Morphology of Atherosclerotic Plaques
Atherosclerotic plaques are not usually concentric, encircling the entire arterial circumference, but eccentric, localized to one side of the artery. This is simply because arterial thrombi are parietal. Eccentricity is the characteristic feature of an atherosclerotic plaque and differentiates them from other vascular lesions.
The lipid core is an inconsistent but distinctive feature of atherosclerotic plaques. Mainstream theory holds that the core contains the lipid which elicited the overlying plaque. Of course, we now know this to be false. Duguid thought that the core was part of the original thrombus which escaped organization, simply being out of reach of the body’s power of organization. Morgan wrote in 1956, “it is certain that as a mural thrombus retracts, a variable number of red cells are trapped in the fibrinous network and squeezed between its laminae, and must ultimately, in broken-down form, constitute an element in the deposit, capable of providing a source of cholesterol" . Perhaps a more important origin for the necrotic core is intraplaque hemorrhage. Morgan writes, “It is therefore safe to assume that the average atheromatous abscess has been subjected to repeated micro-hemorrhages over a long period and that its pultaceous contents are partly haemic in origin” . I believe that intraplaque hemorrhage is the more important source, and that this occurs because of mechanical stress caused by contraction of arterial smooth muscle. Lipid cores do not occur in atherosclerotic plaques which develop in synthetic vascular grafts because they do not have smooth muscle.
Figure 5. This is an ordinary atherosclerotic plaque. Important features are: the plaque is eccentric, that is the plaque thickens the bottom of the artery making the lumen off-center; it contains a lipid rich core; and is surrounded by a layer of smooth muscle, which has the ability to contract. Courtesy of Kerry Johnson.
Figure 6. Atherosclerotic plaque in a synthetic graft. Note the eccentric location. There is a thin layer of thrombus (darker pink) along the luminal side of the plaque.
Next Section: Role of Lipoproteins in Atherogenesis (Part 6)
1. Morgan, AD. The Pathogenesis of Coronary Occlusion. Blackwell Scientific Publications, Oxford, p.117.
2. Morgan, AD. The Pathogenesis of Coronary Occlusion. Blackwell Scientific Publications, Oxford, p.84.