Platelets come contact collagen exposed on the subendothelial matrix at sites of vascular injury that triggers their activation and the formation of a hemostatic plug. Glycoprotein VI and integrin α2β1 are major collagen receptors on the platelet surface. Although the spreading of platelets on the collagen is important to function, it has been difficult to study the dynamics of spreading over a relevant time scale. Here we focus on the early stages of murine platelet spreading on collagen and/or fibrinogen under both static conditions and flow and then probe their dynamics by quantitative visualization using real-time reflection interference contrast microscopy. In this study, we found under static conditions the spreading area of platelets on collagen initially increased quickly by following the power law At0.7 before slowing. Interestingly, we observed in microchannels under flow that single platelets as well as aggregates that were spread on the collagen contracted over time under shear. This contraction was not observed under static conditions in our system. This effect might help to maintain the hemostatic plug under the shear force. Future studies will be aimed at investigating the spatial-temporal cytoskeletal dynamics of platelets and the functions of collagen receptors on the platelets surface using platelets derived from genetically engineered mice.

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