Epithelium covers body surfaces, lines body cavities, and constitutes glands
Epithelium is an avascular tissue composed of cells that cover the exterior body surfaces and line internal closed cavities (including the vascular system) and body tubes that communicate with the exterior (the alimentary, respiratory, and genitourinary tracts). Epithelium also forms the secretory portion (parenchyma) of glands and their ducts. In addition, specialized epithelial cells function as receptors for the special senses (smell, taste, hearing, and vision).
The cells that make up epithelium have three principal characteristics:
• They are closely apposed and adhere to one another by means of specific cell-to-cell adhesion molecules that form specialized cell junctions (Fig. 4.1).
• They exhibit functional as well morphologic polarity; i.e., different functions are associated with three distinct morphologic surface domains: a free surface or apical domain, a lateral domain, and a basal domain. (The properties of each domain are determined by specific membrane proteins.)
• Their basal surface is attached to an underlying basement membrane, a noncellular, protein-polysaccharide-rich layer demonstrable at the light microscopic level by histochemical methods (see Fig. 1.2, page 7).
In special situations, epithelial cells lack a free surface (epithelioid tissues)
In some locations, cells are closely apposed to one another but lack a free surface. Although the close apposition of these cells and the presence of a basement membrane would classify them as epithelium, the absence of a free surface more appropriately classifies such cell aggregates as epithelioid tissues. Epithelioid organization is typical of most endocrine glands; examples of such tissue
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