STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Platelets in a blood clot. Coloured scanning electron micrograph (SEM) of platelet cells (thrombocytes). Platelets are formed in the bone marrow and circulate in the blood stream. 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 fibres, catching platelets and other blood cells and eventually forming a solid clot. Magnification x4600 when printed 10 centimetres wide.
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