The endothelium is the thin layer of cells that lines blood and lymph vessels. The cells of the endothelium are called endothelial cells. Until quite recently the endothelium was thought to be an inert lining of the blood vessels but it is now becoming clear that endothelial cells play a number of essential and complex roles within the body.

Endothelial cells perform the following functions.

• They act as a selective barrier to the passage of molecules and cells between the blood and the surrounding bodily tissue. Endothelial cells make up the blood-brain barrier - the barrier between the central nervous system and the rest of the body.
• They play an essential role in summoning and capturing white blood cells (leukocytes) to the site of an infection.
• They play an important role in the mechanics of blood flow.
• They regulate coagulation of the blood at the site of a trauma.
• They regulate the growth of the veinous muscular cells.
• They control the contraction and relaxation of veins.
• They secrete and modify several veinous signalling molecules.

With respect to multiple sclerosis it is the role that they play within the immune system that is perhaps the most important. At the site of an infection, immune system cells already on the scene secrete signalling molecules, called cytokines, which migrate to the endothelial cells in the surrounding blood vessels.

When the endothelial cells receive these siganls they express molecules, called adhesion molecules, on their surfaces. These molecules are "sticky" for complementary molecules expressed on the surfaces of leukocytes and act as hooks to fish them out of the blood.

There are a variety of adhesion molecules including the selectins which are expressed by endothelial cells, integrins which are expressed by leukocytes and other cells, Intercelluar Adhesion Molecules (ICAMs), Vascular Cell Adhesion Molecules (VCAMs) and many others. Chemotaxis, leukocyte capture and their interaction with cytokines is extremely complex and a lot of research work is currently being done in this field.

Endothelium links:
Endothelium
Properties of the endothelium

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