ABSTRACT
Platelets play a pivotal role in haemostasis, interacting with the coagulation cascade on both a macro and a molecular level. Platelets circulate as tiny anuclear cells, which are produced by the megakaryocytes in the bone marrow. Platelets are released from the bone marrow and circulate for 7-10 days before being cleared by the reticuloendothelial system. The normal platelet count ranges from 150-450109/l, and very low platelet counts are associated with bleeding, with very high platelet counts being associated with thrombosis. Platelets bind to damaged endothelium through a number of receptors. The most important receptor is glycoprotein Ib (GP Ib), which is complexed with glycoprotein V and IX. GP Ib binds to the subendothelium through a large fibrillary protein called von Willebrand factor (vWF) (Figure 11.1). The complex of GP Ib, V and IX is present in high concentrations on the platelet surface at approximately 20000-40000 receptors per platelet. Other less abundant platelet receptors also participate in adhesion. In particular, glycoprotein Ia/IIa binds to collagen exposed on the damaged blood vessel wall, glycoprotein IIb/IIIa binds to fibrinogen and glycoprotein Ic/IIa binds to fibronectin.
