Endothelial cells in vivo rest on the subendothelium, a complex array of acellular, connective tissue structures. Ultrastructural studies of the subendothelium in vivo have shown that it is composed of at least five different components; amorphous basement membrane, microfibrils, elastic fibers, fibrillar collagen, and mucopolysaccharides (1). The subendothelium is thought to have several important physiologic functions. It provides most of the rigidity of capillary walls (2), serves as an anchor for the overlying endothelial cell layer, and may act as a filter (3). The subendothelium is also involved in pathologic processes. Loss of endothelial cells and exposure of the subendothelium to blood is an important step in the development of thrombosis since the subendothelium can initiate platelet adhesion and aggregation (1). Alterations in the structure and function of the subendothelium may contribute significantly to the pathogenesis of atherosclerosis (1) and the vascular disease of diabetes mellitus. For example, one of the earliest changes seen in diabetes mellitus is thickening of capillary basement membranes (4). In this paper we report that cultured human endothelial cells synthesize material which is morphologically and immunologically like amorphous basement membrane and biochemically like basement membrane collagen. Cultured endothelial cells probably also synthesize material which is morphologically similar to microfibrils and elastic fibers. Materials and Methods Cell Culture Techniques and Culture Media. Human endothelial cells were derived from umbilical cord veins and cultured using methods and materials previously described (5). Endothelia
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