When a blood vessel is injured, platelets (green) – tiny blood cells without nuclei – are the first on the scene, sticking to the vessel walls surrounding the injury site and assembling a meshwork of fibrin molecules (red) between them. This platelet and fibrin bundle (or clot) then contracts, as shown in the video, which serves two purposes: closing the wound to prevent blood loss, and preventing the vessel itself from becoming blocked by the clot. Although scientists were aware of the major molecular players controlling this remarkable feat of compression, the precise mechanism at work was a mystery. Now state-of-the-art microscopy has uncovered the dynamics of clot contraction, revealing that platelets extend thin, arm-like projections, called filopodia (visible in the video), that grab and pull on the fibrin cables and draw the cells together.
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