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SUMMARY OF KEY POINTS
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Epithelial Tissue SUMMARY OF KEY POINTS
An epithelium is a tissue in which cells are bound tightly together structurally and functionally to form a sheetlike or tubular structure with little extracellular material between the cells.
Cells in epithelia each have an apical side facing the sheet’s free surface and a basal side facing a basement membrane and underlying connective tissue.
Epithelia are often specialized for absorption or transcytosis, pinocytosis of material at the apical side and exocytosis at the basolateral side (or vice versa).
Cells of most epithelia exhibit continuous renewal, with the locations of stem cells and rates of cell turnover variable in various specialized epithelia.
Basement Membrane The basement membrane of all epithelia is a thin extracellular layer of specialized proteins, usually having two parts: a basal lamina and a more fibrous reticular lamina.
The basal lamina is a thin meshwork of type IV collagen and laminin produced by the epithelial cells.
The reticular lamina contains type III collagen and anchoring fibrils of VII collagen, all secreted by cells of the immediately adjacent connective tissue.
Together, these components attach epithelia to connective tissue, regulate (filter) substances passing from connective tissue into epithelia, provide a guide or scaffold during tissue regeneration after injury, and compartmentalize epithelial cells from other tissues.
Intercellular Junctions Intercellular junctions are well developed in epithelia and consist of three major types, with different functions.
Tight or occluding junctions are formed by interacting transmembrane proteins such as claudin and occludin; linear arrangements of these linked proteins surround the apical ends of the cells and prevent paracellular passage of substances (between the cells.)
Adherent or anchoring junctions, formed by interacting proteins of the cadherin family, are points of strong attachment holding together cells of the epithelium.
Adherent junctions may form zonula adherens that encircle epithelial cells just below their tight junctions and attach indirectly to actin filaments, or scattered, spot-like attachment sites called desmosomes or maculae adherens, which attach to keratin intermediate filaments.
Hemidesmosomes composed of transmembrane integrins attach cells to proteins of the basal lamina.
Gap or communicating junctions are points of cell contact where both plasma membranes have numerous hexameric complexes of transmembrane connexons, each forming a channel allowing passage of small molecules from one cell to the other.
Apical Structures of Epithelial Cells Microvilli are small membrane projections with cores of actin filaments that generally function to increase epithelial cells’ apical surface area for absorption.
Stereocilia are long microvilli with specialized mechanosensory function in cells of the inner ear and for absorption in tissues of the male reproductive tract.
Cilia are larger projecting structures with a well-organized core of microtubules (in a 9 + 2 arrangement called the axoneme) in which restricted, dynein-based sliding of microtubules causes ciliary movement that propels material along an epithelial surface.
Morphological Types of Epithelia