Boundary events: contact-dependent and contact-facilitated signaling between platelets.

The theme of this review is that formation of a stable hemostatic plug requires adhesive interactions and signaling events that continue beyond the initial phases of platelet aggregation. These interactions and events are facilitated and, in some cases made possible, by the persistent close contacts between platelets that can only occur after the onset of aggregation. The molecules that are involved include integrins, cell adhesion molecules, receptor tyrosine kinases, and ligands that are either attached to or shed from the surface of activated platelets. The picture that emerges is one in which events after aggregation are nearly as complex as those that precede aggregation and the initiation of platelet plug formation.