Atherosclerotic Plaque

Atherosclerotic plaques (and other vessel wall pathology) can occlude arteries 

Atherosclerosis often develops at branch points or curving portions along extracranial and intracranial large arteries, locations where blood flow is slowed and more turbulent. The internal carotid artery is particularly at risk.  Exactly where disease tends to occur, though, and how quickly it progresses apparently reflect the individual's genetic background.  Additional factors that may stimulate plaque growth include hypertension and cigarette smoking.Atherosclerosis involves focal accumulation of lipid, smooth muscle cells, foamy macrophages, and, eventually, cholesterol crystals under the surface lining (endothelium) of the artery.  With time, such an accumulation can form an elevated plaque that protrudes into the vessel's lumen and significantly reduces blood flow.  Perhaps the analogy of a kitchen drain pipe becoming gradually plugged with cooking grease and sludge will help you visualize what is happening.

These are cross-sections of the common carotid artery through its bifurcation into the internal and external carotids and also of the smaller internal carotid. Can you find regions where the walls of the vessels are thickened by brownish-yellow fatty material, therefore reducing the lumen diameter? This material is part of an atherosclerotic plaque. 

This is a microscopic section showing a large artery, in this case the basilar artery, which contains an enormous atherosclerotic plaque filled with lipid and cells. Can you find the tiny lumen through which blood can flow?

If occluding the vascular lumen weren't bad enough, plaques can do a number of additional things that further compromise the brain's circulation.  For instance, a plaque can ulcerate (break open), and the damage to its endothelial lining stimulates the development of a thrombus (blood clot) which even further narrows the vessel. What's more, pieces of thrombus or fragments of an exposed plaque core can be swept along by the blood flowing through the vessel, becoming emboli (discussed next).

Atherosclerotic plaques can form in the walls of small arteries as well. However, there are several additional kinds of pathology that particularly affect the walls of small arteries leading to their collapse and blockage of blood flow.

This is a microscopic section of an advanced atheromatous plaque that has not ruptured.  Notice how the structure of the vessel wall is altered in the region of the plaque.  Look at the deep regions of the plaque.  Can you see elongated unstained crystal-like profiles?  These are cholesterol crystals that have accumulated as the plaque developed.  Note that lipid and other cellular debris is also present in this area.