Epithelial Cells & Tissues

OBJECTIVES

This chapter should help the student to:

· Know the structural and functional characteristics that distinguish epithelial tissues from the 3 other basic tissue types.
· Know the types of epithelial tissues and give examples of body sites where each may be found.
· Know the functional capabilities of each epithelial tissue type and relate them to tissue structure.
· Describe the specialized functions of the various epithelial cell types and give examples of body sites where each may be found.
· Recognize the various epithelial types in photomicrographs or slides and predict their function from their structure.
· Know the criteria used to classify glands.
· Know the names of the types of glands commonly found in humans and give examples of body sites where each may be found.
· Recognize glands in photomicrographs or diagrams and identify the gland type.

SYNOPSIS

I. THE 4 BASIC TISSUE TYPES

A tissue is a complex assemblage of cells and cell products having a common function. The many body tissues are grouped according to their cells and cell products into 4 basic types: epithelial, connective, muscular, and nervous tissues.

II. GENERAL FEATURES OF EPITHELIAL TISSUES

Epithelial tissues usually occur as structurally minor but functionally important components of complex organs. Glands derive from the invagination and ingrowth of lining epithelia into underly ing connective tissue. Composed mainly of epithelial cells, glands are considered a type of epithelial tissue.

A. Diversity: Epithelial tissues range from one to several cell layers in thickness, forming sheets, solid organs, or glands. Their functions range from protection to secretion and absorption.

B. Metaplasia: When faced with a chronic change of environmental conditions, epithelia are capable of metaplasia; ie, they may change from one type to another.

C. Lining and Covering: Epithelia cover or line all body surfaces and cavities except articular cartilage in joint cavities. Their function is analogous to that of cell membranes: They (1) separate self from nonself; (2) divide the body interior into functional compartments; and (3) form passive and active barriers which monitor, control, and modify substances that traverse them.

D. Basal Lamina: Epithelia rest on an extracellular basal lamina (or basement membrane) that separates them from an underlying connective tissue layer, the lamina propria

E. Renewal: Epithelia are continuously renewed and replaced. The epithelial cells closest to the basal lamina undergo continuous mitosis, and their progeny replace the surface cells.

F. Avascularity: Blood vessels in the subjacent connective tissue rarely penetrate the basal lamina to invade epithelia.

G. CellPacking: Epithelial tissues have very little intercellular substance. The cells are densely packed, closely apposed, and joined by specialized junctions.

H. Derivation: Ectoderm, mesoderm, and endoderm can all give rise to epithelia (Table 4-1).

III. CLASSIFICATION OF EPITHELIAL TISSUES

A. Classifyng Principles: Epithelia are classified according to the number of their cell layers and the shape of the cells in the surface layer.

1. Number of cell layers a. Simple epithelia have one cell layer. b. Stratified epithelia have 2 or more cell layers. c. Pseudostratified epithelia have all their cells resting on the basal lamina, but not all the cells extend to the surface. The nuclei lie at different depths, giving the appearance of multiple cell layers.
2. Shape of the surface cells a. Squamous cells are hat and platelike. b. Cuboidal cells are polygonal and about as tall as they are wide. c. Columnar cells are polygonal and taller than they are wide.



B. Specific Epithelial Types:

1. Simple squamous epithelium is a single layer of flat, platelike cells that functions as a semipermeable barrier between compartments. It lines blood vessels (endothelium) and body cavities (mesothelium) and forms the parietal layer of the renal corpuscles.
2. Simple cuboidal epithelium is a single layer of blocklike cells. It forms the walls of conduits that carry secretory and excretory products, and it regulates ion and water con centration in certain specialized (striated) salivary ducts. It may act as a protective barrier in some locations. It lines the kidney tubules and the smaller(intercalary and interlobular) ducts of many glands, and it covers the free surface of the ovary and the inner surface of the lens capsule.
3. Simple columnar epithelium is a single layer of roughly cylindric cells whose apical (free) surfaces may be covered with cilia or microvilli. It functions in secretion, absorption, and, when ciliated, propulsion of mucus. It often acts as a protective barrier. It lines the stomach, intestines, rectum, uterus, and oviducts, as well as the larger ducts of certain glands and the papillary ducts of the kidneys.
4. Pseudostratified columnar epithelium is a single layer of cells of variable shape and height, with nuclei at 2 or more levels (III.A.l.c). Cells reaching the surface are often ciliated. This epithelium forms a protective barrier and, when ciliated, moves surface mucus and trapped debris. Ciliated pseudostratified columnar epithelium, or respiratory epi thelium, lines the larger diameter respiratory passageways. Reudostratified columnar epi thelium also lines parts of the male reproductive tract, where its apical surfaces are often covered with nonmotile stereocilia (IV.A.4).
5. Stratified squamous epithelium occurs in 2 forms: a. The keratinized (cornified) type is a multilayered sheet of cells. The superficial cells are squamous, dead, and tilled with the scleroprotein keratin: they lack nuclei. Deeper layers have polygonal cells in progressive stages of keratinization. The deepest layer has cuboi dal to columnar cells and lies on the basal lamina. Keratinized stratified squamous epithelium is found mainly in the skin and forms a highly specialized barrier against friction, abrasion, infection, and water loss. b. The nonkeratinized (noncornified) type is similar in structure but is thinner and lacks heavily keratinized cells. Its surface cells are ffattened but nucleated. Nonkeratinized stratified squamous epithelium, also called mucous membrane, forms a protective bar rier that is less resistant to water loss than the Ireratinized type. It lines wet cavities subject to abrasion leg, mouth, esophagus, vagina and anal canal, vocal folds).
6. Stratifed cuboidal epithelium usually has 2-3 layers of cuboidal cells. This relatively rare epithelium lines the ducts of some glands leg, salivary, sweat).
7. Stratified columnar epitbelium is similar to stratified cuboidal epithelium, but its super~i cial cells are columnar and may be ciliated. Also rare, it lines the larger ducts of some large glands, forms the conjunctiva, and occurs in small, isolated patches in some mucous mem branes. It sometimes covers the respiratory surface of the epiglottis.
8. Transitional epithelium is a stratified epithelium that lines most urinary passages (renal pelvis, ureters, bladder, proximai portions of the urethra). Its surface cells are large and often binucleate. When the bladder is empty, the surface cells appear domelike, giving the epithelium a "cobblestone" appearance; when the bladder is full, the surface cells stretch and Batten.

IV. POLARITY & SPECIALIZATIONS OF EPITHELIAL CELLS

Polarity (structural and functional asymmetry) is characteristic of most epithelial cells. It is best seen in simple epithelia, where each cell has 3 types of surfaces: an apical (Free) surface, lateral surfaces that abut neighboring cells, and a basal surface attached to the basal lamina.

A. Specializations of the Apical Surface: The cell's apical surface is on the organ's external or internal (lumen) surface. It is specialized to carry out functions that occur at these interfaces, including secretion, absorption, and movement of luminal contents.


1. Cilia (see Chapter 3). These membrane-covered extensions of the cell surface typically occur in tufts or cover the entire apical surface. They beat in waves, often moving a surface coat of mucus and trapped materials. Ciliated epithelia include ciliated pseudostratified columnar (respiratory) epithelium and the ciliated simple columnar epithelium of the oviducts.
2. Flagella (see Chapter 3) are also concerned with movement. Spermatotoa, derived from seminiferous epithelia, are the only flagellated human cells.
3. Microvilli are plasma membrane-covered extensions of the cell surface. Their cores (unlike those of cilia and flagella) are composed of numerous parallel actin microfilaments; these are anchored in a dense mat of filaments in the apical cytoplasm called the terminal web. By interacting with cytoplasmic myosin, the microfilaments can contract, shortening the microvilli. The apical surface of absorptive cells is usually covered with microvilli, which greatly increase the apical surface area when extended. Microvillus-covered epithelia, said to exhibit a striated border, or brush border, include the absorptive simple columnar epithelium lining the small intestines and the absorptive simple cuboidal epithelium lining the proximal tubules of the kidney.
4. Stereocilia are not true cilia but very long microvilli. They are found in the male reproduc tive tract (epididymis. ductus deferens). where they have an absorptive function, and in the internal ear (hair cells of the maculae and organ of Corti). where they have a sensory function.

B. Specializations of the Lateral Surfaces: Epithelial cells attach tightly to one another by specialized intercellular junctions. Junctions occur in 3 major forms: Zonulae are bandlike junctions are similar in shape to maculae but differ in composition and function. A junctional complex (formerly called a terminal bar) is any combination of intercellular junctions close to the cell apex that looks dark in the light microscope.


1. Zonula occludens. Zonulae occludentes (tight junctions, occluding junctions) are located near the cell apex and seal off the intercellular space, allowing the epithelium to isolate certain body compartments leg, they help keep intestinal bacteria and toxins out of the bloodstream). Their structure, best seen in freeze-fracture preparations, results from the fusion of 2 trilaminar plasma membranes of adjacent cells to form a pentalaminar structure (as seen in TEM); this fusion may require specific "tight-junction proteins." In some tissues, tight junctions can be disrupted by removing calcium ions or treating with protease.
2. Zonula adherens. Zonulae adherentes (sometimes called belt desmosomes) are usually just basal to the tight junctions. The membranes of the adhering cells are typically 20-90 nm apart at a zonula adherens; the gap may be wider there than in nonjunctional areas. An electron-dense plaque containing myosin, tropomyosin, alpha actinin, and vinculin is found on the cytoplasmic surface of each of the membranes participating in the junction. Actin containing microfilaments arising from each cell's terminal web insert into the plaques and appear to stabilize the junction.
3. Macula adherens. A macula adherens, or desmosome, consists of 2 dense, granular attach ment plaques composed of several proteins and borne on the cytoplasmic surfaces of the opposing cell membranes. Transverse thin EM sections show dense arrays of tonoilaments (cytokeratin intermediate filaments) that insert into the plaques or make hairpin turns and return to the cytoplasm. The gap between the attached membranes is often over 30 nm. Sometimes fibrillar or granular material (probably glycoprotein) is seen as a dense central line in the intercellular space. Desmosomes, distributed in patches along the lateral mem branes of most epithelial cells, form particularly stable attachments but do not hamper the Bow of substances between the cells.
4. Gap junction. A gap junction (nexus) is a disk- or patch-shaped structure, best appreciated by viewing both freeze-fracture and transverse thin EM sections. The intercellular gap is 2 nm, and the membrane on each side contains a circular patch of connexons, Each connexon Is a protein hexamer with a central 1.5-nm hydrophilic pore. The connexons in one mem brane link with those in the other to form continuouspores that bridge the intercellular gap, allowing passage of ions and small molecules (<800 daltons). As sites of electrotonic coupling (reduced resistance to ion flow), gap junctions are important in intercellular com munication and coordination; they are found in most tissues.

C. Specializations of the Basal Surface: The basal surface contacts the basal lamina. Because it is the surface closest to the underlying blood supply, it often contains receptors for blood borne factors such as hormones.


1. A basal lamina underlies all true epithelial tissues. a. Structure, The basal lamina is a sheetlike structure, usually composed of type IV collagen, proteoglycan, and laminin, a glycoprotein that aids in binding cells to the basal lamina. The basal lamina exhibits electron-lucent and electron-dense layers termed the lamina lucida (lamina rara) and the lamina dense, respectively. The lamina densa is a 20- to 100-nm-thick fibrillar network; the amount of lamina lucida is variable. Basal lamina components are contributed by the epithelial cells, the underlying connective tissue cells, and (in some locations) muscle, adipose, and Schwann cells. In some sites, a layer of type III collagen fibers (reticular fibers), produced by the connective tissue cells and termed the reticular lamina, underlies the basal lamina. Basal laminae accompanied by reticular laminae are often thick enough to be seen with the light microscope as PAS- positive layers and are sometimes termed basement membranes, b. Functions. The basal lamina forms a sievelike barrier between the cpithelium and con nective tissue. It aids in tissue organization and cell adhesion and (through trans membrane linkages with cytoskeletal components) helps maintain cell shape. it has a role in maintaining specific cell functions, probably through its effect on shape. Muscle basal laminae are critical in establishing neuromuscular junctions.
2. Hemidesmosomes are located on the inner surface of basal plasma membranes in contact with the basal lamina. They help to attach epithelial cells to the basal lamina. The best examples are found in the basal layers of stratified squamous epithelium.
3. Sodium-potassium ATPase is a plasma membrane-bound enzyme localized preferentially in the basal and basolateral regions of epithelial cells. It transports sodium out of and potassium into the cell (VI.A. i).

D. Intracellular Polarity: The nucleus and organelles are often found in characteristic regions of epithelial cells, a feature particularly important to glandular cells. For example, in protein secreting cells, the RER is preferentially located in the basal cytoplasm, the nucleus in the basal to middle region just above the RER, and the Golgi complex just above the nucleus. Mature secretory vesicles collect in the apical cytoplasm.

V. GLANDS

Glands are single cells or groups of cells specialized for secretion.

A. Exocrine and Endocrine Glands: All glands arise in early development from lining or covering epithelia. Exocrine glands, described below, are those that keep their connection with the epithelium in the form of a duct. Endocrine glands (ductless glands) lose their connection with the surface and release their secretions into the bloodstream. Endocrine glands are com pared with exocrine glands in Table 4-2; they are described in more detail in Chapters 20 and 21.

B. Classification of Exocrine Glands: Exocrine glands may be classified according to their structure, secretory product, or mode of secretion.


1. By structure. Structural classification is based on the number of cells, the type of duct system, and the shape of the secretory portion of the gland.


a. Number of cells. Unicellular glands are single secretory cells scattered among other cell types in epithelia (eg, mucus-secreting goblet cells). Multicellular glands occur mainly as solid glands, whose secretions are carried by ducts to the body surface (eg, sweat glands) or to a lumen (eg, salivary glands).
b. Duct system. The duct system may be simple (unbranched) or compound (branched). Simple ducts may be straight or coiled. c, Secretory portion. The secretory portion of the gland may be tubular (test tube shaped); alveolar or acinar (flask-shaped); or tubuloacinar (with acini branching off the straight tubular portion).


2. By secretory product


a. Mucous secretion, or mucus, is a thick secretion containing proteins, chiefly highly glycosylated glycoproteins called mucins or mucin precursors called mucinogens, Other leg, membrane) glycoproteins commonly have short, N-linked oligosaccharides attached to asparagine. Mucous glycoproteins have longer, O-linked oligosaccharide chains at tached to hydroxyl groups of serine orthreonine. This attachment is mediated by special glycosyltransferases in the Golgi complex of the mucus-secreting cells. Examples of mucus-secreting glands include goblet cells and the sublingual salivary glands.
b. Serous secretion is a watery secretion containing proteins and glycoproteins. The ex ocrine pancreas and parotid salivary glands produce serous secretions.
c. Seromucous secretion is a mixed secretion of intermediate thickness. The submandibular salivary glands contain both serous and mucous secretory cells and produce seromucous secretions.


3. By mode of secretion.


a. In merocrine secretion (eccrine secretion), the secretory product exits by exocytosis, with no loss of cytoplasm or membrane. Most secretory cells release their products in this manner. Specific examples include the pancreas and pituitary.
b. In apocrine secretion, the secretory product collects in the cell apex and the entire apex is released, with some loss of cytoplasm and membrane. Apocrine sweat glands of the skin and mammary glands both employ this type of secretion.
c. In holocrine secretion, storage of large amounts of secretory products in the cytoplasm is followed by cell lysis. The entire cell is released into the duct. The skin's sebaceous glands are the classic examples.

VI. MAJOR TYPES OF EPITHELIAL CELLS

A. Epithelial Cells Specialized for Transport:


1. Ion-transporting cells. Some epitheiial cells are specialized for transcellular transport; ie. they can pump ions across their entire thickness, apex to base. Sheets of such cells form active barriers that control ion and waterconcentrations in body compartments. Tight junc tions are often found between the cells and appear to restrict backflow. Ion-transporting cells typically have highly infolded basal plasma membranes that interdigitate with numerous mitochondria. Commonly, the ion pump is specific for sodium tie, it is Na/K+-ATPase), and chloride ions and water follow the sodium ion flow passively. Some ion-transporting epithelia exploit this mechanism to concentrate other solutes by moving water from one compartment to another. Important ion-transporting epithelia are found in the kidney tubules. the stnated ducts of the salivary glands, the gallbladder, the choroid plexus and the ciliary body of the eye.
2. Cells that transport by pinocytosis. Epithelial cells specialized for pinocytosis have tight junctions and abundant pinocytotic vesicles. The vesicles transport substances across the cell from the luminal surface to the basal surface or vice versa. The best example is the endo thelial cells lining the blood vessels, where transcellular transport is rapid (2-3 minutes).

B.  Epithelial Cells Specialized for Absorption: Specialized absorptive cells lining the digestive tract (especially the small intestine) have numerous microvilli on their apical surfaces to increase the exposed area. Small nutrient molecules diffuse into the microvilli, and contraction of the microfilaments shortens the microvilli, bringing the nutrients into the cytoplasm. Other nutrients are pinocytosed between microvilli. Absorptive cells with similar specializations occur in the proximal tubules of the kidney.

C.  Epithelial Cells Specialized for Secretion:


1. Protein-secreting cells. Cells that synthesize proteins for segregation and secretion have abundant basophilic RER, a well-developed Golgi complex, and, frequently, an accumula tion of secretory granules in the cell apex. Proteins secreted by epithelial cells include the digestive enzymes, produced by pancreatic acinar cells and the chief cells of the stomach: serum albumin, produced by liver hepatocytes; and protein hormones leg, parathyroid hor mone, produced by the chief cells of the parathyroid gland).
2. Polypeptide-secreting cells. Secreted polypeptides have fewer amino acids than the secreted proteins just mentioned. Polypeptide-secreting cells have a small amount of RER, a supranuclear Golgi complex, and an accumulation of 100- to 400-nm secretory granules in their bases. These APUD cells (amine precursor uptake and decarboxylation) charac teristically concentrate important bioactive amines such as epinephrine, norepinephrine, and serotonin in their cytoplasm. They may absorb these amines from the bloodstream or synthe size them from amino acid precursors by means of amino acid decarboxylases, also found in high concentrations in these cells. Most APUD cells are unicellular glands scattered among other epithelial cells. They are believed to derive mainly from the embryonic neural crest. The number, variety, and wide distribution of cells with these characteristics has generated the concept of the diffuse neuroendocrine system (DNES~ DNES is becoming the pre ferred designation, but DNES and APUD refer to the same polypeptide-secreting cells. Some APUD polypeptides have paracrine effects on neighboring cells; others are released into the bloodstream and have endocrine effects on distant cells. Some important APUD polypeptides are glucagon, from pancreatic islet A cells: insulin. from pancreatic islet B cells; gastrin. from the stomach, small intestine, and pancreatic islet G cells; and somatostatin, from the stomach, small intestine, and pancreatic islet D cells. Tumors composed of APUD cells are called apudomas.
3. Mucous cells occur as unicellular, sheet, or solid glands. Histologic features include a light staining, foamy appearance caused by numerous large mucus-containing vesicles concen trated near the cell apex; PAS-positive staining from an abundance of oligosaccharide resi dues, predominantly acidophilic staining with H&E; a large supranuclear Golgi complex with distinctive glycosyltransferases (V.B.Z.a); and nuclei and sparse RER in the base of the cell.
4. Serous cells have characteristics of protein-secreting cells. They are usually smaller, darker staining, and more basophilic than mucus-secreting cells. Serous cells include pancreatic acinar cells and secretory cells of the parotid salivary glands.
5. Steroid-secreting cells. Endocrine cells specialized to secrete steroid hormones are polygo nal or rounded, with a central nucleus and pale-staining, acidophilic cytoplasm that often contains numerous lipid droplets. Their abundant SER contains enzymes for cholesterol synthesis and for converting steroid hormone precursors (eg, progesterone) into specific hormones leg, androgens, estrogens, and progesterone). Their mitochondria typically have tubular rather than shelflike cristae and contain enzymes that convert cholesterol to progesterone. Steroid hormones include testosterone, produced by interstitial cells of the testes: estrogen, from follicle cells of the ovaries; progesterone, from granulosa lutein cells of the corpus iuteum; and cortisone and aldosterone, from cells of the adrenal cortex.

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