ABSTRACT
Epithelial barriers are essential for life. These are cellular structures established and maintained at interfaces that separate the internal environment of an organism from the external world. For example, epithelia cover the external surface of the body and line the gastrointestinal (GI), respiratory, and reproductive tracts. All epithelia present an asymmetric complement of surface components, intracellular organelles, and cytoskeletal arrangements that are organized in accordance with the external environment they experience. Most importantly, all epithelia must maintain the homeostatic conditions of the body. Thus, epithelial barriers must perform a number of critical functions that are distinct for specic internal/external interfaces. Some of the best-known functions performed by epithelia relate to their ability to restrict the casual movement of water and solutes, unidirectional movement of specic molecules, sense changes at their surface, and transmit that information into the body (e.g., neuroepithelial cells), as well as exclude and respond to pathogens, toxins, and innocuous agents (e.g., myoepithelial cells). As one might expect, epithelia of the body are under constant stress by external challenges; epithelial cells have a relatively rapid turnover rate typically in the range of a few days to weeks.
