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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
  • An epithelium in which the basement membrane has one cell ...

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