Endothelium is a type of
epithelium that lines the interior surface of
blood vessels and
lymphatic vessels, forming an interface between circulating
blood or
lymph in the
lumen and the rest of the vessel wall. It is a thin layer of simple
squamous cells called
endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells.