Clot Contraction Drives the Translocation of Procoagulant Platelets to Thrombus Surface.

Objective- After activation at the site of vascular injury, platelets differentiate into 2 subpopulations, exhibiting either proaggregatory or procoagulant phenotype. Although the functional role of proaggregatory platelets is well established, the physiological significance of procoagulant platelets, the dynamics of their formation, and spatial distribution in thrombus remain elusive. Approach and Results- Using transmission electron microscopy and fluorescence microscopy of arterial thrombi formed in vivo after ferric chloride-induced injury of carotid artery or mechanical injury of abdominal aorta in mice, we demonstrate that procoagulant platelets are located at the periphery of the formed thrombi. Real-time cell tracking during thrombus formation ex vivo revealed that procoagulant platelets originate from different locations within the thrombus and subsequently translocate towards its periphery. Such redistribution of procoagulant platelets was followed by generation of fibrin at thrombus surface. Using in silico model, we show that the outward translocation of procoagulant platelets can be driven by the contraction of the forming thrombi, which mechanically expels these nonaggregating cells to thrombus periphery. In line with the suggested mechanism, procoagulant platelets failed to translocate and remained inside the thrombi formed ex vivo in blood derived from nonmuscle myosin ( MYH9)-deficient mice. Ring-like distribution of procoagulant platelets and fibrin around the thrombus observed with blood of humans and wild-type mice was not present in thrombi of MYH9-knockout mice, confirming a major role of thrombus contraction in this phenomenon. Conclusions- Contraction of arterial thrombus is responsible for the mechanical extrusion of procoagulant platelets to its periphery, leading to heterogeneous structure of thrombus exterior.

Traumatic vessel injuries initiating hemostasis generate high shear conditions.

Blood flow is a major regulator of hemostasis and arterial thrombosis. The current view is that low and intermediate flows occur in intact healthy vessels, whereas high shear levels (>2000 s-1) are reached in stenosed arteries, notably during thrombosis. To date, the shear rates occurring at the edge of a lesion in an otherwise healthy vessel are nevertheless unknown. The aim of this work was to measure the shear rates prevailing in wounds in a context relevant to hemostasis. Three models of vessel puncture and transection were developed and characterized for a study that was implemented in mice and humans. Doppler probe measurements supplemented by a computational model revealed that shear rates at the edge of a wound reached high values, with medians of 22 000 s-1, 25 000 s-1, and 7000 s-1 after puncture of the murine carotid artery, aorta, or saphenous vein, respectively. Similar shear levels were observed after transection of the mouse spermatic artery. These results were confirmed in a human venous puncture model, where shear rates in a catheter implanted in the cubital vein reached 2000 to 27 000 s-1. In all models, the high shear conditions were accompanied by elevated levels of elongational flow exceeding 1000 s-1. In the puncture model, the shear rates decreased steeply with increasing injury size. This phenomenon could be explained by the low hydrodynamic resistance of the injuries as compared with that of the downstream vessel network. These findings show that high shear rates (>3000 s-1) are relevant to hemostasis and not exclusive to arterial thrombosis.