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== Introduction ==

An epithelium is defined as a tissue that covers or lines the free surfaces of the body. It is one of four fundamental body tissues, with the other three basic tissues being muscle, nervous and connective tissues. Histologically, an epithelium is characterized as being composed of cells sitting upon a basement membrane. The cells are usually arranged in one or more layers with scant intercellular material, or they may sometimes be arranged in cords as in some endocrine glands.