The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. In small blood vessels and capillaries, endothelial cells are often the only cell-type present. Endothelial cells are involved in many aspects of vascular biology, including:
vasoconstriction & vasodilation, and hence the control of blood pressure
Endothelial cells also control the passage of materials — and the transit of white blood cells — into and out of the bloodstream. In some organs, there are highly differentiated endothelial cells to perform specialized 'filtering' functions. Examples of such unique endothelial structures include the renal glomerulus and the blood-brain barrier.
The corneal endothelium is a layer of fluid-transport epithelial cells in the eye, responsible for the hydration of the cornea. Despite its name, it has a very different origin, function and appearance from vascular endothelia. It is considered a squamous epithelium, but a distinct structure from the corneal epithelium.
Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular diseases, and often leads to atherosclerosis. This is very common in patients with diabetes mellitus, hypertension or other chronic pathophysiological conditions.
Endothelialcells are very flat, have a central nucleus, are about 1-2 µm thick and some 10-20 µm in diameter.
Endothelialcells are selective filters which regulate the passage of gases, fluid and various molecules across their cell membranes.
It has long been realised that the endothelialcells become "injured" either physically by abrasion or toxic insult (such as from nicotine) and large molecules, which are normally confined to the blood, are allowed to escape through the endothelium and become lodged in the smooth muscle cells in the arterial wall.
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