Summary
Cultured human endothelial cells synthesize and secrete a protein(s) which has factor
VIII antigen and von Willebrand factor activity. Subcellular membrane and granule
fractions derived from human platelets also contain the factor VIII antigen and von
Willebrand factor activity. Circulating platelets constitute a significant reservoir
of VIII antigen containing approximately 15 % of the amount present in platelet-poor
plasma. Thus normal platelets contain surface bound as well as intracellularly stored
von Willebrand factor, a protein synthesized by endothelial cells which is required
for normal platelet function.