In biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue. It lies on top of connective tissue, and the two layers are separated by a basement membrane.
In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue.
Epithelium is often defined by the expression of the adhesion molecule e-cadherin (as opposed to n-cadherin, which is used by cells of the connective tissue).
Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation and they commonly as a result present extensive apical-basolateral polarity (e.g. different membrane proteins expressed) and specialisation.