Epithelial Tissue - Junqueira's Histology PDF

C H A P T E R CHARACTERISTIC FEATURES OF EPITHELIAL CELLS 4 Epithelial Tissue 72...

C H A P T E R CHARACTERISTIC FEATURES OF EPITHELIAL CELLS 4 Epithelial Tissue 72 TYPES OF EPITHELIA Covering or Lining Epithelia 80 80 Basement Membranes 72 Secretory Epithelia & Glands 84 Intercellular Adhesion & Other Junctions 73 TRANSPORT ACROSS EPITHELIA 88 SPECIALIZATIONS OF THE APICAL RENEWAL OF EPITHELIAL CELLS 88 CELL SURFACE 77 Microvilli 77 SUMMARY OF KEY POINTS 90 Stereocilia 79 ASSESS YOUR KNOWLEDGE 93 Cilia 79 D espite its complexity, the organs of the human body are composed of only four basic tissue types: epithelial, connective, muscular, and ner- vous tissues. Each tissue is an assemblage of similarly spe- cialized cells united in performing a specific function. The the organ. Except in the brain and spinal cord, the stroma is always connective tissue. Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of basic tissues, each containing extracellular matrix (ECM) organs and cover the body surface. Epithelia (Gr. epi, upon + as well as cells, associate with one another in the vari- thele, nipple) line all external and internal surfaces of the body able proportions and morphologies characteristic of each and all substances that enter or leave an organ must cross this organ. The main features of the basic tissue types are sum- type of tissue. marized in Table 4–1. The principal functions of epithelial tissues include the Connective tissue is characterized by cells producing very following: abundant ECM; muscle tissue is composed of elongated cells specialized for contraction and movement; and nervous tissue Covering, lining, and protecting surfaces (eg, epidermis) is composed of cells with long, fine processes specialized to Absorption (eg, the intestinal lining) receive, generate, and transmit nerve impulses. Most organs Secretion (eg, parenchymal cells of glands) can be divided into the parenchyma, which is composed Specific cells of certain epithelia may be contractile (myo- of the cells responsible for the organ’s specialized functions, epithelial cells) or specialized sensory cells, such as those of and the stroma, the cells of which have a supporting role in taste buds or the olfactory epithelium. TABLE 4–1 Main characteristics of the four basic types of tissues. Extracellular Tissue Cells Matrix Main Functions Epithelial Aggregated polyhedral cells Small amount Lining of surface or body cavities; glandular secretion Connective Several types of fixed and wandering cells Abundant amount Support and protection of tissues/organs Muscle Elongated contractile cells Moderate amount Strong contraction; body movements Nervous Elongated cells with extremely fine processes Very small amount Transmission of nerve impulses 71 04_Mescher_ch04_p071-095.indd 71 16/03/21 5:29 PM  72 CHAPTER 4 Epithelial Tissue › CHARACTERISTIC FEATURES OF The region of the cell contacting the ECM and connective tissue is called the basal pole and the opposite end, usually facing a EPITHELIAL CELLS space, is the apical pole, with the two poles differing significantly The shapes and dimensions of epithelial cells are quite variable, in both structure and function. Regions of cuboidal or colum- ranging from tall columnar to cuboidal to low squamous cells. nar cells that adjoin neighboring cells comprise the cells’ lateral The cells’ size and morphology are generally dictated by their surfaces; cell membranes here often have numerous folds that function. Epithelial cell nuclei vary in shape and may be elliptic increase the area and functional capacity of that surface. (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape. Columnar cells generally have elongated Basement Membranes nuclei, squamous cells have flattened nuclei, and cuboidal or The basal surface of all epithelia rests on a thin extracellular, pyramidal cells have more spherical nuclei (Figure 4–1). felt-like sheet of macromolecules referred to as the basement Because the lipid-rich membranes of epithelial cells are membrane (Figure 4–1), a semipermeable filter for sub- frequently indistinguishable by light microscopy, the number stances reaching epithelial cells from below. Glycoproteins and and shape of their stained nuclei are important indicators of other components in this structure can often be stained and cell shape and density. The nuclei also allow one to determine visualized with the light microscope (Figure 4–2). the number of cell layers in an epithelium, a primary morpho- With the transmission electron microscope (TEM) two logic criterion for classifying epithelia. parts of the basement membrane may be resolved. Nearest Most epithelia are adjacent to connective tissue contain- the epithelial cells is the basal lamina, a thin, electron-dense, ing blood vessels from which the epithelial cells receive nutri- sheetlike layer of fine fibrils, and beneath this layer is a more ents and O2. Even thick epithelia do not themselves normally diffuse and fibrous reticular lamina (Figure 4–3a). The terms contain blood vessels. The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems is called the lamina propria. The area FIGURE 4–2 Basement membranes. of contact between the two tissues may be increased by small evaginations called papillae (L. papula, nipple) projecting from the connective tissue into the epithelium. Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue. Epithelial cells generally show polarity, with organelles and membrane proteins distributed unevenly within the cell. FIGURE 4–1 Epithelia and adjacent connective tissue. Cuboidal or pyramidal cells of epithelia generally have spheri- cal nuclei, while nuclei of squamous epithelial cells are flat- tened. An extracellular basement membrane (red) always This section of kidney shows the well-stained basement mem- lies at the interface of epithelial cells and connective tissue. branes (arrows) of epithelia forming structures within the Nutrients for epithelial cells must diffuse across the basement large, round renal glomerulus and its surrounding tubules. In membrane. Nerve fibers normally penetrate this structure, but kidney glomeruli, the basement membrane, besides having a small blood capillaries (being epithelial themselves) normally supporting function, has a highly developed role as a filter that never enter epithelia. is key to renal function. (X100; Picrosirius-hematoxylin [PSH]) 04_Mescher_ch04_p071-095.indd 72 16/03/21 5:29 PM  Characteristic Features of Epithelial Cells 73 FIGURE 4–3 Basal and reticular laminae of basement membranes. C H A P T E R E 4 C V Epithelial Tissue Characteristic Features of Epithelial Cells H BL RL M N a b (a) The ultrastructural components of the basement membrane (b) Laminin, a major glycoprotein within basal laminae, is shown are revealed by TEM. The dense basal lamina (BL), 20-100 nm here by immunohistochemistry and identifies the basement mem- thick, may appear with thin clear zones on each side and is branes of the stratified epithelium (E) and the simple epithelium anchored to a thicker, more diffuse reticular lamina (RL) contain- lining a small blood vessel (V). Laminin also occurs in the external ing collagen III fibers. Hemidesmosomes (H) bind the basal sur- laminae surrounding nerves (N) and muscle (M) fibers, seen here face of the epithelial cell (C) to the basal lamina. (X54,000) in cross section. (X200) “basement membrane” and “basal lamina” are sometimes used The more diffuse meshwork of the reticular lamina con- interchangeably, but “basal lamina” usually denotes the fine tains type III collagen and is bound to the basal lamina by extracellular layer seen ultrastructurally and “basement mem- anchoring fibrils of type VII collagen, both of which are pro- brane” the entire structure beneath the epithelial cells visible duced by cells of the connective tissue (Figure 4–3). with the light microscope. Besides acting as filters, functions of basement mem- The macromolecules of the basal lamina are secreted from branes include helping to provide structural support for epi- the basal sides of the epithelial cells and form a sheetlike array. thelial cells and attach epithelia to underlying connective ECM components are described more fully in Chapter 5, but tissue. Basal lamina components help organize integrins and those of the basal lamina characteristically include the following: other proteins in the plasma membrane of epithelial cells, maintaining cell polarity and helping to localize endocytosis, Type IV collagen: Monomers of type IV collagen self- signal transduction, and other activities. Basement membrane assemble into a two-dimensional network of evenly proteins also mediate many cell-to-cell interactions involving spaced subunits resembling the mesh of a window screen. epithelia and mark routes for certain cell migrations along epi- Laminin: These are large glycoproteins that attach to thelia. Finally, the basement membrane also serves as a scaf- transmembrane integrin proteins in the basal cell mem- fold that allows rapid epithelial repair and regeneration. brane and project through the mesh formed by the type IV collagen. Intercellular Adhesion & Other Junctions Nidogen and perlecan: Respectively a short, rodlike Several membrane-associated structures provide adhesion protein and a proteoglycan, both of these cross-link lami- and communication between cells. Some are present in other nins to the type IV collagen network, helping to provide tissues but all are particularly numerous and prominent in the basal lamina’s three-dimensional structure, to bind epithelia. Epithelial cells adhere strongly to neighboring cells the epithelium to that structure, and to determine its and basal laminae, particularly in epithelia subject to friction porosity and the size of molecules able to filter through it. or other mechanical forces. As shown in Figure 4–4 and summarized in Table 4–2, Basal laminae often called external laminae but with simi- lateral surfaces of epithelial cells have complexes of several lar composition also exist as thin sleeves surrounding muscle specialized intercellular junctions with different functions: cells, nerves (Figure 4–3b), and fat-storing cells, where they serve as semipermeable barriers regulating macromolecular Tight or occluding junctions form a seal between adja- exchange between the enclosed cells and connective tissue. cent cells. 04_Mescher_ch04_p071-095.indd 73 16/03/21 5:29 PM  74 CHAPTER 4 Epithelial Tissue FIGURE 4–4 Junctional complexes of epithelial cells. Microvilli Tight junction (occluding junction) Adherens junction (anchoring junction) Desmosome (anchoring junction) Intermediate filament Gap junction (communicating junction) Hemidesmosome (anchoring junction) Basal lamina Most cuboidal or columnar epithelial cells have intercellular Both desmosomes and gap junctions are spot-like, not circular, junctional complexes with the different types of junctions shown structures between two cells. Bound to intermediate filaments schematically here. At the apical end, tight junctions (zonulae inside the cells, desmosomes form very strong attachment points occludens) and adherent junctions (zonulae adherens) are typi- that supplement the zonula adherens and play a major role to cally close together and each forms a continuous band around maintain the integrity of an epithelium. Gap junctions, each a the cell. Multiple ridges of the tight junction prevent passive flow patch of many connexons in the adjacent cell membranes, have of material between the cells but are not very strong; the adher- little strength but serve as intercellular channels for flow of mol- ing junctions immediately below them serve to stabilize and ecules. All of these junctional types are also found in certain other strengthen the circular occluding bands and help hold the cells cell types besides epithelia. Hemidesmosomes bind epithelial together. cells to the underlying basal lamina. 04_Mescher_ch04_p071-095.indd 74 16/03/21 5:29 PM  Characteristic Features of Epithelial Cells 75 Epithelial cell junctions, their major structural features and functions, and TABLE 4–2 C H A P T E R medical significance. Tight Junction Adherens Junction Desmosome Gap Junction Junction (Zonula Occludens) (Zonula Adherens) (Macula Adherens) Hemidesmosome (Nexus) Major Occludins, claudins, E-cadherin, catenin Cadherin family Integrins Connexin transmembrane ZO proteins complexes proteins (desmogleins, link proteins desmocollin) 4 Cytoskeletal Actin filaments Actin filaments Intermediate filaments Intermediate None Epithelial Tissue Characteristic Features of Epithelial Cells components (keratins) filaments Major functions Seals adjacent cells to Provides points linking Provides points of Anchors cytoskeleton Allows direct one another, controlling the cytoskeletons strong intermediate to the basal lamina transfer of small passage of molecules of adjacent cells; filament coupling molecules and between them; strengthens and between adjacent cells, ions from one cell separates apical and stabilizes nearby tight strengthening the tissue to another basolateral membrane junctions domains Medical Defects in occludins Loss of E-cadherin in Autoimmunity against Mutations in the Mutations in significance may compromise epithelial cell tumors desmoglein I leads to integrin-β4 gene are various connexin the fetal blood–brain (carcinomas) promotes dyshesive skin disorders linked to some types genes have been barrier, leading to tumor invasion and the characterized by of epidermolysis linked to certain severe neurologic shift to malignancy reduced cohesion of bullosa, a skin types of deafness disorders epidermal cells blistering disorder and peripheral neuropathy Adherent or anchoring junctions are sites of strong different sets of components, which allows these two sides of cell adhesion. the epithelium to display different receptors and other pro- Gap junctions are channels for communication teins and to function differently. Apical cell membranes of between adjacent cells. epithelia are part of the luminal compartment of a tissue or an organ, while the basolateral domains are part of a basal In many epithelia, these junctions are present in a defi- compartment that also encompasses the underlying connec- nite order at the apical end of the cells. Tight junctions, also tive tissue. called zonulae occludens, are the most apical of the junctions. The term “zonula” indicates that the junction forms a band completely encircling each cell. In TEM the adjacent mem- branes at these junctions appear fused or very tightly apposed › › MEDICAL APPLICATION Proteins of tight junctions provide the targets for certain (Figure 4–5). The seal between the two cell membranes is due common bacteria of medical importance. The enterotoxin to tight interactions between the transmembrane proteins: secreted by Clostridium perfringens, which causes “food poi- claudin and occludin. Tight junctions are clearly seen after soning,” binds claudin molecules of intestinal cells, prevents cryofracture of epithelia (Figure 4–6), where they appear as insertion of these proteins during maintenance of tight junc- a band of branching strands in the membrane around each tions, and causes loss of tissue fluid into the intestinal lumen cell’s apical end. The intercellular seal of tight junctions via the paracellular pathway. ensures that molecules crossing an epithelium in either direc- Similarly, Helicobacter pylori, which is important in the tion do so by going through the cells (a transcellular path) etiology of gastric ulcers, binds the extracellular domains of rather than between them (the paracellular pathway). Epithe- tight-junction proteins in cells of the stomach and inserts a lia with one or very few fused sealing strands (eg, proximal protein into these cells, which targets ZO-1 and disrupts sig- renal tubule) are more permeable to water and solutes than naling from the junction. are epithelia with many fused strands (eg, the lining of the urinary bladder). Epithelial tight junctions also serve a related purpose: The second type of junction is the adherens junction or these continuous zones within cell membranes serve as zonula adherens (Figures 4–4 and 4–5), which also encircles the fences restricting movements of membrane lipids and pro- epithelial cell, usually immediately below the tight junction. This teins at the apical cell surface into the lateral and basal sur- is an adherent junction, firmly anchoring a cell to its neighbors. faces, and vice versa. The tight junctions thus maintain two Cell adhesion is mediated by cadherins, transmembrane glyco- distinct membrane domains (apical and basolateral) with proteins of each cell that bind each other in the presence of Ca2+. 04_Mescher_ch04_p071-095.indd 75 16/03/21 5:29 PM  76 CHAPTER 4 Epithelial Tissue FIGURE 4–5 Epithelial cell junctional complex. FIGURE 4–6 View of a tight junction after cryofracture. MV MV (TJ) (AJ) TJ (D) IF Just below the apical microvilli (MV) of this epithelial cell, a cryofracture plane splitting fused cell membranes reveals the Ultrastructural View Of The Apical Region Near Microvilli (Mv) fused strands of transmembrane proteins forming the tight of two epithelial cells, revealing a junctional complex with a junction (TJ, or zonula occludens. (X100,000) tight junction (TJ) or zonula occludens, an adherent junction (AJ) or zonula adherens, and a desmosome (D) associated with intermediate filaments (IF). The functions and major pro- tein components of these junction types are summarized in Table 4–2. (X195,000) › › MEDICAL APPLICATION Various blistering (bullous) diseases, such as pemphigus vul- garis, involving the epidermis or stratified squamous epithelia of the oral mucosa, are due to abnormal desmosome function caused by autoimmune reactions against specific desmogleins At their cytoplasmic ends, cadherins bind catenins that link to that reduce cell-to-cell adhesion. Similar disorders arise with actin filaments with actin-binding proteins. The actin filaments genetic mutations for various junctional proteins. linked to the adherens junctions form part of the “terminal web,” a cytoskeletal feature at the apical pole in many epithelial Gap junctions, shown in Figure 4–7, mediate inter- cells. Together, the tight and adherent junctions encircling the cellular communication rather than adhesion or occlusion apical ends of epithelial cells function like the plastic bands that between cells. Abundant in many epithelia, gap junctions are hold a six-pack of canned drinks together. also functionally important in nearly all mammalian tissues. Another anchoring junction is the desmosome Cryofracture preparations show that gap junctions consist of (Gr., desmos, binding and soma, body) or macula adherens aggregated transmembrane protein complexes that form cir- (L. macula, spot). As the name implies, this junction resem- cular patches in the plasma membrane (Figure 4–7b). bles a single “spot-weld” and does not form a belt around The transmembrane gap junction proteins, connexins, the cell. Desmosomes are disc-shaped structures at the sur- form hexameric complexes called connexons, each of which face of one cell that are matched with identical structures at has a central hydrophilic pore about 1.5 nm in diameter. When an adjacent cell surface (Figures 4–4 and 4–5). Desmosomes two cells attach, connexins in the adjacent cell membranes contain larger members of the cadherin family called desmo- move laterally and align to produce connexons between the gleins and desmocollins. The cytoplasmic ends of these clus- two cells (Figures 4–4 and 4–7a), with each junction having tered transmembrane proteins bind plakoglobins, catenin-like dozens or hundreds of aligned connexon pairs. Gap junc- proteins that link to larger proteins called desmoplakins in an tions permit intercellular exchange of molecules with small electron-dense plaque. Desmoplakins in turn bind interme- (