Endothelial

Endothelial refers to the endothelium, the thin layer of cells that lines the interior surfaces of blood vessels and lymphatic vessels. The endothelium forms a dynamic interface between circulating blood and the vessel wall and plays a central role in vascular homeostasis, inflammation, coagulation, and angiogenesis.

Structure and location: Endothelial cells line arteries, capillaries, veins, and the heart chambers, forming a continuous sheet called the endothelium. Vascular endothelium shows regional specialization: continuous endothelium in most tissues; fenestrated endothelium with pores in tissues requiring rapid exchange (kidney, intestine); and discontinuous (sinusoidal) endothelium with large gaps in liver, spleen, and bone marrow. The endothelium is supported by the glycocalyx and intercellular junctions that regulate barrier function.

Functions: Endothelial cells regulate vascular tone by producing vasodilators and vasoconstrictors, notably nitric oxide, prostacyclin, and endothelin-1. They control blood fluidity and coagulation, and maintain barrier integrity to regulate permeability for nutrients and cells. They modulate inflammation by expressing adhesion molecules and cytokines to recruit leukocytes, and they contribute to angiogenesis by releasing growth factors such as VEGF during development, wound healing, and disease processes.

Clinical relevance: Endothelial dysfunction, characterized by reduced nitric oxide availability, increased permeability, and a pro-inflammatory, pro-thrombotic state, is linked to cardiovascular diseases such as atherosclerosis, hypertension, and diabetes. Risk factors include smoking, hyperlipidemia, and chronic inflammation.

Development and repair: Endothelium arises from mesoderm during embryogenesis. In adulthood, endothelial turnover is limited, but endothelial progenitor cells and local proliferation support repair after vascular injury.