The biosynthesis of extracellular matrix proteins by primary cultures of endothelial cells from human umbilical vein, and by clones from a murlne hemangioendothelioma, was studied and compared to that reported for endothellum cultured from other sources. Umbilical vein endothelial cells secreted two glycoproteins -flbronectln and thrombospondin -which comprise the major proportion of the protein In the culture media of bovine aortic, venous, and corneal endothelial cells. These blosynthetlc

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... ucts were absent from hemangioendothelioma cultures. However, In contrast to bovine endothellum from large vessels and cornea, which secrete primarily Type III procollagen Into the culture medium, both the umbilical vein and hemangioendothelioma cultures secrete Type IV (basement membrane) procollagen. In addition, EC, a novel endothelial collagen type that has been characterized in bovine endothelial cell supernates, was not present In the umbilical vein or tumor-derived endothellum. The production of basement membrane procollagen as the major collagen type In the medium of these cultures probably reflects the nature of the vascular bed from which the endothelial cells originated, rather than differences In species or In cellular Isolation and subculture. We suggest that endothelial cells from different vascular environments could display variations In growth, migration, morphology, and response to exogenous blood-borne factors as a result of their relationship to an extracellular matrix/subendothellum composed of diverse structural glycoproteins. (Arteriosclerosis 2:27-36, January/February 1982) S ynthesis of extracellular matrix components by cells of the blood vessel wall has received much attention in the last several years. We have been particularly interested in protein secretion From the