Platelets in a blood clot. Coloured scanning electron micrograph (SEM) of platelet cells (thrombocytes) in a blood clot. They are oval shaped, but when they meet damaged blood vessels they become activated releasing chemical factors and turning into spiky spheres. The chemical factors cause fibrinogen to come out of solution as fibrin fibres, catching platelets and other blood cells and eventually forming a solid clot. Magnification: x4600 when printed 10 centimetres wide.
