Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
a tissue found in animals and man consisting of specialized cells that line the inner surfaces of blood and lymph vessels, as well as the cavities of the heart. The endothelium is formed from the mesenchyme. It consists of a single layer of closely packed flat cells, which are usually separated from underlying tissues by a basal membrane. The endothelium of lymph vessels has no basal membrane. The cytoplasm of endothelial cells of certain capillaries may have pores that account for the permeability of the capillary wall, as, for example, in the renal glomeruli.
The endothelium of the enlarged capillaries of the blood-forming organs and the endothelium present in the liver, adrenal cortex, and certain other organs has a great capacity for phagocytosis. The endothelial cells of these organs belong to the reticuloendothelial system.