Endothelium

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Endothelial
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Endothelium
microvessel showing endothelial cells, which encircle an erythrocyte (E), forming the innermost layer of the vessel, the tunica intima.
Details
SystemCirculatory system
LocationLining of the inner surface of blood vessels and lymphatic vessels
Identifiers
MeSHD004727
THH2.00.02.0.02003
FMA63916
Anatomical terms of microanatomy]

The endothelium (pl.: endothelia) is a single layer of

squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels.[1] The endothelium forms an interface between circulating blood or lymph in the lumen
and the rest of the vessel wall. Endothelial cells form the barrier between vessels and tissue and control the flow of substances and fluid into and out of a tissue.

Endothelial cells in direct contact with blood are called vascular endothelial cells whereas those in direct contact with

capillaries
.

These cells have unique functions that include fluid filtration, such as in the glomerulus of the kidney, blood vessel tone, hemostasis, neutrophil recruitment, and hormone trafficking. Endothelium of the interior surfaces of the heart chambers is called endocardium. An impaired function can lead to serious health issues throughout the body.

Structure

The endothelium is a thin layer of single flat (

squamous) cells that line the interior surface of blood vessels and lymphatic vessels.[1]

Endothelium is of

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.[2][3]

Terminology

The

Function

Endothelium lines the inner wall of vessels, shown here.
Microscopic view showing endothelium (at top) inside the heart.

The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. This forms a barrier between vessels and tissues and control the flow of substances and fluid into and out of a tissue. This controls 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 endothelium, as in cases of chronic inflammation, may lead to tissue swelling (edema). Altered barrier function is also implicated in cancer extravasation.[6]

Endothelial cells are involved in many other aspects of vessel function, including:

Blood vessel formation

The endothelium is involved in the formation of new blood vessels, called

Angiopoietin-2
works with VEGF to facilitate cell proliferation and migration of endothelial cells.

The general outline of angiogenesis is

  • activating signals binding to surface receptors of vascular endothelial cells.
  • activated endothelial cells release proteases leading to the degradation of the basement membrane
  • endothelial cells are freed to migrate from the existing blood vessels and begin to proliferate to form extensions towards the source of the angiogenic stimulus.

Host immune response

Endothelial cells express a variety of immune genes in an organ-specific manner.[12] These genes include critical immune mediators and proteins that facilitate cellular communication with hematopoietic immune cells.[13] Endothelial cells encode important features of the structural cell immune response in the epigenome and can therefore respond swiftly to immunological challenges. The contribution to host immunity by non-hematopoietic cells, such as endothelium, is called “structural immunity”.[14]

Clinical significance

diabetes mellitus, hypertension, hypercholesterolemia, as well as in smokers. Endothelial dysfunction has also been shown to be predictive of future adverse cardiovascular events including stroke, heart disease, and is also present in inflammatory disease such as rheumatoid arthritis, diabetes, and systemic lupus erythematosus.[16][17]

Endothelial dysfunction is a result of changes in endothelial function.

TNFα and IFNγ, which promotes inflammation.[21][22] This state of endothelial cells promotes accumulation of lipids and lipoproteins in the intima, leading to atherosclerosis, and the subsequent recruitment of white blood cells and platelets, as well as proliferation of smooth muscle cells, leading to the formation of a fatty streak. The lesions formed in the intima, and persistent inflammation lead to desquamation of endothelium, which disrupts the endothelial barrier, leading to injury and consequent dysfunction.[23] In contrast, inflammatory stimuli also activate NF-κB-induced expression of the deubiquitinase A20 (TNFAIP3), which has been shown to intrinsically repair the endothelial barrier.[24]

One of the main mechanisms of endothelial dysfunction is the diminishing of

ERK5 is essential for maintaining normal endothelial cell function.[26] 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.

Angiosarcoma is cancer of the endothelium and is rare with only 300 cases per year in the US.[27] However it generally has poor prognosis with a five-year survival rate of 35%.[28]

Research

Endothelium in cancer

It has been recognised that endothelial cells building tumour vasculature have distinct morphological characteristics, different origin compared to physiological endothelium, and distinct molecular signature, which gives an opportunity for implementation of new biomarkers of tumour angiogenesis and could provide new anti-angiogenic druggable targets.[29]

Endothelium in diet

A

processed sugar increases adhesion endothelial cells and atherogenic promoters.[30] High-fat diets adversely affect the endothelial function.[31]

A Mediterranean diet has been found to improve endothelial function in adults which can reduce risk of cardiovascular disease.[32][33] Walnut consumption improves endothelial function.[34][35]

Endothelium in Covid-19

In April 2020, the presence of viral elements in endothelial cells of 3 patients who had died of

COVID-19 was reported for the first time. The researchers from the University of Zurich and Harvard Medical School considered these findings to be a sign of a general endotheliitis in different organs, an inflammatory response of the endothelium to the infection that can lead or at least contribute to multi-organ failure in Covid-19 patients with comorbidities such as diabetes mellitus, hypertension and cardiovascular disease.[36][37]

History

In 1958, A. S. Todd of the University of St Andrews demonstrated that endothelium in human blood vessels have fibrinolytic activity.[38][39]

See also

List of distinct cell types in the adult human body

References

External links