Tissues: Concept and Classification Tissues Concept and Classification

*Tissues are aggregation or groups of cells organized in 3D, that function in a collective manner to perform one or more specific functions.. * Tissue classification based on structure of cells, composition of noncellular extracellular matrix, and cell function...

* Major types of adult tissues - Epithelial (epithelial tissue) covers body surfaces, lines body cavities, and forms - Connective underlies or supports the other three basic tissues, both structurally and functionally. - Muscle is made up of contractile cells and is responsible for movement. - Nervous receives, transmits, and integrates information from outside and inside the body to control the activities of the body. Histogenesis of tissues Ectoderm Derivatives * Ectoderm is the outermost of the three germ layers. The derivatives of the ectoderm may be divided into two major classes: surface ectoderm and neuroectoderm. - Surface ectoderm gives rise to: • epidermis and its derivatives (hair, nails, sweat glands, sebaceous glands, and the parenchyma and ducts of the mammary glands), • cornea and lens epithelia of the eye, • enamel organ and enamel of the teeth, • components of the internal ear, • adenohypophysis (anterior lobe of pituitary ), and • mucosa of the oral cavity and lower part of the anal canal. - Neuroectoderm gives rise to: • the neural tube and its derivatives, including components of the central nervous system, ependyma ( lining the cavities of the brain and spinal cord), pineal body, posterior lobe of pituitary gland (neurohypophysis),and the sensory epithelium of the eye, ear, and nose; • the neural crest and its derivatives, including componentsof the peripheral nervous system (cranial, spinal, and autonomic ganglia, peripheral nerves, and Schwann cells); glialcells (oligodendrocytes and astrocytes); chromaffin (medullary) cells of the adrenal gland; enteroendocrine (APUD) cells of the diffuse neuroendocrine system; melanoblasts, the precursors of melanocytes; the mesenchyme of the head and its derivatives (such as pharyngeal arches that contain muscles, connective tissue, nerves, and vessels); odontoblasts; and corneal and vascular . Mesodermal Derivatives

- Mesoderm is the middle of the three primary germ layers of an embryo. It gives rise to: • connective tissue, including embryonic connective tissue (mesenchyme), connective tissue proper (loose and dense connective tissue), and specialized connective tissues (cartilage, bone, adipose tissue, blood and hemopoietic tissue), and lymphatic tissue; • striated muscles and smooth muscles; • heart, blood vessels, and lymphatic vessels, including their endothelial lining; • spleen; • kidneys and the gonads (ovaries and testes) with genital ducts and their derivatives (ureters, uterine tubes, uterus, ductus deferens); • , the epithelium lining the pericardial, pleural, and peritoneal cavities; and • the adrenal cortex. Endodermal Derivatives

- Endoderm is the innermost layer of the three germ layers. In the early embryo it forms the wall of the primitive gut and gives rise to epithelial portions or linings of the organs arising from the primitive gut tube. Derivatives of the endoderm include: • alimentary canal epithelium (excluding the epithelium of the oral cavity and lower part of the anal canal, which are of ectodermal origin); • extramural digestive gland epithelium (e.g., the liver, pancreas, and gallbladder); • lining epithelium of the urinary bladder and most of the urethra; • respiratory system epithelium; • thyroid, parathyroid, and thymus gland epithelial components; • parenchyma of the tonsils; • lining epithelium of the tympanic cavity and auditory (Eustachian) tubes. Derivatives of the three germ layers Epithelium Tissue Learning objectives:

- Understand the general features and classification of epithelial tissue. - Be able to correlate different types of epithelia to their functions. - Know the structure of apical , basal, and lateral specializations and their functions. - Understand the L. M structure, E. M structured features and functions of microvilli and cilia. - Understand the ultra-structure features and functions of varieties of intercellular conjunctions. - Understand the position, L. M structure, ultra-structure and function of . - Know the conception of glandular cells, glandular epithelium, and the morphological classification of exocrine glands. Epithelial tissue; General Characteristics & Functions

* Epithelium is an avascular tissue composed of cells that: - Covers exterior body surface and lines internal closed cavities body cavity and body tubes. - Forms the secretory portion (parenchyma) of glands and their ducts. - Specialized epithelial cells function as receptors for the special senses Special Characteristics of Epithelia (General structural features) * Cellularity - cells are in close contact and adhere to one another by means of specific cell-to-cell adhesion molecules with little or no intercellular space between them. * Specialized contacts - may have junctions for both attachment and communication * Polarity - epithelial tissues always have free surface or apical domain, a lateral domain, and a basal domain. * Support by connective tissue - at the basal surface, both the epithelial tissue and the connective tissue contribute to the basement membrane * Avascular - nutrients must diffuse * Innervated * Regenerative - epithelial tissues have a high capacity for regeneration Special Characteristics of Epithelia * In special situations, epithelial cells lack a free surface ( Epithelioid tissues). - Epithelioid organization is typical of most endocrine glands.

- Epithelioid patterns are also formed by accumulations of connective tissue macrophages in response to certain types of injury and infections as well as by many tumors derived from epithelium. * Functions of epithelium Epithelium may serve one or more functions, depending on the activity of the cell types that are present: - Mechanical protection, - Absorption, - Secretion, - Transportation - Filtration (Selective barrier ), - Forms slippery surfaces and - Sensory reception

* Basal cells are the stem cells that give rise to the mature functional cells of the epithelium, thus balancing cell turnover. Classifications of Epithelia * First name of tissue indicates number of layers - Simple – one layer of cells

- Stratified – more than one layer of cells Classifications of Epithelia * Last name of tissue describes shape of cells

- Squamous – cells wider than tall (plate or “scale” like)

- Cuboidal – cells are as wide as tall, as in cubes

- Columnar – cells are taller than they are wide, like columns Classification of epithelial tissue on the base of the function Naming Epithelia

* Naming the epithelia includes both the layers (first) and the shape of the cells (second) - i.e. stratified cuboidal epithelium * The name may also include any accessory structures or specialization of the apical cell surface domain. - Mucous cells - Microvilli - Cilia - * Special epithelial tissues (don’t follow naming convention - Psuedostratified - Transitional (urothelium) is a stratified epithelium

* The cells in some exocrine glands are more or less pyramidal. Simple Squamous Epithelium

* Description

- single layer of flat cells with disc-shaped nuclei

* Special types - Endothelium (inner covering) . lining of the blood and lymphatic vessels. - Endocardium . lining of ventricles and atria of the heart. - Mesothelium (middle covering) . Lines peritoneal, pleural, and pericardial cavities . Covers visceral organs of those cavities Simple Squamous Epithelium

* Function - Transepithelial transport by passive diffusion and filtration - Secretes lubricating substances in serous membranes * Location - Renal corpuscles - Alveoli of lungs - Lining of heart, blood and lymphatic vessels - Lining of ventral body cavity (serosae/serous memb.) Simple Squamous Epithelium Simple Squamous – Endothelium (En) Simple Squamous – Endothelium (Ed) by TEM Endothelium Simple squamous epithelial line loop of Henle Simple squamous (Surface of mesothelium)

Cell margins

nuclei

Bio 348 Lapsansky - 2007 Mesothelium and endothelium Mesothelium Simple Squamous Epithelium (mesothelium)

Surface view of lining of peritoneal cavity • Section of intestine showing serosa Simple Squamous (Mesothelium) by TEM Simple Cuboidal Epithelium

* Description - single layer of cube-like cells with large, spherical central nuclei * Function - secretion and absorption * Location - kidney tubules, secretory portions of small glands, ovary surface Simple Cuboidal Epithelium Simple Cuboidal by TEM

Simple Columnar Epithelium

* Description - single layer of column-shaped (rectangular) cells with oval nuclei . Some bear cilia at their apical surface . Some bear microvilli ------. May contain goblet cells . Some consist of mucus cells * Function - Absorption; secretion of mucus, enzymes, and other substances (The height of the cells often reflects the level of secretory or absorptive activity). - Ciliated type propels mucus or reproductive cells by ciliary action Simple Columnar Epithelium

* Location - Consist of mucus cells . Lines - Bear microvilli . Lines digestive tract, gallbladder (striated border) , kidney tubule (brush border) and ducts of some gland (striated ). - Ciliated form . Uterine tubes, and uterus. Simple Columnar mucus secreting ( stomach) Simple Columnar Epithelium (Human duodenum.) Simple Columnar Epithelium (striated border) Simple Columnar Epithelium (striated border) Simple Columnar (striated border) with Goblet Cells (Mu) by TEM Simple Columnar Epithelium (Human gallbladder) Striated border Ciliated Simple Columnar Epithelium Light micrograph of human oviduct. The lumen is lined by simple columnar epithelium composed of ciliated and nonciliated cells Stratified Epithelia

- Contain two or more layers of cells - Regenerate from below - Major role is protection - Are named according to the shape of cells at apical layer Stratified Squamous Epithelium

* Description - Many layers of cells – squamous in shape - Deeper layers of cells appear cuboidal or columnar -Thickest epithelial tissue – adapted for protection Stratified Squamous Epithelium

* Specific types - Keratinized – contain the protective protein keratin .Surface cells are dead and full of keratin - Non-keratinized – forms moist lining of body openings * Function - Protects underlying tissues in areas subject to abrasion * Location - Keratinized – forms epidermis - Non-keratinized – forms lining of esophagus, mouth, and vagina Stratified Squamous Epithelium

Non-keratinized vs. Keratinized Stratified Squamous Epithelium: Keratinized of human eyelid. Keratinized

Stratified Squamous Epithelium: Nonkeratinized Epithelium of human oral cavity Stratified Squamous (Lining mucosa of vagina) Stratified Squamous (Lining mucosa of Esophagus) Stratified Cuboidal Epithelium

* Description - generally two layers of - cube-shaped cells * Function - protection * Location - Forms largest ducts of sweat glands - Forms ducts of mammary glands and salivary glands Stratified Columnar Epithelium

* Description - several layers; basal cells usually cuboidal; superficial cells elongated * Function - protection and secretion * Location - Rare tissue type - Found in male urethra largest ducts of salivary glands, conjunctiva Stratified Columnar Epithelium Conjunctiva of human eyelid Stratified Columnar Epithelium

Male urethra Epithelium of palpebral conjunctiva Special classifications of epithelium

Two special categories of epithelium are pseudostratified and transitional. Pseudostratified Columnar Epithelium

* Description - All cells originate at basement membrane - Only tall cells reach the apical surface - May contain goblet cells and bear cilia - Nuclei lie at varying heights within cells .Gives false impression of stratification * Function - secretion of mucus; propulsion of mucus by cilia

* Locations - Non-ciliated type . Ducts of male reproductive tubes . Ducts of large glands - Ciliated variety . Lines trachea and most of upper respiratory tract Ciliated Pseudostratified Columnar Epithelium (Human trachea) Pseudostratified Columnar Epithelium

- Single cell layer - All cells attach to basement membrane but not all reach free surface - Nuclei at varying depths Pseudostratified Ciliated Epithelium with Goblet Cells Pseudostratified columnar epithelium with stereo- Microvilli (ducts epididymis) Pseudostratified Epithelium with Stereovilli

* Description - Basal cells usually cuboidal or columnar - Superficial cells dome-shaped or squamous * Function - stretches and permits distension of urinary bladder * Location - Lines ureters, urinary bladder and part of urethra Transitional Epithelium Contracted urinary bladder Transitional Epithelium Distended urinary bladder Transitional Epithelium

Stretched state Relaxed state

Photomicrograph : Transitional epithelium lining of the bladder, relaxed and stretched state (500x)

* Epithelial metaplasia; - Is a reversible conversion of one mature epithelial cell type to another mature epithelial cell type. - Metaplasia results from reprogramming of epithelial stem cells that changes the patterns of their gene expression. . Columnar to- squamous (squamous metaplasia) . Squamous-to-columnar epithelial metaplasia (columnar metaplasia). - Metaplasia is usually a reversible phenomenon. - If abnormal stimuli persist for a long time, squamous metaplastic cells may transform into squamous cell carcinoma. . ( of the lung, cervix, and bladder often originate from squamous metaplastic epithelium). . Squamous columnar epithelium may give rise to glandular adenocarcinomas.

Squamous metaplasia of the uterine cervix. Photomicrograph of a cervical canal lined by simple columnar epithelium. Note that the center of the image is occupied by an island containing squamous stratified epithelium. This metaplastic epithelium is surrounded on both sides by simple columnar epithelium. Since metaplasia is triggered by reprogramming of stem cells, metaplastic squamous cells have the same characteristics as normal stratified squamous epithelium. 240. (Courtesy of Dr. Fabiola Medeiros.) Squamous metaplasia

columnar metaplasia

Transformation of stratified squamous epithelium of Esophagus to simple columnar epithelium with goblet cells (intestinal metaplasia) Cell polarity

* Apical surface * Lateral surface * Basal surface

Specific biochemical characteristics are associated with each cell surface. Apical domain and its modifications

* Microvilli, cytoplasmic processes containing a core of filaments, It is about 0. 2μm in diameter; * Stereocilia (stereovilli), microvilli of unusual length, it can be as long as 120 μm; and * cilia, cytoplasmic processes containing bundle of microtubules, it is about 5-10μm long and 0. 2μm in diameter. Apical surface features (Microvilli)

- Microvilli are short, irregular, bleb like projections In some cell types. . In other cell types, they are tall, closely packed, uniform projections. Electron micrograph showing variation in microvilli of different cell types:

Uterine gland; very small projection, bleb like

placenta; irregular, branching microvilli

Intestinal absorptive cell with very uniform and regularly arranged microvilli Uniform microvilli

* Description - finger-like extensions of plasma membrane - The internal structure of microvilli contains a core of actin filaments that are cross-linked by several actin-bundling proteins. * Location - Abundant in epithelia of small intestine (striated border) and kidney (brush Border). * Function - Maximize surface area across which small molecules enter or leave - Act as stiff knobs that resist abrasion the number and shape of the microvilli of a given cell type correlate with the cell’s absorptive capacity. Microvilli (TEM--transmission electron micrograph) Molecular structure of microvilli Microvilli Apical surface features (stereovilli)

* Description - Stereocilia are unusually long, immotile microvilli. - stereocilia are supported by internal bundles of actin filaments that are cross- linked by fimbrin.

* Location - limited to the epididymis, the proximal part of the ductus deferens of the male reproductive system, (absorptive structures ) - and the sensory (hair) cells of the inner ear, (are uniform). . Serve as sensory mechanoreceptors

. Stereocilia (stereovilli) Molecular structure of stereocilia Stereocilia of the sensory epithelium Serve as sensory mechanoreceptors

They are uniform in diameter and organized into ridged bundles of increasing heights. Stereocilia of sensory epithelia lack both ezrin and α-actinin. Dynamic turnover of an internal architecture of sterocilia

Diagram illustrates the mechanism by which the core of actin filaments is remodeled. Actin polymerization and espin cross-linking into the barbed (plus) end of actin filaments occurs at the tip of the stereocilia. Disassembly and actin filament depolymerization occurs at the pointed (minus) end of actin filament near the base of the stereocilium. When the rate of assembly at the tip is equivalent to the rate of disassembly at the base, the actin molecules undergo an internal rearward flow or treadmilling, thus maintaining the constant length of the stereocilium. (Reprinted with permission from Rzadzinska AK, Schneider ME, Davies C, Riordan GP, Kachar B. An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal. J Cell Biol 2004;164:887–897.) stereocilia (stereovilli) are unusually long, immotile microvilli. Cilia

- In general, cilia are classified as . motile, . primary, and . nodal.

Motile cilia * Description - They are hair like, highly motile extensions of the apical plasma membrane. . 250 or more cilia (in each cell), arranged in parallel rows.

* Location - ciliated epithelia, such as the trachea, bronchi, or oviducts, Ductus efferentes.

* Function; Cilia beat in a rhythmical wave –like manner - the cilia sweep mucus and trapped particulate material toward the oropharynx where it is swallowed with saliva and eliminated from the body. - In the oviducts, cilia help transport ova and fluid toward the uterus. - drive the spermatozoa toward the epididymis Cilia

With L.M - Hair –like protrusions - No internal structure is detectable - At the base of each cilium a dense granule (Basal body) is seen

With E.M - Have a complex internal structure Contains a core A characteristic arrangement of nine pairs of microtubules encircling one middle pair of microtubules called “ Axoneme’’ = 9+2 microtubule complex . - centriole derived, microtubule organizing center (MTOC) .

- Each cilium is covered by an extention of plasmalemma . Tapering tip . Cylindrical shaft . Transitional zone . Basal body (located in the apical cytoplasm) Dynein arms - Are arranged along the length of the microtubule . - They are formed by a protein called “ dynein’’ and contain ATPase (ATP splitting enzyme)activity. - Extremely large protein - Dynein arms form temporary cross bridges between microıtubule A of one doublet and microtubule B of the adjacent doublet. - During sliding they undergo a cyclic break and reattachment . - Formation of cross bridges is ATP dependent. “Nexin links’’ - Attach each microtubule A to the microtubule B of the adjacent doublet. - Nexin links are composed of an elastic material called “nexin’’ - Responsible for recovery stroke Basal Bodies

- Cylindrical structures about 0.2μ in diameter and 0.4μ in length - Nine groups of three microtubules fused into triplets form the wall of the basal body - Basal body resembles a centriol - Basal body contains some accessory structures such as rootlet and basal foot. - Accessory structures anchor the cilium in apical cytoplasm Mechanism of ciliary movement

* Cilia beat in a rhythmical wave-like manner’’ * Sliding Microtubule Mechanism - A cilium bends a long the axis by a type of ” Sliding Microtubule Mechanism’’ between microtubules, which is generated by the ATPase activity of the dynein arms. - It is similar to that seen between myofilaments in striated muscle fibers - Ciliary movement Requires : ATP , Ca++ ions and Mg++ ions . Dynein –Walking Model * Dynein appears to “ walk’’ along the adjacent doublet Experiment: - Proteolytic enzymes digest nexin links and radial links - The addition of ATP produces a sliding movement of the bridge along the B tubule of the adjacent doublet. - Doublets move relative to each other powered by the motor activity of axonemal dynein - Radial linkers convert the sliding of microtubules into bending of cilia Dynein – Walking Model Epithelial cells lining the respiratory tract have many very well-developed cilia. (a) By light microscopy cilia (C) on the columnar cells appear as a wave of long projections, interrupted by nonciliated, mucus-secreting goblet cells (G). X400. Toluidine blue. (b) SEM of the apical surfaces of this epithelium shows the density of the cilia (C) and the scattered goblet cells (G). X300. (c) TEM of cilia (C) sectioned longitudinally reveals the central and peripheral microtubules of the axonemes, with cross sections (inset) clearly showing the 9 + 2 array of the microtubule doublets. At the base of each cilium is a basal body (B) anchoring the axoneme to the apical cytoplasm. Much shorter microvilli (MV) can be seen between the cilia. X59,000. Inset: X80,000. Ciliated surface of the respiratory mucosa

basal bodies (BB), striated rootlets (SR), basal foot (BF), axomene (Ax), transitional zone (TZ) Molecular structure of cilia

. Dynein arms ; 24-nm intervals. . Nexin; 86-nm intervals. . Central sheath projection; 14-nm intervals. . Radial spokes; 29-nm intervals. Motile Cilia CLINICAL POINT: - Kartagener’s syndrome, which is caused by a structural abnormality that results in absence of dynein arms, and misoriented basal feet pointing in different directions, causing failure of the mucociliary transport system. -Young’s syndrome, which is characterized by malformation of the radial spokes and dynein arms, also affects ciliary function in the respiratory tract. Primary cilia (monocilia) * Description - Are solitary projections (monocilia) found on almost all eukaryotic cells. - Primary cilia are immotile. - Different arrangements of microtubules in the axoneme and lack of microtubule associated motor proteins. - passively bend by a flow of the fluid. * Location -They are found in the epithelial cells of the rete testis, biliary tract,,, kidney tubules, , ependymal cells. -The connecting stalk of photoreceptor cells in the retina, the vestibular hair cells of the ear, and fibroblast.e extracellular environment. * Function; - primary cilia contribute to cell signaling (cell's antenna) . Hedgehog signal transduction and is crucial for normal development. . Wnt signaling through the primary cilium - primary cilia receive chemical (chemosensors), osmotic (osmosensors) , light (mediate light sensation), and mechanical stimuli (mechanosensors) f Primary cilia in the connective tissue and the kidney tubule The chemosensation receptor-based signalling model. Receptors such as platelet-derived growth factor receptor-α (PDGFRα) are located within the axonemal membrane (left panel). Ligands in the tubular flow bind to their receptors, inducing cellular responses through downstream signalling pathways such as the MEK/ERK cascade. IFT, intraflagellar transport; MEK, mitogen activated protein kinase (MAPK)– extracellular signal-regulated kinase (ERK) kinase. Part a modified with permission from REF. 130 (2006) Elsevier. The mechanosensation-based cilia signalling model. The polycystin-1– polycystin-2 complex (PC1–PC2), which is sensitive to shear stress, is localized within the ciliary membrane (left panel). Fluid-induced ciliary bending activates this Ca2+channel. The Ca2+ influx (right panel) causes Ca2+ release from ryanodine sensitive intracellular stores and subsequent downstream responses such as activating protein-1 (AP1)- dependent gene transcription by the Ca2+-dependent kinase PKCa. Mutations in PC1 or PC2 might disable cilia-mediated mechanosensation, which is normally required for tissue morphogenesis, and thus can cause polycystic kidney disease. Primary cilium in the kidney tubule is a primary sensor for the fluid flow. The ciliopathy spectrum. Schematic illustration of common features of ciliopathies, and severity of each ciliopathy along a spectrum from perinatal lethal to isolated retinal dystrophy. Key shows which phenotype is represented by each symbol. Primary cilia Nodal cilia

* Description -They have a similar axonemal internal architecture as primary cilia; - hey are distinct in their ability to perform rotational movement. * Location - Are found in the embryo on the bilaminar embryonic disc at the time of gastrulation. . They are concentrated in the area that surrounds the primitive node, hence their name nodal cilia... * Function - They play an important role in early embryonic development. . They create a right to left fluid flow that is responsible for determining the normal L– R asymmetry of the internal organs of the mammalian body. - The nodal flow results in a leftward gradient of a hypothetical morphogen. - nodal vesicular parcels filled with sonic hedgehog and retinoic acid molecules and wnt transcription factors. Nodal cilia Occur in the “embryonic node” -- very early stage of development

Generate oriented flow of signal molecule A scanning electron micrograph of mouse embryo node cilia helped researchers determine the cilia's tilt direction, a factor that contributes to embryo asymmetry Models to explain the function of nodal flow in L–R asymmetry. (a) The ‘morphogen’ hypothesis. Clockwise beating of motile cilia transports a morphogen or NVPs towards the left side of the node. (b) The ‘two-cilia’ hypothesis. Fluid flow generated by the motile cilia is sensed by immotile cilia on the perinodal crown cells (shown here as deflections; blue arrow). Nodal pit cells are depicted in light brown, whereas perinodal crown cells are in dark brown. The motile cilia are tilted posteriorly. Basal bodies are indicated with red dots and the direction of nodal flow is shown with the black arrow. A, anterior; P, posterior Nodal cilia Intraflagellar transport mechanism within the cilium

Assembly and maintenance of cilia depends on the intraflagellar transport mechanism (IFT) that utilizes raft-like platforms. They move up and down between the outer doublets of microtubules and plasma membrane of the elongating cilium. Cargo molecules (including inactive cytoplasmic dynein) are loaded onto the IFT platform while it is docked near the base of the cilium. Using kinesin II as a motor protein, the fully loaded platform is moved upward toward the plus end of microtubules at the tip of the cilium (anterograde transport). The cargo is then unloaded at the tip of the cilium (the site of axoneme assembly). Here, particles turn around, and the platform powered by cytoplasmic dynein heads back to the base of the cilium (retrograde transport) after picking up turnover products (including inactivated kinesin II). Inset. Electron micrograph of a longitudinal section of a Chlamydomonas flagellum with two groups of IFT platforms. 55,000. (Reprinted with permission from Pedersen LB, Veland IR, Schrّ der JM, and Christensen ST. Assembly of primary cilia. Dev Dyn 2008;237:1993–2006.) The lateral domain and its Specializations in cell to cell adhesion Epithelial Cell Junctions (Terminal bars) * the lateral domain is characterized by the presence of unique proteins,that are part of junctional specializations. Terminal bars in pseudostratified epithelium. Junctional complex Lateral Surface Features – Cell Junctions

* Occluding junctions (zonula occludens, tight junctions) - Occluding junctions are symmetrical structures on opposite sides of two adjacent cells - Its form a belt-like seal around the apical surfaces of two adjacent cells. - Separating the apical domain from the baso lateral domain. -The major components of zonula occludens are cells trans membrane protein called claudin which forms linear fibrils in the occluding junction. - They serve as selective permeability barriers, separating fluids on either side that have a different chemical composition. Zonula occludens

zonula occludens appears not as a continuous seal but as a series of focal fusions between the cells. These focal fusions are created by transmembrane proteins of adjoining cells that join in the intercellular space. Molecular structure of zonula occludens Several proteins are involved in the formation of zonula occludens strands.

Junctional adhesion molecule (JAM), Occludin and claudin have four transmembrane domains with two extracellular loops, but JAM has only a single transmembrane domain, and its extracellular portion possesses two immunoglobulinlike loops. ZO-1 is a tumor suppressor, and ZO-2 is required in the epidermal growth factor–receptor signaling mechanism. Major Proteins Localized within the Zonula Occludens Junction Two transcellular and paracellular pathways for transport of substances across the epithelia.

PDZ= post synaptic density protein (PSD95), Dorsophila disc large tumor suppressor (Dig1), and Zonula occludens-1 protein (ZO-1) . PDZ domains play a key role in anchoring receptor proteins in the membrane to cytoskeletal components. . play a key role in the formation and function of signal transduction complexes. CLINICAL POINT: - Proteins of the zonula occludens provide the targets for certain common bacteria of medical importance. . The enterotoxin secreted by Clostridium perfringens, which causes “food poisoning,” binds claudin molecules of intestinal cells, prevents insertion of these proteins during maintenance of tight junctions, and causes loss of tissue fluid into the intestinal lumen via the paracellular pathway. . Similarly, Helicobacter pylori, which is important in the etiology of gastric ulcers, binds the extracellular domains of tight-junction proteins in cells of the stomach and inserts a protein into these cells, which targets ZO-1 and disrupts signaling from the junction.

. Many pathogenic agents, such as cytomegalovirus and cholera toxins, act on ZO-1 and ZO-2, causing the junction to become permeable. Anchoring Junctions

Anchoring junctions provide lateral adhesions between epithelial cells, using proteins (Transmembrane proteins known as cell adhesion mole - cules (CAMs) that link into the cytoskeleton of adjacent cells. . - Two types of anchoring cell-to-cell junctions canbe identified on the lateral cell surface: • Zonula adherens, which interacts with the network of actin filaments inside the cell; and • Macula adherens or desmosome, which interacts with intermediate filaments. fuzzy plaque

TEM of Zonula Adherens (red arrows)

TEM of a Desmosome (blue arrow) Cell adhesion molecules play important roles in cell-to- cell and cell-to-extracellular matrix adhesions.

* Transmembrane proteins known as cell adhesion molecules (CAMs) an essential part of every anchoring junction on both lateral and basal cell surfaces.

* To date, about 50 CAMs have been identified, and they are classified on the bases of their molecular structure into four major families: - cadherins, - integrins, - selectins, and - the immunoglobin superfamily. Cell adhesion molecules (CAM)

IgSF members mediate homotypic cell-to-cell adhesions and are represented by the: . Intercellular cell adhesion molecule (ICAM),, . Cell–cell adhesion molecule (C- CAM), Vascular cell adhesion molecule (VCAM), . Down syndrome cell adhesion molecule (DSCA),M), platelet endothelial cell adhesion molecules (PECAM), . Junctional adhesion molecules (JAM). Lateral Surface Features – Cell Junctions

* Adherens junctions (zonula adherens) – anchoring junction - The adherens junction (AJ) is an element of the cell–cell junction in which cadherin receptors bridge the neighboring plasma membranes via their homophilic interactions. . Cadherins associate with cytoplasmic proteins, called catenins, which in turn bind to cytoskeletal components, such as actin filaments and microtubules. . These molecular complexes further interact with other proteins, including signaling molecules, rendering the AJs into highly dynamic and regulatable structures. . The AJs of such nature contribute to the physical linking of cells, as well as to the regulation of cell–cell contacts, which is essential for morphogenesis and remodeling of tissues and organs. . AJs function not only as a physical ligand between cells but also as an active regulator for cell rearrangement. Zonula adherens The cadherin molecule: Ca++ - dependent cell-cell adhesion 1- Single pass transmembrane protein: E and N cadherin, 750 aa. 2- Extracellular: five domains, 3 with ca++ binding, 100aa per domain. 3- Intracellular: attachment sequence: catenin, X, actin cytoskeleton. 4- Homophilic binding Fascia adherens. Lateral Surface Features – Cell Junctions

* Desmosomes (macula adherens) – two disc-like plaques connected across intercellular space - Desmosomes are intercellular junctions of epithelia and cardiac muscle. - They are less than 0.4 μm in diameter. - The intercellular space is between 20 and 35 nm wide. - They resist mechanical stress , they are said to be hyper-adhesive; . desmosomes are specialised for strong adhesion and their failure can result in diseases of the skin and heart. - They also dynamic structures whose adhesiveness can switch between high and low affinity adhesive states during processes such as embryonic development and wound healing, the switching being signalled by protein kinase C. - Desmosomes may also act as signalling centres, regulating the availability of signalling molecules and thereby participating in fundamental processes such as cell proliferation, differentiation and morphogenesis. - The linkage between the intermediate filaments and the desmosomal adhesion molecules (cadherins) is mediated by desmoplankin, plakoglobin and plakophilin. Lateral Surface Features – Cell Junctions * Desmosomes (macula adherens) – two disc-like plaques connected across intercellular space Plaques of adjoining cells are joined by proteins called cadherins

CDH1 - E-cadherin (epithelial) CDH2 - N-cadherin (neural) CDH12 - cadherin 12, type 2 (N-cadherin 2) CDH3 - P-cadherin (placental) CDH4 - R-cadherin (retinal) CDH5 - VE-cadherin (vascular endothelial) CDH6 - K-cadherin (kidney) CDH7 - cadherin 7, type 2 CDH8 - cadherin 8, type 2 CDH9 - cadherin 9, type 2 (T1-cadherin) CDH10 - cadherin 10, type 2 (T2-cadherin) CDH11 - OB-cadherin (osteoblast) CDH13 - T-cadherin - H-cadherin (heart) CDH15 - M-cadherin (myotubule) CDH16 - KSP-cadherin CDH17 - LI cadherin (liver-intestine) CDH18 - cadherin 18, type 2 CDH19 - cadherin 19, type 2 CDH20 - cadherin 20, type 2 CDH23 - cadherin 23, (neurosensory epithelium) TEM of a Desmosome (spotlike junction) Molecular structure of the macula adherens (desmosome)

Each attachment plaque is composed of several constitutive proteins, mainly desmoplakins , plakoglobins, and plakophilin Cytoskeleton of cells

*Intermediate filament cytoskeleton. Human *Cytoskeletal filaments. There are three main types keratinocyte intermediate filament cytoskeleton. of filaments: microtubules, intermediate filaments, and microtubules. Intermediate filaments diameter (10 nm), is between that of smaller microfilaments (7 nm) and larger microtubules (25 nm). Intermediate filament classification Communicating Junctions

*Communicating junctions, also called gap junctions or nexuses (passageway between two adjacent cells)., _- -A gap junction consists of an accumulation of transmembrane channels (10-nm-long cylindrical transmembrane channel with a diameter of 2.8 nm ) or pores in a tightly packed array (termed plaques)., . Each channel formed by two half channels called connexons embedded in the facing membranes. - Each connexon formed by 6 subunits of the connexin protein family. - Conformational changes in connexins leading to opening or closing gap junction channels. - Allow cells to exchange materials signaling molecules from one cell to another and to communicate. GAP JUNCTIONS Structure of a gap junction

* About 21 members of the connexin family of proteins have been identified. GAP JUNCTIONS : Passenger molecules Atomic force microscopic (AFM) image of a gap junction. CLINICAL POINT: * Several diseases result from mutations in genes encoding connexins, which are named according to molecular size. - Recessive mutations in connexin-26, with a molecular size of 26 kD, lead to the most common cause of inherited human deafness, which often affects the elderly. . Connexin-26 is usually involved in K+ transport in cells that support cochlear hair cells. - An X-linked form of Charcot-Marie-Tooth disease is due to mutations in connexin- 32 and causes degeneration of myelin sheaths in central and peripheral nervous systems. - A mutation in connexin-50 leads to cataracts in the lens of the eye.

Morphologic Specializations of the Lateral Cell Surface (Interdigitating cytoplasmic processes of adjoining cells.)

* Lateral cell surface folds (plicae) create interdigitating cytoplasmic processes of adjoining cells. - These infoldings increase the lateral surface area of the cell and are particularly prominent in epithelia that are engaged in fluid and electrolyte transport. Lateral interdigitation Electron micrograph showing infoldings or interdigitation at the lateral surface of two adjoining cells The basal domain and its specialization in cell-to- extracellular matrix adhesion * The basal domain of epithelial cells is characterized by several features: - The basement membrane is a specialized structure located next to the basal domain of epithelial cells and the underlying connective tissue stroma. - Cell-to-extracellular matrix junctions anchor the cell to the extracellular matrix; they are represented by focal adhesions and hemidesmosomes. - Basal cell membrane infoldings increase the cell surface area and facilitate morphologic interactions between adjacent cells and extracellular matrix proteins. The basement membrane Structure and Function Amorphous, dense layer of variable thickness at the basal surfaces of epithelia. Basal Feature: The Basal Lamina * Noncellular supporting sheet between the epithelium and the connective tissue deep to it * Consists of proteins secreted by the epithelial cells and fibroblasts cells. * Functions: - Acts as a selective filter, determining which molecules from capillaries enter the epithelium - Acts as scaffolding along which regenerating epithelial cells can migrate * Basal lamina and reticular layers of the underlying connective tissue form the basement membrane * Hemidesmosomal and focal adhesions junctions… holding it all down! Basal lamina. The basal lamina appears as a sheet-like structure of densely woven fibers on scanning electron microscopy. Tracheal basement membrane Normal skin . Normal skin stained of normal skin. The basement membrane using standard hematoxylin and eosin (H&E) stained with periodic acid-schiff stain. staining. Photomicrographs showing serial sections of intestinal glands of the colon

Basal lamina in the kidney glomerulus Demonstration of basement membrane material in splenic vessels Basement membrane types Smooth muscle external lamina The basal lamina is the structural attachment site for overlying epithelial cells and underlying connective tissue. Basement membrane, Basal lamina Electron micrograph of two adjoining epithelial cells with their basal lamina. Four layers of the basement membrane zone. (1) The basal keratinocyte layer, (2) the lamina lucida, (3) the lamina densa, and (4) the sublamina densa (lamina reticularis). With the development of new EM preparation techniques, the lamina lucida appears to be an artifact of fixation; in the living state, the basal lamina is composed of a single layer of the lamina densa. Electron micrograph of epithelial cells preserved by low-temperature, high-pressure freezing. 55,000. (Courtesy of Douglas R. Keene.) The basal lamina contains molecules that come together to form a sheet-like structure.

* Analyses of basal laminae indicate that they consist of approximately 50 proteins that can be classified into four groups: - collagens, (type IV collagen., type XV collagen and type XVIII collagen, type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina. - laminins, possess binding sites for different integrin receptors i in the basal domain of the overlying epithelial cells. . They are involved in many cell to – extracellular matrix interactions. . There are approximately 15 different variations of laminin molecules. - Entactin/ nidogen, small, rodlike sulfated glycoprotein serves as a link between laminin and the type IV collagen network in almost all basal laminae. - Proteoglycans, Most of the volume of the basal lamina is probably attributable to its proteoglycan content. . Proteoglycans consist of a protein core to which heparan sulfate (e.g., perlecan, agrin), chondroitin sulfate (e.g., bamacan), or dermatan sulfate side chains are attached. . They also carry a high negative charge; this quality suggests that proteoglycans play a role in regulating the passage of ions across the basal lamina. Diagram of the Basal Lamina

Basal cell memb w/ Integrins

Laminin

Nidogen

Heparan Sulfate PGs Collagen Type IV Collagen IV BMZ-specific binding - [α1(IV)]2 α 2(IV) protomers are found in all basal laminae. - Those containing α 3(IV) α 4(IV) α 5(IV) protomers occur mainly in the kidney and lungs, and - those containing [α 5(IV)]2 α 6(IV) protomers are restricted to the skin, esophagus, and Bowman capsule in the kidney. Collgen IV self-assembly. Collagen IV protomers self-assemble into dimers via the NC1 domain, forming an NC1 hexamer. Dimers assemble together via the 7S or “hockey blade” portion of the molecule to form a tetramer spider connector. The collagen IV network is stabilized by supercoiling of collagenous triple helix domains, and their interactions with NC-1 domains. Short non-collagenous sequences within the molecules’ rod-like central collagenous domain, provide collagen IV with significant flexibility (supercoiling). The collagen IV suprastructure appears as a dense plaque on electron microscopy, known as the lamina densa. The molecular structure of type IV collagen determines its role in the formation of the basal lamina network suprastructure Nidogen domains Nidogen BMZ-specific binding Perlecan domains Perlecan BMZ-specific binding

* Basal lamina self-assembly is initiated by - the polymerization of laminins (calcium dependent polymerization ) on the basal cell domain and - interaction with the type IV collagen suprastructure.. Molecular components of the basal lamina Several structures are responsible for attachment of the basal lamina to the underlying connective tissue.

* Anchoring fibrils (type VII collagen) entrap type III collagen (reticular) fibers in the underlying connective tissue. - Mutations in the collagen VII gene result in dystrophic epidermolysis bullosa, an inherited blistering skin disease in which the epithelium is detached below the basement membrane.

* Fibrillin microfibrils are 10 to 12 nm in diameter and attach the lamina densa to elastic fibers. - A mutation in the fibrillin gene (FBN1) causes Marfan’s syndrome and other related connective tissue disorders.

* Discrete projections of the lamina densa on its connective tissue side interact directly with the reticular lamina to form an additional binding site with type III collagen. Basal laminae functions:

- Structural attachment - Compartmentalization - Filtration - Polarity - Tissue scaffolding - Regulation and signaling. - Basal lamina of endothelial cells has recently been found to be involved in the regulation of tumor angiogenesis. Morphologic Modifications of the Basal Cell Surface Cell-to–Extracellular Matrix Junctions

* Anchoring junctions maintain the morphologic integrity of the epithelium– connective tissue interface. * The two major anchoring junctions are: - Focal adhesions, dynamic link which anchor actin filaments of the cytoskeleton into the basement membrane; and - Hemidesmosomes, which anchor the intermediate filaments of the cytoskeleton into the basement membrane. - Anchoring filaments are formed mainly by laminin-5 and type XVII collagen molecules. They Electron micrograph of the basal portion of epithelial cell. Molecular structure of hemidesmosome

- BP 230 is called bullous pemphigoid antigen 1 (BPAG1), and - The collagen XVII molecule is called bullous pemphigoid antigen 2 (BPAG2) or BP 180. . Mutation of the genes encoding laminin-5 chains results in junctional epidermolysis bullosa, another hereditary blistering skin disease. - CD151 (32 kilodaltons), a glycoprotein that participates in the clustering of integrin receptors to facilitate cell to extracellular matrix interactions. Molecular structure of focal adhesions create a dynamic link between the actincytoskeleton and extracellular matrix proteins.

- Tyrosine kinase (focal adhesion kinase ) closely associated with focal adhesion molecules, - Focal adhesions are also important sites of signal detection and transduction. Basal infoldings Electron micrograph of the basal portion of a kidney tubule cell showing the infolding of the plasma membrane. Summary of Junctional Features Glandular Epithelium Glandular Epithelium * Typically, glands are classified into two major groups according to how their products are released. - Exocrine glands, they secrete their products into ducts that empty at the surface of covering and lining epithelium or directly onto a free surface. . cells that secrete; sweat, ear wax, saliva, digestive enzymes onto free surface of epithelial layer. . connected to the surface by tubes (ducts). - unicellular glands or multicellular glands. - Endocrine glands are ductless; . secrete hormones into the bloodstream. - hormones help maintain homeostasis. - Paracrine signaling. - Autocrine signalin. Exocrine Vs. Endocrine Glands Formation of glands from covering epithelia Types of glands and their mechanism of secretion. Cells of exocrine glands exhibit different mechanisms of secretion.

- secretion: . Salivary glands, Sudoriferous sweat glands of the skin - secretion: . Lactating , ciliary (Moll’s) glands, ceruminous glands, Apocrine sweat glands - secretion: . Sebaceous (oil) glands of the skin, tarsal (Meibomian) glands of the eyelid. Mechanisms of secretion

Unicellular Exocrine Glands (The Goblet Cell)

* Goblet cells produce mucin - Mucin + water → mucus - Protects and lubricates many internal body surfaces Goblet Cell with PAS stain (red) Multicellular exocrine glands

* Have two basic parts - Epithelium-walled duct - Secretory unit * Their structural organization allows subclassification according to - the arrangement of the secretory cells (parenchyma) . Tubular . Alveolar or acinar . Tubuloalveolar - the presence or absence of branching of the duct elements. . Simple . Compound. * Classification by type of secretion produced: . Mucous . Serous- proteins (digestive, anti-bacterial, etc) . Muco-serous/Sero-mucous (Mixed) . Lipid Types of multicellular exocrine glands; Cellular sheet

- Is simplest arrangement of a multicellular gland . - lining of the stomach and its gastric pits Types of multicellular exocrine glands Simple Simple coiled tubular glands Simple branched tubular glands Simple acinar glands Simple branched acinar gland Compound glands ( Ducts from several secretory units converge into large ducts) Compound branched tubular gland Compound acinar gland Compound tubulo-acinar gland Lobulation of Compound exocrine glands Duct types of Compound exocrine glands Compound exocrine glands with ducts (arrows) Serous secretory unit (high magnification)

Mucous Gland with duct (arrow)

Myoepithelial cells

(a) The TEM shows two cells Containing secretory granules, with an Associated (M).X20,000. (b) A myoepithelial cell immunostained Brown with antibodies against actin show Its association with cells of stained by H&E. X200. * Contraction of the myoepithelial cell compresses The acinus and aids in the expulsion of secretory products into the duct. Mixed Gland with Serous Demilunes (SD) Serous gland ( e.g. parotid gland ( e.g. sublingual salivary gland Mixed gland (e.g. Submandibular salivary gland Epithelial cell renewal

* Most epithelial cells have a finite life span less than that of the whole organism. - Continuously renewing cell populations. - Stable cell populations. Autoradiograph of (crypt). DYSPLASIA Nuclear/cytoplasmic ratio

- The normal cells have much larger cells in comparison to their nucleus than do carcinoma cells. - The nucleus of carcinoma cells is a much larger component of its cell than in normal cells The end