Figure 7.

Coated platelets do not aggregate with each other but are recruited into aggregates by noncoated platelets: a possible mechanism. Capacities of platelets from two distinct subpopulations for binding each other are illustrated. Irregular shapes (A and B) depict normal activated platelets. Round shapes (B and C) depict coated platelets. Colored molecules on the coated-platelet surface represent receptors and their ligands, which form the coat of the coated platelet. (A) The interaction between two noncoated platelets occurs via fibrinogen (red) binding with GPIIbIIIa receptors (black). (B) Interaction between coated and noncoated platelets occurs when a GPIIbIIIa receptor on the noncoated platelet surface binds the free terminus of a fibrinogen molecule retained on the coated-platelet surface. (C) The interaction between two coated platelets cannot occur, since they have no active or free GPIIbIIIa receptors capable of binding the fibrinogen retained on the coated-platelet surface. The curve between platelets symbolizes the impossibility of their forming an aggregate by interaction with each other.