Update on the cellular and molecular basis of capillary permeability

The attenuated layer of endothelial cells lining the blood vessels forms the critical barrier controlling the exchange of molecules from the blood to the interstitial fluid. The interactions of normally circulating blood molecules with the endothelial glycocalyx can either restrict transcapillary exchange in general or selectively increase transendothelial transport of a specific group of ligands. Investigations into the mechanisms responsible for the effects of serum have identified specific receptors, some of which appear to be involved in receptor-mediated transcytosis and endocytosis via noncoated plasmalemma vesicles (also known as caveolae). Such studies suggest that regional differences in endothelial expression of cell surface glycoproteins can be exploited for the development of tissue-directed drug therapies.