Platelet aggregation at sites of vascular injury is essential for hemostasis and arterial thrombosis. It has long been assumed that platelet aggregation and thrombus growth are initiated by soluble agonists generated at sites of vascular injury. By using high-resolution intravital imaging techniques and hydrodynamic analyses, we show that platelet aggregation is primarily driven by changes in blood flow parameters (rheology), with soluble agonists having a secondary role, stabilizing formed aggregates. We find that in response to vascular injury, thrombi initially develop through the progressive stabilization of discoid platelet aggregates. Analysis of blood flow dynamics revealed that discoid platelets preferentially adhere in low-shear zones at the downstream face of forming thrombi, with stabilization of aggregates dependent on the dynamic restructuring of membrane tethers. These findings provide insight into the prothrombotic effects of disturbed blood flow parameters and suggest a fundamental reinterpretation of the mechanisms driving platelet aggregation and thrombus growth.

Platelet aggregation, the process by which platelets adhere to each other at sites of vascular injury, has long been recognized as critical for hemostatic plug formation and thrombosis. Until relatively recently, platelet aggregation was considered a straightforward process involving the noncovalent bridging of integrin ␣ IIb ␤ 3 receptors on the platelet surface by the dimeric adhesive protein fibrinogen. However, with recent technical advances enabling real-time analysis of platelet aggregation in vivo, it has become apparent that this process is much more complex and dynamic than previously anticipated. Over the last decade, it has become clear that platelet aggregation represents a multistep adhesion process involving distinct receptors and adhesive ligands, with the contribution of individual receptorligand interactions to the aggregation process dependent on the prevailing blood flow conditions. It now appears that at least 3 distinct mechanisms can initiate platelet aggregation, with each of these mechanisms operating over a specific shear range in vivo. The identification of shear-dependent mechanisms of platelet aggregation has raised the possibility that vascular-bed-specific inhibitors of platelet aggregation may be developed in the future that are safer and more effective than existing antiplatelet agents. IntroductionThe propensity of platelets to clump together at sites of vascular injury was first recognized more than 100 years ago. [1][2][3][4] This phenomenon, most accurately described as platelet cohesion although more commonly referred to as platelet aggregation, was quickly identified as important for hemostatic plug formation. 5 It was also recognized at the time that platelets played a key role in the development of thrombosis, 6 but, it was not until almost a century later that it became widely accepted that platelets played a pivotal role in development of cardiovascular disease. 7 As a consequence, inhibitors of platelet aggregation have become increasingly important parts of the armamentarium for the prevention and treatment of many atherothrombotic disorders. 8,9 For more than 3 decades, the factors mediating platelet aggregation appeared conceptually straightforward, requiring a platelet stimulus (agonist), a soluble adhesive protein (fibrinogen), and a membrane-bound platelet receptor (integrin ␣ IIb ␤ 3 or GPIIb-IIIa), leading to a simple unified model of platelet aggregation ( Figure 1). Although these core elements remain fundamental, recent technical advances allowing real-time analysis of platelet aggregation in vivo have demonstrated a much more complex and dynamic process than previously anticipated. It is now widely accepted that one of the key elements influencing the platelet aggregation process is blood flow, with evidence that distinct aggregation mechanisms operate under different shear conditions. [10][11][12][13] This has raised the interesting possibility that vascular bed-specific inhibitors of platelet aggregation may be developed in the future and has stimulated a...

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The formation of blood clots--thrombosis--at sites of atherosclerotic plaque rupture is a major clinical problem despite ongoing improvements in antithrombotic therapy. Progress in identifying the pathogenic mechanisms regulating arterial thrombosis has led to the development of newer therapeutics, and there is general anticipation that these treatments will have greater efficacy and improved safety. However, major advances in this field require the identification of specific risk factors for arterial thrombosis in affected individuals and a rethink of the 'one size fits all' approach to antithrombotic therapy.