Tissue Level of Organization 81

TISSUES

OUTLINE

4.1 Epithelial Tissue 81 4.1a Characteristics of Epithelial Tissue 81 4.1b Functions of Epithelial Tissue 82 4 4.1c Specialized Structure of Epithelial Tissue 82 4.1d Classification of Epithelial Tissue 84 4.1e Types of Epithelium 85 4.1f Glands 92 Tissue 4.2 Connective Tissue 95 4.2a Characteristics of Connective Tissue 95 4.2b Functions of Connective Tissue 95 4.2c Development of Connective Tissue 96 Level of 4.2d Classification of Connective Tissue 98 4.3 Body Membranes 108 4.4 Muscle Tissue 109 Organization 4.4a Classification of Muscle Tissue 109 4.5 Nervous Tissue 111 4.5a Characteristics of Neurons 112 4.6 Tissue Change and Aging 112 4.6a Tissue Change 112 4.6b Tissue Aging 113

MODULE 3: TISSUES

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he human body is composed of trillions of cells, which are orga- Epithelial (ep-i-thē ́lē -ă l; epi = upon, thēl ē = nipple) tissue T nized into more complex units called tissues. Tissues are groups of covers or lines every body surface and all body cavities; thus it similar cells and extracellular products that carry out a common func- forms both the external and internal lining of many organs, and tion, such as providing protection or facilitating body movement. The it constitutes the majority of glands. An epithelium (pl., epithelia) study of tissues and their relationships within organs is called histology. is composed of one or more layers of closely packed cells between There are four principal types of tissues in the body: epithe- two compartments having different components. There is little to lial tissue, connective tissue, muscle tissue, and nervous tissue. no extracellular matrix between epithelial cells; additionally, no Immediately following the connective tissue discussion, section blood vessels penetrate an epithelium. 4.3 (Body Membranes) has been inserted because these structures are composed of an epithelial sheet and an underlying connective 4.1a Characteristics of Epithelial Tissue tissue layer. Tissues are formed from the three primary germ layers All epithelia exhibit several common characteristics: (ectoderm, mesoderm, and endoderm). The four tissue types vary in terms of the structure and function of their specialized cells, as ■ Cellularity. Epithelial tissue is composed almost entirely of well as the presence of an extracellular matrix (mā triks;́ matrix = cells. The cells of an epithelium are bound closely together womb) that not only is produced by the cells but surrounds them. by different types of intercellular junctions (discussed later). The extracellular matrix is composed of varying amounts of water, A minimal amount of extracellular matrix separates the protein fibers, and dissolved macromolecules. Its consistency cells in an epithelium. ranges from fluid to quite solid. Epithelial, muscle, and nervous ■ Polarity. Every epithelium has an apical (ā p ́i-kă l) surface tissues have relatively little matrix between their cells. In contrast, (free or top surface), which is exposed either to the external connective tissue types contain varying amounts of extracellular environment or to some internal body space, and lateral matrix that differ in the volume of space occupied, the relative surfaces having intercellular junctions. Additionally, each amounts of the extracellular matrix components, and the consis- epithelium has a basal (bā ś ă l) surface (fixed or bottom tency (fluid to solid) of the extracellular matrix. surface) where the epithelium is attached to the underlying As we examine each of the four classes of tissues in this chapter, connective tissue. it may help you to refer to table 4.1, which summarizes their ■ Attachment. At the basal surface of an epithelium, the characteristics and functions. This chapter is a transition between epithelial layer is bound to a thin basement membrane, chapter 2, which investigated the nature of cells, and later chapters, a complex molecular structure produced by both the which examine tissue interactions in organs and organ systems. epithelium and the underlying connective tissue. ■ Avascularity. All epithelial tissues lack blood vessels. Epithelial cells obtain nutrients either directly across the 4.1 Epithelial Tissue apical surface or by diffusion across the basal surface from the underlying connective tissue. Learning Objectives: ■ Innervation. Epithelia are richly innervated to detect changes in 1. Identify the structure and function of each type of the environment at a particular body or organ surface region. epithelial tissue. Most nervous tissue is in the underlying connective tissue. 2. Explain where each type of epithelial tissue is found in ■ High regeneration capacity. Because epithelial cells have an the body. apical surface that is exposed to the environment, they are 3. Describe the specialized features of an epithelium. frequently damaged or lost by abrasion. However, damaged 4. Classify exocrine glands. or lost epithelial cells generally are replaced as fast as they

Table 4.1 Tissue Types Type General Characteristics General Functions Primary Example Subtypes and Their Locations Germ Layer Derivative Epithelial tissue Cellular, polar, attached, Covers surfaces; lines Ectoderm, Simple columnar epithelium: Inner lining of avascular, innervated, high insides of organs and body mesoderm, digestive tract regeneration capacity cavities endoderm Stratiﬁ ed squamous epithelium: Epidermis of skin Transitional epithelium: Inner lining of urinary bladder Connective tissue Diverse types; all contain cells, Protects, binds together, Mesoderm Adipose connective tissue: Fat protein ﬁ bers, and ground and supports organs Dense regular connective tissue: Ligaments substance and tendons Dense irregular connective tissue: Dermis of skin Hyaline cartilage: Articular cartilage in some joints Fluid connective tissue: Blood Muscle tissue Contractile; receives stimulation Facilitates movement of Mesoderm Skeletal muscle: Muscles attached to bones from nervous system and/or skeleton or organ walls Cardiac muscle: Muscle layer in heart endocrine system Smooth muscle: Muscle layer in digestive tract Nervous tissue Neurons: Excitable, high Neurons: Control activities, Ectoderm Neurons: Brain and spinal cord metabolic rate, extreme process information Glial cells: Brain and spinal cord longevity, nonmitotic Glial cells: Support and Glial cells: Nonexcitable, mitotic protect neurons

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are lost because epithelia have a high regeneration capacity. lamina, which contains protein fibers and carbohydrates. Together, The continual replacement occurs through the mitotic these components of the basement membrane strengthen the attach- divisions of the deepest epithelial cells (called stem cells), ment and form a selective molecular barrier between the epithelium which are found within the epithelium near its base. and the underlying connective tissue. The basement membrane has the following functions: 4.1b Functions of Epithelial Tissue ■ Providing physical support for the epithelium Epithelia may have several functions, although no single epithe- ■ Anchoring the epithelium to the connective tissue lium performs all of them. These functions include: ■ Acting as a barrier to regulate the movement of large ■ Physical protection. Epithelial tissues protect both exposed molecules between the epithelium and the underlying and internal surfaces from dehydration, abrasion, and connective tissue destruction by physical, chemical, or biological agents. Intercellular Junctions ■ Selective permeability. All epithelial cells act as “gatekeepers,” in that they regulate the movement of materials Epithelial cells are strongly bound together by specialized connec- into and out of certain regions of the body. All substances tions in the plasma membranes of their lateral surfaces called inter- that enter or leave the body must pass through an epithelium. cellular junctions. There are four types of junctions: tight junctions, Sometimes an epithelium exhibits a range of permeability; adhering junctions, desmosomes, and gap junctions (figure 4.1b). that is, it may be relatively impermeable to some substances, Each of these types of junctions has a specialized structure. while at the same time promoting and assisting the passage of other molecules by absorption or secretion. The structure Tight Junctions A tight junction, also called a zonula (zō ń ū lă) and characteristics of an epithelium may change as a result occludens (“occluding belt”), encircles epithelial cells near their of applied pressure or stress; for example, walking around apical surface and completely attaches each cell to its neighbors. without shoes may increase the thickness of calluses on the Plasma membrane proteins among neighboring cells fuse, so the bottom of the feet, which could alter or reduce the movement apical surfaces of the cells are tightly connected everywhere around of materials across the epithelium. the cell. This seals off the intercellular space and prevents sub- ■ Secretions. Some epithelial cells, called exocrine glands, stances from passing between the epithelial cells. The tight junction are specialized to produce secretions. Individual gland cells forces almost all materials to move through, rather than between, may be scattered among other cell types in an epithelium, the epithelial cells in order to cross the epithelium. Thus, epithelial or a large group of epithelial secretory cells may form a cells control whatever enters and leaves the body by moving across gland to produce specific secretions. the epithelium. For example, in the small intestine, tight junctions ■ Sensations. Epithelial tissues contain some nerve endings prevent digestive enzymes that degrade molecules from moving to detect changes in the external environment at their between epithelial cells into underlying connective tissue. surface. These sensory nerve endings and those in the underlying connective tissue continuously supply Adhering Junctions An adhering junction, also called a zonula information to the nervous system concerning touch, adherens (“adhesion belt”), is formed completely around the cell. pressure, temperature, and pain. For example, receptors This type of junction occurs when extensive zones of microfilaments in the epithelium of the skin respond to pressure by extend from the cytoplasm into the plasma membrane, forming a stimulating adjacent sensory nerves. Additionally, supporting and strengthening belt within the plasma membrane several organs contain a specialized epithelium, called a that completely encircles the cell immediately adjacent to all of its neuroepithelium, that houses specific cells responsible for neighbors. Typically, adhering junctions are located deep to the tight the senses of sight, taste, smell, hearing, and equilibrium. junctions; the anchoring of the microfilament proteins within this belt provides the only means of junctional support for the apical surface of the cell. The ultra-strong tight junctions are needed only near the apical WHAT DO YOU THINK? surface and not along the entire length of the cell. Once neighboring ●1 Why do you think epithelial tissue does not contain blood vessels? cells are fused together by the tight junctions near the apical surface, Can you think of an epithelial function that could be compromised the adhering junctions support the apical surface and provide for a if blood vessels were running through the tissue? small space between neighboring cells in the direction of the basal surface. Thus, the junction affords a passageway between cells for 4.1c Specialized Structure of Epithelial Tissue materials that have already passed through the apical surface of the Because epithelial tissues are located at all free surfaces in the body, epithelial cell and can then exit through the membranes on the lateral they exhibit distinct structural specializations. An epithelium rests surface and continue their journey toward the basement membrane. on a layer of connective tissue and adheres firmly to it which secures the epithelium in place and prevents it from tearing. Between the Desmosomes A desmosome (dez ḿ ō -sō m; desmos = a band, soma = epithelium and the underlying connective tissue is a thin extracel- body), also called a macula adherens (“adhering spot”), is like a but- lular layer called the basement membrane. The basement mem- ton or snap between adjacent epithelial cells. Each cell contributes brane can be seen as a single layer beneath epithelium using the half of the complete desmosome. It is a small region that holds cells light microscope (figure 4.1a). However, it consists of three layers together and provides resistance to mechanical stress at a single when observed using an electron microscope: the lamina lucida, the point, but it does not totally encircle the cell. In contrast to tight junc- lamina densa, and the reticular lamina. The two laminae closest to tions, which encircle the cell to secure it to its neighbors everywhere the epithelium contain collagen fibers as well as specific proteins around its periphery, the desmosome attaches a cell to its neighbors and carbohydrates some of which are secreted by the epithelial only at potential stress points. The neighboring cells are separated cells. Cells in the underlying connective tissue secrete the reticular by a small space that is spanned by a fine web of protein filaments.

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Apical (free) surface Epithelium

Lateral surface Basement membrane

Basal surface

Connective tissue

Blood vessel

(a) Epithelium–connective tissue junction

Tight junction Membrane protein Plasma membrane

Microfilament Hemidesmosome Adhering junction Desmosome Protein filaments Gap junction Protein plaque Pore Intercellular space Intermediate filaments Connexon Plasma membrane Adjacent plasma Intercellular space membranes (b) Types of intercellular junctions

Figure 4.1 Polarity and Intercellular Junctions in an Epithelium. An epithelium exhibits polarity and has intercellular junctions only on the lateral surfaces of its individual cells. ( a) The apical surface is the cell's free surface exposed to a body cavity, an organ lumen, or the exterior of the body. The basal surface of the cell adheres to the underlying connective tissue by a basement membrane. Hemidesmosomes help anchor cells to the basement membrane in some epithelia. ( b) The lateral surfaces of the cell contain intercellular junctions. Types of intercellular junctions are tight junctions, adhering junctions, desmosomes, and gap junctions.

These filaments anchor into a thickened protein plaque located at the traveling between neighboring cells. Ions, glucose, amino acids, and internal surface of the plasma membrane. On the cytoplasmic side of other small solutes can pass directly from the cytoplasm of one cell each plaque, intermediate filaments of the cytoskeleton penetrate the into the neighboring cell through these channels. The flow of ions plaque to extend throughout the cell the support and strength supplied between cells coordinates such cellular activities as the beating of between the cells by the desmosome. The basal cells of some epithelial cilia. Gap junctions are also seen in certain types of muscle tissue, tissue exhibit structures called hemidesmosomes, half-desmosomes where they help coordinate contraction activities. that anchor them to the underlying basement membrane. WHATW DID YOU LEARN? Gap Junctions A gap junction is formed across the intercellular gap between neighboring cells. This gap (about 2 nanometers in ●1 Describe the basement membrane, its origins, and its functions. length) is bridged by structures called connexons (kon-neks on).́ ●2 Which intercellular junction ensures that epithelial cells act as Each connexon consists of six transmembrane proteins, arranged “gatekeepers”? in a circular fashion to form a tiny, fluid-filled tunnel or pore. ●3 What type of intercellular junction provides resistance to Gap junctions provide a direct passageway for small molecules mechanical stress at a single point?

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Apical surface Nucleus

Lateral surface Figure 4.2 Basement membrane Basal surface Squamous cell Classification of Epithelia. Simple epithelium Two criteria are used to classify epithelia: the number of cell Nucleus layers and the shape of the cell Apical surface at the apical surface. (a) An epithelium is simple if it is one Cuboidal cell cell layer thick, and stratified if it has two or more layers of cells. (b) Epithelial cell shapes include squamous (thin, flattened cells), Lateral surface cuboidal (cells about as tall as they are wide), and columnar Nucleus (cells taller than they are wide). Basement membrane Basal surface

Stratified epithelium Columnar cell (a) Epithelium classified by layers (b) Epithelium classified by shapes

4.1d Classification of Epithelial Tissue them do not reach its apical surface. Those cells that do reach the The body contains many different kinds of epithelia, and the clas- apical surface often bear cilia to move mucus along the surface. sification of each type is indicated by a two-part name. The first This so-called ciliated pseudostratified epithelium lines the nasal part of the name refers to the number of epithelial cell layers, and cavity and the respiratory passageways. the second part describes the shape of the cells at the apical surface of the epithelium. Classification by Cell Shape Epithelia are also classified by the shape of the cell at the apical Classification by Number of Cell Layers surface. In a simple epithelium, all of the cells display the same Epithelia may be classified based on number of cell layers as shape. However, in a stratified epithelium there is usually a dif- either simple or stratified (figure 4.2a). A simple epithelium is ference in cell shape between the basal layer and the apical layer. one cell layer thick, and all of these epithelial cells are in direct Figure 4.2b shows the three common cell shapes observed in contact with the basement membrane. Often, the apical surface epithelia: squamous, cuboidal, and columnar. (Note that the cells is covered by a thin layer of fluid or mucus to prevent desicca- in this figure all appear hexagonal when looking at their apical tion and help protect the cells from abrasion or friction. A simple surface, or “en face”; thus these terms describe the cells’ shapes epithelium is found in areas where stress is minimal and where when viewed laterally, or from the side.) filtration, absorption, or secretion is the primary function. Such Squamous (skwā ḿ ŭ s; squamosus = scaly) cells are flat, wide, locations include the linings of the air sacs in the lungs, intes- and somewhat irregular in shape. The nucleus looks like a flattened tines, and blood vessels. disc. The cells are arranged like irregular, flattened floor tiles. A stratified epithelium contains two or more layers of Cuboidal (kū -boy ́dă l; kybos = cube, eidos = resemblance) cells are epithelial cells. Only the cells in the deepest (basal) layer are about as tall as they are wide. The cells do not resemble perfect in contact with the basement membrane. A stratified epithelium “cubes,” because they do not have squared edges. The cell nucleus resembles a brick wall, where the bricks in contact with the is spherical and located within the center of the cell. Columnar ground represent the basal layer and the bricks at the top of the (kol ́ŭ m ń ă r; columna = column) cells are slender and taller than wall represent the apical layer. The multiple cell layers of a strati- they are wide. The cells look like a group of hexagonal columns fied epithelium make it strong and capable of resisting stress and aligned next to each other. Each cell nucleus is oval and usually protecting underlying tissue. In contrast to a simple epithelium, oriented lengthwise and located in the basal region of the cell. a stratified epithelium is found in areas likely to be subjected to Another shape that occurs in epithelial cells is called tran- abrasive activities or mechanical stresses, where two or more sitional (tran-zish ́ŭ n-ă l; transitio = to go across). These cells can layers of cells are better able to resist this wear and tear (e.g., the readily change their shape or appearance depending upon how internal lining of the esophagus, pharynx, or vagina). Cells in stretched the epithelium becomes. They are found where the the basal layer continuously regenerate as the cells in the more epithelium cycles between distended and relaxed states, such as superficial layer are lost due to abrasion or stress. in the lining of the bladder, which fills with urine and is later Finally, a pseudostratified (soo ́dō -strat ́i-fı̄ d; pseudes = emptied. When the transitional epithelium is in a relaxed state, false, stratum = layer) epithelium looks layered (stratified) the cells are described as polyhedral, which means “many-sided” because the cells’ nuclei are distributed at different levels and reflects the ranges in shape that are possible in this type of between the apical and basal surfaces. But although all of these epithelium. When transitional epithelium is stretched, the sur- epithelial cells are attached to the basement membrane, some of face cells resemble squamous cells.

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yolk. This epithelium is extremely delicate and highly special- Study Tip! ized to allow rapid movement of molecules across its surface by diffusion, osmosis, or filtration. Simple squamous epithelium is In your anatomy lab, you may be asked to identify a particular found only in protected regions where moist surfaces reduce fric- type of epithelium under the microscope. This can be a daunting task, tion and abrasion. For example, in the lining of the lung air sacs especially for a student who has never examined tissues under the (alveoli), the thin epithelium is well suited for the exchange of microscope before. Ask the following questions to help identify each oxygen and carbon dioxide between the blood and inhaled air. type of epithelium: This type of epithelium is also found lining the lumen (inside 1. Is the epithelium one layer or many layers thick? If it is one space) of blood vessel walls, where it allows for rapid exchange layer thick, you are looking at some type of simple epithelium. of nutrients and waste between the blood and the interstitial fluid If it is many layers thick, you are looking at some type of stratified epithelium or one of the unusual types of epithelium surrounding the blood vessels. (such as pseudostratified or transitional). Simple squamous epithelia that line closed internal body cavities and all circulatory structures have special names. The 2. What is the shape of the cells? If the cells (or at least the apical layer of cells) are flattened, you are looking at some simple squamous epithelium that lines the lumen of the blood type of squamous epithelium. and lymphatic vessels and the heart and its chambers is termed endothelium (en-dō -thē -ĺ ē -ŭ m; endon = within, thele = nipple). Your answer to question 1 gives you the first part of the epithelium’s Mesothelium (mez-ō -thē ́lē -ŭ m; mesos = middle) is the simple name (e.g., simple). Your answer to question 2 gives you the second squamous epithelium of the serous membrane (discussed part of the epithelium’s name (e.g., squamous). Put these answers in chapter 1) that lines the internal walls of the pericardial, together, and you will have the name of the tissue (simple squamous pleural, and peritoneal cavities as well as the external sur- epithelium, in this case). faces of the organs within those cavities. Mesothelium gets its name from the primary germ layer mesoderm, from which it is derived. 4.1e Types of Epithelium Using the classification system just described, epithelium can be Simple Cuboidal Epithelium broken down into the primary types shown in table 4.2. In this A simple cuboidal epithelium consists of a single layer of cells section, we describe the characteristics of these types of epithelium that are as tall as they are wide (table 4.3b). A spherical nucleus and show how each appears under the microscope. is located in the center of the cell. A simple cuboidal epithelium functions primarily to absorb fluids and other substances across Simple Squamous Epithelium its apical membrane and to secrete specific molecules. It forms the A simple squamous epithelium consists of a single layer of flat- walls of kidney tubules, where it participates in the reabsorption of tened cells (table 4.3a). When viewed “en face,” the irregularly nutrients, ions, and water that are filtered out of the blood plasma. shaped cells display a spherical to oval nucleus, and they appear It also forms the ducts of exocrine glands, which secrete materials. tightly bound together in a mosaic-like pattern. Each squamous Simple cuboidal epithelium covers the surface of the ovary and cell resembles a fried egg, with the nucleus representing the also lines the follicles of the thyroid gland.

Table 4.2 Types of Epithelium Type Structure SIMPLE EPITHELIUM One cell layer thick; all cells are tightly bound; all cells attach directly to the basement membrane Simple squamous One layer of ﬂ attened cells Simple cuboidal One layer of cells about as tall as they are wide Simple columnar, nonciliated One layer of nonciliated cells that are taller than they are wide; cells may contain microvilli Simple columnar, ciliated One layer of ciliated cells that are taller than they are wide

STRATIFIED EPITHELIUM Two or more cell layers thick; only the deepest layer directly attaches to the basement membrane Stratifi ed squamous, keratinized Many layers thick; cells in surface layers are dead, fl at, and fi lled with the protein keratin Stratifi ed squamous, nonkeratinized Many layers thick; no keratin in cells; surface layers are alive, fl at, and moist Stratifi ed cuboidal Two or more layers of cells; apical layer of cells is cuboidal-shaped Stratifi ed columnar Two or more layers of cells; cells in apical layer are columnar-shaped

OTHER TYPES OF EPITHELIUM Cell layers vary, from single to many Pseudostratiﬁ ed columnar One layer of cells of varying heights; all cells attach to basement membrane; ciliated form contains cilia and goblet cells; nonciliated form lacks cilia and goblet cells Transitional Multiple layers of polyhedral cells (when tissue is relaxed) or ﬂ attened cells (when tissue is distended); some cells may be binucleated

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Table 4.3 Simple Epithelia

Amnion Kidney tubules

Simple squamous cell Basement membrane

Lumen of kidney tubule

Simple cuboidal cell

LM 400x LM 1000x

Simple squamous cell Basement membrane

Lumen of kidney tubule

Simple cuboidal cell

(a) Simple Squamous Epithelium (b) Simple Cuboidal Epithelium

Structure Single layer of thin, ﬂ at, irregularly shaped cells Structure Single layer of cells about as tall as they are wide; resembling ﬂ oor tiles; the single nucleus of each cell spherical, centrally located nucleus bulges at its center

Function Rapid diffusion, ﬁ ltration, and some secretion in serous Function Absorption and secretion membranes

Location Air sacs in lungs (alveoli); lining of heart chambers and Location Thyroid gland follicles; kidney tubules; ducts and lumen of blood vessels (endothelium); serous membranes secretory regions of most glands; surface of ovary of body cavities (mesothelium)

Simple Columnar Epithelium has two forms; one type has no cilia, while the apical surface of the A simple columnar epithelium is composed of a single layer of tall, other type is lined with cilia. narrow cells. The nucleus is oval and located within the basal region of Nonciliated simple columnar epithelium often contains the cell. Active movement of molecules occurs across this type of epi- microvilli and a scattering of unicellular glands called goblet cells thelium by either absorption or secretion. Simple columnar epithelium (table 4.3c). Recall that microvilli are tiny, cytoplasmic projections

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Mucosa of small intestine Uterine tube

Cilia

Simple columnar Goblet cell epithelial cell

Microvilli (brush border) Nonciliated simple Basement membrane columnar cell

Basement membrane

LM 400x LM 100x

Cilia

Goblet cell Simple columnar epithelial cell Microvilli (brush border) Basement membrane Nonciliated simple columnar cell

Basement membrane

Structure Single layer of tall, narrow cells; oval-shaped nucleus in Structure Single layer of tall, narrow, ciliated cells; oval- basal region of cell; nucleus oriented lengthwise in cell; shaped nucleus oriented lengthwise in the basal apical regions of cells have microvilli; may contain goblet region of the cell; goblet cells may be present cells that secrete mucin Function Absorption and secretion; secretion of mucin Function Secretion of mucin and movement of mucus along apical surface of epithelium by action of cilia; oocyte movement through uterine tube Location Lining of most of digestive tract; lining of stomach does Location Lining of uterine tubes and larger bronchioles of not contain goblet cells respiratory tract

on the apical surface of the cell that increase the surface area that upon hydration (being mixed with water) forms mucus for for secretion and absorption. You cannot distinguish individual lubrication. Nonciliated simple columnar epithelium lines most of microvilli under the microscope; rather, the microvilli collectively the digestive tract, from the stomach to the anal canal. appear as a darkened, fuzzy structure known as a brush border. In ciliated simple columnar epithelium, cilia project from the Goblet cells secrete mucin (mū sin;́ mucus = mucus), a glycoprotein apical surfaces of the cells (table 4.3d). Mucus covers these apical

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This type of epithelium forms the walls of the larger ducts of most Study Tip! exocrine glands, such as the sweat glands in the skin. Although the function of stratified cuboidal epithelium is mainly protective, If you are having trouble distinguishing cilia from microvilli, recall it also serves to strengthen the wall of gland ducts. that cilia appear under the light microscope like fine hairs extending from the apical surface of the cell, while microvilli are extensive folds Stratified Columnar Epithelium of the plasma membrane that appear as a fuzzy, darkened brush border A stratified columnar epithelium is relatively rare in the body. It at the apical surface. consists of two or more layers of cells, but only the apical surface cells are columnar in shape (table 4.4d). This type of epithelium is found in the large ducts of salivary glands and in the membranous segment of the male urethra. surfaces and is moved along by the beating of the cilia. Goblet cells typically are interspersed throughout this epithelium. This type of Pseudostratified Columnar Epithelium epithelium lines the luminal (internal) surface of the uterine tubes, Pseudostratified columnar epithelium is so named because upon where it helps move an oocyte from the ovary to the uterus. A cili- first glance, it appears to consist of multiple layers of cells. However, ated simple columnar epithelium is also present in the bronchioles this epithelium is not really stratified, because all of its cells are in (smaller air tubes) of the lung. direct contact with the basement membrane. It may look stratified, Stratified Squamous Epithelium but it is actually pseudostratified due to the fact that the nuclei are scattered at different distances from the basal surface but not all of A stratified squamous epithelium has multiple cell layers, and the cells reach the apical surface (table 4.5a). The columnar cells only the deepest layer of cells is in direct contact with the base- within this epithelium always reach the apical surface; the shorter ment membrane. While the cells in the basal layers have a varied cells are stem cells that give rise to the columnar cells. shape often described as polyhedral, the superficial cells at the There are two forms of pseudostratified columnar epithe- apical surface display a flattened, squamous shape. Thus, strati- lium: Pseudostratified ciliated columnar epithelium has cilia fied squamous epithelium is so named because of its multiple cell on its apical surface, whereas pseudostratified nonciliated layers and the shape of its most superficial cells. This epithelium is columnar epithelium lacks cilia. Both types of this epithelium adapted to protect underlying tissues from damage due to activities perform protective functions. The ciliated form houses goblet that are abrasive and cause friction. Stem cells in the basal layer cells, which secrete mucin that forms mucus. This mucus traps continuously divide to produce a new stem cell and a committed foreign particles and is moved along the apical surface by the cell that gradually moves toward the surface to replace the cells beating of the cilia. Pseudostratified ciliated columnar epithe- lost during protective activities. This type of epithelium exists in lium lines much of the larger portions of the respiratory tract, two forms: nonkeratinized and keratinized. including the nasal cavity, part of the pharynx (throat), the The cells in nonkeratinized stratified squamous epithelium larynx (voice box), the trachea, and the bronchi. The cilia in remain alive all the way to its apical surface, and they are kept this epithelium help propel dust particles and foreign materials moist with secretions such as saliva or mucus. Keratin, a fibrous away from the lungs and to the nose and mouth. In contrast, intracellular protein, is not present within the cells. Thus, because the nonciliated form of this epithelium has no goblet cells. It is a all of the cells are still alive, the flattened nuclei characteristic rare epithelium that occurs primarily in part of the male urethra of squamous cells are visible even in the most superficial cells and the epididymis. (table 4.4a). Nonkeratinized stratified squamous epithelium lines the oral cavity (mouth), part of the pharynx (throat), the esopha- Transitional Epithelium gus, the vagina, and the anus. A transitional epithelium varies in appearance, depending on In keratinized (ker ắ -ti-nı̄ zd; keras = horn) stratified squa- whether it is in a relaxed or a stretched state (table 4.5b). In a mous epithelium, the apical surface is composed of layers of cells relaxed state, the basal cells appear almost cuboidal, and the apical that are dead; these cells lack nuclei and all organelles and are cells are large and rounded. During stretching, the transitional filled with tough, protective keratin. It is obvious that the superfi- epithelium thins, and the apical cells continue to flatten, becom- cial cells lack nuclei when they are viewed under the microscope ing almost squamous. In this distended state, it may be difficult to (table 4.4b). New committed cells produced in the basal region distinguish a transitional epithelium from a squamous epithelium. of the epithelium migrate toward the apical surface. During their However, one distinguishing feature of transitional epithelium is migration, they fill with keratin, lose their organelles and nuclei, the presence of a handful of binucleated (double-nucleus-contain- and die. However, the keratin in these dead cells makes them ing) cells. This epithelium lines the urinary bladder, an organ that very strong. Thus, there is a tradeoff with the appearance of changes shape as it fills with urine. It also lines the ureters and keratin, in that the tissue becomes very strong, but the cells must the proximal part of the urethra. Transitional epithelium permits die as a result. The epidermis (outer layer) of the skin consists of stretching and ensures that toxic urine does not seep into the keratinized stratified squamous epithelium. underlying tissues and structures of these organs. Stratified Cuboidal Epithelium WHAT DO YOU THINK? A stratified cuboidal epithelium contains two or more layers of cells, and the apical cells tend to be cuboidal in shape (table 4.4c). ●2 What types of epithelium are well suited for protection?

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Table 4.4 Stratifi ed Epithelia

Vagina Epidermis of skin

Squamous epithelial cell

Keratinized stratified squamous epithelial cells

Nonkeratinized stratified squamous epithelium

Living stratified Basement membrane squamous epithelial cells Basement membrane

Connective tissue Connective tissue LM 125x LM 100x

Squamous epithelial cell

Keratinized stratified squamous epithelial cells

Nonkeratinized stratified squamous epithelium

Living stratified squamous epithelial cells Basement membrane Basement membrane Connective tissue Connective tissue

(a) Nonkeratinized Stratifi ed Squamous Epithelium (b) Keratinized Stratifi ed Squamous Epithelium Structure Multiple layers of cells; basal cells typically are cuboidal Structure Multiple layers of cells; basal cells typically are or polyhedral, while apical (superfi cial) cells are cuboidal or polyhedral, while apical (superfi cial) cells squamous; surface cells are alive and kept moist are squamous; more superfi cial cells are dead and fi lled with the protein keratin Function Protection of underlying tissue Function Protection of underlying tissue Location Lining of oral cavity, part of pharynx, esophagus, Location Epidermis of skin vagina, and anus

(continued on next page)

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Table 4.4 Stratifi ed Epithelia (continued)

Duct of sweat gland Male urethra

LM 100x

Cuboidal cell Columnar cell

Stratified cuboidal Stratified columnar epithelium epithelium

Basement membrane

Connective tissue

LM 500x

Cuboidal cell Columnar cell

Stratified cuboidal Stratified columnar epithelium epithelium

Basement membrane

Connective tissue

(c) Stratiﬁ ed Cuboidal Epithelium (d) Stratiﬁ ed Columnar Epithelium Structure Two or more layers of cells; cells at the apical surface Structure Two or more layers of cells; cells at the apical surface are cuboidal are columnar Function Protection and secretion Function Protection and secretion Location Found in large ducts in most exocrine glands and in Location Rare; found in large ducts of some exocrine glands some parts of the male urethra and in some regions of the male urethra

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Table 4.5 Other Epithelia

Nasal cavity lining Urinary bladder lining

Goblet cell Cilia

Pseudostratified ciliated Binucleated columnar epithelium epithelial cell Columnar cell Transitional epithelium (relaxed)

Basal cell

Basement membrane

Basement membrane Connective tissue LM 600x Connective tissue LM 180x

Goblet cell Cilia

Pseudostratified ciliated Binucleated columnar epithelium epithelial cell Columnar cell Transitional epithelium (relaxed)

Basal cell

Basement membrane

Basement membrane Connective tissue Connective tissue

(a) Pseudostratiﬁ ed Columnar Epithelium (b) Transitional Epithelium

Structure Single layer of cells with varying heights that appears Structure Epithelial appearance varies, depending on whether multilayered; all cells connect to the basement the tissue is stretched or relaxed; shape of cells membrane, but not all cells reach the apical surface. at apical surface changes; some cells may be Ciliated form has goblet cells and cilia (shown); binucleated nonciliated form lacks goblet cells and cilia Function Protection; ciliated form also involved in secretion of Function Distention and relaxation to accommodate urine mucin and movement of mucus across surface by ciliary volume changes in bladder, ureters, and urethra action Location Ciliated form lines most of respiratory tract, including Location Lining of urinary bladder, ureters, and part of nasal cavity, part of pharynx, larynx, trachea, bronchi. urethra Nonciliated form is rare; lines epididymis and part of male urethra

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Microvilli

Secretory vesicles containing mucin

Figure 4.3 Goblet Cell: A Unicellular Exocrine Gland. (a) Photomicrograph and Rough ER (b) diagram of a goblet cell in the small intestine.

Mitochondria

Golgi apparatus

Nucleus TEM 30,000x

(a) (b)

4.1f Glands an epithelium that is predominantly nonsecretory. Unicellular exo- As epithelial tissue develops in the embryo, small invaginations crine glands typically do not contain a duct, and they are located from this epithelium into the underlying connective tissue give rise close to the surface of the epithelium in which they reside. The to specialized secretory structures called glands. Glands are either most common type of unicellular exocrine gland is the goblet cell individual cells or multicellular organs composed predominantly (figure 4.3). For example, the respiratory tract is lined mainly by of epithelial tissue. Glands perform a secretory function by produc- pseudostratified ciliated columnar epithelium, which also contains ing substances either for use elsewhere in the body or for elimina- some mucin-secreting goblet cells. Mucus then coats the inner sur- tion from the body. Glandular secretions include mucin, hormones, face of the respiratory passageway to cover and protect its lining enzymes, and waste products. and to help warm, humidify, and cleanse the inhaled air before it reaches the gas exchange surfaces in the lungs. Endocrine and Exocrine Glands Multicellular exocrine glands are composed of numerous Glands are classified as either endocrine or exocrine, depending cells that work together to produce a secretion and secrete it upon whether they have a duct connecting the secretory cells to onto the surface of an epithelium. A multicellular exocrine gland ́ ı̄ ́ ŭ acinus = the surface of an epithelium. consists of acini (as i-n ; sing., as i-n s; grape), sacs that Endocrine (en ́dō -krin; endon = within, krino = to separate) produce the secretion, and one or more smaller ducts, which merge glands lack ducts and secrete their products directly into the inter- to eventually form a larger duct that transports the secretion to the stitial fluid and bloodstream. The secretions of endocrine glands, epithelial surface (figure 4.4). Acini are the secretory portions, called hormones, act as chemical messengers to influence cell while ducts are the conducting portions of these glands. activities elsewhere in the body. Endocrine glands are discussed Most multicellular exocrine glands are enclosed within a tra- in depth in chapter 20. fibrous capsule. Extensions of this capsule, called septa or beculae, Exocrine (ek ś ō -krin; exo = outside) glands typically partition the gland internally into compartments called originate from an invagination of epithelium that burrows into lobes. Further subdivisions of the septa within each lobe form ́ū the deeper connective tissues. These glands usually maintain microscopic lobules (lob l). The septa contain ducts, blood their contact with the epithelial surface by means of a duct, an vessels, and nerves supplying the gland. The connective tissue epithelium-lined tube through which secretions of the gland are framework of the gland is called the stroma. The stroma supports ă ́ ă discharged onto the epithelial surface. This duct may secrete and organizes the parenchyma (p -reng ki-m ), the functional materials onto the surface of the skin (e.g., sweat from sweat cells of the gland that produce and secrete the gland products. glands or milk from mammary glands) or onto an epithelial These cells are usually simple cuboidal or columnar epithelial surface lining an internal passageway (e.g., enzymes from the cells. Multicellular exocrine glands are found in the mammary pancreas into the small intestine or saliva from the salivary glands, pancreas, and salivary glands. glands into the oral cavity). Classification of Exocrine Glands Exocrine Gland Structure Multicellular exocrine glands may be classified according to An exocrine gland may be unicellular or multicellular. A unicel- three criteria: (1) form and structure (morphology), which is lular exocrine gland is an individual exocrine cell located within considered an anatomic classification; (2) type of secretion;

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Stroma Septum and (3) method of secretion. The latter two are considered Capsule Parenchyma physiologic classifications.

Lobules (within lobe) Form and Structure Based on the structure and complexity of Duct their ducts, exocrine glands are considered either simple or com-

Secretory acini pound. Simple glands have a single, unbranched duct; compound glands exhibit branched ducts. Exocrine glands are also classified according to the shape or organization of their secretory portions. If the secretory portion and the duct are of uniform diameter, the gland is called tubular. If the secretory cells form an expanded sac, the gland is called Lobe Secretory vesicles acinar (as ́i-nar). Finally, a gland with both secretory tubules and (a) secretory acini is called a tubuloacinar gland. Figure 4.5 shows the several types of exocrine glands as classified by morphology.

Secretion Types Exocrine glands are classified by the nature of their secretions as serous glands, mucous glands, or mixed glands. Serous (sēr ́ŭ s; serum = whey) glands produce and secrete a nonviscous, watery fluid, such as sweat, milk, tears, or digestive juices. Acinus Duct This fluid carries wastes (sweat) to the surface of the skin, nutrients (secretory portion) (conducting portion) (milk) to a nursing infant, or digestive enzymes from the pancreas Figure 4.4 (b) to the lumen of the small intestine. Mucous (mūk ́ ŭ s) glands secrete General Structure of Exocrine Glands. (a) Exocrine glands have a mucin, which forms mucus when mixed with water. Mucous glands connection called a duct that leads to an organ or body surface. Inside the are found in such places as the roof of the oral cavity and the sur- gland, the duct branches repeatedly, following the connective tissue septa, face of the tongue. Mixed glands, such as the two pairs of salivary until its finest divisions end on secretory acini. (b) The acinus is the glands inferior to the oral cavity, contain both serous and mucous secretory portion of the gland, and the duct is the conducting portion. cells, and produce a mixture of the two types of secretions.

Duct

Secretory portion

Simple tubular Simple branched tubular Simple coiled tubular Simple acinar Simple branched acinar

(a) Simple glands

Duct

Secretory portions

Compound tubular Compound acinar Compound tubuloacinar

(b) Compound glands Figure 4.5 Structural Classification of Multicellular Exocrine Glands. (a) Simple glands have unbranched ducts, whereas (b) compound glands have ducts that branch. These glands also exhibit different forms: Tubular glands have secretory cells in a space with a uniform diameter, acinar glands have secretory cells arranged in saclike acini, and tubuloacinar glands have secretory cells in both types of regions.

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Disintegrating cells Secretions with contents Secretory contents becoming the secretion

Secretory vesicle Nucleus of secretory cell

Nucleus Cells Pinching off of dividing apical portion of Secretory vesicles secretory cell releasing their contents via exocytosis (a) Merocrine gland (b) Holocrine gland (c) Apocrine gland Figure 4.6 Modes of Exocrine Secretion. Exocrine glands use different processes to release their secretory products. (a) Merocrine glands secrete products by means of exocytosis at the apical surface of the secretory cells. (b) Holocrine gland secretion is produced through the destruction of the secretory cell. Lost cells are replaced by cell division at the base of the gland. (c) Apocrine gland secretion occurs with the “decapitation” of the apical surface of the cell and the subsequent release of secretory product and some cellular fragments.

Secretion Methods Glands also can be classified by their Apocrine (ap ́ō -krin; apo = away from or off) glands are mechanism of discharging secretory product as merocrine glands, composed of cells that accumulate their secretory products within holocrine glands, or apocrine glands (figure 4.6). the apical portion of their cytoplasm. The secretion follows as this Merocrine (mer -́ ō -krin; meros = share) glands package apical portion decapitates. The apical portion of the cytoplasm their secretions in structures called secretory vesicles. The begins to pinch off into the lumen of the gland for the secretory secretory vesicles travel to the apical surface of the glandular product to be transported to the skin surface. Mammary glands cell and release their secretion by exocytosis. The glandular and ceruminous glands are apocrine glands. cells remain intact and are not damaged in any way by producing the secretion. Lacrimal (tear) glands, salivary glands, some sweat glands, the exocrine glands of the pancreas, and the gastric glands of the stomach are examples of merocrine Study Tip! glands. Some merocrine glands are also called eccrine glands, The hol part of “holocrine” sounds like the word “whole.” Holocrine to denote a type of sweat gland in the skin that is not con- gland secretions are produced when the whole cell ruptures, dies, and nected to a hair follicle (see chapter 5). becomes the secretion. The apo part of “apocrine” sounds like “a part.” = Holocrine (hō l ́ō -krin; holos whole) glands are formed Secretions produced when a part of the cell is pinched off come from from cells that accumulate a product and then the entire cell apocrine glands. disintegrates. Thus, a holocrine secretion is a mixture of cell fragments and the product the cell synthesized prior to its destruction. The ruptured, dead cells are continuously replaced by other epithelial cells undergoing mitosis. Without this regenerative WHATW DID YOU LEARN? capacity, holocrine glands would quickly lose all of their cells ●4 What two main characteristics are used to classify epithelial tissues? during their secretory activities. Holocrine secretions tend to ●5 Why is one epithelium referred to as “pseudostratified”? be more viscous than merocrine secretions. The oil-producing glands (sebaceous glands) in the skin are an example of holocrine ●6 What are the two basic parts of a multicellular exocrine gland? glands. (So the oily secretion you feel on your skin is actually ●7 Why is epithelial cell regeneration important to the continued composed of ruptured, dead cells!) functioning of a holocrine gland?

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4.2 Connective Tissue tissue. This differs markedly from epithelial tissue, whose cells crowd closely together with little to no extracellular matrix sur- Learning Objectives: rounding them. 1. Describe the structure and function of connective tissue. 2. Identify the characteristics of embryonic connective tissue. Protein Fibers 3. Compare connective tissue proper, supporting connective Most connective tissue contains protein fibers throughout. These tissue, and fluid connective tissue. fibers strengthen and support connective tissue. The type and 4. Explain where each type of connective tissue is found in abundance of these fibers indicate to what extent the particular the body. connective tissue is responsible for strength and support. Three types of protein fibers are found in connective tissue: collagen Connective tissue is the most diverse, abundant, widely dis- fibers, which are strong and stretch-resistant; elastic fibers, tributed, and microscopically variable of the tissues. Connective which are flexible and resilient; and reticular fibers, which form tissue is designed to support, protect, and bind organs. As its an interwoven framework. name implies, it is the “glue” that binds body structures together. The diversity of connective tissue is obvious when examining Ground Substance some of its types. Connective tissue includes the fibrous tendons Both the cells and the protein fibers reside within a material and ligaments, body fat, the cartilage that connects the ends of called ground substance. This nonliving material is produced ribs to the sternum, the bones of the skeleton, and the blood. by the connective tissue cells. It primarily consists of protein and carbohydrate molecules and variable amounts of water. The 4.2a Characteristics of Connective Tissue ground substance may be viscous (as in blood), semisolid (as in Although the types of connective tissue are diverse, all of them cartilage), or solid (as in bone). Together, the ground substance share three basic components: cells, protein fibers, and ground and the protein fibers form an extracellular matrix. Most substance (figure 4.7). Their diversity is due to varying propor- connective tissues are composed primarily of an extracellular tions of these components as well as to differences in the types matrix, with relatively small proportions of cells. and amounts of protein fibers. 4.2b Functions of Connective Tissue Cells As a group, the many types of connective tissue perform a wide Each type of connective tissue contains specific types of cells. variety of functions, including the following: For example, connective tissue proper contains fibroblasts, fat contains adipocytes, cartilage contains chondrocytes, and bone ■ Physical protection. The bones of the cranium, sternum, contains osteocytes. Most connective tissue cells are not in and thoracic cage protect delicate organs, such as the direct contact with each other, but are scattered throughout the brain, heart, and lungs; fat packed around the kidneys

Ground substance

Elastic fibers Extracellular matrix

Collagen fibers Protein Figure 4.7 fibers Connective Tissue Components and Organization. Connective tissue is Reticular fibers composed of cells and an extracellular matrix of protein fibers and ground Mesenchymal cell substance. Blood vessel

Macrophage

Adipocyte (fat cell)

Fibroblast

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and at the posterior side of the eyes within the skull WHAT DO YOU THINK? protects these organs. 3 Why does connective tissue contain fewer cells than epithelium? ■ ● Support and structural framework. Bones provide the Can you think of some reasons related to the functions of framework for the adult body and support the soft tissues; connective tissue? cartilage supports such body structures as the trachea, bronchi, ears, and nose; connective tissue sheets form 4.2c Development of Connective Tissue capsules to support body organs such as the spleen The primary germ layer mesoderm forms all connective tissues. and kidneys. There are two types of embryonic connective tissue: mesen- ■ Binding of structures. Ligaments bind bone to bone; chyme and mucous connective tissue. In the developing embryo, tendons bind muscle to bone; dense irregular connective mesenchyme (mez en-ḱ ı̄ m; mesos = middle, enkyma = infusion) tissue binds skin to underlying muscle and bone. is the first type of connective tissue to emerge. It has star-shaped ■ Storage. Fat is the major energy reserve in the body; bone (stellate) or spindle-shaped mesenchymal cells dispersed within is a large reservoir for calcium and phosphorus. a gel-like ground substance that contains fine, immature protein ■ Transport. Blood carries nutrients, gases, hormones, wastes, fibers (table 4.6a). In fact, there is proportionately more ground and blood cells between different regions of the body. substance than mesenchymal cells in this type of embryonic con- ■ Immune protection. Many connective tissue types contain nective tissue. Mesenchyme is the source of all other connective white blood cells (leukocytes), which protect the body tissues. Adult connective tissues often house numerous mesen- against disease and mount an immune response when chymal (stem) cells that support the repair of the tissue following the body is exposed to something foreign. A derivative of damage or injury. one type of leukocyte, called a macrophage, phagocytizes A second type of embryonic connective tissue is mucous (“eats up”) foreign materials. Additionally, the connective tissue (Wharton’s jelly). The immature protein fibers in extracellular matrix is a viscous material that interferes mucous connective tissue are more numerous than those within with the movement and spread of disease-causing mesenchyme (table 4.6b). Mucous connective tissue is located organisms. within the umbilical cord only.

CLINICAL VIEW What Are You Planning to Do with Although this technology is hopeful, it is not without drawbacks. First, Your Baby’s Umbilical Cord? each cord blood sample contains relatively few stem cells. Although a method to increase the growth and number of stem cells is currently being Years of medical research have led to an amazing discovery: A baby’s investigated, at present the limited amount of cells available remains a blood contains stem cells that are the same as those found in a child’s problem. Second, harvesting and storing cord blood can be costly. bone marrow, and these cells can be used to treat a variety of life- Donor registries are not yet available in all parts of the United States, threatening diseases. What’s more, these important stem cells are easy and private banks are expensive. Presently, the typical charge for the to collect. The leftover blood in the placenta and umbilical cord is a initial processing at a private bank is as much as $1000, a cost not ready, and often discarded, source. covered by insurance plans. And lifelong storage will certainly add to Cord blood can easily be harvested immediately following the the overall expense. birth of a baby. The specimen can be shipped to a cord blood bank The odds of a child needing a cord blood stem cell transplant are for testing, processing, and storage. The cells are carefully banked in approximately 4 in 10,000, or about .04%. Certainly the odds of a a cryogenic vault for optimal preservation should there be a future person needing a transplant sometime during his or her life are much need. To date, conditions successfully treated with cord blood stem greater than just the .04% of childhood, but calculating the need over cells include lymphoma, leukemia, anemias resulting from severe bone a typical adult life span is very difficult. However, the increasing risk marrow damage (especially complications of cancer chemotherapy), with age, plus other possible applications of cord blood stem cells not and sickle-cell disease. yet discovered, seem to favor the banking of this valuable resource.

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Table 4.6 Types of Embryonic Connective Tissue

Amnion Umbilical cord

Immature protein fibers Immature Mesenchymal Mesenchymal cells protein cell fiber

Ground Ground substance substance

LM 400x LM 250x

Immature protein fibers Immature Mesenchymal Mesenchymal cells protein cell fiber

Ground Ground substance substance

(a) Mesenchyme (b) Mucous Connective Tissue Structure Ground substance is a viscous gel with some immature Structure Mesenchymal cells scattered within a viscous, gel-like protein ﬁ bers; mesenchymal cells are stellate or ground substance; immature protein ﬁ bers present spindle-shaped Function Common origin for all other connective tissue types Function Support of structures in umbilical cord attaching fetus to mother Location Throughout the body of the embryo, fetus, and adult Location Umbilical cord of fetus

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Connective Tissue Classification

Common origin (mesenchyme)

Connective tissue proper Supporting connective tissue Fluid connective tissue

Loose connective Dense connective Cartilage Bone Blood tissue (fewer fibers, tissue (more fibers, (semisolid matrix) (solid matrix) more ground less ground substance) substance)

1. Areolar 1. Regular 1. Hyaline 1. Compact 2. Adipose 2. Irregular 2. Fibrocartilage 2. Spongy 3. Reticular 3. Elastic 3. Elastic

Figure 4.8 Connective Tissue Classification. Mesenchymal cells are the origin of all connective tissue cell types. The three classes of connective tissue are connective tissue proper, supporting connective tissue, and fluid connective tissue.

4.2d Classification of Connective Tissue ■ Adipocytes (ad ́i-pō -sı̄ t; adip = fat) are also called fat cells. The connective tissue types present after birth are classified They often appear in small clusters within some types of into three broad categories: connective tissue proper, supporting connective tissue proper. If a larger cluster of these cells connective tissue, and fluid connective tissue. Figure 4.8 pro- dominates an area, the connective tissue is called adipose vides an overview of these tissue types and the subcategories connective tissue. = = within them, each of which is described in detail next. ■ Fixed macrophages (mak ŕ ō -fāj; makros large, phago to eat) are relatively large, irregular-shaped cells with numerous Connective Tissue Proper surface folds and projections. They are derived from one type Connective tissue proper includes those types of connective tissue of leukocyte (called a monocyte) and dispersed throughout that exhibit a variable mixture of both connective tissue cell types the extracellular matrix, where they phagocytize damaged and extracellular protein fibers suspended within a viscous ground cells or pathogens. When the fixed macrophages encounter substance. These connective tissue types differ with respect to foreign materials, the cells release chemicals that stimulate their numbers and types of cells and the relative properties and the immune system and lure numerous wandering cells proportions of their fibers and ground substance. involved in body defense to the foreign materials. ■ Mesenchymal cells are a type of embryonic stem cell Cells of Connective Tissue Proper Two classes of cells form contained within connective tissue. As a result of local injury the connective tissue proper: resident cells and wandering cells or connective tissue damage, mesenchymal stem cells divide. (table 4.7) . Resident cells are permanently contained within One of the cells produced is the replacement mesenchymal the connective tissue. These stationary cells help support, main- cell, and the other becomes a committed cell that moves into tain, and repair the extracellular matrix. Wandering cells move the damaged or injured area and differentiates into the type of throughout the connective tissue and are involved in immune connective tissue cell that is needed. protection and repair of damaged extracellular matrix. The number of wandering cells at any given moment varies depending on local The wandering cells of the connective tissue proper are pri- = conditions. marily types of leukocytes (loo ́kō -sı̄ t; leukos white), also called The resident cells of the connective tissue proper include the white blood cells. As you will learn in greater detail in chapter 21, following types: there are several different types of leukocytes, and each type performs certain functions that help us overcome illness or fight ■ Fibroblasts (fı̄ ́brō -blast; fibra = fiber, blastos = germ) are foreign invaders in our bodies. Connective tissue proper contains large, relatively flat cells with tapered ends. They are the specific types of wandering cells, including the following: most abundant resident cells in connective tissue proper. They produce the fibers and ground substance components ■ Mast cells. These small, mobile cells contain a granule- of the extracellular matrix. filled cytoplasm. They are usually found close to blood

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Table 4.7 Cells of Connective Tissue Proper Type of Cell Appearance Function

Resident Cells Stationary Maintain and repair extracellular matrix; store materials

Fibroblasts Abundant, large, relatively fl at cells, often with tapered ends Produce fi bers and ground substance of the extracellular matrix Adipocytes Fat cells with a single large lipid droplet; cellular components Store lipid reserves pushed to one side Fixed macrophages Large cells derived from monocytes in blood; reside in Phagocytize foreign materials extracellular matrix after leaving the blood Mesenchymal cells Stellate or spindle-shaped embryonic stem cells Divide in response to injury to produce new connective tissue cells Wandering Cells Move through connective tissue spaces Repair damaged extracellular matrix; active in immune response Mast cells Small cells with a granule-fi lled cytoplasm Release histamine and heparin to stimulate local infl ammation Plasma cells Small cells with a distinct nucleus derived from activated Form antibodies that bind to foreign substances, bacteria, B-lymphocytes viruses Free macrophages Mobile phagocytic cells formed from monocytes of the blood Phagocytize foreign materials Other leukocytes White blood cells that enter connective tissue Attack foreign materials (lymphocytes) or directly combat bacteria (neutrophils)

vessels; they secrete heparin to inhibit blood clotting, Collagen (kol ́lă-jen; koila = glue, gen = producing) fibers and histamine to dilate blood vessels and increase are long, unbranched extracellular fibers composed of the protein blood flow. collagen. They are strong, flexible, and resistant to stretching. ■ Plasma cells. When B-lymphocytes (a type of white Collagen forms about 25% of the body’s protein, making it the blood cell) are activated by exposure to foreign materials, most abundant protein in the body. In fresh tissue, collagen fibers the cells mature into plasma cells. These cells are small appear white, and thus they are often called white fibers. In tissue “factories” that synthesize disease-fighting proteins called sections stained with hematoxylin and eosin to give contrast, they antibodies (an t́ ı̄ -bod-ē; anti = against, bodig = corpus). appear pink. In tissue sections, collagen forms coarse, sometimes Antibodies attach to foreign material and prevent it wavy bundles. The parallel structure and arrangement of collagen from causing further damage. Usually, plasma cells are bundles in tendons and ligaments allows them to withstand enor- found in the intestinal walls, spleen, and mous forces in one direction. lymph nodes. Elastic (ĕ -las tik;́ elastreo = drive) fibers contain the pro- ■ Free macrophages. These mobile, phagocytic cells are tein elastin and are thinner than collagen fibers. They stretch formed from monocytes (a type of white blood cell) that easily, branch, rejoin, and appear wavy. The coiled structure of migrate out of the bloodstream. They wander through elastin allows it to stretch and recoil like a rubber band when connective tissue and engulf and destroy any bacteria, the deforming force is withdrawn. Elastic fibers permit the skin, foreign particles, or damaged cells and debris they lungs, and arteries to return to their normal shape after being encounter. stretched. Fresh elastic fibers have a yellowish color and are ■ Other leukocytes. In addition to the leukocytes just called yellow fibers. In tissue sections, elastic fibers are only mentioned, other leukocytes migrate through the blood visible when stained with special stains, such as Verhoff stain, vessel walls into the connective tissue where they spend which makes elastic fibers appear black. most of their time. The majority of these leukocytes are Reticular (re-tik ́ū -lă r; reticulum = small net) fibers are neutrophils, a type of white blood cell that seeks out and thinner than collagen fibers. They contain the same protein phagocytizes bacteria. The rest are lymphocytes, which subunits that collagen has, but their subunits are combined attack and destroy foreign materials. in a different way and they are coated with a glycoprotein (a protein with some carbohydrate attached to it). These fibers Fibers of Connective Tissue Proper As mentioned previ- form a branching, interwoven framework that is tough but flex- ously, the three types of protein fibers in connective tissue ible. Reticular fibers are especially abundant in the stroma, a proper are collagen fibers, elastic fibers, and reticular fibers. structural connective tissue framework in organs such as the Fibroblasts synthesize the components of all three fiber types, lymph nodes, spleen, and liver. The meshlike arrangement of and then secrete these protein subunits into the interstitial the reticular fibers permits them to physically support organs fluid. The subunits combine or aggregate within the matrix and and resist external forces that may damage the organ’s cells form the completed fiber. and blood vessels.

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Ground Substance of Connective Tissue Proper The cel- CLINICAL VIEW lular and fibrous components of the connective tissue proper are suspended within the ground substance, a colorless, featureless, Pathogenesis of Collagen viscous solution. Ground substance usually has a gelatinous, almost rubbery consistency due to the mixture of its component Collagen is an important protein that strengthens and sup- molecules, which vary both in their size and in their proportions ports almost all body tissues, especially the connective tissues. of proteins and carbohydrates. The different molecules in the The pathogenesis (development of disease conditions) of certain ground substance are called glycosaminoglycans, proteoglycans, connective tissue diseases may be traced to errors in collagen and structural glycoproteins. production. If the collagen does not form properly, the connective tissues are weak and subject to problems. Often these conditions Categories of Connective Tissue Proper are caused by a lack of dietary vitamin C (ascorbic acid), which is Connective tissue proper is divided into two broad categories: loose essential to collagen production. For example, scurvy, a disease connective tissue and dense connective tissue (table 4.8). This caused by a def iciency in vitamin C, is marked by weakness, ulceration classification is based on the relative proportions of cells, protein of gums with loss of teeth, and hemorrhages in mucous membranes fibers, and ground substance. and internal organs. Bone growth is abnormal, capillaries rupture easily, and wounds and fractures do not heal. All of these signs of Loose Connective Tissue Loose connective tissue contains scurvy are directly related to abnormal production of collagen. relatively fewer cells and protein fibers than dense connective Scurvy was especially prevalent many years ago among tissue. The protein fibers in loose connective tissue are loosely sailors who took long sea voyages and whose diets while at arranged rather than tightly packed together. Usually, this tis- sea lacked vitamin C. These sailors eventually learned that bring- sue occupies the spaces between and around organs. Loose ing citrus fruits (such as limes and lemons) along on their voyages connective tissues support the overlying epithelia and provide prevented scurvy. (This also explains how sailors received the nick- cushioning around organs, support and surround blood vessels name “limeys.”) Nowadays, physicians try to treat collagen production and nerves, store lipids, and provide a medium for the diffusion disorders by promoting vitamin C supplementation in their patients’ of materials. Thus, loose connective tissues act as the body’s diets. Besides citrus fruits, foods high in vitamin C include broccoli, “packing material.” There are three types of loose connective cauliflower, peppers, mustard greens, spinach, and tomatoes. tissue: areolar connective tissue, adipose connective tissue, and reticular connective tissue.

CLINICAL VIEW Marfan Syndrome

Marfan syndrome is a rare genetic disease of connective tissue that is characterized by skeletal, cardiovascular, and visual abnormalities. It is caused by an abnormal gene on chromosome 15. Patients with Marfan syndrome are tall and thin. Their skeletal system deformities include abnormally long arms, legs, fingers, and toes; malformation of the thoracic cage and/or vertebral column as a result of excessive growth of ribs; and easily dislocated joints resulting from weak ligaments, tendons, and joint capsules. Cardiovascular system problems involve a weakness in the aorta and abnormal heart valves. Abnormalities in fibrillin, a protein that helps support blood vessels and other body structures, and in both collagen and elastin, are responsible for these clinical effects. Vision abnormalities develop because the thin fibers that hold the optic lens are weak, allowing the lens to slip out of place. Patients usually exhibit symptoms of Marfan syndrome by age 10; those affected often die of cardiovascular problems before they reach 50 years of age. Several individuals have speculated that Individual with Marfan syndrome. Abraham Lincoln suffered from Marfan syndrome because he exhibited many characteristics of the disease. Recently, however, researchers have stated there isn’t enough conclusive evidence to prove the Lincoln-Marfan link.

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Table 4.8 Connective Tissue Proper Type Structure Function Location

Loose Connective Tissue Relatively fewer cells and fi bers than in dense connective tissue; fi bers are loosely arranged Areolar connective Fibroblasts; lesser amounts of collagen and Binds and packs around organs Surrounding nerves, vessels; tissue elastic fi bers; viscous ground substance subcutaneous layer Adipose connective Adipocytes Protects; stores fat; insulates Subcutaneous layer; surrounding tissue kidney and selected other organs Reticular connective Meshwork of reticular fi bers Forms stroma of lymphatic organs Stroma of spleen, liver, lymph nodes, tissue bone marrow Dense Connective Tissue Higher proportion of fi bers to ground substance; protein fi bers densely packed together Dense regular Densely packed collagen fi bers are parallel to Provides great strength and Tendons and ligaments connective tissue direction of stress fl exibility primarily in a single direction Dense irregular Densely packed collagen fi bers are interwoven; Provides tensile strength in all Dermis; capsules of organs connective tissue fi bers are irregularly clumped together and directions project in all directions Elastic connective Elastic and collagen fi bers are arranged Provides framework and supports Walls of large arteries tissue irregularly organs

Areolar (ă-r ē ́ō -lă r) connective tissue is highly variable in diverse locations as a fat capsule surrounding the kidney, the appearance and the least specialized connective tissue in the pericardial and abdominopelvic cavities, and the subcutane- body (table 4.9a). It has a loosely organized array of collagen ous layer. and elastic fibers and an abundant distribution of blood vessels. Fat is a primary energy store for the body. The amount of Areolar connective tissue contains all of the cell types of connec- stored fat fluctuates as the adipose cells either increase (called tive tissue proper, although the predominant cell is the fibroblast. lipogenesis) or decrease (called lipolysis) their amount of stored A viscous ground substance occupies the spaces between fibers fat. But although there is a constant turnover of the stored fat, an and accounts for most of the volume of areolar connective tissue. equilibrium is usually reached, and the amount of stored fat and The ground substance cushions shocks, and the loosely organized the number of adipocytes are normally quite stable in an indi- fibers ensure that this type of connective tissue can be distorted vidual. Although adipocytes cannot divide, mesenchymal cells without damage. Additionally, the elastic properties of this tissue can provide additional fat cells if the body has excess nutrients. promote independent movements. For instance, the dermis of the Thus, even after a surgical procedure to reduce the amount of skin contains a superficial layer of areolar connective tissue, and body fat, such as liposuction, the mesenchymal stem cells may thus tugging on the skin of the leg, for example, does not affect the replace adipocytes to store excess fat in the body. underlying muscle. Reticular connective tissue contains a meshwork of reticu- Areolar connective tissue is found nearly everywhere in the lar fibers, fibroblasts, and leukocytes (table 4.9c). This connective body. It surrounds nerves, blood vessels, and individual muscle tissue forms the stroma of many lymphatic organs, such as the cells. It is also a major component of the subcutaneous layer deep spleen, thymus, lymph nodes, and bone marrow. to the skin. Adipose connective tissue (commonly known as “fat”) is Dense Connective Tissue Dense connective tissue is com- a loose connective tissue composed primarily of cells called adipo- posed primarily of protein fibers and has proportionately less cytes (table 4.9b). Adipocytes (white fat) usually range from 70 μm ground substance than does loose connective tissue. Dense con- to 120 μm in diameter. In life, adipocytes are filled with one lipid nective tissue is sometimes called collagenous tissue because droplet. On a histology slide, the lipid has been extracted during collagen fibers are the dominant fiber type. There are three preparation, so all that is left is the plasma membrane of the adi- categories of dense connective tissue: (1) dense regular connec- pocyte, with the nucleus pushed to the side of a round, clear space tive tissue, (2) dense irregular connective tissue, and (3) elastic looking much like a ring. connective tissue. Adipose connective tissue serves as packing around In dense regular connective tissue, col la gen f ibers a re packed structures and provides padding, cushions shocks, and acts as tightly and aligned parallel to an applied force (table 4.10a). This an insulator to slow heat loss through the skin. Adipose con- tissue type is found in tendons and ligaments, where stress is nective tissue is commonly found throughout the body in such applied in a single direction. Dense regular connective tissue has

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Table 4.9 Connective Tissue Proper: Loose Connective Tissue

Subcutaneous layer Papillary layer of dermis (hypodermis)

Elastic fiber

Fibroblast Adipocyte Adipocyte Blood vessel nucleus Collagen fiber

Ground substance

LM 240x LM 250x LM 200x

Elastic fiber

Fibroblast Adipocyte

Collagen fiber Adipocyte nucleus Blood vessel

Ground substance

(a) Areolar Connective Tissue (b) Adipose Connective Tissue Structure Abundant vascularized ground substance is gel-like; Structure Closely packed adipocytes (fat cells); nucleus scattered ﬁ broblasts; many blood vessels squeezed to one side by large fat droplet

Function Surrounds and protects tissues and organs; loosely Function Stores energy; protects, cushions, and insulates binds epithelia to deeper tissues; provides nerve and blood vessel packing

Location Subcutaneous layer under skin; surrounds organs Location Subcutaneous layer; covers and surrounds some organs

few blood vessels, and thus it takes a long time to heal following connective tissue (table 4.10b). Dense irregular connective tissue injury, since a rich blood supply is necessary for good healing. provides support and resistance to stress in multiple directions. In dense irregular connective tissue, individual bundles of An example of dense irregular connective tissue is the collagen fibers extend in all directions in a scattered meshwork. deep portion of the dermis, which lends strength to the skin and These bundles of collagen fibers appear in clumps throughout the permits it to withstand applied forces from any direction. Dense tissue, rather than arranged in parallel as seen in dense regular irregular connective tissue also forms a supporting layer around

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collagen fibers (table 4.10c). The elastic fibers provide resilience and the ability to deform and then return to normal shape. Examples of structures composed of elastic connective tissue are the vocal cords, the suspensory ligament of the penis, and some ligaments of the spinal column. Elastic connective tissue also is Stroma of spleen present as wavy sheets in the walls of large and medium arteries.

WHAT DO YOU THINK?

●4 What type of connective tissue have you damaged when you sprain your ankle?

Reticular fibers

Leukocytes Study Tip! Ask the following questions to help distinguish the types of connective tissue proper under the microscope: 1. Is the connective tissue loose or dense? Loose connective tissue has fewer protein fibers and relatively more ground Ground substance substance. Dense connective tissue has more protein fibers and relatively little ground substance. 2. If the tissue is dense, are the protein fibers in clumps or in LM 280x parallel? Protein fibers in clumps indicate dense irregular connective tissue. Protein fibers that run in parallel, like lasagna noodles stacked on top of one another, indicate dense regular connective tissue. Elastic connective tissue may resemble dense regular connective tissue, but its fibers are not as neatly arranged. 3. If the tissue is loose, what types of cells are present? Areolar Reticular fibers connective tissue primarily contains fibroblasts, whereas adipose connective tissue contains adipocytes. The presence of numerous leukocytes may indicate reticular connective tissue.

Leukocytes Supporting Connective Tissue Cartilage and bone are types of supporting connective tissue because they form a strong, durable framework that protects and supports the soft body tissues. The extracellular matrix in Ground substance supporting connective tissue contains many protein fibers and a ground substance that ranges from semisolid to solid. In general, cartilage has a semisolid extracellular matrix, while bone has a solid extracellular matrix. Cartilage Cartilage has a firm, gel-like extracellular matrix composed of both protein fibers and ground substance. Mature (c) Reticular Connective Tissue cartilage cells are called chondrocytes (kon ́drō -sı̄ t; chondros = Structure Ground substance is gel-like liquid; scattered gristle or cartilage, cytos = a hollow [cell]). They occupy small arrangement of reticular ﬁ bers, ﬁ broblasts, and = leukocytes spaces called lacunae (lă -koo ne;́ lacus a hollow, a lake), within the extracellular matrix. The physical properties of cartilage vary Function Provides supportive framework for spleen, lymph with the extracellular matrix contents. Cartilage is stronger and nodes, thymus, bone marrow more resilient than any previously discussed connective tissue type, and it provides more flexibility than bone. Collagen fibers Location Forms stroma of lymph nodes, spleen, thymus, bone marrow within the matrix give cartilage its tensile strength; its resilience is attributed to elastic fibers and variations in the kinds and amounts of ground substance components, including water. Cartilage is found in areas of the body that need support cartilage (called the perichondrium) and around bone (called the and must withstand deformation, such as the tip of the nose or the periosteum), except at joints. In addition, it forms a thick, fibrous external part of the ear (auricle). capsule that supports and houses internal organs, such as the liver, Chondrocytes produce a chemical that prevents blood ves- kidneys, and spleen. sel formation and growth within the extracellular matrix. Thus, Elastic connective tissue has branching elastic fibers and mature cartilage is avascular, meaning without blood vessels. more fibroblasts than loose connective tissue in addition to packed Therefore, the chondrocytes must exchange nutrients and waste

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Table 4.10 Connective Tissue Proper: Dense Connective Tissue

Reticular layer Tendon of dermis

Ground substance Collagen fiber bundles

Collagen fibers Fibroblast nucleus

Fibroblast nucleus Ground substance

LM 250x LM 200x

Ground substance Collagen fiber bundles

Single collagen fiber

Fibroblast nucleus Collagen fibers

Fibroblast nucleus Ground substance

(a) Dense Regular Connective Tissue (b) Dense Irregular Connective Tissue Structure Densely packed, parallel collagen fi bers; fi broblast Structure Predominantly collagen fi bers, randomly arranged nuclei squeezed between layers of fi bers; scarce and clumped together; fi broblasts in spaces among ground substance fi bers; more ground substance than in dense regular connective tissue Function Attaches muscle to bone and bone to bone; resists Function Withstands stresses applied in all directions; durable stress applied in one direction Location Forms tendons, most ligaments Location Dermis; periosteum covering bone; perichondrium covering cartilage, organ capsules

products with blood vessels outside of the cartilage by diffu- and secures the perichondrium to the cartilage and to other sion. Cartilage usually has a covering called the perichondrium structures. The cellular layer contains stem cells (chondroblasts) (per-i-kon ́drē -ŭ m; peri = around, chondros = cartilage). Two necessary for the growth and maintenance of the cartilage. distinct layers form the perichondrium: an outer, fibrous region of dense irregular connective tissue and an inner, cellular layer. Types of Cartilage Three major types of cartilage are found in The fibrous layer provides protection and mechanical support, the body: hyaline cartilage, fibrocartilage, and elastic cartilage.

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extracellular matrix (table 4.11a). However, the collagen within the matrix is not readily observed by light microscopy because it is primarily in the form of submicroscopic fibrils. It provides support through flexibility and resilience. Hyaline cartilage is surrounded by a perichondrium. If the hyaline cartilage tissue is stained by hema- Aorta wall toxylin and eosin and then observed under the microscrope, the tissue resembles carbonated grape soda, where the lacunae represent the bubbles in the soda. Hyaline cartilage has many functions in addition to its primary one of supporting soft tissue. It forms most of the fetal skeleton and is a model for most future bone growth. The cartilage at the articular ends of long bones allows the bones in a joint to move freely and easily. Hyaline cartilage is found in many Ground substance other areas of the body, including the nose, trachea, most of the larynx, costal cartilage (the cartilage attached to the ribs), and the articular ends of long bones. Fibrocartilage (fı̄ -brō -kar ti-lij;́ fibro = fiber) has numerous Fibroblast nucleus coarse, readily visible fibers in its extracellular matrix (table 4.11b). The fibers are arranged as irregular bundles between Elastic fibers large chondrocytes. There is only a sparse amount of ground substance, and often the chondrocytes are arranged in parallel rows. The densely interwoven collagen fibers contribute to the extreme durability of this type of cartilage. It has no perichon- LM 160x drium because stress applied at the surface of the fibrocartilage would destroy this tissue layer. Fibrocartilage is found in the intervertebral discs (circular structures between adjacent vertebrae), the pubic symphysis (a pad of cartilage between the anterior parts of the pelvic bones), and the menisci (C-shaped cartilage pads) of the knee joint. In these locations, fibrocartilage acts as a shock absorber and resists compression. Ground substance Elastic cartilage is so named because it contains numerous elastic fibers in its matrix (table 4.11c). The higher concentration of elastic fibers in this cartilage causes it to appear yellow in fresh Fibroblast nucleus sections. The chondrocytes of elastic cartilage are almost indis- tinguishable from those of hyaline cartilage. They are typically Elastic fibers closely packed and surrounded by only a small amount of extracellular matrix. The elastic fibers are both denser and more highly branched in the central region of the extracellular matrix, where they form a weblike mesh around the chondrocytes within the lacunae. These fibers ensure that elastic cartilage is extremely resilient and flexible. Elastic cartilage is surrounded by a perichondrium. Elastic cartilage is found in the epiglottis (a structure in the larynx that prevents swallowed food and fluids from entering the (c) Elastic Connective Tissue trachea) and in the external ear. You can see for yourself how flex- Structure Predominantly freely branching elastic fi bers; ible elastic cartilage is by performing this experiment: Fold your fi broblasts occupy some spaces between fi bers outer ear over your finger, hold for 10 seconds, and release. You will notice that your ear springs back to its original shape because the elastic cartilage resists the deformational pressure you applied. Function Allows stretching of some organs (This also explains why our ears aren’t permanently misshapen if we sleep on them in an unusual way!) Location Walls of elastic arteries; trachea; bronchial tubes; true vocal cords; suspensory ligaments of penis Bone Bone connective tissue (or osseous connective tissue) makes up the mass of most of the body structures referred to as “bones.” Bone is more solid than cartilage and provides greater The cartilage types exhibit differences in density and dispersal of support. Chapter 6 provides a detailed description of the histology chondrocytes within the extracellular matrix. of bone connective tissue. Hyaline (hı̄ ́ă-lin, -lēn; hyalos = glass) cartilage is the most About one-third of the dry weight of bone is com- common type of cartilage and also the weakest. It is named for posed of organic components (collagen fibers and different its clear, glassy appearance under the microscope. The chondro- protein-carbohydrate molecules), and two-thirds consists of cytes within their lacunae are irregularly scattered throughout the inorganic components (a mixture of calcium salts, primarily

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Table 4.11 Supporting Connective Tissue: Cartilage

Articular cartilage on bone Intervertebral disc

Lacuna Collagen fibers

Lacuna Chondrocyte

Extracellular matrix

Chondrocyte

LM 250x LM 250x

Collagen fibers Lacuna Lacuna Chondrocyte

Extracellular matrix

Chondrocyte

(a) Hyaline Cartilage (b) Fibrocartilage Structure Glassy-appearing matrix; lacunae house chondrocytes; Structure Readily visible, parallel collagen ﬁ bers in matrix; usually covered by perichondrium lacunae house chondrocytes; no perichondrium

Function Smooth surfaces for movement at joints; model for Function Resists compression; absorbs shock in some joints bone growth; supports soft tissue Location Most of fetal skeleton; covers articular ends of long Location Intervertebral discs; pubic symphysis; menisci of knee bones; costal cartilage; most of the larynx, trachea, joints nose

calcium phosphate). Bone derives its remarkable properties from the joints of long bones) are covered by a dense irregular connec- its combination of components: Its organic portions provide some tive tissue called the periosteum (per-ē -os t́ ē -ŭ m; osteon = bone), flexibility and tensile strength, and its inorganic portions provide which is similar to the perichondrium of cartilage. compressional strength. The minerals are deposited onto the col- There are two forms of bone connective tissue: compact lagen fibers, resulting in a structure that is strong and durable but bone and spongy bone. Both types are typically found in all not brittle. Almost all bone surfaces (except for the surfaces of bones of the body. Compact bone appears solid but is in fact

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Table 4.12 Supporting Connective Tissue: Bone

External ear Compact bone

Osteon

Chondrocytes

Lamellae of osteon Elastic fibers Central canal

Canaliculi

Osteocyte in lacuna

LM 200x LM 160x

Osteon Chondrocytes

Lamellae Elastic fibers of osteon Central canal

Canaliculi

Osteocyte in lacuna

Structure Contains abundant elastic fi bers; elastic fi bers form Structure Compact bone: Calcifi ed matrix arranged in osteons weblike mesh around lacunae; perichondrium (concentric lamellae arranged around a central present canal containing blood vessels) Spongy bone: Lacks the organization of compact Function Maintains structure and shape while permitting bone; contains macroscopic spaces; bone arranged extensive fl exibility in a meshwork pattern Location External ear; epiglottis of the larynx Function Supports soft structures; protects vital organs; provides levers for movement; stores calcium and phosphorus. Spongy bone is the site of hemopoiesis. Location Bones of the body

perforated by a number of vascular canals. It usually forms the yet lightweight. This design allows our bones to be both strong hard outer shell of the bone. Spongy bone (or cancellous bone) and lightweight at the same time. is located within the interior of a bone. Instead of being com- Compact bone has an ordered histologic pattern pletely solid, spongy bone contains spaces, and the bone con- (table 4.12). It is formed from cylindrical structures called nective tissue forms a latticework structure that is very strong, osteons, or Haversian systems. Osteons run parallel to the shafts

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of long bones. Each osteon contains concentric rings of bone Table 4.13 Fluid Connective Tissue: Blood called lamellae (lă -mel ́ē ; lamina = plate), which encircle a central canal (Haversian canal). Blood vessels and nerves travel through the central canals of osteons. Lacunae between neighboring concentric lamellae house bone cells, called osteocytes (os ́tē -ō -sı̄ t). Diffusion of nutrients and waste products cannot occur through the hard matrix of bone, so osteocytes must com- Blood smear municate with one another, and ultimately with the blood vessels in the central canal, through minute passageways in the matrix called canaliculi (kan-ă -lik ́ū -lı̄ ; canalis = canal). Together, all of the canaliculi form a branching network throughout compact Platelet bone for the exchange of materials between the blood vessels and the osteocytes within the lacunae. Bone serves a variety of functions. Bones provide levers for Erythrocytes movement when the muscles attached to them contract, and they (red blood cells) protect soft tissues and vital body organs. The hard matrix of bone stores important minerals, such as calcium and phosphorus. Finally, many areas of spongy bone contain hemopoietic (hē ḿ ō - = Lymphocyte poy-et ́ik; hemat blood) cells, which form a type of reticular (white blood connective tissue that is responsible for producing blood cells (a cell) process called hemopoiesis). Thus, the connective tissue that produces our blood cells is stored within our spongy bone. Neutrophil (white blood cell) Fluid Connective Tissue LM 720x Blood is a fluid connective tissue composed in part of cells and cell fragments called formed elements. These formed elements are erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (table 4.13). The erythrocytes transport oxygen and carbon dioxide between the lungs and the body tissues, while Platelet some leukocytes mount an immune response and others respond to foreign pathogens such as bacteria, viruses, fungi, and parasites. Platelets are involved in blood clotting. Erythrocytes Besides the formed elements, blood contains dissolved pro- (red blood cells) tein fibers in a watery ground substance. Together, the dissolved protein fibers and the watery ground substance form an extracellular matrix called plasma. Plasma transports nutrients, wastes, Lymphocyte and hormones throughout the body. The dissolved protein fibers (white blood are modified to become insoluble and form a clotting meshwork cell) if a blood vessel or tissue becomes damaged and bleeds. Blood is discussed in greater detail in chapter 21. Neutrophil (white blood cell) WHATW DID YOU LEARN?

●8 What is the extracellular matrix? What are its main components? Blood ●9 What three categories are used to classify connective tissue types? Structure Contains erythrocytes, leukocytes, and platelets; ●10 Identify the three types of protein fibers in connective tissue proper. soluble (dissolved) protein fi bers and a watery ●11 Compare loose connective tissue to dense connective tissue with ground substance form a fl uid extracellular matrix respect to fiber density and distribution and amount of ground called plasma substance. Function Erythrocytes transport oxygen and some carbon dioxide. Leukocytes initiate and control immune response. Plasma contains clotting elements to stop blood loss. Platelets help with blood clotting. Plasma transports nutrients, wastes, and hormones 4.3 Body Membranes throughout the body. Learning Objectives: Location Primarily within blood vessels (arteries, veins, capillaries) and the heart; leukocytes are also located 1. Explain the structure and function of mucous, serous, in lymphatic organs and can migrate to infected or cutaneous, and synovial membranes. infl amed tissues in the body 2. Describe where in the body the different types of membranes are found.

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The major cavities of the body have membranes that line both the internal surfaces of the cavities and the external 4.4 Muscle Tissue surfaces of some of the viscera housed within those cavities. We discuss these membranes here because they consist of an Learning Objectives: epithelial sheet and an underlying connective tissue layer. The 1. Describe the structure and function of skeletal, cardiac, four types of body membranes are mucous, serous, cutaneous, and smooth muscle. and synovial membranes. 2. Identify the body locations where each type of muscle The two principal kinds of internal membranes are tissue is found. mucous and serous membranes. A mucous membrane, also called a mucosa (mū -kō ś ă ), lines body passageways and com- Muscle tissue is composed of specialized cells (fibers) that partments that eventually open to the external environment; respond to stimulation from the nervous system by undergoing these include the digestive, respiratory, reproductive, and uri- internal changes that cause them to shorten. As muscle tissue nary tracts. Mucous membranes perform absorptive, protective, shortens, it exerts physical forces on other tissues and organs to and/or secretory functions. A mucous membrane is composed produce movement; these movements include voluntary motion of an epithelium and underlying connective tissue called the of body parts, blood circulation, respiratory activities, propulsion lamina propria. Often, it is covered with a thin layer of mucus of materials through the digestive tract, and waste elimination. derived from goblet cells, multicellular mucous glands, or both. In order to perform these functions, muscle cells are very differ- The mucus prevents the underlying layer of cells from drying ent from typical cells with respect to their cellular organization, out (a process called desiccation ), provides lubrication, and cellular organelles, and other properties. traps bacteria and foreign particles to prevent them from invad- 4.4a Classification of Muscle Tissue ing the body. The three histologic types of muscle in the body are skeletal muscle, A serous membrane, also termed a serosa (se-rō ś ă ), is cardiac muscle, and smooth muscle. The contraction mechanism is composed of a simple squamous epithelium called mesothelium somewhat similar in all three, but they vary in their appearance, and a thin underlying layer of loose connective tissue. The meso- location, physiology, internal organization, and means of control thelium is so named because it is derived from mesoderm. Serous by the nervous system. Specific details about the muscular system membranes produce a thin, watery serous fluid, or transudate are discussed in chapter 10. (tran ́soo-dā t; trans = across, sudo = to sweat), which is derived from blood plasma. Serous membranes are composed of two Skeletal Muscle Tissue parts: a parietal layer that lines the body cavity and a visceral Skeletal muscle tissue is composed of cylindrical muscle cells layer that covers organs. The parietal and visceral layers are called muscle fibers (table 4.14a). Individual skeletal muscle in close contact; a thin layer of serous fluid between them cells are slender and often long (sometimes the length of the entire reduces the friction between their opposing surfaces. Examples muscle). Such long cells need more than one nucleus to control of serous membranes include the pericardium, the peritoneum, and carry out all cellular functions, so each skeletal muscle fiber and the pleura. is multinucleated; some contain hundreds of nuclei. These multiple The largest body membrane is the cutaneous (kū -tā ń ē -ŭ s; nuclei form when smaller embryonic muscle cells fuse early in the cutis = skin) membrane, more commonly called the skin. The cuta- development of the skeletal muscle fiber. The nuclei in skeletal neous membrane is composed of a keratinized stratified squamous muscle fibers are located at the edge of the cell (called the periph- epithelium (called the epidermis) and a layer of connective tissue ery), immediately internal to the plasma membrane. (termed the dermis) upon which the epithelium rests (see chapter 5). Skeletal muscle is described as striated and voluntary. Under It differs from the other membranes discussed so far in that it the light microscope, the cells of skeletal muscle exhibit alternat- is relatively dry. Its many functions include protecting internal ing light and dark bands, termed striations (strı̄ -ā sh́ ŭ n; striatus = organs and preventing water loss. furrow), that reflect the overlapping pattern of parallel thick and Some joints of the skeletal system are lined by a synovial thin contractile protein filaments inside the cell. Additionally, (si-nō v́ ē -ă l; syn = together, ovum = egg) membrane, which is com- skeletal muscle is considered voluntary because it usually does posed of a well-vascularized areolar, fibrous, or adipose connective not contract unless stimulated by the somatic (voluntary) nervous tissue under a superficial highly cellular lining . Some of the cells system. Skeletal muscle attaches to the bones of the skeleton and secrete a synovial fluid that reduces friction in the joint cavity also forms muscles associated with the skin, such as the muscles and distributes nutrients to the cartilage on the joint surfaces of of facial expression and those forming body sphincters that help the bone. control waste removal. When skeletal muscles contract and relax, they produce heat for the body, which is why we become warmer WHAT DO YOU THINK? when we “work out” at the gym or fitness center. ●5 What type of body membrane is found on the external surface of Cardiac Muscle Tissue your forearm? Cardiac muscle tissue is confined to the thick middle layer of the heart wall (called the myocardium). Macroscopically, cardiac WHATW DID YOU LEARN? muscle tissue resembles skeletal muscle in that both contain visible striations (table 4.14b). However, several obvious cellular ●12 What is the function of mucous membranes? differences distinguish the two types. First, the typical cardiac ●13 Distinguish between the parietal and visceral layers of the serous muscle cell is much shorter than a typical skeletal muscle fiber. membrane. Second, a cardiac muscle cell contains only one or two centrally

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Table 4.14 Muscle Tissue

Skeletal muscle Heart wall

Intercalated discs

Cardiac muscle cell

Nuclei Striations

Nuclei Skeletal muscle fiber Striations

Intercalated LM 500x discs

Cardiac muscle cell

Striations

Nuclei

Skeletal muscle fiber Striations

(a) Skeletal Muscle Tissue (b) Cardiac Muscle Tissue Structure Fibers are long, cylindrical, striated, parallel, and Structure Cells are short, bifurcated, and striated, with one unbranched; ﬁ bers are multinucleated with nuclei or two centrally located nuclei; intercalated discs along periphery between cells Function Moves skeleton; responsible for voluntary body Function Involuntary contraction and relaxation pump blood in movements, locomotion, heat production heart Location Attaches to bones or sometimes to skin (e.g., facial Location Heart wall (myocardium) muscles); also found in the voluntary sphincters—lips, urethra, anus

located nuclei. Third, the cardiac muscle cell often bifurcates electrical stimulus (nerve impulse) through many cardiac muscle (branches), thus resembling a Y in shape. Finally, cardiac muscle cells at once, allowing the entire muscle wall to contract as a unit. cells are connected by intercalated discs (in-ter ́kă-l ā-ted; interca- When you view cardiac muscle tissue under the microscope, lates = inserted between), which are strong gap junctions between the intercalated discs appear as thick, dark lines between the cells. The intercalated discs promote the rapid transport of an the cells.

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initiate the contraction. Thus, cardiac muscle tissue is both striated and involuntary. Smooth Muscle Tissue Muscularis of Smooth muscle tissue is so named because it lacks the stria- small intestine tions seen in the other two types of muscle tissue, so the cells appear smooth (table 4.14c). Smooth muscle tissue is also called visceral muscle tissue because it is found in the walls of most viscera, such as the stomach, urinary bladder, and blood vessels. The contraction of smooth muscle helps propel and control the movement of material through these organs. Smooth muscle cells are fusiform (spindle-shaped), which means they are thick in the middle and tapered at their ends. The cells are also rela- Nuclei of tively short. Each cell has one centrally placed nucleus. Smooth smooth muscle is considered involuntary because we do not have vol- muscle cells untary control over it. For example, you cannot voluntarily stop your stomach from digesting your food or your blood vessels from transporting your blood.

Study Tip! Ask the following questions to help distinguish the three types of muscle tissue under the microscope: 1. What is the shape of the cell? Skeletal muscle fibers are long Nuclei of and cylindrical; smooth muscle cells are fusiform; and cardiac smooth muscle cells are short and bifurcated. muscle cells 2. Are the nuclei centrally located or at the periphery of the cell? Skeletal muscle nuclei are located at the periphery, while cardiac and smooth muscle cells have centrally located nuclei. 3. How many nuclei are present? A skeletal muscle fiber contains many nuclei at the periphery of the cylindrical fiber. A smooth muscle cell has one nucleus at the center of the cell. A cardiac muscle cell has one or two nuclei at its center. 4. Do the cells have striations? Smooth muscle cells have no striations, while cardiac and skeletal muscle cells do have striations. 5. Do you see intercalated discs? Only cardiac muscle has intercalated discs.

WHATW DID YOU LEARN?

●14 What type of muscle tissue has long, cylindrical, multinucleated (c) Smooth Muscle Tissue cells with obvious striations? Structure Cells are fusiform (spindle-shaped), short, ●15 Why is smooth muscle referred to as involuntary? nonstriated, and contain one centrally located nucleus Function Involuntary movements and motion; moves 4.5 Nervous Tissue materials through internal organs Learning Objectives: Location Walls of hollow internal organs, such as vessels, airways, stomach, bladder, uterus 1. Explain the structure and function of nervous tissue. 2. Describe where nervous tissue is found in the body. Nervous tissue is sometimes termed neural tissue. It consists Cardiac muscle is responsible for the rhythmic heart con- of cells called neurons (noor on),́ or nerve cells, and a larger num- tractions that pump blood throughout the blood vessels of the ber of different types of glial cells (or supporting cells) that support, body. Cardiac muscle cells are considered involuntary because protect, and provide a framework for neurons (table 4.15). This they do not require nervous system activity to initiate a contrac- tissue will be discussed in detail in chapter 14, but we provide a tion; instead, specialized cardiac muscle cells in the heart wall brief description here.

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Table 4.15 Nervous Tissue 4.5a Characteristics of Neurons Neurons are specialized to detect stimuli, process information quickly, and rapidly transmit electrical impulses from one region of the body to another. Each neuron has a prominent cell body, or soma, that houses the nucleus and most other organelles. The cell Brain body is the “head” that controls the rest of the cell and produces proteins for the cell. Extending from the cell body are branches called nerve cell processes. The short, branched processes are dendrites (den ́drı̄ tes; dendritēs = relating to a tree), which receive Dendrite incoming signals from other cells and transmit the information to the cell body. The long nerve cell process extending from a cell body is the axon (ak son;́ axon = axis), which carries outgoing signals to other cells. Due to the length of an axon, neurons are usually the longest cells in the body; some are longer than a meter. Much of the nervous tissue in the body is concentrated in the brain and spinal cord, the control centers for the nervous system. Cell body of neuron WHATW DID YOU LEARN?

●16 What general name is applied to the supporting cells in nervous tissue? Axon Nuclei of glial cells 4.6 Tissue Change and Aging LM 1000x Learning Objectives: 1. Describe how tissues may change in form, size, or number. 2. Identify the changes that occur in tissues with age. Dendrite Some tissue subtypes, although well established in an adult, may undergo changes. 4.6a Tissue Change Sometimes a mature epithelium changes to a different form of mature epithelium, a phenomenon called metaplasia (met-ă-pl ā ź ē - Cell body ă ; metaplasis = transformation). Metaplasia may occur as an epi- of neuron thelium adapts to environmental conditions. For example, smokers typically experience metaplastic changes in the tracheal epithelium. The smoke and its by-products are the environmental stressors that change the normal pseudostratified ciliated columnar epithelium Axon lining the trachea to a stratified squamous epithelium. Thus, the Nuclei of act of smoking causes metaplastic changes in the epithelium of the glial cells airway. Tissues can grow or shrink in two ways: by a change in cell size or by a change in cell number. Increase in the size of existing Nervous Tissue cells is called hypertrophy; increase in the number of cells in the Structure Contains neurons with rounded or stellate cell tissue due to mitosis is called hyperplasia. When growth proceeds bodies and an axon and dendrites extending from out of control, a tumor that is composed of abnormal tissue devel- the cell body; glial cells lack such extensive ﬁ brous ops, and the condition is termed neoplasia (nē -ō -plā ź ē -ă ; neo = processes new, plasis = molding). Function Neurons: Responsible for control; information Shrinkage of tissue by a decrease in either cell number or cell processing, storage, and retrieval; internal ́ ō ē communication size is called atrophy (at r -f ). Atrophy may result from normal Glial cells: Support and protect neurons aging (senile atrophy) or from failure to use an organ (disuse atrophy). When people do not perform normal activities, their muscles Location Brain, spinal cord, nerves exhibit disuse atrophy as the cells become smaller.

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CLINICAL VIEW Gangrene diseases that restrict blood flow, primarily to the hands and feet, the areas most commonly affected by dry gangrene. G angrene is the necrosis (death) of the soft tissues of a body part due Wet gangrene is caused by a bacterial infection of tissues that have lost to a diminished or obstructed arterial blood supply to that region. The their blood and oxygen supply. The cells in the dying tissue rupture and body parts most commonly affected are the limbs, fingers, or toes. release fluid (hence the name “wet” gangrene). The wet environment Gangrene may also occur as a consequence of either a bacterial infec- allows bacteria to flourish, and they often produce a foul-smelling pus. The tion or direct mechanical injury. Gangrene is a major complication for most common bacteria associated with wet gangrene are Streptococcus, diabetics, who often suffer from diminished blood flow to their upper Staphylococcus, Enterobacter, and Klebsiella. Wet gangrene must be treated and lower limbs as a consequence of their disease. There are several quickly with antibiotics and removal of the necrotic tissue. different types of gangrene. Gas gangrene is often mistaken for wet gangrene. However, the bacteria Intestinal gangrene usually occurs following an obstruction of the typically associated with gas gangrene are Clostridrium, a type of bacterium blood supply to the intestines. If the intestines are without sufficient that is called anaerobic because it can live and grow in the absence of blood, the tissue undergoes necrosis and gangrene. Untreated intestinal oxygen. This type of gangrene usually affects muscle tissue. As the bacteria gangrene leads to death. invade the necrotic tissue, a release of gases from the tissue produces gas Dry gangrene is a form of gangrene in which the involved body part is bubbles. These bubbles make a crackling sound in the tissue, especially desiccated, sharply demarcated, and shriveled, usually due to constricted if the patient is moved. Symptoms of fever, pain, and edema (localized blood vessels as a result of exposure to extreme cold. Dry gangrene can swelling) occur within 72 hours of the initial trauma to the region. The be a complication of frostbite or result from a variety of cardiovascular treatment for gas gangrene is similar to that for wet gangrene.

Dry gangrene of the foot. Gas gangrene in a recently amputated limb.

4.6b Tissue Aging the body’s structural and functional building blocks. As individu- All tissues change as a result of aging. Proper nutrition, good health, als age, epithelia become thinner, and connective tissues lose their normal circulation, and relatively infrequent wounds promote con- pliability and resilience. Because the amount of collagen in the tinued normal tissue functioning past middle age. Thereafter, the body declines, tissue repair and wound healing lose efficiency. support, maintenance, and replacement of cells and extracellular Bones become brittle; joint pains and broken bones are common. matrix become less efficient. Physical damage, chemical changes, Muscle and nervous tissue begin to atrophy. Diet and circulation and physiologic changes can alter the structure and chemical com- problems contribute to these tissue declines. Eventually, cumula- position of many tissues. For example, adequate intake of protein is tive losses from relatively minor damage or injury contribute to required to enable the cells to continue synthesizing new proteins, major health problems.

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CLINICAL VIEW: In Depth

Tissue Transplantation bacterium Clostridium, which later infected the transplant recipients and caused deaths in some cases. The tissue banks in question came Grafting is the process of surgically transplanting healthy tissue to under investigation, and these tragic circumstances illustrated the replace diseased, damaged, or defective tissue. The healthy tissue may potential complications of tissue allografts. In 2005, a New Jersey be a person’s own tissue or tissue from another individual or animal. The facility that was called a “tissue bank” stole tissue from cadavers problem with using tissue from a different donor is that the patient’s awaiting burial or cremation. This tissue was implanted into patients. body may reject the tissue as “foreign.” In essence, the body’s immune Forged death certificates and organ donor consent forms were used system attacks the tissue because it recognizes that the tissue came to try to legitimize this activity. In the spring of 2006, health pro- from another body. There are four types of tissue grafts. fessional organizations in Minnesota created a national model for the ethical procurement and use of human anatomic donations. This An autograft (au‘to¯-graft; autos = self, graef = implant) is a tissue trans- sets the stage for establishing best practices in the use of donated plant from one site on a person to a different site on the same person. human organs. Autografts are often performed with skin, as healthy skin from one part of the body is grafted to another part of the body where the skin Although most tissue allografts are successful, transplantation of has been damaged by burns or chemicals. Because an autograft uses a entire organs is much more problematic. The patient and the organ person’s own tissue, the body does not reject the tissue as “foreign.” donor must be as genetically similar as possible; traits such as blood However, autografts may not be feasible in certain situations, such as type and other blood factors must “match.” The closer the match, the when the amount of skin damaged is so great that a transplant would less likely it is that the allograft will be rejected. The recipient of the not be possible. Most burn victims have damaged too much of their transplanted organ(s) must take powerful immunosuppressant drugs, own skin to be able to provide autografts for all of their burned areas. which help prevent the body from rejecting the organ. Unfortunately, these same drugs work by suppressing the immune system, making A syngenetic (sin-je˘-net‘ik; syn = together) graft, also called an iso- the transplant patient more susceptible to illness. Even with immu- graft, is a tissue transplant from one person to a genetically identical nosuppressant drugs, rejection of allografts is common. Typically, person (i.e., an identical twin). It is unlikely that the body will reject graft rejection occurs after 15 to 25 days, when the transplant site the syngenetic graft because it came from a genetically identical has become infiltrated with graft-rejection cells that recognize and individual. However, very few of us have an identical twin, so this destroy the foreign cells. type of graft is not possible for most people. A heterograft (he˘‘ter-o¯-graft; heteros = other), also called a xenograft An allograft (al‘o¯-graft; allos = other) is the transplantation of tis- (ze¯‘no¯-graft; xeno = foreign), is a tissue transplant from an animal sue from a person who is not genetically identical. Many tissue types into a human being. For example, porcine (pig) and bovine (cow) have been used as allografts, including skin, muscle, bone, and car- tissue have been successfully used to replace heart valves, blood ves- tilage. In fact, the term allograft also refers to the transplantation sels, and bone. A chimpanzee kidney and a baboon heart have been of organs or parts of organs, such as heart valves, kidneys, and the transplanted into human patients. Porcine nervous tissue cells were liver. Orthopedists, physicians who treat musculoskeletal injuries, have transplanted into the brain of an individual with Parkinson disease frequently used musculoskeletal graf ts from cadavers for such purposes in the hope that the healthy cells would stop or reverse the progress of as knee replacements or ligament reconstruction. In these cases, the the disease. Rejection of these animal tissues usually occurs frequently bone, cartilage, and joint capsule from a cadaver are transplanted and quickly, which is not surprising, because tissue from a completely into another individual. These types of allografts are typically very different species is being transplanted into a human. However, a few successful. However, in 2002 the Food and Drug Administration (FDA) of these transplants have worked for a short time, and recent research reported that some for-profit tissue banks had sent contaminated is investigating the reasons for the lack of rejection of this tissue. tissue samples to physicians. These tissues were contaminated with the

Clinical Terms

adhesions Inflammatory bands that connect opposing serous is taken by forceps in an endoscope within a hollow organ; surfaces. in an open biopsy, a body cavity is opened for sample biopsy Microscopic examination of tissue removed from the body removal; and in an excisional biopsy, a lump is removed for the purpose of diagnosing a disease. In a skin or muscle from a tissue or organ. biopsy, a small piece of skin or muscle is removed, and the lesion (lē zh́ ŭ n) Any localized wound, injury, or infection wound is sutured; in a needle biopsy, a tissue sample is that affects tissue over a specific area rather than spread removed from skin or an organ through an inserted needle; throughout the body. in an aspiration biopsy, cells are sucked into a syringe liposuction A method of removing unwanted subcutaneous fat through a needle; in an endoscopic biopsy, a tissue section using a suction tube.

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Chapter Summary

■ There are four tissue types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

4.1 Epithelial Tissue 81 ■ Epithelial tissue covers the surface of the body, lines body cavities, and forms secretory structures called glands. 4.1a Characteristics of Epithelial Tissue 81 ■ The characteristics of an epithelium include cellularity, polarity, attachment to a basement membrane, avascularity, innervation, and high regeneration ability. ■ Epithelial cells have an apical (free) surface, junctions on lateral membranes that bind neighboring cells, and a basal surface, which is closest to the basement membrane. 4.1b Functions of Epithelial Tissue 82 ■ Epithelial tissues provide physical protection, control permeability, produce secretory products, and contain nerve cells that detect sensations. Gland cells are derived from epithelial cells and produce secretions. 4.1c Specialized Structure of Epithelial Tissue 82 ■ The four types of epithelial cell junctions are tight junctions, adhering junctions, desmosomes, and gap junctions. 4.1d Classiﬁ cation of Epithelial Tissue 84 ■ Epithelia are classified by two criteria: (1) number of cell layers, and (2) shape of apical surface cells. ■ A simple epithelium has only one cell layer overlaying the basement membrane. A stratified epithelium is two or more layers of cells thick, and only the deepest (basal) layer is in direct contact with the basement membrane. Pseudostratified columnar epithelium appears stratified but is not; all cells are in contact with the basement membrane. 4.1e Types of Epithelium 85 ■ In a simple epithelium, the surface cells are thin and flat (squamous epithelium), about as tall as they are wide (cuboidal epithelium), or taller than they are wide (columnar epithelium). ■ The shape of transitional epithelium cells changes between relaxed and distended states. 4.1f Glands 92 ■ Endocrine glands secrete hormones into the bloodstream. Exocrine glands secrete their products through ducts onto the epithelial surface. ■ Multicellular exocrine glands are classified by the structure of their ducts and the organization of the secretory portion of the gland. ■ Serous glands produce nonviscous, watery fluids; mucous glands secrete mucin that forms mucus; and mixed glands produce both types of secretions.

4.2 Connective ■ Connective tissue binds, protects, and supports the body organs. Tissue 95 4.2a Characteristics of Connective Tissue 95 ■ Connective tissue contains cells, protein fibers, and a ground substance. The protein fibers and ground substance together form the extracellular matrix. 4.2b Functions of Connective Tissue 95 ■ Connective tissue provides physical protection, support and structural framework, binding of structures, storage, transport, and immune protection. 4.2c Development of Connective Tissue 96 ■ All connective tissues are derived from two types of embryonic connective tissue: mesenchyme and mucous connective tissue. 4.2d Classiﬁ cation of Connective Tissue 98 ■ Loose connective tissue has a high volume of ground substance; it is easily distorted and serves to cushion shocks. ■ Dense connective tissue consists primarily of large amounts of extracellular protein fibers. ■ Supporting connective tissue (cartilage and bone) provides support and protection to the soft tissues and organs of the body. ■ Blood is a fluid connective tissue. Its cells are called formed elements, and the dissolved protein fibers and watery ground substance form an extracellular matrix called plasma.

4.3 Body ■ Mucous membranes line cavities that communicate with the exterior. Membranes 108 ■ Serous membranes line internal cavities and are delicate, moist, and very permeable. ■ The external body surface is covered by the cutaneous membrane, which is dry, keratinized, and relatively thick. ■ Synovial membranes line the inner surface of synovial joint cavities.

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Chapter Summary (continued)

4.4 Muscle Tissue 109 ■ Muscle tissue is composed of muscle cells, sometimes termed muscle fibers, which are capable of contractions resulting in cellular shortening along their longitudinal axes and producing movement, either of the skeleton or specific body parts. 4.4a Classiﬁ cation of Muscle Tissue 109 ■ Skeletal muscle tissue is composed of long, multinucleated, cylindrical fibers that are striated and voluntary. ■ Cardiac muscle tissue is located within the wall of the heart. It is composed of branched, short cells with one or two centrally located nuclei. It is striated and involuntary. ■ Smooth muscle tissue is found in the walls of organs; it has short, tapered cells that are nonstriated and involuntary.

4.5 Nervous Tissue 111 ■ Nervous tissue is composed of two specific cell types: neurons and glial cells. Neurons receive stimuli and transmit impulses in response. Glial cells interact with each other to form an extensive supporting framework for neurons and nervous tissue. Additionally, glial cells help provide nutrient support to the neuron. 4.5a Characteristics of Neurons 112 ■ Neurons have a prominent cell body, dendrites, and a long process called the axon. 4.6 Tissue Change 4.6a Tissue Change 112 and Aging 112 ■ Metaplasia is a change from one mature epithelial type to another in response to injury or stress. ■ Hypertrophy is an increase in cell size, whereas hyperplasia is an increase in cell number. 4.6b Tissue Aging 113 ■ When tissues age, repair and maintenance become less efficient, and the structure and chemical composition of many tissues are altered.

Challenge Yourself

Matching ______2. What is the predominant cell type in areolar Match each numbered item with the most closely related lettered connective tissue? item. a. mesenchymal cell b. fibroblast 1. smooth muscle a. a characteristic of all epithelia c. adipocyte 2. merocrine secretion b. contains intercalated discs d. satellite cell 3. ground substance c. lines the small intestine ______3. Preventing desiccation and providing surface lumen lubrication within a body cavity are the functions of 4. simple columnar ______membranes. epithelium d. scattered arrangement of a. cutaneous protein fibers 5. goblet cell b. mucous e. part of extracellular matrix c. serous 6. dense regular d. synovial connective tissue f. unicellular exocrine gland ______4. Which of the following is a correct statement about a 7. endot he l iu m g. parallel arrangement of simple epithelium? protein fibers 8. cardiac muscle a. It protects against mechanical abrasion. h. salivary glands, for example b. It may contain the protein keratin. 9. dense irregular c. It is adapted for diffusion and filtration. i. lines blood vessel lumen connective tissue d. It is formed from multiple layers of epithelial cells. j. has no striations 10. avascular ______5. Which of the following is not a function of an epithelium? a. It is selectively permeable. b. It serves as a packing and binding material. c. The cells can produce secretory products. Multiple Choice d. It is designed for physical protection. Select the best answer from the four choices provided. ______6. Which connective tissue type is composed of cells ______1. Which type of tissue contains a calcified ground called chondrocytes and may be surrounded by a substance and is specialized for structural support? covering called perichondrium? a. muscle tissue a. cartilage b. nervous tissue b. dense irregular connective tissue c. areolar connective tissue c. bone d. bone connective tissue d. areolar connective tissue

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______7. Aging effects on tissue include which of the following? 6. What are the main structural differences between dense a. Tissue is less able to maintain itself. regular and dense irregular connective tissue? b. Tissue has a decreased ability to repair itself. 7. In what regions of the body would you expect to find hyaline c. Epithelium becomes thinned. cartilage, fibrocartilage, and elastic cartilage, and why would d. All of these are correct. these supporting connective tissues be located in these ______8. Which epithelial tissue type lines the trachea (air regions? tube)? 8. Name the four types of body membranes, and cite a location a. simple columnar epithelium of each type. b. pseudostratified ciliated columnar epithelium 9. A significant structural feature in the microscopic study of c. simple squamous epithelium cardiac muscle cells is the presence of gap junctions between d. stratified squamous epithelium neighboring cells. Why are these junctions so important? ______9. Which muscle type consists of long, cylindrical, 10. What are the similarities and differences between skeletal striated cells with multiple nuclei located at the muscle, cardiac muscle, and smooth muscle? periphery of the cell? a. smooth muscle Developing Critical Reasoning b. cardiac muscle 1. During a microscopy exercise in the anatomy laboratory, c. skeletal muscle a student makes the following observations about a tissue d. All of these are correct. section: (1) The section contains some different types of scattered protein fibers—that is, they exhibit different widths, ______10. A gland that releases its secretion by exocytosis into some are branched, some are long and unbranched, and secretory vesicles is called a ______gland. their staining characteristics differ (some are observed only a. apocrine with specific stains). (2) Several cell types with different b. merocrine morphologies are scattered throughout the section, but these c. holocrine cells are not grouped tightly together. (3) The observed d. All of these are correct. section has some “open spaces”—that is, places between cells and the observed fibers in the section that appear clear with Content Review no recognizable features. What type of tissue is the student 1. What are some common characteristics of all types of observing? Where might this tissue be found in the body? epithelium? 2. Your father is suffering from painful knee joints. He has been 2. Describe the types of intercellular junctions between told that he either has the early stages of arthritis or some epithelial cells and where each is located. inherent joint problems. His friend recommends that he take a chemical supplement with his meals (chondroitin sulfate), 3. List the epithelial type that is found (a) lining the lumen of which has been shown to help some people with joint aches the stomach, (b) lining the oral cavity, (c) lining the urinary and pains. This supplement stimulates growth and recovery bladder, and (d) lining the tiny air sacs of the lungs. of degenerated cartilage on the surfaces of bones in joints. 4. What are the three secretion methods of exocrine glands, Based on your knowledge of connective tissues, do you think and how does each method work? the chondroitin sulfate supplements could help your father’s 5. What characteristics are common to all connective tissues? knee problems?

Answers to “What Do You Think?”

1. If epithelium contained blood vessels, the “gatekeeper” The number of protein fibers in connective tissue is related function of selective permeability would be compromised. to the strength and support the connective tissue gives. Materials would be able to enter the body by entering the The ground substance can serve as a packing and binding bloodstream without passing through the epithelium. material and can suspend the cells and protein fibers. 2. All types of stratified epithelium (stratified squamous, 4. You have damaged dense regular connective tissue when stratified columnar, stratified cuboidal) and transitional you sprain your ankle. epithelium are suited for protection, because they have 5. A cutaneous membrane is found on the external surface of multiple layers of cells. your forearm. 3. Connective tissue has fewer cells because it contains other materials, such as protein fibers and ground substance.

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

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