The Endothelial Junction

Abstract

The endothelium of most blood vessels is a single layer of tightly packed cells which line the vascular lumen and border on the basal lamina and, in some arteries and arterioles, on the processes of vascular smooth muscle cells. Like singlelayered epithelia, the endothelial cells are polar, with an apical, i.e., adluminal, and a basal, i.e., abluminal, plasma membrane region which appear to be segregated from each other by a special membrane region containing occluding, i.e., “tight,” junctions (for reviews see Ref. 89). Again similarly to polar epithelia, the endothelium is capable of vectorial sorting, secretion, and virus budding as well as endocytotic and transcytotic processes (for examples see Refs. 47,66,72). Obviously, tight sealing of endothelial cells to each other is a prerequisite for their physiological functioning, and situations in which the coherence of the endothelial layer is locally and/or transiently interrupted are usually associated with pathological processes (e.g., Refs. 16,17,52,54,77,86,92). The elucidation of the molecular organization of the “endothelial junction” is crucial to our understanding of the structures and molecules involved in the intercellular adhesion of the endothelial cells. This junctional zone is characterized by a relatively close apposition of the adjacent plasma membranes which are flanked by a so-called “parajunctional zone” of cortical cytoplasm (for definition see Ref. 90). So far only two specific junctions have been identified within this junctional complex. These are the occluding (tight) and the communicating (gap) junctions which are morphologically similar to those present in other tissues (reviewed in Refs. 89-92; for certain freeze-cleave aspects see also Refs. 98,110). Typical desmosomes or other adhering junction structures have not been demonstrated in higher vertebrates.