Vascular Endothelial Cell Biology: An Update

Abstract

The vascular endothelium, a monolayer of endothelial cells (EC), constitutes the inner cellular lining of arteries, veins and capillaries and therefore is in direct contact with the components and cells of blood. The endothelium is not only a mere barrier between blood and tissues but also an endocrine organ. It actively controls the degree of vascular relaxation and constriction, and the extravasation of solutes, fluid, macromolecules and hormones, as well as that of platelets and blood cells. Through control of vascular tone, EC regulate the regional blood flow. They also direct inflammatory cells to foreign materials, areas in need of repair or defense against infections. In addition, EC are important in controlling blood fluidity, platelet adhesion and aggregation, leukocyte activation, adhesion, and transmigration. They also tightly keep the balance between coagulation and fibrinolysis and play a major role in the regulation of immune responses, inflammation and angiogenesis. To fulfill these different tasks, EC are heterogeneous and perform distinctly in the various organs and along the vascular tree. Important morphological, physiological and phenotypic differences between EC in the different parts of the arterial tree as well as between arteries and veins optimally support their specified functions in these vascular areas. This review updates the current knowledge about the morphology and function of endothelial cells, particularly their differences in different localizations around the body paying attention specifically to their different responses to physical, biochemical and environmental stimuli considering the different origins of the EC.

Keywords: angiogenesis; endothelium; glycocalyx; shear stress; thrombosis.

Figure 1

(A) Immunostaining of an endothelial cell monolayer (cell nuclei in blue, von Willebrand factor in red, vinculin in green); (B) Endothelial cell borders from the confluent endothelial cell monolayer are stained according to Ranvier with AgNO₃ (400-fold primary magnification).

Figure 2

Hemodynamic stresses (τ_W: wall shear stress, σ: circumferential wall stress) acting on the endothelial cell monolayer. (Q: blood volume flow, η: blood viscosity, r vessel radius, P_T: transmural pressure difference).

Figure 3

Factors inducing vasodilation and/or vasoconstriction. (The red circle represents a blood vessel).

Figure 4

Platelet dependent and independent, receptor-mediated activation of endothelial cells.

Figure 5

Buckling of human venous endothelial cells in vitro on extracellular matrix pre-secreted by bovine corneal endothelial cells and displayed by confocal laser scanning microscopy (CLSM) induced by addition of an iodinated radiographic contrast medium (Iopromide-370, 30% v/v) to the cell culture medium [108].

Figure 6

Segmental constriction of a nailfold capillary documented at three time points (final magnification 1:570) [109].

Figure 7

Angiogenesis in brief. VEGF initiates assembly of endothelial cells, PDGF-BB recruits pericytes, whereas angiopoietin-1 and TGF-β stabilize the nascent vessel.

Figure 8

Capillary projection before occlusion and at different phases of capillary remodeling (Reprinted with the permission of [142], S. Karger AG).

Figure 9

Overlay of the former and the remodeled capillary beds by means of a Helmert transformation. (The picture in red shows the capillary bed before occlusion, superimposed by the picture in green with the newly grown capillary (Reprinted with the permission of [127], S. Karger AG)).