Endothel

Endothel, or endothelium, is the thin layer of specialized cells that lines the interior surface of blood vessels and the lymphatic system. It forms a continuous, single-cell-thick barrier between the bloodstream and surrounding tissues and covers the heart’s chambers and valves as well.

An endothelium consists of endothelial cells anchored to a basement membrane, often with a surrounding glycocalyx that helps regulate permeability and interactions with circulating cells. Endothelia vary by tissue type: continuous endothelium is common in most vessels; fenestrated endothelium, with small pores, is found in organs involved in rapid exchange such as the kidney and endocrine glands; discontinuous (disrupted) endothelium is seen in the liver and bone marrow, allowing large molecules to pass.

Key functions include maintaining vascular tone and blood flow through the release of vasoactive substances such as nitric oxide and endothelin; regulating vascular permeability to control fluid and solute exchange; providing an anti-thrombotic, anti-adhesive surface under normal conditions while enabling immune cell trafficking during inflammation; and supporting angiogenesis and tissue repair through growth factors like VEGF. The endothelium also participates in coagulation, fibrinolysis, and lipid metabolism.

Endothelial dysfunction, characterized by reduced nitric oxide availability and increased expression of adhesion molecules, is linked to cardiovascular risk and diseases such as hypertension, atherosclerosis, diabetes, and sepsis. Endothelial cells originate from the mesoderm and exhibit turnover and repair through resident cells and circulating endothelial progenitors.

In summary, the endothelal layer is a multifunctional interface essential for vascular homeostasis, tissue exchange, and immune regulation.