The Vascular Function of Platelets

Platelets are small anucleate cells that play a critical role in haemostasis and thrombosis. Platelets ordinarily circulate in the bloodstream in a quiescent state but undergo ‘explosive’ activation following damage to the vessel wall, leading to the rapid formation of a platelet aggregate or vascular plug and occlusion of the site of damage to prevent blood loss. To facilitate a rapid response, platelets are enriched in signalling proteins and surface receptors, and activation is associated with the generation of the positive feedback or amplification mediators ADP and thromboxane (Tx)A2 and release of the adhesion/coagulation proteins, that further support aggregate formation and the generation of thrombin through the coagulation cascade. The importance of platelets in haemostasis is illustrated by the excessive bleeding associated with defects in platelet function or platelet number. Conversely the activation of platelets in diseased blood vessels (e.g. at sites of plaque rupture) can lead to arterial thrombotic disorders such as stroke and myocardial infarction, two of the major causes of morbidity and mortality. Platelets are also involved in many other physiological functions and are part of the innate immune system. In this chapter we aim to describe the features of platelets that enable them to perform their physiological roles in the vasculature..