Platelets (thrombocytes) are small disc-like, anucleated cells that derive from the cytoplasm fragmentation of megakaryocytes. Platelets normally circulate in blood for approximately seven days. Platelets are the primary defense against bleeding by forming the initial plug when a blood vessel is injured. Platelets also participate in blood coagulation by providing the phospholipid surface on which the coagulation reactions occur. In order to become functional, platelets must be activated. Physiologically Thrombin is the most potent platelet activator (agonist). The slide show entitled Activation of Platelets by Thrombin shows how a platelet is affected by Thrombin.
