Endothelial Information and Courses from MediaLab, Inc.

Atherosclerosis is a clogging, narrowing and hardening of the body's large and medium-sized blood vessels. Atherosclerosis can lead to hypertension, stroke, myocardial infarction (heart attack), renal problems, etc. Not surprisingly, cardiovascular risk markers tend to reflect a person's degree of atherosclerosis.Atherosclerosis is actually a chronic inflammatory response within the walls of arteries. Small lipoproteins like LDL are able to diffuse through the endothelial wall of blood vessels and accumulate. The inflammatory component of atherosclerosis results from the migration of leukocytes (mainly macrophages) that enter the blood vessel walls. These macrophages seek to remove the deposited LDL as well as intermediate-density lipoproteins (IDL). As macrophages phagocytose these lipoproteins, they become foam cells that get trapped in the endothelial space. This eventually leads to "hardening" or "furring" of the arteries and plaque formation. Arteriosclerosis is a general term describing any hardening (loss of elasticity) of medium or large arteries whereas atherosclerosis is a hardening of an artery specifically due to plaque. The risk to patients with significant atherosclerosis is that eventually a narrowing of the artery (stenosis) can cause a reduction in oxygen delivery to tissues and plaque rupture can lead to an acute coronary event.

As discussed earlier, a break in the vessel endothelium leads to exposure of collagen and the vessel's subendothelial surface. Ruptured endothelial cells leak ADP and Serotonin, which are the chemical triggers that induce platelet adhesion, the next step in the sequence of hemostatic events. Circulating platelets are drawn to the area by those liberated chemical signals, and begin to physically attach themselves to the rough, damaged surfaces of the breach. As platelets continue to arrive and bind to the exposed collagen and basement membrane, a rudimentary barrier begins to form, as the platelets themselves serve to fill in the breached vessel wall. Platelets possess an inherent “sticky” property which enables them to adhere to one another, and not just to the damaged vessel endothelium. The process by which platelets bind to one another is referred to as platelet aggregation, and is vital because it allows for a platelet plug to be formed. The platelet plug is the structure responsible for plugging the hole in the vessel wall.

Reticular cells (adventitial cells) provide structural support for the endothelial cells that line the venous sinus and the developing blood cells within the hematopoietic cord. The cytoplasm of the reticular cells is capable of extending itself in fiberlike strands deep into the hematopoietic cords. These strands provide a meshwork for the blood cells. Other types of cells which furnish support in the cord include macrophages and fat cells.

Another representation of the cells in a hematopoiectic cord which focuses in the appearance of various cells as well as their location within the cord is shown here. HC: hematopoiectic cordVS: venous sinusAdv: adventitial cellEnd: endothelial cellGP: granulopoiesisF: fat cellsEi: erythroipoietic islandHist: histiocyte (macrophage)Meg: megakaryocyteA: arteriole

It is also important to note that in addition to red cells, ABO antigenic determinants (epitopes) are found in many tissues, body fluids, and other cells including endothelial cells and platelets. Because ABO antigens are so widely expressed, ABO antigens are also a major consideration in solid organ and bone marrow transplants.