endothelium

Endothelium is the thin layer of specialized squamous cells that lines the interior surface of all blood vessels and lymphatic vessels. It forms the tunica intima and is continuous with the endocardium of the heart. Endothelial cells originate from mesoderm and create a dynamic, metabolically active interface between blood and the vessel wall.

Endothelium varies by organ. Continuous endothelium lines most vessels; fenestrated endothelium occurs in kidneys, endocrine glands, and the small intestine; discontinuous (sinusoidal) endothelium is found in liver, spleen, and bone marrow. A glycocalyx on the luminal surface contributes to barrier function and mechanotransduction.

Functions include regulating vascular tone via nitric oxide, endothelin-1, and prostacyclin; maintaining hemostasis through antithrombotic properties and thrombin regulation; controlling permeability; mediating leukocyte recruitment through adhesion molecules; and promoting angiogenesis via VEGF and angiopoietins.

In the lymphatics, endothelium lines lymphatic vessels, supporting fluid balance and immune cell movement. Pathology: endothelial dysfunction—reduced NO, a pro-inflammatory state—contributes to atherosclerosis, hypertension, and diabetes. Markers such as von Willebrand factor and soluble adhesion molecules reflect endothelial health.