Epithelium is a tissue existing in a multiplicity of forms which are specialized to carry out one or more characteristic tasks. The unique function of epithelia lies in providing a boundary . . . [that] line inner surfaces and cover outer surface . . . In some organs, such as skin, the epithelium serves as a protective barrier, but the additional activities in which most epithelia engage are in some instances their primary role. Characteristic functions include transport, absorption, and secretion.
