Staying on track: collagen fibers orientation strongly affects thrombi formation at high shear

Objective Subendothelial matrix exhibits a distinctive organization, with collagen fibers beneath the endothelium oriented primarily parallel to the flow and comprising mainly types III and I collagen in arteries and arterioles. However, the significance of such organization in initiating rapid thrombus formation remains unclear. Approach and Results To investigate the role of collagen fibers orientation in triggering thrombus formation we utilized the in vitro microfluidic model of thrombosis. Human whole blood was perfused through a system with collagen fibers oriented either parallel or perpendicular to the flow, and the primary stages of platelet adhesion and thrombi growth were analyzed using a high-speed fluorescence microscopy. At the shear rates of 200 and 1000 s -1 no significant difference in thrombogenicity was observed between the collagen fibers orientations. However, at a high shear rate of 2000 s -1 , thrombi on the parallel fibers were higher and covered a much larger area than those on the perpendicular ones. High-speed microscopy revealed that platelets were able to achieve stable adhesion only after translocating for over several seconds under all studied conditions. Analysis of single platelets dynamics revealed that platelets interacted longer and translocated farther during their interaction with parallel fibers at high shear, suggesting that flow-aligned fibers facilitate stable adhesion by enabling platelets to translocate along them and stay in contact with collagen. Importantly, only a small fraction of collagen fibers belonging to type III collagen admixture - a common component of a standard type I collagen preparation - accumulated plasma vWF and supported platelet adhesion at high shear rate. Conclusion Type III, but not type I collagen fibers oriented parallel to the flow facilitate stable platelet adhesion at high shear rates. Thus, axial orientation of subendothelial collagen fibers observed in vivo may have physiological relevance for triggering rapid thrombus formation at high shear rates.