Epithelial tissues cover or line all body parts both externally and internally. With cells tightly packed together and with little intercellular space, the epithelial tissues form effective barriers between underlying tissues and external media. Thus, the epithelial tissues are extremely important in regulating the exchange of materials between different body parts. The portions of cell membranes found on the outer surface of an epithelial tissue are specialized to act as the regulators of the tissue strictly managing the flow of materials into and out of the epithelium. In contrast, those portions of cell membranes that are adjacent only to other epithelial cell membranes allow a less restricted flow of materials between epithelial cells. The outside surfaces of epithelial tissues often are highly specialized in other ways possessing cilia, hairs, or glands, which may arise from individual cells or groups of cells that have invaginated and which are capable of covering the outer surface of an epithelial tissue with mucous or waxy secretions. Epithelial tissues are separated from underlying tissues by extra cellular basement membranes.
