The endothelium is the thin layer of cells that lines the interior surface of blood vessels
and lymphatic vessels, forming an interface between circulating blood or lymph in the
lumen and the rest of the vessel wall.
The cells that form the endothelium are called endothelial cells.
Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas
those in direct contact with lymph are known as lymphatic endothelial cells.
Vascular endothelial cells line the entire circulatory system, from the heart to the
smallest capillaries.
These cells have unique functions in vascular biology.
These functions include fluid filtration, such as in the glomeruli of the kidney, blood
vessel tone, hemostasis, neutrophil recruitment, and hormone trafficking.
Endothelium of the interior surfaces of the heart chambers is called endocardium.
Structure It is mesodermal in origin, Both blood and
lymphatic capillaries are composed of a single layer of endothelial cells called a monolayer.
In straight sections of a blood vessel, vascular endothelial cells typically align and elongate
in the direction of fluid flow.
Terminology The foundational model of anatomy makes a
distinction between endothelial cells and epithelial cells on the basis of which tissues
they develop from, and states that the presence of vimentin rather than keratin filaments
separate these from epithelial cells.
Many considered the endothelium a specialized epithelial tissue.
Function Endothelial cells are involved in many aspects
of vascular biology, including: Barrier function - the endothelium acts as
a semi-selective barrier between the vessel lumen and surrounding tissue, controlling
the passage of materials and the transit of white blood cells into and out of the bloodstream.
Excessive or prolonged increases in permeability of the endothelial monolayer, as in cases
of chronic inflammation, may lead to tissue edema/swelling.
Blood clotting.
The endothelium normally provides a non-thrombogenic surface because it contains, for example,
heparan sulfate which acts as a cofactor for activating antithrombin, a protease that inactivates
several factors in the coagulation cascade.
Inflammation Formation of new blood vessels
Vasoconstriction and vasodilation, and hence the control of blood pressure
Repair of damaged or diseased organs via an injection of blood vessel cells
Angiopoietin-2 works with VEGF to facilitate cell proliferation and migration of endothelial
cells Clinical significance
Endothelial dysfunction, or the loss of proper endothelial function, is a hallmark for vascular
diseases, and is often regarded as a key early event in the development of atherosclerosis.
Impaired endothelial function, causing hypertension and thrombosis, is often seen in patients
with coronary artery disease, diabetes mellitus, hypertension, hypercholesterolemia, as well
as in smokers.
Endothelial dysfunction has also been shown to be predictive of future adverse cardiovascular
events, and is also present in inflammatory disease such as rheumatoid arthritis and systemic
lupus erythematosus.
One of the main mechanisms of endothelial dysfunction is the diminishing of nitric oxide,
often due to high levels of asymmetric dimethylarginine, which interfere with the normal L-arginine-stimulated
nitric oxide synthesis and so leads to hypertension.
The most prevailing mechanism of endothelial dysfunction is an increase in reactive oxygen
species, which can impair nitric oxide production and activity via several mechanisms.
The signalling protein ERK5 is essential for maintaining normal endothelial cell function.
A further consequence of damage to the endothelium is the release of pathological quantities
of von Willebrand factor, which promote platelet aggregation and adhesion to the subendothelium,
and thus the formation of potentially fatal thrombi.
See also
References
External links Endothelium at eMedicine Dictionary
Anatomy photo: Circulatorycapillaries1/capillaries3 - Comparative Organology at University of
California, Davis, "Capillaries, non-fenestrated" Histology image: 21402ooa - Histology Learning
System at Boston University Endothelium Journal of Endothelial Cell Research,
Informa Healthcare Endothelium and inflammation
Platelet Activation, University of Washington