoietin, 87, 216 Embryo diurnal variations in, 72 Esophageal cardiac glands, 177 development of, 249-251 formation of, 84-86 Esophageal glands, 177 formation of, 249-251 structure of, 49, 72-73 Esophageal reflux, 177

301 Esophagus, 176-177 Fascia adherens, 98 Follicle-stimulating hormone adventitia of, 177 Fascicles (FSH), 226, 240-243, 267 features of, 176-177 of nerve fibers, 110 Follicular antrum, 240 glands of, 177 of skeletal muscle, 89 Follicular dendritic cells, 136, junction with stomach, 177 of tendon, 97 139 mucosa of, 176-177 Fast twitch muscle fibers, 95 Folliculostellate cells, 267 muscularis externa of, 177 Fat, 49-50 Foot processes, submucosa of, 177 absorption of, 183-184 of astrocytes, 113 Estrogens, 241, 253 brown, 50 Foreign body giant cell, 47 Ethmoid sinuses, 160 metabolism of, 50 Foreskin, 234 Euchromatin, 22, 28 multilocular, 50 Forming face, 12 Eustachian tube, 288 unilocular, 50 Fossa navicularis, 218 Excretion, Fat cells, 21, 48 Fovea, 273, 280 defined, 5 Fatty marrow, 80 Fovea centralis, 277, 280 Excretory ducts, 229-231 Female pronucleus, 250 Foveolae, 178 Exocervix, 247 Female reproductive system Freckles, 152 Exocrine glands, 39 239-249 Free nerve endings, 110 formation of, 40-42 Fenestrae, Free ribosomes, 10 Exocytosis, 10 defined, 126 Frontal sinuses, 160 Exopeptidases, Fenestrated capillaries, 126-127, Fundus, pancreatic secretion of, 197 209 of stomach, 178 External anal sphincter, 191 Fertilization, 243, 250 Fungiform papillae, 171 External auditory meatus, 287 Fibrillin, 45, 150 G-cells External ear, 287 Fibrinogen, 67 pancreatic, 199 External elastic lamina, 125 Fibroblasts, 47, 50 of stomach, 180-181 External genitalia in tendon, 97 GABA (gamma-aminobutyric female, 249 Fibronectin, 32, 46 acid), 112-113 male, 233-235 Fibrous astrocytes, 113 Galactocerebroside, 7, 15, 108 External lamina, Fibrous cartilage, 53 Gallbladder, 196-197 of muscle fiber, 91, 94, 98-99 Fibrous joints, 56 Ganglia, 107-108 External mesaxon, 109 formation of, 63 Ganglion cells, External scleral sulcus, 274 Fibrous synovial membranes, retinal, 277-278, 280 Extrafusal fibers, 96 59 Gangliosides, 7, 15 Extraglomerular mesangium, Fila olfactoria, 161 Gap junctions, 36-37, 113, 120 209, 215 Filamin, 18, 19, 32 of cardiac muscle, 99 Extrahepatic ducts, 196 Filiform papillae, 171 of smooth muscle, 100 Extramedullary hemopoiesis, Filtration barrier, 209-210 Gastric cardiac glands, 178 80 Filtration slits, 208-209 Gastric glands, 179-180 Extrapulmonary bronchi, 162- Fimbria, 243 kinetics of, 180-181 163 Fimbrin, 19, 37, 182 Gastric inhibitory peptide, 185 Eye Flat cone bipolar cells, 280 Gastric intrinsic factor, 180 corneoscleral coat of, 273- Flat midget bipolar cells, 280 Gastric lining, 178 275 Flower spray ending, 96 cells of, 178 lens of, 281-282 Fluid transport, 38 Gastric pits, 178 retina of, 277-281 Fluid-phase micropinocytosis, 9 Gastrin, 180-181, 185 structure of, 273 Focal adherens, 36 Gastrointestinal mucus 191-192 uveal layer of, 275-277 Foliate papillae, 171 Gaucher's disease, 14, 15 vitreous body of, 282 Follicles, 41 Gelosin, 19 Eyelash, 282 lymphatic, 136 Gemmules, 106 Eyelids, 282-283 ovarian, 239-241 General connective tissue, F-cells, thyroid, 260-262 defined, 43 of synovial membrane, 58 Genital corpuscles, 111

302 Germ cells 223, 228-229, 241, intestinal, 184 Helicine arteries, 234 250, Golgi complex, 12-13, 26 Helicotrema, 291 meiosis of, 222-223 Golgi type I neurons, 107 Heme, 70 Germinal center, Golgi type II neurons, 107 Hemidesmosome, 36 of lymphatic tissue, 136 Gomphosis, 56 Hemoglobin, 68, 69-70 Germinal epithelium, 33-34, Granular endoplasmic Hemopoiesis 225 reticulum (GER), 10, 11, 26 in bone marrow, 80-88 Gingiva, 170, 174, 176 Granular leukocytes, 71-73 embryonic and fetal, 79-80 Gingival ligament, 176 formation of, 84-86 in lymphatic tissue, 134 Gingival sulcus, Granule cell layer, Hemopoietic cells, 79-88 of tooth, 176 of cerebellar cortex, 116 Hemopoietic compartment, 79, Gitterzellen, 114 Granulocytopoiesis, 84-86 81 Glands Granulomere, 70 of bone marrow, 79-83 classification of, 39-41 Granulosa cells, 240 Hemopoietic connective tissue, defined, 39 Granulosa lutein cells, 241 43, 80-81 Glands of Littré, 218 Gray matter, 107 Hemorrhoidal plexus, 191 Glands of Zeis, 282 of brain, 115-117 Hemosiderin, 20, 28, 141 Glans clitoridis, 249 spinal cord, 115 Henle's layer, 155 Glans penis, 233 Ground substance, 43, 45, 52 Heparan sulfate, 32, 209 Glassy membrane, 155, 242, Growth, Heparin, 48 275 defined, 6 Hepatic lobule, 192-193 Glia limitans, 113 Growth hormone (GH), 266 Hepatic macrophages, 194 Glial cells 113-115 Guanylin, 186 Hepatic portal system, 192 of pineal, 259 Gums, 176 Hepatic sinusoids, 193-194 of retina, 278 Gut-associated lymphatic Hepatocytes, 192, 194-195 Glial fibrillary acid protein tissue, 186-187 Herring bodies, 269 (GFAP), 19, 113 H band, Heterochromatin, 22, 28 Glial filaments, 19, 27 of skeletal muscle, 90-91 Heterophagic vacuole, 14 Globin, 70 Hair, 154-156 Heterophagy, 14 Globulins, cycle, 156 Heterophils, 71 normal blood values of, 67 follicle of, 155 Hilum, Glomerular capsule, 208-210 structure of, 154-155 renal, 206 Glomerular epithelium, 208- Hair bulb, 155 Hilus, 210 Hair cells, 292-293 of lymph node, 138 Glomerulus, vestibular, 289-291 Histamine, 48, 180, 185 renal epithelium of, 208-210 Haploid cells, 22 Histiocytes. filtration by, 209-210 Hard keratin, 154 See Macrophages, 47-48, 75- Glucagon, 198-199 Hard palate, 171 76 Glucocorticoids, 263 Haustra, 190 Histones, 22 Glucose, 20-21, 28 Haversian canals, 54 Holocrine secretion, 41, 156 normal blood values of, 67 Haversian systems, 54 Homologous chromosomes 22, polymers of, 20-21 Hay fever, 48, 73 222, Glycine, 112 HbA, 70 Horizontal cells, Glycocalyx, 183 HbA2, 70 of retina, 280 Glycogen, 20-21, 28 HbF, 70 Hormone receptors, 7-9, 258 in liver, 21, 194-195 HbS, 70 Hormones, 258 Glycolipids, Head cap, Howell-Jolly bodies, 69 in cell membrane, 7 of sperm, 227 Howship's lacunae, 55 Glycosaminoglycans, 46 Head, Human chorionic of bone, 56 of sperm, 228 gonadotrophin (hCG), 253 Glycosphingolipids, 7 Heart, 120-123 Humoral immunity, 74 Goblet cells, 40, 162 muscle of, 120-122 Hurler's syndrome, 76

303 Huxley's layer, 155 of organ of Corti, 292-293 of retina, 280 Hyaline cartilage, 52 Inner limiting membrane, Interprismatic substance, 175 Hyalocytes, 282 of retina, 277, 278 Interstitial cell-stimulating Hyaloid canal, 282 Inner mitochondrial hormone (ICSH), 225, 267 Hyalomere, 70 membrane, 16 Interstitial cells of Cajal, 188- Hyaluronic acid, 46, 282 Inner phalangeal cells, 189 Hydrochloric acid, of organ of Corti, 292 Interstitial cells, secretion of, 179-180 Inner pillar cells, 292 of Leydig, 224 Hydrogen peroxide, Inner spiral sulcus, 275 Interstitial growth, synthesis of, 15, 25 Inner tunnel, of cartilage, 59 Hydroxyapatite, 56 of organ of Corti, 292 Interstitial lamellae, 5-hydroxytryptamine, 180-181 Inorganic materials, of bone, 54, 62 Hypersegmentation, 76 in protoplasm, 5 Interterritorial matrix, Hypochromia, 69 Insulin, 199 of cartilage, 52 Hyponychium, 154 Integral membrane proteins, 7 Intervertebral disks, 57 Hypophysis, 265-272 Integrins, 36, 46 Interzonal fibers, 30 organization of, 265 Integument, Intestinal glands, 183, 186 subdivisions of, 265-269 defined, 149 of colon, 189 vascularization of, 265-266 Interalveolar septum, 165 Intestine. See also Large Hypothalamohypophyseal tract, Intercalated disks, 98-99 intestine, 189-190 269 Intercalated ducts Small intestine Hypothalamus, 268 pancreatic, 198 epithelial cells of, 183-185 control adenohypophysis, of salivary glands, 173 epithelium of, 183-185 268 Intercellular adhesion glands of, 186, 188 Hypoxia, molecules, 128 Intimal ridges, erythropoiesis, 87 secretion of, 128 penile, 234 I bands, Intercellular secretory Intracristal spaces, skeletal muscle, 90-91 canaliculi, 174, 197 of mitochondria, 16 IgA, of salivary glands, 173 Intraepithelial glands, 39, 40, in saliva, 173 Interdigitating cells, 141 218 Ileocecal junction, 189 Interfascicular Intrafusal fibers, 96 Ileum, 182 oligodendrocytes, 113-114 Intraglomerular mesangial cells, Immediate hypersensitivity Interferons, 47, 74, 146, 209 reactions, 48, 73 Interleukins, 47, 55, 74 Intralobular duct system, 173, Immunity Interlobular bile ducts, 196 198 cell-mediated, 74-75 Interlobular ducts, of salivary glands, 173 humoral, 74 pancreatic, 198 Intramembranous ossification, Implantation, 251 Intermediate filaments, 19, 27, 59-60 Implantation fossa, 100 Intramural part, of sperm, 227 Intermediate sinus, of oviduct, 243 Inclusions, of lymph node, 139 Intraperiod lines, cytoplasmic, 6, 20-21, 28 Intermediate squamous cells, of nerve fibers, 109 Incus, 287-288 of cervical canal, 249 Intrapulmonary bronchi, 163 Infundibular stalk, Internal anal sphincter, 191 Invaginating midget bipolar of hypophysis, 265, 269 Internal elastic lamina, 125 cells, 280 Infundibulum, 243 Internal mesaxon, 109 Iodopsin, 279 Inhibin, 226, 241, 243, 267 Internal scleral sulcus, 274 Ion-channel proteins, 8 Inhibiting hormones, 268 Internodal segments, Iris, 276-277 Inner cell mass, 251 in nerve fibers, 108 Iron, Inner ear, 288-293 Internuncial neurons, 115 normal blood values of, 67 Inner epithelial root sheath, 155 Interphase, 29 Irritability, Inner hair cells, Interplexiform cells, defined, 5

304 Ischemic phase, Killer lymphocytes, 74 Lateral nail fold, 154 of menstrual cycle, 247 Kinesin, 20, 106 Lateral surfaces, Isogenous groups, Kinetochore, 30 of epithelium, 33 defined, 52 Kinocilium, 290 Lens, 281-282 Isthmus, Krause, Lens fibers, 281 of oviduct, 243 end-bulb of, 111 Lens substance, 281 Jejunum, 182 Kupffer cells, 194 Leptocytes, 69 Joints Labia majora, 249 Leptotene, 222 cartilaginous, 56 Labia minora, 249 Leukemoid reaction, 77 development of, 63 Labial glands, 170, 172 Leukocytes, 48, 67, 71-76 fibrous, 56 Labial salivary glands, 170, 172 abnormalities of, 76 synovial, 57-58 Lacrimal duct, 283 classification of, 71 Junctional complexes, Lacrimal glands, 283 formation of, 84-88 epithelial, 34-37 Lacrimal sac, 283 types of, 48-49 Junctional feet, Lactation, 254-255 Leukocytosis, 76 in skeletal muscle, 92 Lactic acid, Leukopenia, 76 Junctional folds, 95 normal blood values of, 67 Light cells, Juxtaglomerular apparatus, 215- Lactiferous ducts, 254 of thyroid, 262 216 Lactiferous sinuses, 253 Light, Juxtaglomerular cells, 213, 216 Lactoferrin, sensing of, 278-280 Kartagener's syndrome, 38 in neutrophils, 72 Light pole, Karyokinesis, 30 Lacunae of lymphatic tissue, 136 Karyolymph, 6 of bone, 54 Limbus, 274-275 Karyosomes, 22 of cartilage, 52, 53 Lingual salivary glands, 171-172 Karyotype, 22 endometrial, 246, 251 Lingual tonsils, 138, 171 Keratan sulfate, 46 of Morgagni, 218 Lining epithelium, Keratin, 19, 27, 33, 151 Lamellae, of digestive tract, 170 hard, 154 of bone, 54 Lip, 170 soft, 151 Lamina cribrosa, 273 Lipases, 197 Keratinized stratified squamous Lamina densa, Lipid droplets, 21, 28 epithelium, 33, 149 epithelial, 32 Lipids, Keratinocytes, 149 Lamina lucida, absorption of, 183-184 Keratocytes, epithelial, 32 metabolism of, 195 of cornea, 274 Lamina propria, 170 Lipofuscin, 14, 20, 28 Keratohyalin granules, 150 cells in, 181 in eye, 281 Kidney of small intestine, 186-187 in neurons, 20, 105 collecting tubules of, 214-215 of stomach, 181 Lipocytes, 194 corpuscle of, 208-210 Lamina splendens, 57 Liposaccharide from E. coli, distal tubule of, 212-213 Laminin, 32, 46 a mitogen, 75 features of, 206-217 Lamins, 23 Liquor folliculi, 240 functions of, 206-217, 219- Langerhans' cells, 152 Liver 192-195 221 Lanugo, 154 bile ducts of, 192-193, 195- juxtaglomerular apparatus of, Large arteries, 124 196 215-216 Large granular lymphocytes, 74 cells of, 192-195 loop of Henle of, 213-214 Large intestine 189-190 embryonic hemopoiesis in, macroscopic features of, 206- mucosa of, 189-190 79-80 207 muscularis externa of, 190 functions of, 194-195 microscopic features of, 207- serosa of, 190 lobes of, 192-193 217 submucosa of, 190 organization of, 192-194 nephron of, 207-215 Large veins, sinusoids of, 193-194 proximal tubule of, 210-212 features of, 129 Liver acinus, 193 vascularization of, 216-217 Larynx, 162

305 Lobation, Lymphokines, Marginal bundle, of leukocyte nucleus, 71 produced by T-cells, 74 of platelets, 70-71 Lobuli testis, 224 Lymphopenia, 76 Marginal cells, Loop of Henle, 207, 213-214 Lysosomes, 13-15, 26 of cochlear duct, 292 Loose connective tissue, in eosinophils, 72 Marginal sinus, types of, 43 in monocytes, 75 of lymph node, 139 Low columnar, in neurons, 105 Marrow cavity, 53 defined, 32 in neutrophils, 72 Marrow Lower esophageal sphincter, Lysozyme embryonic hemopoiesis in, 177 in neutrophils, 72 79-80 Lungs in saliva, 173 hemopoiesis in, 81-88 alveoli of, 165-166 M line, lymphocyte conditioning in, bronchi of, 162-164 of skeletal muscle, 90, 93 87 bronchioles of, 164 M-cells, 187 structure of, 80 Lunule, 154 of synovial membrane, 58 types of, 80 Luteinization, 241 Macrocytic erythrocytes, 69 vasculature of, 80-81 Luteinizing hormone (LH), Macroglia, 113 Mast cells, 48 225, 240-243, 267 Macrophage-activating factor, Matrix, Lymph capillaries, 131 74 of bone, 56 Lymph nodes, Macrophages, 47-48, 135 Matrix granules, sinuses of, 139 alveolar, 48 in mitochondria, 17 structure of, 138-140 in connective tissue, 47,48 Maturation and hypertrophy, vascularization of, 139-140 hepatic, 48 zone of, 61 Lymph vascular system, 120, Macropolycytes, 76 Mature follicle, 241 130-132 Macula adherens, 35 Maxillary sinuses, 160 Lymphatic nodules, 136, 138, Macula densa, Median eminence, 187 renal, 213 of hypophysis, 265, 268-269 Lymphatic organs 137-145 Macula lutea, 277 Mediastinum testis, 224 lymph nodes, 138-140 Macula pellucida, 241 Medium arteries, 125 spleen, 140-142 Macula sacculi, 289-291 Medium veins, 129 thymus, 142-145 Macula utriculi, 289-291 Medulla, tonsils, 137-138 Magnesium, adrenal, 264 Lymphatic sinuses, 139 normal blood values of, 67 of hair, 154 Lymphatic system, 130-132 Major calyces, of lymph node, 139 Lymphatic tissues 134-136 renal, 206 renal, 206 cells of, 134-135 Major dense lines, of thymus, 143 types of, 134-136 of nerve fibers, 108-109 Medullar pyramids, Lymphatic trunks, 131 Major salivary glands, 173-174 renal, 206 Lymphoblasts, 135 Male pronucleus, 250 Medullar rays, Lymphocyte blastogenic factor, Male reproductive system, 224- renal, 206-207 74 238 Medullary collecting ducts, Lymphocytes, 49, 71, 134-135 Malleus, 287-288 renal, 214-215 antigenic stimulation of, 73- Mammary glands Medullary sinus, 75 features of, 253-256 of lymph node, 139 characteristics of, 73-75 hormonal control of, 255 Megakaryoblasts, 86 classification of, 73-75, 134- in pregnancy and lactation, Megakaryocytes, 70, 86 135 254 Meibomian glands, 282 large, 74, 135 structure of, 253-256 Meiosis, 222-223 medium, 135 Mammotrophs, 266 Meissner's corpuscles, 111 small, 73, 75, 134 MAPs (microtubule-associated Meissner’s plexus, 188 Lymphocytosis, 76 proteins), 20, 106 in children, 77

306 Melanin, 20, 28, 151 Microtubules, 19, 20 of esophagus, 176-177 in eye, 281 of centriole, 17 of large intestine, 189-190 in neurons, 105 Microtubule-associated of small intestine, 182-187 Melanocyte-stimulating proteins, 20, 106 of stomach, 178-181 hormone (MSH), 266, 268 Microvilli, 37, 38 Mucosa-associated lymphoid Melanocytes, 149, 151 intestinal, 182-183 tissue, 148, 186-187 Melanosomes, 20, 152 Midbody, 31 Mucous glands, 41 Melatonin, 259 Middle ear, 287-288 Mucous membrane, Membrane bones, 59 Middle ear cavity, 287 of digestive tract, 170 Membrane lipids, 7 Middle piece, Mucous neck cells, 179, 180- Membrane proteins, 7-9 of sperm, 227-228 181 Membrane pump proteins, 7-8 Midget ganglion cells, Mucus, 40, 191-192 Membrane space, retinal, 280 Müller's cells, 278 of mitochondria, 16 MIF, 74 Multicellular endocrine glands, Membrane-coating granules, Milk, 254 40, 258-272 150 Mineralocorticoids, 263 Multicellular exocrine glands, Membranous labyrinth, 288 Minor calyces, 39-40 Membranous urethra, 218 renal, 206 Multilocular fat, 50 Memory cells, 75 Minor salivary glands, 172 Multipolar neurons, 107 Meninges, 117 Mitochondria, 16-17, 26 Multivesicular body, 15, 26 Meniscus, 57 Mitochondrial matrix, 16-17 Muscle Menstrual cycle, 246-247 Mitogens, 75 cardiac, 97-99 Menstrual phase, Mitosis, 29-31 classification of, 89 of menstrual cycle, 247 Mitotic spindle, 30 skeletal, 89-97 Merkel's cells, 152-153 Mixed compound smooth, 99-101 Merkel's discs, 111 tubuloalveolar glands, 174 Muscle fibers, 89-97 Merocrine secretion, 41 Mixed glands, 41, 174 cardiac, 97-99 Mesangial cells, 209 Mixed nerves, 110 contraction and stretching of, Mesaxon, 109 Modiolus, 291 94 Mesenchyme, 43, 49 Moll's glands, 157, 282-283 myofibrils, 92-94 Mesentery, 189 Monocytes, 48-49, 75-76 structure of, 90-101 Mesothelial cells, characteristics of, 75-76 types of, 94-95 of epicardium, 121 formation of, 87 Muscle spindles, 96-97 Mesothelium, 33 Monocytosis, 76 Muscular arteries, Metabolism, Mononuclear, features of, 125 defined, 5 defined, 49 Muscularis externa Metacentric, Mononuclear leukocytes, 49, of digestive tract, 170 defined, 22 71, 73-76 of esophagus, 177 Metamyelocytes, 85 Mononuclear phagocyte of large intestine, 190 Metaphase, 30 system, 48, 75-76 of small intestine, 188-189 Metaphase I, Morula, 250 of stomach, 181-182 meiotic, 223 Mossy fibers, Muscularis mucosae, 170 Metaphase II, cerebellar, 117 of small intestine, 187 meiotic, 223 Motilin, 185 of stomach, 181 Metarterioles, 126 Motor end-plate, 95-96, 110 Myelin, 108 Metarubicyte, 83 Motor nerves, 95, 110 components of, 108-109 Microcytic erythrocytes, 69 Motor unit, 95, 110 Myelin sheath, 108-109 Microfibrils, 44, 282 mRNA, 10, 23 Myelinated fiber, 108-109 Microfilaments, 18, 27 Mucoid connective tissue, 43, in central nervous system, Microglia, 114 49 113-114 Micropinocytosis, 9 Mucosa Myeloblasts, 84 Micropinocytotic vesicles, 9 of digestive tract, 170 Myelocytes, 85

307 Myeloperoxidase, Neurilemma, 108-109 Nuclear pore complex, 23, 28 in eosinophils, 72 Neuroepithelial bodies, 164 Nuclear pores, 23, 28 Myenteric plexus, 188-189 Neuroepithelial cells, Nucleic acids, 5 Myocardium, 120-121 of taste buds, 172 Nucleoid, Myoepithelial cells, 41, 163, Neurofibrils, 105 defined, 15 177, 253-254, 283, 287 Neurofilaments, 19, 27, 105 Nucleolonema, 23 of salivary glands, 173-174 Neuroglia, 113-115 Nucleolus, 21, 23, 28 of sweat glands, 157 Neurohypophysis, 269 Nucleolus-associated Myofibrils, 91-94 Neurokeratin, 108 chromatin, 23 cardiac, 98 Neuromuscular spindles, 96-97 Nucleolus-organizing regions, of smooth muscle, 99 Neurons, 105 23, 30 structure of, 92-94 distribution of, 107 Nucleus, 6, 28, 107 Myofibroblasts, 47 lipofuscin in, 105 of lens, 281 Myofilaments, 92-94, 100 types of, 107 Nucleus pulposus, 57 Myoid cells, 144 Neuropeptides, 112-113 Null cells, 74 Myoid region, Neurophysin, 269 Nutrient artery, of rod cells, 279 Neurotendinous endings of of marrow, 80 Myomesin, 93 Golgi (tendon organ), 97 Odontoblasts, 174-175, 200 Myometrium, 244-245 Neurotensin, 185 Olfactory cells, 161 Myoneural junction, 95-96 Neurotubules, 105 Olfactory epithelium, 160-161 Myosin, 18 Neutropenia, 76 Olfactory hairs, 161 in cardiac muscle, 98 Neutrophilia, 76 Olfactory knob, 161 in platelets, 70 Neutrophils, 48, 71 Olfactory vesicle, 161 in skeletal muscle, 92-94 characteristics of, 71-72 Oligodendrocytes, 113-114 in smooth muscle, 100 formation of, 84-86 Oligomucous cells, 186 Myosin II, 18 Nexus junctions, 36-37 Oocyte, 240-241 Nail bed, 154 of cardiac muscle, 99 Opsonization, 47 Nail matrix, 154 of smooth muscle, 100 Optic disc, 273, 277 Nails, 154 Nipple, 254 Optic nerve, 273 Naked nerve endings, 110 Nissl substance, 105 Ora serrata, 273, 277 Nasal cavity, 160 Nitric oxide, Oral cavity, 170-173 Nasolacrimal duct, 283 secretion of, 128 Oral mucosa, 170-171 Nasopharynx, 162 Nodal cells, Organ of Corti, 292-293 Natural killer (NK) cells, 75 cardiac, 122 Organelles, 6-20, 26-27 Neck Nodes of Ranvier, 108-109 Oropharynx, 176 of sperm, 228 Nodular lymphatic tissue, 134, Osmolality, of tooth, 174, 176 136, 138 normal blood values of, 67 Negative image, 12, 48, 135 Nonkeratinized stratified Osseous spiral lamina, 292, 293 Nephron, 207-216 squamous epithelium, 33 Ossicles, 287 Nerve cell membranes, Nonpyramidal cells, Ossification components of, 7, 108-109 of cerebral cortex, 116 endochondral, 60-63 Nerve endings, 110-111 Norepinephrine, 264-265 intramembranous, 59-60 Nerve process, 106 Normoblast, 83-84 zone of, 61 Nerves cells (Neurons) defined, 84 Osteoblasts, 55, 60 features of, 105, 107-108 Nose, Osteocalcin, 56 fibers of, 108-109 epithelium of, 160-161 Osteoclasts, 55 peripheral, 110 Nuclear bag fibers, 96 Osteocytes, 54-55, 60 speed of transmission of, 109 Nuclear chain fibers, 96 Osteoid, 60 synapses of, 111- 113 Nuclear envelope, 22-23 Osteonectin, 56 Neumann's sheath, 174 reforming of, 30 Osteons, 54 Neural crest, 130, 152 Nuclear lamina, 23, 28 definitive, 62 Neural retina, 277-278 Nuclear matrix, 21 primitive, 62

308 replacement of, 62 Pancreatic islets, 198-199, 202- PCO2, Osteopontin, 56 203 normal blood values of, 67 Osteoprogenitor cells, 59-60 Pancreatic polypeptide, 199 Pectinate line, 191 Otoconia, 290 Paneth cells, 186 Pectinate muscles, 120 Otolithic membrane, 290 Panniculus adiposus, 50 Pectiniform septum, 233 Outer epithelial root sheath, Papanicolaou smears, 248-249 Pelger-Huët anomaly, 76 155 Papillae Penile urethra, 218 Outer hair cells, of hair, 155 Penis, 233-235 of organ of Corti, 292-293 of lip, 170 vascularization of, 234 Outer limiting membrane, of tongue, 171 Pepsin, 180 of retina, 277-278 Papillary ducts, Pepsinogen, 180 Outer mitochondrial renal, 214 Pericardium, 121 membrane, 16 Papillary layer, Perichondrium, 52 Outer phalangeal cells, of dermis, 153 Perichoroidal space, 275 of organ of Corti, 292 Papillary muscles, 123 Pericytes, 126, 128 Outer pillar cells, 292 Parabasal cells, Perikaryon, 105 Ova, 223, 239, 241 of cervical canal, 248 Perilymph, 288 Oval window, 287 Paracrine secretion, 40, 258 Perimetrium, 244 Ovarian cycle, 242-243 Parafollicular cells, Perimysium, 90, 98 Ovary of thyroid, 262 Perineural satellite cortex of, 239 Paraganglia, 264-265 oligodendrocytes, 113-114 features of, 239-243 Parasympathetic ganglia, 117- Perineurium, 110 follicles of, 239-242 118 Perinuclear space, 22 function of, 239-243 Parathyroid hormone (PTH), Periodontal membrane, 174- structure of, 239-243 213, 260 176 Oviduct, 243-244 Parathyroids, 259-260 Periosteal bud, 61 epithelium of, 243 Parenchyma, 43 Periosteal collar, 61 features of, 243-244 Parietal cells, Periosteum, 53, 55, 60 mucosa of, 243-244 of stomach, 179-181 Peripheral couplings, muscular coat of, 244 Parietal layer, of cardiac muscle, 98 serosa of, 244 of glomerulus, 208 Peripheral membrane proteins, structure of, 243-244 Parietal pleura, 166-167, 169 8 Ovulation, 241 Parotid salivary gland, 173 Peripheral nerve endings, 110- Oxygen, Pars cavernosa, 218 111 normal blood values of, 67 Pars distalis, Peripheral nerves, 110 Oxyntic glands, 179 of hypophysis, 265-268 Perisinusoidal space, Oxyphil cells, 259-260 Pars fibrosa, hepatic, 193 Oxytocin, 245, 269 of cell, 23 Peritendineum, 97 Pachytene, 222 Pars granulosa, Peritoneal macrophages, 48 Palate, 170-171 of cell, 23 Peritubular tissue, 225 Palatine salivary glands, 170, Pars intermedia, Peroxisomes, 15-16, 27 172 of hypophysis, 265, 268 Peyer's patches, 136, 187 Palatine tonsils, 137 Pars maculata, pH, Palm, of distal renal tubule, 213 normal blood values of, 67 skin of, 149 Pars membranacea, Phagocytins 72 Pampiniform plexus, 233 of urethra, 218 Phagocytosis, 9, 14 Pancreas, 197-199 Pars nervosa, Phagosomes, 14 duct system of, 197-198 of hypophysis, 265, 269 Pharyngeal tonsil, 138 endocrine, 198-199 Pars prostatica, Pharynx, 176 exocrine, 197-198 of urethra, 218, 231 Phosphatidylcholine, 7 features of, 197-199, 202-203 Pars tuberalis, Phosphatidylethanolamine, 7 of hypophysis, 265, 268 Phosphatidylserine, 7

309 Phospholipid bilayer, 7 Pokeweed mitogen (PWM), Primary spermatocytes, 227 Phospholipids as mitogen, 75 Primary synaptic clefts, 95 in cell membrane, 7 Polar bodies, 223 Primitive erythroblasts, 79 Phosphorus, Polarity, Primitive osteons, 62 normal blood values of, 67 defined, 33 Primordial follicle, 239-240 Photoreceptors, 277-280 Pollens, Principal piece, Phytohemagglutinin (PHA), as mitogen, 75 of sperm, 228 as mitogen, 75 Polychromatophilic Prismatic rod sheath, 175 Pia mater, 117 erythroblasts, 83 Procentriole, 17, 18 Piebaldism, 152 Polychromatophilic Procentriole organizer, 18 Pigment epithelium, 277, 281 erythrocytes, 69, 84 Procollagen peptidase, 47 Pigment granules, 20, 28 Polycythemia, Proerythroblasts, 83 Pigmentation, 149, 151-152 defined, 77 Profilin, 19 Pilosebaceous apparatus, 156 Polymorphonuclear leukocytes, Progesterone, 241, 253 Pineal gland, 258-259 71-73 Prolactin, 266 Pinealocytes, 258-259 Polyribosomes, 10 Proliferation, Pinna, 287 Polysomes, 10 zone of, 61 Pinocytosis, 9 Porins, 16 Proliferative phase, Pituicytes, 269 Portal areas (triads), 192-193 of menstrual cycle, 246 Pituitary, 265-272 Portal lobule, 193 Prolymphocytes, 135 organization of, 265-269 Postcapillary venules, 140 Promegakaryocytes, 86 subdivisions of, 265-269 Posterior chamber, Promonocytes, 87 vascularization of, 265-266 of eye, 276 Promyelocytes, 84 Placenta, Posterior lingual salivary Prophase, 30 structure of, 251-253 glands, 172 meiotic, 222-223 veins of, 252 Postsynaptic membrane, 112- Prorubricytes, 83 Planum semilunatum, 290 113 Prostacyclin, 128 Plaque, Potassium secretion of, 128 epithelial, 35-36 normal blood values of, 67 Prostate, 232 Plasma, 67 transport of, 211-213, 215 Prostatic concretions, 232 Plasma cells, 48, 135 PP cells, Prostatic urethra, 218, 231 Plasmalemma, pancreatic, 199 Prostatic utricle, 231 fluid mosaic model of, 6-10 Precapillary arterioles, 126 Protein, 5 function of, 6-10, 26 Predentin, 174 absorption of, 183-184, 211 organization of, 6-10 Pregnancy 249-253 metabolism of, 195 Platelet-activating factor, cleavage, 250-251 normal blood values of, 67 secretion of, 87 fertilization, 250 synthesis of, 10 Platelets, 67, 70-71, 86 implantation, 251 Proteoglycans, 32, 46 characteristics of, 70 placenta in, 251-253 Protoplasm number of, 71 Premenstrual phase, components of, 5 structure of, 70-71 of menstrual cycle, 247 organization of, 6 Pleura, 166-167 Prepuce, 234 properties of, 5-6 connective tissue of, 167 Presynaptic membrane, 111- Protoplasmic astrocytes, 113 Pleural macrophages, 48 113 Protropin, 266 Plicae, Prickle cells, 150 Proximal convoluted tubule, of oviduct, 243 Primary follicle, 240 210-212 Plicae circulares, 182 Primary lysosomes, 13-14, 26, Proximal nail fold, 154 Plicae semilunares, 190 75 Proximal tubule, 207 Pneumonocytes, 48 Primary oocyte, 239-240 anatomy of, 210-212 Podocalyxin, 208 Primary ossification center, 60 function of, 210-212, 220 Podocytes, 208-209 Primary osteons, 60 Pseudostratified columnar Poikilocytosis, 69 Primary placental villi, 251-252 epithelium, 32, 34

310 Pseudounipolar neurons, 107- Reproduction, of hair, 155 108 cellular, 6 of nail, 154 Pubic symphysis, 53, 56-57 Reproductive system of tooth, 174-176 Pulmonary epithelial cells, 165- female, 239-257 Rouleaux, 69 166 male, 224-238 Round window, 287 Pulmonary veins, 166 Reserve cartilage, 61 Rubricytes, 83 Pulp, Residual body, 14, 228, 281 Ruffini, of tooth, 174, 175 Resorption, corpuscles of, 111 Pulp cavity, 174 zone of, 61 Ruffled border, 55 Punctum adherens, 35 Respiratory bronchioles, 164 Rugae, Pupil, 276 Respiratory system, 160-169 gastric, 178 Purkinje fibers, 122 extrapulmonary bronchi, 162- Saccule, 12, 289 Purkinje cells, 163 Saliva, 172-173 of cerebellar cortex, 116 intrapulmonary bronchi, 163- Salivary glands, 172-174 Pyloric glands, 181 164 major, 172, 173-174 Pylorus, larynx, 162 minor, 172 of stomach, 178 nose, 160 Saltatory conduction, 109 Pyramidal cells, pharynx, 162 Sarcolemma, 91, 98 of cerebral cortex, 116 pleura, 166-167 Sarcomere, Radial zone, tissue of, 164-166 of skeletal muscle, 90, 93-94 of synovial joint, 57 trachea, 162-163 Sarcoplasm, 91 Ranvier, Restricted stem cells, 82 Sarcoplasmic reticulum, 91-92 nodes of, 108-109 Rete testis, 225, 229 of cardiac muscle, 98 Receptors Reticular cells, 50, 80 Sarcotubules, 91 signal-recognition, 8-9, 112- of bone marrow, 80 Satellite cells, 108 113 of lymphatic tissue, 134 of muscle fiber, 91 types of, 110-111 Reticular connective tissue, 43, Scala tympani, 291 Rectal columns of Morgagni, 50, 80 Scala vestibuli, 291 191 Reticular fibers, 44, 50, 80 Schlemm's canal, 275 Rectoanal junction, 191 Reticular layer, Schmidt-Lantermann incisures, Rectum, 191 of dermis, 153 109 Red cells. Reticulocytes, 69, 84 Schwann cells, 108-109 See Erythrocytes 68-70 Retina, 273, 277-281 Sclera, 273-274 Red marrow, 80 anatomy of, 277-281 Scleral spur, 274 Red muscle fibers, 94-95 melanin in, 281 Scrotum, 233 Red pulp, vascularization of, 281 Sebaceous glands, 156 of spleen, 141 Rhodopsin, 278 Sebum, 156 Regulated exocytosis, 10 Ribonuclease, 197 Secondary follicle, 240-241 Reinke, Ribonucleoprotein, 10 Secondary lysosomes, 14, 26 crystals of, 224 in mitochondria, 17 Secondary oocyte, 241, 250 Relaxin, 245 in reticulocytes, 69 Secondary ossification center, Releasing hormones, 268, Ribosomal RNA, 23 61 Renal capsule, 206 Ribosomes, 10, 27 Secondary placental villi, 252 Renal column, 206 in endoplasmic reticulum, 10- Secondary spermatocytes, 227 Renal corpuscle, 207-210 11 Secondary synaptic clefts, 95 Renal interstitium, 215 RNA (ribonucleic acid), Secondary tympanic Renal lobe, 207 ribosomal, 10 membrane, 287 Renal lobule, 207 types of, 10 Secretin, 185, 198 Renal papilla, 206 Rod bipolar cells 280 Secretion Renal pelvis, 206 Rods, defined, 5, 39-41 Renal sinus, 206 structure of, 278-279 glandular, 39-41 Renin, 216 Root Secretory granules, 12, 28

311 Secretory phase, Simple cuboidal epithelium, 32, function of, 99, 103-104 of menstrual cycle, 246-247 34 myofilaments of, 100-101 Secretory piece, 173, 187 Simple epithelia, organization of, 101 Secretory sheet, 39-40, 178 defined, 32 phasic, 101, 230 Semen, 232-233 types of, 32-33 tonic, 100 Semicircular canals, 289-291 Simple glands, 39-40 Socket, Seminal fluid, 232-233 defined, 40 of tooth, 174 Seminal vesicles, 231 Simple squamous epithelium, Sodium Seminiferous tubules, 224-227 32-34 normal blood values of, 67 Sensory ganglia, 108 Simple tubular glands, 39-40 transport of, 184, 211-214 Sensory nerve endings, 96-97, Sinoatrial node, 122 Soft keratin, 151 110-111, 173 Sinuses, Somatic efferent fibers, 110 Sensory nerves, 105, 108, 110 paranasal, 160 Somatodendritic synapse, 111 Septal cells, Sinusoids Somatosomatic synapse, 111 pulmonary, 165 of marrow, 81 Somatostatin, 181, 185, 199, Septula testis, 224 vascular, 127 268 Septum membranaceum, 123 Skeletal muscle, 89-97 Somatotrophs, 266 Serosa fibers of, 94-95 Somatotropin (GH), 266 of digestive tract, 170 innervation of, 95-97 Sound, of large intestine, 190 junction with tendons, 97 transmission of, 287-288, of small intestine, 189 organization of, 89-97 291-293 of stomach, 182 Skin, Space of Disse, 193 Serotonin, 181, 259 appendages of, 153-157 Special connective tissue, 43, Serous demilunes, dermis of, 153 50, 52-66, 67-78, 79-88 of salivary glands, 174 epidermis of, 149-153 Specific granules, Serous glands, 41, 173 pigmentation of, 151-152 in neutrophils, 72, 85 of tongue, 171 structure of, 149-159 Spectrin, 68 Sertoli cells, 225-226 vascularization of, 153 Spermatic cord, 233 Serum, 67 Slit pores, 208-209 Spermatids, 227 Sex chromosomes, 22 Slow twitch muscle fibers, 94- Spermatocytes, 227 Shaft, 95 Spermatogenesis, 226-227 of hair, 154 Small arteries, 125 Spermatogenic cells, 225, 226- Sharpey's fibers, 55, 175 Small granule cells, 227 Sheath of Schwann, 108-109 of trachea, 162 Spermatogonia, 226-227 Sheets, Small intestine, 182-189 Spermatozoa, epithelial, 32-34 absorption in, 182 structure of, 228 Short cells, epithelial lining of, 182-186 Spermiogenesis 227-228 of trachea, 163 features of, 182-189 Sphenoid sinuses, 160 Sickle cell anemia, 69 glands of, 183, 186 Spherocytes, 69 hemoglobin of, 70 ileocecal junction of, 189 Sphincter Siderocytes, 69 lamina propria of, 186-187 anal, 191 Signal-recognition particle mucosa of, 182-187 of capillary, 126 (SRP), 11 muscularis externa of, 188- esophageal, 177 Signal-recognition receptor, 11 189 of iris, 276 Simple branched acinar glands, muscularis mucosae of, 187 of urethra 218 39-40 serosa of, 189 Sphingomyelin, 7, 15 Simple branched tubular submucosa of, 188 Spinal cord, glands, 39-40, 178-179, 181 Small veins, 129 gray matter of, 115 Simple coiled tubular glands, Smooth endoplasmic reticulum neurons of, 115 39-40, 156-157 (SER), 11, 26 Spinal ganglia, 107-108 Simple columnar epithelium, Smooth muscle, 99-101 Spindle capsules, 96-97 32, 34, 182 fibers of, 99-100

312 Spine, Stratified cuboidal epithelium, Superficial squamous cells, joints of, 57 33-34 of cervical canal, 249 Spiral crest, 292 Stratified epithelium Supporting (sustentacular) cells, Spiral ganglion, defined, 32-33, 34 160, 172, 292 of cochlea, 291 types of, 33-34 Suprachoroid lamina, 275 Spiral lamina, Stratified squamous epithelium, Surfactant, 165 of cochlea, 291 33, 34 Sutures, Spiral ligament, Stratum basale, 149-150 of skull, 56 of cochlea, 291 endometrial, 245-247 Sweat glands, 156-157 Spiral limbus, 293 Stratum corneum, 151 Sympathetic chain ganglia, 108, Spiral prominence, 292 Stratum disjunctum, 151 117 Spleen, 140-142 Stratum functionale, 245-247 Symphyses, 56 embryonic hemopoiesis in, Stratum germinativum, 149-150 Symport mechanism, 8 79 Stratum granulosum, 150, 240 Synapses hemopoietic cells in, 79 Stratum lucidum, 150 anatomy of, 111-113 hemosiderin-containing cells Stratum Malphigii, 150 function of, 111-113 in, 141 Stratum spinosum, 150 types of, 111-113 red pulp of, 140-141 Stratum vasculare, Synapsis, 222 structure of, 140-142 of myometrium, 245 Synaptic cleft, 95, 112-113 vascularization of, 141-142 Stria vascularis, 288, 291-292 Synaptic troughs, 95 white pulp of, 140-141 Striated border, 37, 182-183 Synaptic vesicles, 96, 111-113 Splenic cord, 141 Striated ducts, Synchondrosis, 56 Splenic follicles, 140 of salivary glands, 173-174 Syncytial trophoblast, 251 Splenic sinusoids, 140-141 Striated muscle. Syndesmosis, 56 Spongy bone See Skeletal muscle, 89-97 Synovial cells, 58 structure of, 53 Stroma Synovial fluid, 59, 65-66 Squames, 151 defined, 43 Synovial intima, 58 Squamous, of iris, 276-277 Synovial joints, 57-58 defined, 32-33 Subarachnoid space, 117 formation of, 63 Stapedius, 288 Subcapsular limiting plate, Synovial membrane, 58-59 Stapes, 287-288 hepatic, 193 Synovial villi, 58 Stellate cells Subcapsular sinus, T-helper cells, 74 of cerebellar cortex, 116 of lymph node, 139 T-lymphocytes, of cerebral cortex, 116 Subdural space, 117 types of, 74-75 Stem cells, 81-83 Subepicardial layer, 121 T-suppressor cells, 74 Stereocilia, 37, 229-230 Sublingual salivary glands, 174 T-system, Stigma, 241 Submandibular salivary gland, in skeletal muscle, 92 Stomach, 178-182 173-174 T-tubules cardiac glands of, 178 Submetacentric, in cardiac muscle, 98 epithelium of, 178 defined, 22 in skeletal muscle, 92 gastric glands of, 179-181 Submucosa Tables, junction with esophagus, 178 of digestive tract, 170 of bone, 53 junction with intestine, 178 of esophagus, 177 Tactile discs of Merkel, 110, lamina propria of, 181 of large intestine, 190 111 mucosa of, 178-181 of small intestine, 188 Taeniae coli, 190 muscularis externa of, 181 of stomach, 181 Tail, muscularis mucosae of, 181 Subsarcolemmal cisternae, 98 of sperm, 228 pyloric glands of, 181 Substance P, 185 Tangential zone, serosa of, 182 Substantia propria, 274 of synovial joint, 57 submucosa of, 181 Subsynaptic web, 112 Target cells, 69, 258 Stratified columnar epithelium, Sugars, Tarsal glands, 282 33-34 absorption of, 184, 211 Tarsal plate, 282

313 Taste buds, 171-172 Thrombospondin, 46, 128 Triiodothyronine, 261 Taste hairs, 172 Thymic corpuscles, 143 Tripartite single-pass integral Taste pores, 172 Thymus, membrane proteins, 8 Tay-Sachs disease, 14-15 cortex of, 143 Triplets Tectorial membrane, 292-293 growth of, 145 in centriole, 17 Teeth, medulla of, 143 of microtubules, 38 structure of, 174-176 structure of, 142-145 tRNA, 10, 23 Tela choroidea, 114 vascularization of, 143-144 Trophoblast, 251 Telodendria, 106 Thymus-dependent area, syncytial, 251-253 Telogen, of lymph node, 139 Tropocollagen, 44 phase of hair growth, 156 Thyrocalcitonin, 262 Tropomyosin, Telomere, 29 Thyroglobulin, 261 in skeletal muscle, 93 Telophase, 30 Thyroid, Troponin complex, 93 meiotic, 223 cells of, 260, 262 Trypsin, 197 Tendon organs, 97 function of, 261-262 Tubular cristae, 16, 224 Tendons, 50, 97 structure of, 260-262 Tubular digestive tract, 176-191 junction with muscles, 97 Thyroid follicles, 260 Tubules, 10 Tensor tympani, 288 Thyroid-stimulating hormone, glandular, 40 Terminal bars, 34, 183 261-262, 267 Tubuli recti, 229 Terminal boutons, 111 Thyrotrophs, 267 Tubulin, 19 Terminal bronchioles, 163 Thyrotropin, 267 Tubulovesicular system, Terminal cisternae, Thyroxin, 261 of stomach, 179 in skeletal muscle, 91-92 Tight junction, 35 Tumor necrosis factor (TNF), Terminal hair, 154 Tomes' fibers, 174 74 Terminal web, 37, 182 Tongue, 171 Tunica adventitia, 123-124 Territorial matrix, muscles of, 171 of arteries, 124-125 of cartilage, 52 Tonofilaments, 36, 143 of capillaries, 126 Tertiary placental villi, 252 Tonsils, 137-138 of lymphatic vessels, 131 Testes, structure of, 137-138 of veins, 128-129 features of, 224-225 Toxic granulation, 76 Tunica albuginea spermiogenesis in, 227-228 Trabeculae, 53, 117, 138, 140 in female, 239 Testosterone, 224 Trabeculae carnea, 120 in male, 224, 233 Tetraiodothyronine, 261 Trabecular meshwork, 275 Tunica dartos, 233 Theca externa, 240 Trabecular sinus, Tunica intima, 123 Theca folliculi, 240 of lymph node, 139 of arteries, 124-125 Theca interna, 240 Trachea, 162-163 of capillaries, 126 Theca lutein cells, 241 epithelium of, 162 of lymphatic vessels, 131 Thick filaments features of, 162-163 of veins, 129 of skeletal muscle, 92-94 Trail endings, 96 Tunica media, 123 of smooth muscle, 100 Trans-Golgi network, 12 of arteries, 124-125 Thick segment, Transcytosis, 9, 124 of lymphatic vessels, 131 of loop of Henle, 212-214 Transitional epithelium, 33-34, of veins, 128-129 Thick skin, 149 217, 231 Tunica vaginalis, 224 Thin filaments Transitional zone, Tunica vasculosa, 224 of skeletal muscle, 93-94 of synovial joint, 57 Tympanic cavity, 287-288 of smooth muscle, 100 Transmembrane proteins, 7 Tympanic membrane, 288 Thin segment, Transport vesicles, 12 Type I hair cells, 289-291 of loop of Henle, 207, 212- Trichohyalin granules, 155 Type I muscle fibers, 94 214 Triglycerides Type I pneumonocytes, 165 Thin skin, 149 absorption of, 183 Type II hair cells, 289-291 Threshold of excitation, 112 normal blood values of, 67 Type II pneumonocytes, 165- Thrombopoiesis, 86 Trigona fibrosi, 123 166

314 Type IIA muscle fibers, 95, 177 Vasoactive intestinal Yellow marrow, 80 Type IIB muscle fibers, 95 polypeptide, 185 Yolk sac, Type IIC muscle fibers, 95 Vater-Pacini, erythropoiesis in, 79 Unicellular glands, 39-40 corpuscles of, 111 Z line Unilocular fat, 50 Veins, of cardiac muscle, 98 Unipolar neurons, 107 features of, 128-129 of skeletal muscle, 90 Uniport mechanism, 8 Vellus hairs, 154 Zeis, Unit fibrils, 44 Venae cavae, glands of, 282 Unit membrane, 6 features of, 129 Zona ciliaris, 276, 282 of nuclear envelope, 22-23 Venous sinuses Zona fasciculata, Unmyelinated fiber, 108-109 of marrow, 81 adrenal, 262-263 Upper esophageal sphincter, of nose, 160 Zona glomerulosa, 177 Venules, 128 adrenal, 262 Ureter, Vermillion border, 170 Zona pellucida, 240, 250-251 features of, 217-218 Vessel layer, Zona reticularis, Urethra, of choroid, 275 adrenal, 263 female, 218 Vestibular glands, 249 Zone of calcification and cell male, 218 Vestibular labyrinth, 289-291 death, 61 Uric acid, Vestibular membrane, Zone of maturation and normal blood values of, 67 of cochlea, 291-292 hypertrophy, 61 Urinary pole, 208 Vestibule, 249 Zone of ossification, 61 Urinary system, Villi Zone of proliferation, 61 extrarenal passages, 217-218 intestinal, 182-183 Zone of reserve cartilage, 61 features of, 206-221 of placenta, 251-252 Zone of resorption, 61 kidneys, 206-217 Vimentin, 19, 27, 99 Zonula adherens, 35 Uriniferous tubules, 207 Vinculin, 19 Zonula occludens, 35 Uroepithelium, 33, 217 Visceral efferent fibers, 110 Zonule fibers, 276, 282 Uroguanylin, 186 Visceral efferent motor Zygotene, 222 Uterine glands, 245-247 neurons, 108 Zymogen cells, Uterus, Visceral layer, of stomach, 180 endometrium of, 245-247 of glomerulus, 208-209 Zymogen granules, 180, 197 hormonal effects on, 246-247 Visceral pleura, 166-167, 169 myometrium of, 244-245 Vitamin A, 281 perimetrium of, 244 Vitamin D, 149, 159, 206, 260 structure of, 244-248 Vitamins, veins of, 245-246 storage of, 195 Utricle, 289 Vitreal portion, Uvea, 273, 275-277 of rod cells, 279 Vagina, Vitreous body, 273, 282 features of, 248 Volkmann's canals, 54 junction with cervix, 247-249 Vomeronasal organs, 161 Valves von Ebner's glands, 171-172 heart, 123 Waldeyer's ring, 137 in lymph vessels, 131 Water, in veins, 129 in protoplasm, 5 Vas deferens, White fat, 50 features of, 230 White matter, 107 Vasa vasorum, 124, 129 of brain, 115-117 Vascular compartment, spinal, 115 of bone marrow, 81 White pulp, Vascular pole, of spleen, 140-141 renal, 208 Woven bone, 60

315