An epithelium is a layer of cells which forms a boundary with properties adapted for its position in the body and the function it performs. Epithelia cover every exposed surface, like the external covering of the body or the lining of the alimentary canal or gut. Epithelia are also found in the urinary, reproductive or respiratory passages, all of which, like the gut, are open to the exterior. Epithelia line enclosed cavities like the chambers of the brain, or the space enclosed by the blood system and the heart. Glands like the pancreas are largely composed of specialised epithelia.
Epithelia have a number of important characters.
Epithelia are made up of cells bound together by cell junctions of different kinds, depending on the function of the epithelium. They may also be joined by special structures on the surface of the cell, known as cell junctions. There are three types of cell junctions, tight junctions, desmosomes and gap junctions. Tight junctions consist of a partial fusion of the lipid membranes of two adjoining cells. Desmosomes are much stronger and more complex, and include CAMs and proteoglycans, as well as a structure called a dense area within the cell.