The endothelium is a single-layer layer of flat cells that lines all vessels and thus represents an important barrier between intravascular and extravascular space (as the space inside and outside the blood vessels).
Structure
The endothelium forms the innermost cell layer of the intima, the inner layer of the three-layer wall structure of an artery. The cells contain one or more cell nuclei and are relatively flat. They are arranged longitudinally and thus ensure a smooth flow of blood through the vessels.
The endothelium consists of individual cells that are interlocked by dense cell contacts. These contacts include adherent contacts, tight junctions and gap junctions. They separate the intravascular space from the deeper layers of the vessel wall and thus prevent contact between blood cells and extracellular matrix (i.e. the fluid outside the vessels).
At the same time, they also control the passage of plasma components. They thus have an influence on endothelial permeability. Which dissolved substances can pass through the cell contacts is influenced, among other things, by an uppermost layer of sugar chains.
This apical surface is also called glycocalix. In addition, various substances can bind to the glycocalix and thus influence the cell interior. On the opposite side, the basal side of the cell, the endothelial cells are interlocked with the subendothelial layer via local contacts.
Function
The endothelium has several functions that can vary depending on the size and location of the vessel. On the one hand it has the function to create a barrier. The tight junctions as a strong cell contact between the endothelial cells prevent the passive passage of components that are dissolved in the blood.
They therefore form a tight diffusion barrier to protect unwanted concentrations of substances in the subendothelial layer. The apical surface with sugar residues prevents the attachment of blood cells. Only by activating selectins and other molecules can substances bind to it.
Thus, the endothelium also contributes to blood clotting. When intact, it prevents the formation of a blood clot, and after a vascular injury, it promotes coagulation. The endothelium can also regulate the width of the blood vessels.
The endothelial cells are connected to inner muscle cells of the middle layer, the media, by so-called myoendothelial contacts. This contact, which is usually established via gap junctions, exerts a vasodilating influence on the muscles. Locally, the endothelium can also release nitric oxide (NO).
The release of nitric oxide can be triggered by the shear forces caused by the friction of the passing blood when blood pressure is elevated. Another possibility is stimulation by vasodilating agents that bind to surface receptors of the endothelium. This is a vasodilating substance. However, if required, it can also release a vasoconstrictive substance. This is the protein endothelin.
