Epithelial cell overview:
Epithelial tissues are comprised of continuous sheets of adherent epithelial cells that cover the body surfaces and cavities, form glands, and line the inner and outer surfaces of sac- and tube-shaped organs. Epithelial cells characteristically have an apical-basolateral asymmetry or polarity that is marked by compositionally, functionally, and morphologically distinct membrane domains: an apical one that is contiguous with the external milieu or faces the body cavities, and a basolateral domain that abuts the underlying tissues. By separating the external environment from the internal one, a function that depends, in part, on the tight junction, adjacent epithelial cells can perform specialized functions including vectorial water, ion, and peptide transport. Furthermore, their location at the interface of the external and internal milieus makes them ideally suited to form barriers to water, ions, and pathogens, to carry out immune surveillance, and to perform sensory transduction.