Platelets play an important role in initiating clotting in arteries and arterioles. They form an initial plug or clot and are followed by the deposit of a fibrin mesh that forms a firm clot. Platelets are trapped by the material exposed by the fractured plaque and the first phase of thrombosis is initiated. Aspirin or platelet glycoprotein IIa/IIIb receptor blockers are used to prevent this dele¬terious platelet aggregation. Platelets are intriguing blood particles that require much research in order to uncover their therapeutic potential.
B. Superficial Erosion of the Endothelial Lining Covering the Plaque
Evidence of superficial erosion of the intimal lining has been observed in approximately 25% of patients who have sustained a myocardial infarction and died within a few hours. Endothelial cell desquamation through activation of basement membrane degrading metalloproteinases appears to be involved, but the mechanisms are unclear.
C. Hemorrhage into the Plaque
New capillaries and small vessels grow into the plaque and provide a useful function in that they may provide nutrient material for smooth muscle cells that form collagen necessary to strengthen the fibrous cap. These new vessels are, however, fragile and may burst causing a minute hemorrhage within the plaque. The pressure within the plaque may cause disruption of the fibrous cap, and thrombosis completes the occlusion of the artery.

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