In the early development of the embryo the rapidly multiplying cells lie in layers. From each of these three germ layers a class of tissue develops that persists to maturity as a tightly packed layerof cells, contrasting with the connective tissues where the cells are spaced out in an extensive extracellular matrix. The class is the epithelial kind of tissue.
Epithelia cover the outside of the body and line the spaces and tubes within the body.
In the gut, digested foods, but not unwanted and toxic materials have to be absorbed into the blood. In the eye, epithelia serve both transparency and light screening . In many epithelia the cells are secretory . They take raw materials from blood and build up complex materials for release from the cell as a secretion. Epithelial secretions protect, lubricate, digest, or, in the case of hormones, control.