Plugging the leaks

In multicellular organisms, exchange between the cytoplasm of cells and the environment is facilitated by the vascular system in combination with the infolding of the surface epithelium to form the gastrointestinal, respiratory, and excretory (urinary) systems. These internalized, branching tubular pathways into the body of an organism are lined by continuous layers of cells, called epithelia, which preserve the boundary between the environment and the extracellular connective tissues and blood spaces. The epithelia lining these tubes are selectively permeable to molecules destined to be absorbed or secreted, by virtue of cellular transport systems that move these molecules through the cells between the inside and outside of the body. This route of transport of molecules through the epithelial cell cytoplasm is called the transcellular pathway. In addition to this route, molecules can move across epithelia by diffusing in between the cells [the so-called paracellular pathway (1)] (Fig. 1). Although the spaces between epithelial cells are very narrow, typically 20–30 nm, the paracellular pathway nonetheless represents a significant leak between the environment and the connective tissues that must be regulated for the epithelium to remain selectively permeable. Organisms also subdivide their internal spaces into separate physiological compartments that do not communicate with the external environment. These internal compartments include the blood and lymphatic systems, the thoracic, pericardial, and abdominal cavities, and the specialized matrix that accompanies peripheral nerves. These compartments are likewise delimited by epithelioid layers of cells, termed endothelium, mesothelium, and perineurium.