The endothelial cell is thought to arise from the splanchnopleuric mesoderm. Endothelial cells form the inner lining of a blood vessel and provides an anticoagulant barrier between the vessel wall and blood. In addition to its role as a selective permeability barrier, the endothelial cell is a unique multifunctional cell with critical basal and inducible metabolic and synthetic functions. The endothelial cell reacts with physical and chemical stimuli within the circulation and regulates hemostasis, vasomotor tone, and immune and inflammatory responses. In addition, the endothelial cell is pivotal in angiogenesis and vasculogenesis. Endothelial cell injury, activation or dysfunction is a hallmark of many pathologic states including atherosclerosis, loss of semi-permeable membrane function, and thrombosis. Cell facts: (1) Endothelium consists of approximately (1-6) x 10(13) endothelial cells forming an almost 1 kg organ. (2) They uniquely contain Weibel-Palade bodies, 0.1 microm wide, 3 microm long membrane-bound structures that represent the storage organelle for von Willebrand factor (vWF). (3) The endothelial cell is not only a permeability barrier but also a multifunctional paracrine and endocrine organ. It is involved in the immune response, coagulation, growth regulation, production of extracellular matrix components, and is a modulator of blood flow and blood vessel tone.