Platelets and coagulation

Platelet activation and blood coagulation are reciprocal processes, both contributing to hemostasis and thrombosis in an interactive way. Exposed collagen, von Willebrand factor (VWF), and other components of the subendothelialmatrix initiate platelet adhesion, whereas exposed tissue factor simultaneously triggers the coagulation cascade. During the various phases of the coagulation process, activated platelets support and steer coagulation by at least four mechanisms, namely, by (1) exposing the procoagulant phospholipid phosphatidylserine (PS) at their outer surface; (2) releasing coagulation factors like factors V and XIII, as well as anticoagulant factors like tissue factor pathway inhibitor; (3) providing a scaffold for the formation of fibrin fibers; and (4) stimulating retraction of the fibrin clot. In this chapter, we describe these roles of platelets in detail, also focusing on the receptors and signaling pathways involved and on the distinct (anti)coagulant pathways.