Vascular endothelium is a thin monolayer of cells that constitute the lining of blood vessels and organs (1-3). A hallmark of many diseases (e.g., cancer, diabetes mellitus, viral infections, etc) is highlighted in the dysfunctional states of the vascular endothelium. Vascular endothelial cells line the entire circulatory system — heart, large blood vessels, and small capillaries (1). And the cells form the barrier between blood and organ tissues (3). 

Notable functions of the vascular endothelium:

1) Controls and regulates vascular relaxation and constriction;

2) Regulates homeostasis of solutes, fluid, macromolecules, hormones, platelets, and blood cells;

3) Directs “foreign materials” to inflammatory cell types;

4) Regulates blood fluidity;

Are endothelial cells functionally unique?

In the heart, the vascular system is a network of arteries, arterioles, capillaries, venules, and veins (3). In the kidney, the vascular system includes arterioles, capillaries, arterioles, more capillaries, venules and veins (3). The endothelial cells fulfill specific demands of the organs where they reside. Most distinctly, differences in gene expression profiles (such as gap function proteins) are found, as well as differences in extracellular matrix or matrices (3-5). There are three unique functional aspects: permeability (for small molecules including cytokines and ions), leukocyte trafficking (initially mediated by adhesion molecules such as selectins), and hemostasis (clot prevention via anti-coagulating factors) (3, 5).

Interesting facts:

1) Did you know that the endothelial cells anchor to the basal lamina, and together, they form a combined surface area of 3,000-6,000 m² in the human body? (4)

2) Their shapes are diverse but thin (cobblestone shapes). They are oriented along the blood vessel walls in order to minimize fluid stress. (4)

References:

1) Rajendran et al., The vascular endothelium and human diseases. Int J Biol Sci. 2013 Nov 9;9(10):1057-69. 

2) Godo S1, Shimokawa H2. Endothelial Functions. Arterioscler Thromb Vasc Biol. 2017 Sep;37(9):e108-e114.

3) Krüger-Genge et al., Vascular Endothelial Cell Biology: An Update. Int J Mol Sci. 2019 Sep 7;20(18). pii: E4411. 

4) Aird WC., Phenotypic heterogeneity of the endothelium: I. Structure, function, and mechanisms.  Circ Res. 2007 Feb 2;100(2):158-73.

5) Pries A.R., Kuebler W.M. The Vascular Endothelium I. Springer; Berlin/Heidelberg, Germany: 2006. Normal endothelium; pp. 1–40.