Events in thrombus formation Endothelial damage results in exposure of collagen and von Willebrand factor, which mediate the adhesion of platelets. Platelets express surface glycoprotein receptor, which binds to these ligands. Glycoprotein la (GPla) binds directly to collagen. Glycoprotein Ib (GPIb-lX) binds to von Willebrand factor vill: vWF), and this in turn binds to collagen. (nherited deficiency of GPIb-ix results in a bleeding disorder termed Bernard-Soulier syndrome. Deficiency of Vill vWF is termed von Willebrand's disease.) Adherent platelets undergo a shape change and aggregate. ADP is released from granules, which stimulates further aggregation of platelets (as well as adhesion of further platelets), forming a platelet plug. Prostaglandin thromboxane A2 TXA2) is synthesized by platelets and causes platelet aggregation and vasoconstriction. Receptors are expressed on the platelet surface which interact with coagulation factors collectively referred to as platelet factor 3 (PF3). The coagulation system generates the insoluble protein fibrin. Platelets express glycoprotein complex llb-Illa on their surface (GP a), which acts as a receptor for fibrinogen and vWF, helping form a firm hemostatic (Inherited deficiency of GPllb-Illa results in a bleeding disorder termed Glanzmann's thrombasthenia.) Gpllb/Illa inhibitor drugs are used to treat thrombotic diseases.