Arteriosclerosis

Interestingly, all middle-aged and older people have some form of arteriosclerosis. In fact, some experts consider arteriosclerosis part of the normal wear and tear of aging arteries. Most people will not progress beyond arteriosclerosis to atherosclerosis. In some people, however, such as those with hypercholesteremia or people with high-cholesterol diets, hardened plaques form that block the arteries.

Researchers have formed the injury model of atherosclerosis by studying the arteries of people of all ages. In infants, for instance, a "fatty streak" consisting of lipids and immune cells is present in the innermost layer of arteries. Lipids are molecules found in cholesterol. Even at this young age, the arteries are already responding to injury, most likely the presence of cholesterol.

In teenagers and young adults at high risk for atherosclerosis, layers of macrophages (special white cells that ingest foreign material) and smooth muscle cells overlie the interior wall of arteries. In an attempt to reduce the amount of lipid within the artery, macrophages ingest the lipid. Because under a microscope the lipid within the macrophage appears to be "bubbly," or foamy, a macrophage that has ingested lipid is called a foam cell.

As time passes, more and more macrophages ingest more and more lipid, and more foam cells are present within the artery. To rid the artery of the ever increasing load of lipid, foam cells migrate through the artery wall to return the lipid to the liver, spleen, and lymph nodes. As the foam cells penetrate the artery walls, they further injure the artery. This injury attracts platelets, special components of the blood that lead to the formation of blood clots. When you cut your finger, platelets rush to the site of injury and begin forming a clot to stop the flow of blood. In an artery, platelets also rush to the site of injury and begin the process of clot formation. But instead of helping the situation, the clot further complicates the growing plaque in the artery, which at this point consists of lipid-laden macrophages, lipids, muscle cells, and platelets.

The platelets, in turn, release chemicals called growth factors. Several different growth factors are released by the platelets. One causes the cells in the artery to release proteins and other substances that eventually lead to the formation of a matrix, a collection of tough protein fibers, that further hardens the artery. Another growth factor prompts the development of blood vessels in the plaque. At this stage, the plaque is fully formed. It is large enough to block the flow of blood through the artery. At its center is a pool of lipid and dead artery cells, and it is covered by a cap of connective tissue.