Lipid and cellular constituents of unstable human aortic plaques.

Unstable plaques are undergoing thrombosis which, in most instances, is due to fissuring and rupture of the plaque cap. This process (deep intimal injury) is a complication of plaques with a lipid-rich core. The cap tear allows blood to enter the core from the lumen, leading initially to intraplaque thrombosis and, subsequently, in some cases intraluminal thrombosis. Cap tears reflect the interplay between the force exerted on the tissue and its inherent mechanical strength. Factors which elevate and concentrate circumferential wall stress on the cap during systole include an increasing proportion of the total plaque volume occupied by the lipid core, thinning of the cap and a loss of internal collagen struts within the core. Factors which lead to an inherent reduction in the mechanical strength of cap tissue include a reduction in collagen and glycosaminoglycan concentrations, an increase in the number and density of macrophages, and a concomitant reduction in smooth muscle cells in the cap tissue. It is therefore possible to define a vulnerable plaque as one in which the lipid core is disproportionately large, the cap thin, and in which monocytes preponderate over smooth muscle cells.