Epithelium, Epithelial cells line the free external and internal surfaces of the body

Epithelial cells line the free external and internal surfaces of the body. Epithelia have a paucity of intercellular substance and are interconnected by junctional complexes.
Epithelial cells line the free external and internal surfaces of the body. Epithelia have a paucity of intercellular substance and are interconnected by junctional complexes. 

Components of the junctional complex include the zonula occludens (tight junction), which prevents leakage between the adjoining cells and maintains apical/basolateral polarity; zonula adherens, which links the actin networks within adjacent cells; and macula adherens (desmosome), which links the intermediate filament networks of adjacent cells. 

Epithelial cells also form a firm attachment to the basal lamina, which they secrete. Gap junctions or nexi permit passage of small molecules directly between cells. Apical specializations are prominent in epithelia and include microvilli that increase surface area; stereocilia, which are nonmotile modified microvilli; and cilia and flagella, which are motile structures.

Cilia and flagella have the classic “9 + 2” microtubular arrangement emanating from basal bodies. The basal surface may be modified with infoldings that house numerous mitochondria as found in proximal and distal tubule cells of the kidney and striated duct cells of the salivary glands. Those cells are involved in extensive ion transport.

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