endotelium

Endotelium, also known as the endothelium, is a thin monolayer of specialized endothelial cells that lines the luminal surface of all blood vessels, as well as lymphatic vessels and the heart chambers. It forms a continuous barrier between circulating blood and surrounding tissues and plays a central role in vascular biology.

Anatomically, the endothelium rests on a basement membrane and is coated by a glycocalyx that modulates permeability and interacts with blood components. In microvessels, endothelial cells are often accompanied by pericytes, which help regulate capillary stability and blood flow. Endothelial cells exhibit regional specialization: the endothelium can be continuous, fenestrated, or discontinuous, reflecting organ-specific functions such as filtration in the kidney, absorption in the intestine, or rapid exchange in the liver.

Functions of the endotelium are diverse and tightly regulated. It acts as a selective barrier controlling transudation of fluids, solutes, and cells. It modulates vascular tone through the release of vasodilators like nitric oxide and prostacyclin and vasoconstrictors such as endothelin-1. It participates in hemostasis by balancing anticoagulant and procoagulant properties and influences immune cell trafficking via adhesion molecules and chemokines. The endothelium also contributes to angiogenesis and tissue remodeling in development and repair and metabolizes various vasoactive substances.

Developmentally, endothelial cells originate from mesodermal angioblasts and arise through vasculogenesis and angiogenesis, forming the intricate vascular network. Pathologically, endothelial dysfunction is linked to atherosclerosis, hypertension, diabetes, sepsis, and microvascular complications, highlighting the endothelium’s critical role in vascular health.