The vascular endothelium was, in the past, considered to be relatively pas- sive, merely acting as a filter between the blood and the vessel wall. It is now clear that endothelial cells actively contribute to the maintenance of vascular homeostasis. Endothelial cells synthesize and secrete activators as well as inhibitors of both the coagulation system and the fibrinolysis system, and mediators that influence the adhesion and aggregation of blood platelets. They also release molecules that control cell proliferation and modulate vessel wall tone (1 –3 ) Many of these, and other, processes can be studied in vitro using cultured cells, and umbilical veins are the most readily available source of human vascular endothelial cells. The method described here is based on that of Jaffe et al. (4 , 5 ) and yields primary cultures that contain 95–98% endothelial cells and >98% after the first passage. Contaminating cells in primary cultures are not smooth muscle (if the procedure has been followed correctly), as can be demonstrated by examining the level of staining with antibody to smooth muscle α-actin, but may include monocytes/macrophages at a very low level.
