Transient build-up of a mural thrombus promotes intrathrombus coagulation reactions

Platelet accumulation after nonpenetrating vessel injury exhibits a characteristic kinetic profile with initial formation of a large thrombus that subsequently shrinks to a smaller aggregate covering the injury. Although the molecular mechanisms of platelet aggregation are well characterized, the physiological relevance of this transient large thrombus formation has never been studied. Using a low-concentration FeCl₃ model of the carotid artery, we confirmed that arterial thrombus formation presented a peak-shape kinetic and ended by the formation of a small stable remaining aggregate. Further characterization of this remnant with electron microscopy and immunofluorescence indicated that it was composed of the highly activated platelets and was particularly rich in fibrin. Combining in silico and in vivo approaches, we investigated the possible significance of the initial accumulation of an apparently superfluous number of platelets. Both the numerical model and experiments with thrombocytopenic mice indicated that the thrombus with the height of more than 10%-20% of the vessel lumen provided a shelter to facilitate intrathrombus coagulation even under high-shear blood flow, and it was essential to generate sufficient amounts of thrombin to create the underlying fibrin-rich patch. The critical size of thrombus required for such protection in silico depended on such parameters as injury length, tissue factor density, and shear rate. In agreement with the computational model, our in vivo data revealed that P2Y₁₂ or αIIbβ3 blockers administered to mice during the build-up of an initial subocclusive thrombus inhibited the formation of fibrin in the remaining aggregate, emphasizing the importance of thrombus for intrathrombus coagulation. We propose that the physiological role of the initial formation of a large enough platelet thrombus upon injury is to provide a protective environment allowing coagulation to proceed even at high shear rates and low tissue factor densities and ultimately to produce a stable fibrin-rich aggregate.