Originating in the mesoderm, it is a single, thin layer of flattened polygonal cells lining the interior surfaces of the vertebrate heart, blood and lymph vessels. In fact, endothelial cells line the entire circulatory system. These cells are involved in blood clotting, the formation of new blood vessels, and the control of blood pressure (vasoconstriction and vasodilation), as well as inducing inflammation and edema.
See Mesoderm, Pre-eclampsia