Iiclassification Table

III. POLARITY of an epithelial cell is made evident by specializations that are found in various regions of the cell (Figure 2-1).

A. The apical region

1. Microvilli contain a core of actin filaments that are anchored to the terminal web. The actin filaments are cross-linked by villin. Microvilli of intestinal epithelium are coated with a glycocalyx that consists of terminal oligosaccharides of integral membrane proteins. The glycocalyx has enzymatic activity involved in carbohydrate digestion.

2. Stereocilia are long microvilli found on epididymal epithelium and hair cells of the inner ear.

3. Cilia are motile cell processes that contain a core of microtubules (a and p tubulin) called the axoneme. The axoneme consists of nine doublet microtubules uniformly spaced around two central microtubules (9 + 2 arrangement). Nexin connects the nine doublet microtubules. Each doublet has short arms that consist of dynein ATPase, which splits ATP to provide energy for cilia movement. At the base of each cilium is a basal body that consists of nine triplet microtubules and no central microtubules (9 + 0 arrangement).

B. The lateral region

1. The zonula occludens (or tight junction) extends around the entire perimeter of the cell.

a. The outer leaflets of the cell membrane of the two adjoining celts fuse at various points.

b. The zonula occludens prevents or retards the diffusion of material across an epithelium via the paracellular pathway (i.e., through the intercellular space). Various epithelia have been classified either as "tight" or "leaky" based on the permeability of the zonula occludens.

C. The zonula occludens can be rapidly formed and disassembled (e.g., during leukocyte migration across endothelium).

Table 2-1

Classification of Epithelium

Type of Epithelium

Location In Body

Type I pneumocytes of alveoli, parietal layer of Bowman's capsule, endothelium of blood and lymph vessels, mesothelium of body cavities, corneal endothelium

Simple squamous

Type I pneumocytes of alveoli, parietal layer of Bowman's capsule, endothelium of blood and lymph vessels, mesothelium of body cavities, corneal endothelium

Simple squamous

Simple cuboidal

Simple columnar

Stratified squamous

Lining of respiratory bronchioles, thyroid follicular cells, germinal epithelium of ovary, lens of eye, pigment epithelium of retina, ependymal cells of choroid plexus

Lining of pulmonary bronchioles, lining of gastrointestinal tract, lining of anal canal above anal valves, lining of uterus and uterine tubes, lining of large excretory ducts of glands

Epidermis of skin, lining of oral cavity and esophagus, lining of anal canal below anal valves, lining of vagina, corneal epithelium, lining of female urethra, lining of fossa navicularis of the penile urethra

Stratified columnar Lining of prostatic, membranous, and penile urethra up to fossa navicularis

Psudostratified Lining of trachea and primary bronchi; lining of efferent ductules, epididymis, columnar and ductus deferens

Transitional

Lining of renal calyces, renal pelvis, ureters, and urinary bladder

2. The zonula adherens extends around the entire perimeter of the cell.

a. The cell membranes of the two adjoining cells are separated by an intercellular space filled with an amorphous material.

b. There is a dense area on the cytoplasmic side of each cell that consists of actin filaments, which are linked by a-actinin and vinculin to a transmembrane protein called E-cadherin [or adherens cell adhesion molecule (A-CAM)].

3. The macula adherens (desmosome) occurs at small discrete sites.

a. The cell membranes of the two adjoining cells are separated by an intercellular space filled with a thin dense line of material. An attachment plaque on the cytoplasmic side of each cell anchors tonofilaments.

b. Several protein components of the desmosome have been identified:

(1) Desmoglein I and desmocollin 1 and II are calcium-binding proteins that mediate calcium-dependent cell adhesion.

(2) Desmoplakin I and II are located in the attachment plaque.

4. The gap junction (nexus) occurs at small discrete sites for the metabolic and electrical coupling of cells.

a. The cell membranes of the two adjoining cells are separated by an intercellular space that is bridged by connexons.

(1) Connexons consist of a transmembrane protein (connexin) complcx.

(2) Connexons contain central pores that allow passage of ions, cyclic adenosine monophosphate (cAMP), amino acids, steroids, and small molecules (< 1200 d) between cells. The opening and closing of the pores is regulated by intracellular levels of calcium.

b. Gap junctions are also found between osteocytes, astrocytes, cardiac muscle cells, smooth muscle cells, and endocrine cells.

C. Cancer cells generally do not have gap junctions, so the cancer cells cannot communicate their mitotic activity to each other, which may explain their uncontrolled growth.

Actin

Integral protein

Dyneln ATPase

Actin

Integral protein

Dyneln ATPase

■amOHMHI

Figure 2-1. Diagram of a hypothetical epithelial cell demonstrating the specializations in the apical, lateral, and basal regions.

■amOHMHI

Laminin

Type IV collagen .

Figure 2-1. Diagram of a hypothetical epithelial cell demonstrating the specializations in the apical, lateral, and basal regions.

C. The basal region

1. Basal infoldings are invaginations of the cell membrane that contain ion pumps (Na+-K+-ATPase) found in close association with mitochondria, which provide the substrate ATP. Basal infoldings are found in the proximal and distal convoluted tubules of the kidney and in ducts of salivary glands.

2. Hemidesmosomes are junctions that anchor epithelial cells to the underlying basal lamina via a transmembrane protein called integrin. As a result, hemidesmosomes provide a connection between the cytoskeleton of the epithelial cell and the extracellular matrix.

3. Basal lamina a. The principal constituents are fibronectin (binds to integrin of the hemidesmosome), heparan sulfate, laminin, and type IV collagen.

b. Functions of the basal lamina include:

(1) Forming a barrier between epithelium and connective tissue

(a) In normal conditions, lymphocytes may pass the basal lamina during immune surveillance.

(b) In cancerous conditions, neoplastic cells may pass the basal lamina during malignant invasion.

(2) Serving as a filter (e.g., renal glomerulus)

(3) Playing a role in regeneration (epithelial, nerve, or muscle cells use the basal lamina as a scaffolding during regeneration or wound healing)

IV. CLINICAL CONSIDERATIONS

A. Immotile cilia syndrome (Kartagener syndrome) is a genetic disease involving mutations in genes that code for ciliary proteins (e.g., tubulin, dynein). This results in situs inversus (organ reversal due to failure of cells to migrate properly during embryo-genesis), recurrent sinus/pulmonary infections (inability to move mucous), and sterility in males (retarded sperm movement).

B. Bullous pemphigoid is an autoimmune disease in which antibodies against desmoso-mal proteins are formed. This results in widespread skin and mucous membrane blistering as desmosomes fall apart.

C. Carcinomas is a malignant neoplasm derived from epithelium.

D. Adenocarcinoma is a malignant neoplasm derived from glandular epithelium.

V. SELECTED PHOTOMICROGRAPHS

A. Microvilli and cilia (Figure 2-2; see III A)

Figure 2-2. (A) Electron micrograph of microvilli comprising the microvillus border of an epithelial cell. Nore the actin core (arrows) extending into ihe terminal web within the cytoplasm. (Courtesy of Dr. Jack Brinn, East Carolina University, School of Medicine.) (B) Electron micrograph of microvilli in cross-section demonstrating the actin core (arrow 1) and the fuzzy glycocalyx (arrow 2). (Courtesy of Dr. A. Ichikawa. Reprinted with permission from Fawcett DW: A Textbook of Histology, 12th ed. New York, Chapman Hall, 1994, p 74. Courtesy of Don W. Fawcett, M.D.) (C) Electron micrograph of cilia. Nore the microtubule core (arrow) extending into the basal body within the cytoplasm. (Courtesy of Dr. Jack Brinn, Easl Carolina University, School of Medicine.) (D) Electron micrograph of cilia in cross-section. Note the arrangement of microtubules in a 9 + 2 arrangement and the dynein arm (arrow). (Reproduced with permission from Simionescu M: J Cell Biol 70:608, 1976 by copy-

Figure 2-2. (A) Electron micrograph of microvilli comprising the microvillus border of an epithelial cell. Nore the actin core (arrows) extending into ihe terminal web within the cytoplasm. (Courtesy of Dr. Jack Brinn, East Carolina University, School of Medicine.) (B) Electron micrograph of microvilli in cross-section demonstrating the actin core (arrow 1) and the fuzzy glycocalyx (arrow 2). (Courtesy of Dr. A. Ichikawa. Reprinted with permission from Fawcett DW: A Textbook of Histology, 12th ed. New York, Chapman Hall, 1994, p 74. Courtesy of Don W. Fawcett, M.D.) (C) Electron micrograph of cilia. Nore the microtubule core (arrow) extending into the basal body within the cytoplasm. (Courtesy of Dr. Jack Brinn, Easl Carolina University, School of Medicine.) (D) Electron micrograph of cilia in cross-section. Note the arrangement of microtubules in a 9 + 2 arrangement and the dynein arm (arrow). (Reproduced with permission from Simionescu M: J Cell Biol 70:608, 1976 by copy-

B. Zonula occludens (Figure 2-3; see III B 1)

Figure 2-3. (A) In this electron micrograph, fusion between the outer leaflets of two cell membranes can be observed just beneath the microvilli (MV). This type of cell junction is called a zonula occludens (ZO) or tight junction. (Courtesy of Dr. Jack Brinn, East Carolina University, School of Medicine.) (B) A freeze-fracture replica of a zonula occludens or tight junction. A belt-like band of anastomosing strands (ZO) can be observed. The strands are seen as ridges of intramembranous particles on the P-face or complimentary grooves on the E-face. Microvilli (MV) are apparent. (Reprinted with permission from Gilula N: Cell junctions. In Cell Communication. Edited by Cox R. New York: John Wiley, 1974, pp 1-29. Reprinted by permission of John Wiley & Sons, Inc.)

Figure 2-3. (A) In this electron micrograph, fusion between the outer leaflets of two cell membranes can be observed just beneath the microvilli (MV). This type of cell junction is called a zonula occludens (ZO) or tight junction. (Courtesy of Dr. Jack Brinn, East Carolina University, School of Medicine.) (B) A freeze-fracture replica of a zonula occludens or tight junction. A belt-like band of anastomosing strands (ZO) can be observed. The strands are seen as ridges of intramembranous particles on the P-face or complimentary grooves on the E-face. Microvilli (MV) are apparent. (Reprinted with permission from Gilula N: Cell junctions. In Cell Communication. Edited by Cox R. New York: John Wiley, 1974, pp 1-29. Reprinted by permission of John Wiley & Sons, Inc.)

Figure 2-4. (A) In this electron micrograph, the cell junction indicated by the bracket is called a gap junction or nexus. (B) A freeze-fracture replica of a gap junction or nexus (arrows). Each intramembranous particle located exclusively on the P-face corresponds to a connexon. Gap junctions are constructed from transmembrane proteins (called connexins) that form structures called connexons. Two connexons bridge across the intercellular space to form a channel (or pore) connecting two cells. (Reprinted with permission from Gilula N: Cell junctions. In Cell Communication. Edited by Cox R. New York: John Wiley, 1974, pp 1-29. Reprinted by permission of John Wiley & Sons, Inc.)

Figure 2-4. (A) In this electron micrograph, the cell junction indicated by the bracket is called a gap junction or nexus. (B) A freeze-fracture replica of a gap junction or nexus (arrows). Each intramembranous particle located exclusively on the P-face corresponds to a connexon. Gap junctions are constructed from transmembrane proteins (called connexins) that form structures called connexons. Two connexons bridge across the intercellular space to form a channel (or pore) connecting two cells. (Reprinted with permission from Gilula N: Cell junctions. In Cell Communication. Edited by Cox R. New York: John Wiley, 1974, pp 1-29. Reprinted by permission of John Wiley & Sons, Inc.)

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