Endotel

Endothel, or endothelium in English, refers to the thin, continuous layer of specialized cells that lines the interior surface of all blood vessels, the heart, and lymphatic vessels. It forms the tunica intima and serves as a barrier between circulating blood and underlying tissues. Endothelial cells are flattened and tightly connected, creating a single-cell-thick lining that extends throughout the vascular system.

The endothelium performs a wide range of functions. It regulates vascular tone by releasing vasodilators such as nitric oxide and prostacyclin, and vasoconstrictors such as endothelin. It controls permeability to fluids, solutes, and cells, aided by intercellular junctions and the endothelial glycocalyx. It maintains a non-thrombogenic surface through factors like thrombomodulin and heparan sulfate while producing fibrinolytic agents such as tissue plasminogen activator. The endothelium also participates in immune responses by expressing adhesion molecules that control leukocyte trafficking and by modulating inflammation.

There is considerable endothelial heterogeneity across tissues. The brain contains a highly selective blood-brain barrier with tight junctions and low permeability, while the kidney glomerulus and choroid plexus have more permeable or fenestrated endothelia. Other organs typically display continuous endothelium with specialized features. Endothelial cells respond to shear stress from blood flow and are important in angiogenesis, regulated by factors such as vascular endothelial growth factor and angiopoietins.

Clinical significance: Endothelial dysfunction is linked to cardiovascular diseases, including hypertension and atherosclerosis, as well as sepsis and diabetes. It can be assessed indirectly through vascular reactivity tests or biomarkers, and therapies often aim to preserve or restore endothelial health.