Recent studies suggest that platelet stimulation leads to the induction of specific fibrinogen receptors, which have been identified as the platelet membrane glycoprotein IIb-IIIa complex. The binding of fibrinogen initiates the reversible primary phase of aggregation. With a strong stimulus, thrombospondin is released from the platelet alpha-granules. By interacting with fibrinogen, thrombospondin serves to stabilize the platelet aggregates, which leads to a secondary irreversible phase of aggregation.