EPITHELIUM This CAI program will cover the types of epithelia; the polarity of epithelial cells including the characteristics of the 3 membranes apical, basal, and lateral; and a brief description of cell renewal which will be covered in detail in the CAI on the histology of the gastrointestinal tract. The epithelium provides a continuous lining for inner and outer surfaces of the body and is actively involved in secretion. Epithelial cells are joined together by junctional complexes and rest on an extracellular matrix basement membrane). Epithelial cells also possess a distinct polarity, expressed in the plasma membrane as differences in the composition of the apical (free,) basal (attached to the basement membrane,) and lateral surface. Polarity is also expressed in terms of cellular organelles found in different regions of the cell.
EPITHELIAL TYPES Two criteria are used to classify the epithelial types in the body 1) the number of layers (one layer=simple epithelium, 2 or more layers=stratified epithelium), and 2) the shape of the cells at the free surface (squamous, cuboidal, or columnar).
1. Which of the following is (are) true statements concerning the simple epithelia of the body?simple squamous epithelial cells are found in the endothelium and mesothelium where they are involved in the rapid transport of materials simple cuboidal cells are prevalent in glands simple columnar cells are prevalent in the gastrointestinal tract where they are involved in secretion and absorption A and B only A, B and C Correct. No, the answer is E. Simple squamous epithelial cells are flattened cells suitable for the lining of structures involved in rapid transport of metabolites or gases. Simple cuboidal cells are prevalent in glands where they may be modified into acini composed of pyramidally-shaped acinar cells. Simple columnar cells are found throughout the digestive tract from stomach to large intestine and are involved in absorption and/or secretion.
2. Which of the following statements is (are) true of the stratified epithelia of the body?stratified squamous epithelia are primarily protective in function and may be found in keratinized or non-keratinized forms pseudostratified epithelium is found in the trachea and only gives a multilayered appearance while all cells are attached at the basement membrane surface transitional epithelium is found in organs, such as the urinary bladder, which undergo considerable distension A and C only A, B and C Very good. No, the answer is E.
Stratified squamous epithelium is located in the epidermis, anus, vagina and esophagus and primarily provides a protective function. Keratin is a structural protein present in most epithelial cells. In the epidermis of some regions of the body, the surface epithelial cells become keratinized (lack all distinguishable organelles including nuclei). In other regions, while keratin is present, the nuclei are distinguishable even in the surface cells (non-keratinized epithelia). In a pseudostratified epithelium all cells contact the basement membrane, but not all reach the free surface. This arrangement gives the appearance of a stratified epithelium, but only consists of a single layer of cells. The transitional epithelium lines organs which undergo distension such as the urinary bladder. The appearance of the surface may cells vary considerably depending upon the point of fixation (stretch or relaxation).
POLARITY OF EPITHELIAL CELLS (Apical Surface) 3. The apical membranemay contain proteins with protruding carbohydrate moieties forming the surface coat or glycocalyx may contain proteins for the binding of enzymes to the apical surface may be specialized by the presence of microvilli or cilia A and C only A, B and C Correct. No, the answer is E.
The apical membrane may be covered by a PAS (periodic-Acid-Schiff)-positive surface coat or glycocalyx. The apical membrane may also contain proteins, such as ligatin in the small intestine, which bind brush border enzymes to the surface. Microvilli and cilia are specialized structures found on the apical plasmalemma.
4. The terminal webis the region of the epithelial cell just below the microvilli contains only actin is another term for brush or striated border A and B only A, B and C Right. No, the answer is A.
The terminal web is the region of the epithelial cell just below the microvilli which contains numerous microfilaments (e.g. the noncontractile intermediate filaments). It also contains actin and myosin in association with the apical specializations of the cell (i.e. microvilli) and with the junctional region (specifically the zonula adherens) of the cell. It contains actin filaments which descend through the core of each microvillus and myosin which is aligned perpendicularly to the actin. The terminal web is located below the microvilli and is therefore not a synonymous term for brush or striated border.
5. Microvillitips form regions of endocytosis are capable of movement by interaction of microvillus actin with actin and myosin in the terminal web are found most frequently on the apical surfaces of secretory cells A and B only A, B and C Very good. No, the answer is B.
In a teleological sense, microvilli increase the surface area of epithelial cells for absorption and are found in organs with epithelia specialized for absorption. The bases of microvilli are the sites of membrane invagination to form vesicles in which large molecules can be transported across the membrane in a process known as endocytosis. This process often involves the protein, clathrin, which coats the endocytic vesicles. Microvilli are capable of movement and contain actin which can interact with myosin in the terminal web region.
6. Ciliaare found in the small and large intestine are arranged along a basic structural pattern of 9 central and 2 peripheral pairs of microtubules microtubules are composed of the protein dynein movement involves the reversible cross-bridging of microtubules and the ATPase activity of dynein B and D Right. No, the answer is D.
Cilia are found in the uterus, oviduct, and portions of the tracheobronchial tree. The basic arrangement of a cilium is 9 peripheral and 2 centrally-placed pairs of microtubules. The microtubules are composed of alpha and and beta tubulin. One tubule of each peripheral pair possesses crossarms composed of the protein dynein radiating from it. Ciliary beating probably involves a mechanism similar to the actin-myosin interaction in skeletal muscle and involves reversible cross-bridging of microtubules and the ATPase activity of dynein.
7. Kinociliaare nonmotile cilia found in the sensory epithelia of the eye and ear are identical in microtubule arrangement to motile cilia may function in sensory transduction A and C only A, B and C Very good. No, the answer is D. The microtubular pattern of the kinocilia is described as"9 + 0" because of the absence of the central pair. This arrangement differs from the "9 + 2" arrangement of motile cilia.
POLARITY OF EPITHELIAL CELLS (LATERAL SURFACE) The lateral surfaces of epithelial cells are marked by distinctive intercellular junctions. The tight junction (zonula occludens) is the most apically placed and serves as a permeability barrier. Below the tight junction is the belt-like junction (zonula adherens) which serves an adhesive function. The next part of the junctional complex is the plaque-like zone of intercellular attachment, the desmosome (macula adherens), which allows the transmission of mechanical forces between cells. In addition, there are gap or communicating junctions which allow ions and other small molecules to enter between adjacent cells. In addition to the junctional complexes, lateral cell surface also possess enzymes essential for transepithelial transport, such as Na +
/K+
-ATPase.
8. Tight junctionsare permeable to large molecular weight proteins as viewed by freeze-fracture techniques consist of interweaving strands of intramembranous particles in the cytoplasmic (P) half of the membrane which correspond to grooves seen in the outer (E) half of the membrane have identical structure and permeability in different cell types B and C only A and B only Correct. No, the answer is B. Tight junctions are impermeable to large molecular weight proteins. In freeze fracture preparations, the tight junctions are seen as interweaving strands on the P face and grooves on the E face. Different cell types may be joined by tight junctions which differ in structure and also in the degree of permeability of the junction. Therefore, tight junctions have been classified as "tight" (i.e. urinary bladder) or "leaky" (i.e. proximal renal tubule).
9. Desmosomesconsist of a pair of dense attachment plaques just beneath the lateral cell membranes are also known as the maculae adherens are associated with tonofilaments composed of keratins A and B only A, B and C Right. No, the answer is E. The desmosome or spot (macula)-weld consists of a pair of dense attachment plaques just beneath the lateral membranes. The desmosomes are associated with tonofilaments composed of keratins which are a major cytoskeletal element of epithelial cells.
10. The zonula adherensis another term for the spot desmosome is an intermediate junction which surrounds the cell just basal to the zonula occludens resembles the fascia adherens of the intercalated disc found in cardiac muscle B and C only A, B and C Right. No, the answer is D. The zonula adherens surrounds the entire cell unlike the spot-weld arrangement of the macula adherens (spot desmosome). The fascia adherens of the intercalated disc resembles the zonula adherens.
11. Gap junctionscontains a 2nm. space which is bridged by a variable number of cytoplasmic particles provide a low-resistance channel between adjacent cells for the passage of ions and other small molecules are found only between epithelial cells A and B only B and C only Very good. No, the answer is D. Gap junctions contain a 2nm. space which is bridged by a variable number of cytoplasmic particles. These particles are visible as "pimples" on the cytoplasmic (P) face and as "pits" on the outer (E) face of freeze fracture preparations. Gap junction function as low-resistance channels for passage of ions and small molecules. They are found in other tissues such as muscle in addition to their presence in various epithelial cell types.
POLARITY OF EPITHELIAL CELLS (BASAL SURFACE) The membrane of the basal surface contains proteins which always differ from those found in the apical membrane, but may be similar to those found in the lateral membranes.
12. The basal surface of epithelial cellsmay be involved in endocytosis may contain receptors for various hormones and neurotransmitters may be associated with hemidesmosomes contains brush border enzymes Very good. No. The first three foils are correct.
The basal surface of epithelial cells in the small intestine and placenta are involved in immunoglobulin transport by endocytosis. This surface also contains receptors for various hormones and neurotransmiters. For example,the parietal cell of the stomach possesses receptors on its basal surface for acetylcholine, histamine, and gastrin. In the stratified squamous epithelia (i.e. epidermis) thre are hemidesmosomes which anchor the basal surfaces of epithelial cells.
13. The basement membraneunderlies almost all epithelia is sometimes composed of 3 layers basal lamina, and internal and external lamina rara is composed of the basal lamina which contains Type IV collagen is elaborated and secreted completely by cells of the connective tissue Very good. No. The first three foils are correct. The basement membrane differes in structure in different epithelia, but under lies virtually all epithelia of the body. It is sometimes composed of the basal lamina (lamina densa) composed of Type IV collagen and 2 other layers, the internal and external lamina rara. Some of the components of the basement membrane are formed by epithelial cells, but interaction with the underlying cells of mesenchymal origin may be essential.
14. The basement membrane probably is functionally involvedas a scaffolding for migration of epithelial cells such as in the small intestinal epithelium for stabilization during embryogenesis as a selective permeability barrier, i.e. in the glomerulus in growth control, i.e. in tumor growth Very good. No. All foils are correct.
15. Polarity of epithelial cells may be reflected in the intracellular arrangement of organelles as well as the presence of membrane polarity.True False Very good. No, the statement is true. For example, in the case of cells actively involved in secretion, the rough endoplasmic reticulum is often found near the base of the cell, the Golgi apparatus in the middle of the cell above the nucleus, and the secretory and condensing vacuoles near the apical surface.
16. Many epithelial cells of the body undergo cell renewal and turnover with new cells being supplied by stem cells.Very good. No, the statement is true.