Term
| What are the general features of epithelium? |
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Definition
- The cells are derived from ectoderm, endoderm, and mesoderm
- The cells form cohesive sheets that cover or line body surfaces
- There is very little extracellular space
- They do not have a direct blood or lymphatic supply
- Cohesive and barrier properties of epithelia are maintained by cell adhesion molecules and junctional complexes
- They are attached to a basal lamina (basement membrane)
- They have structural and functional polarity |
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Term
| What are the functions of epithelia? |
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Definition
| Protection, surface transport, absorption, secretion, sensory reception, and transepithelial transport |
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Term
| What are the apical surface specializations of epithelia? |
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Definition
Cilia - presence of cilia on apical cells is associated with surface transport of material
Microvilli/stereocilia - microvilli serve to increase surface area for absorption
Keratin - a keratinized epithelium will prevent water loss through the epithelium thus protecting against desiccation |
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Term
| Describe cilia and flagella |
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Definition
| They are motile projections from the cell surface visible with the light microscope; cilia function in surface transport of materials along epithelial surface |
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Term
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Definition
| They are cell surface projections found on many cells; they serve to increase the absorptive surface area of the cell and it appears in addition, that the movement of these structures themselves may be generated with the help of the cellular contractile proteins, actin and myosin; microvilli are a particularly prominent feature of the epithelia that line the small intestine and the proximal convoluted tubule of the kidney; the striated border or brush border of these epithelial linings are composed of large number of microvilli |
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Term
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Definition
| Very long, branched microvilli found with a very restricted distribution, e.g. epididymis |
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Term
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Definition
| Surface coat consisting of complex carbohydrates associated with proteins present on the cell surface; in the light microscope it appears as a fuzzy layer on the surface of cells |
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Term
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Definition
| Stratified squamous epithelia may be keratinized; in this case there is an apical keratinized or cornified layer; serves to protect against desiccation |
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Term
| What are the 3 structures of the terminal bar? |
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Definition
| Tight-junctions, zonula adherens, and desmosomes |
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Term
| Describe tight-junctions (zonula occludens) |
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Definition
| These junctions are continuous belts (zonules) around the entire apical perimeter of the cells; the plasma membranes of the adjacent cells appear to fuse, thereby preventing the free flow of solutes through the epithelial boundary |
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Term
| Describe zoluna adherens (belt desmosomes, intermediate junctions) |
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Definition
| These junctions are located just beneath the zoluna occludens and form a continuous belt around the cells; no membrane fusion occurs in these locations; on the cytoplasmic faces of these juctions, numerous filaments are found are found continuous with the filaments of the terminal web; these juctions are mainly responsible for adhesion of adjacent cells |
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Term
| Describe desmosomes (macula adherens) |
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Definition
| A snap-fastener or button between cells (macula = spot); dense material accumulates in adhesion plaques beneath the plasma membranes of each cell and extracellular matrix is secreted between the cells; tonofilaments embed into the adhesion plaques from the cytoplasm and serve as roots to mechanically reinforce the area of attachment between two cells |
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Term
| Describe gap junctions (nexus, macula communicans) |
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Definition
An area where the adjacent cell membranes are separated by a 2 nm gap; the junction is a plaque, not a zonule in the plane of the membrane; there is a hexagonal lattice of subunits comprising the junction
Each of these subunits may be resolved by X-ray diffraction to form a channel connecting the cytoplasm of the two cells, through which small molecules may pass; the gap junction is a probable site of intercellular communication, not only between epithelial cells, but also between cells comprising other tissues and organs |
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Term
| What are the three regions of the basement membrane? |
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Definition
Lamina lucida, lamina densa, and lamina reticularis
In some tissues, such as the renal glomerulus, the basement membranes of adjacent epithelia become fused; in these cases, a well-defined lamina densa is associated with a lamina lucida interna and lamina lucida externa (replacing the lamina reticularis) |
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Term
| What are the major macromolecules of the basement membrane? |
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Definition
| Type IV collagen, laminin, and heparan sulfate proteoglycans (perlecan) |
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Term
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Definition
Mediate attachment between epithelial cells and basement membrane; structurally similar to half a desmosome, but biochemically unique
There is a disruption of these structures in a variety of blistering diseases, such as epidermolysis bullose and bullous pemphigoid |
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Term
| What are the differences between endocrine and exocrine glands? |
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Definition
Endocrine glands lack ducts and not all of these are derived from epithelia
Exocrine glands release their secretions into ducts that are continuous with the surface epithelium of an organ (for example, skin or gut) |
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Term
| What are the classifications of multicellular glands based on the structure of the ducts? |
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Definition
Simple - single duct
Compound - ducts that branch |
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Term
| What are the classifications for mutlicellular glands based on the structure of the secretory portion? |
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Definition
Tubular - secretory portion is tube-like
Acinar (alveolar) - secretory portion is spheroid
Tubuloacinar - contain tubular and acinar portions |
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Term
| What are the classifications of exocrine glands based on secretions? |
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Definition
Mucous glands - produce a viscous, mucin-rich secretion (goblet cells, salivary glands)
Serous glands - produce a watery, protein-rich secretion (pancreas, salivary glands)
Mixed glands - produce both serous and mucous secretion; in seperate secretory units or mixed (mucous acini with serous demilune) |
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Term
| What are the 3 different modes of exocrine gland secretion? |
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Definition
Merocrine - secretion leaves grandular cells via exocytosis with no loss of other cellular material (pancreas, salivary glands)
Holocrine - secretion leaves the cell by destruction of the glandular cell; the secretion filled cell is destroyed and resulting in release (sebaceous gland)
Apocrine - intermediate type with secretion involving loss of some apical cytoplasm (mammary gland) |
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