Endothelial cells constitute the natural interface between the blood and the underlying tissue. Changes in endothelial cell function appear to play a key role in the pathogenesis of atherosclerosis. Endothelial cells synthesize and secrete activators as well as inhibitors of both the coagulation system and the fibrinolysis system in addition to mediators that influence the adhesion and aggregation of blood platelets. Endothelial cells also release molecules that control cell proliferation and modulate vessel wall tone. Human aortic endothelial cells (HAEC) produce antithrombotic and thrombotic factors such as t-PA and PAI-1. They respond to TNFalpha by modifying growth characteristics, producing cytokines such as GM-CSF.