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  • Platelet Adhesion and Aggregation Developed and produced for by Mechanisms in Medicine Inc. Animation description Platelet adhesion and aggregation. In flowing blood, red cells predominate in the axial stream, while the biconvex disc-shaped platelets are marginated along the vessel wall where they are well-positioned to monitor the integrity of the endothelium. The normal endothelium provides a non-adhesive surface to circulating platelets. However, when vessel wall injury occurs, for example, by cutting or severing of a vessel, or as shown here, by a puncture, and there is endothelial damage, the initial response of platelets is that of adhesion to collagen fibres in the exposed subendothelium. Collagen is one of the most thrombogenic components of the subendothelial matrix responsible for the initiation of platelet adhesion. A number of adhesive receptors on the platelet surface membrane interact either directly or indirectly with collagen. Initial binding of platelets is considered to occur via the integrin α2β1 (GPIa-IIa) receptor, which allows for further binding to collagen via the GPVI receptor, initiating transmembrane and, subsequently, intracellular signalling. Adhesion of platelets to the exposed subendothelium is influenced by shear rates. At high shear, α2β1 and GPVI are not sufficient to initiate binding to collagen, and binding of the GPIb-IX-V receptor to von Willebrand factor -- abbreviated here as VWF -- that is immobilized on collagen, becomes essential in ...