The image below illustrates the evolution of atherosclerotic plaques and also indicates that there are two possible forms of evolution. Slowly growing plaques expand gradually due to accumulation of lipid in foam cells and migration and proliferation of smooth muscle cells. These plaques tend to stabilize and are not prone to rupture. The so-called fibrin cap on the lesion matures. In contrast, other plaques grow more rapidly as a result of more rapid lipid deposition. These have thin fibrin caps that are prone to rupture. Once a plaque ruptures, it can trigger an acute thrombosis (clot) by activating platelets and the clotting cascade.