Vascular endothelium
Endothelium vasorum
Also known as: endothelium, endothelial cells
The vascular endothelium is the single layer of flattened endothelial cells that lines every blood vessel and the heart. It behaves like an organ in its own right, actively controlling vascular tone, permeability, clotting, inflammation and the growth of new vessels.
Overview
Structure. Endothelial cells form a simple squamous epithelium of mesodermal origin, aligned with the direction of blood flow. Adjacent cells are joined by tight junctions and by adherens junctions built from VE-cadherin. A sugar-rich coat, the glycocalyx, covers the surface facing the lumen. Weibel–Palade bodies inside the cells store von Willebrand factor and P-selectin.
Control of vascular tone. In response to shear stress, acetylcholine, bradykinin and other signals, endothelial cells release vasodilators. These include nitric oxide (made by eNOS from L-arginine), prostacyclin (PGI₂) and endothelium-derived hyperpolarising factor. They also release vasoconstrictors, chiefly endothelin-1. Endothelial ACE, which is abundant in the lungs, converts angiotensin I to angiotensin II and breaks down bradykinin.
Control of clotting. Healthy endothelium resists clotting. It expresses thrombomodulin, which helps activate protein C, and heparan sulfate, which boosts antithrombin. It releases tissue factor pathway inhibitor, tPA, prostacyclin and NO, and breaks down ADP using CD39. When activated, it switches to a pro-clotting state, releasing von Willebrand factor and PAI-1 and expressing tissue factor.
Inflammation. In inflammation, endothelial cells display E- and P-selectins, ICAM-1 and VCAM-1. These molecules allow leukocytes to roll along the vessel wall, stick to it and squeeze through it, mainly in postcapillary venules.
Anatomy
- Histology
Simple squamous cells 0.2–2 µm thick, joined by tight and adherens junctions, with a glycocalyx and Weibel–Palade bodies. They are continuous, fenestrated or discontinuous depending on the vascular bed.
Function
- Forms a selective barrier between blood and tissues.
- Regulates vascular tone (NO, prostacyclin, endothelin-1, ACE).
- Keeps blood fluid (anticoagulant and profibrinolytic surface) and starts haemostasis when injured.
- Controls the recruitment of leukocytes in inflammation.
- Drives angiogenesis in response to VEGF.
Clinical significance
Endothelial dysfunction
Endothelial dysfunction means the endothelium produces too little nitric oxide and becomes pro-inflammatory and pro-clotting. It is the earliest detectable stage of atherosclerosis and is promoted by smoking, diabetes, hypertension and dyslipidaemia. It can be measured as impaired flow-mediated dilation of the brachial artery.
Pre-eclampsia and anti-VEGF therapy
In pre-eclampsia, the placenta releases anti-angiogenic factors such as sFlt-1, which damage the mother's endothelium throughout the body. Drugs that block VEGF, such as bevacizumab, can similarly cause hypertension and protein in the urine.
Sepsis
In sepsis, widespread endothelial activation makes capillaries leaky, promotes disseminated intravascular coagulation and causes vasodilation.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.