Endothelial cell modulation of haemostasis

Abstract

Endothelial cells were isolated from the umbilical vein in human umbilical cords and were grown in tissue culture for periods up to 5 mth. These human endothelial cells grew as monolayers of closely apposed, polygonal large cells whereas both fibroblasts and smooth muscle cells grew as overlapping layers of parallel arrays of slender, spindleshaped cells. By EM, cultured endothelial cells were seen to contain cytoplasmic inclusions (Weibel-Palade bodies) characteristic of in situ endothelial cells. These were also found in endothelial cells lining umbilical veins but were not seen in smooth muscle cells or fibroblasts in culture or in situ. Cultured endothelial cells contained abundant smooth muscle actomyosin as well as ABH antigens appropriate to the donor's blood type; these antigens were not detectable in cultured smooth muscle cells or fibroblasts. The cultured human endothelial cells synthesize and secrete a protein(s) which contains the same polypeptide subunit as factor VIII antigen. Von Willebrand factor activity has also been identified in medium from cultured human endothelial cells. Subcellular membrane and granule fractions derived from human platelets also contain factor VIII antigen and von Willebrand factor activity but not factor VIII procoagulant activity. A factor VIII binding protein has been identified in solubilized platelet membrane and granule fractions. Circulating platelets constitute a significant reservoir of plasma factor VIII antigen. Thus normal platelets contain surface bound.as well as internally stored von Willebrand factor, a protein synthesized by endothelial cells which is necessary for normal platelet function in vivo. These studies suggest that endothelial cells play an important role in the surface orientated modulation of hemostatic events.