Vascular endothelial cells contribute to the maintenance of vascular homeostasis. Vascular endothelial cells produce and secrete activators and inhibitors of the coagulation and fibrinolysis system. In addition, they mediate the adhesion and aggregation of blood platelets. Endothelial cells also release molecules that regulate cell proliferation and control vessel wall tone. Human umbilical vein endothelial cells (HUVEC) and human umbilical artery endothelial cells (HUAEC) are the types of cells most used in the study of endothelial cell processes in vitro. HUAEC have a “cobblestone” morphology and show positive staining for vWF/Factor VIII and CD-31. They also have the ability to take up acetylated low-density lipoprotein. Cells pretreated with IL-1 or TNF-alpha also selectively express E-selectin and VCAM.