Functions of the Endothelial Cell Surface

The primary interactions between the blood components and the vessel wall take place at the luminal surface of vascular endothelium. The blood­ endothelial interface is a composite of the endothelial cell coat proper and some plasma proteins temporarily associated with the former. The endothelial cell coat actually represents the ectodomains of membrane proteins and glycopro­ teins, and proteoglycans. It is still uncertain whether proteoglycans are true membrane constituents or are only attached to the glycocalyx. Endothelial cell surface (ECS) occupies a special strategic position. It is continuously exposed to the dynamic internal milieu, and it senses, transmits, and participates in adjustments to deviations in homeostasis. To fulfill these very complex and vital activities ECS has been endowed with an intricate chemical and functional machinery that is responsible for the blood-tissue exchanges, active monitoring of plasma molecules, and maintenance of a nonthrombogenic surface. The inability, so far, to obtain a pure fraction of endothelial plasma mem­ brane explains the lack of detailed information on its chemistry, including the molecular organization of the glycocalyx. Data currently available emerged mostly from indirect investigations using membrane markers, and from physiologic inquiries on some activities significantly associated with endothe­ lial membrane and implicitly with its cell coat. Our present information comes from studies on both microvascular and large vessel endothelia, and is often affected by the unavoidable limitations of the in-culture conditions. The main purpose of this review is to consider some of the significant advances in (a) the chemical mapping of ECS, (b) the cell surface-associated plasma proteins, and (c) some cell surfaceand membrane-associated activi-