STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Human red blood cells and platelets. Coloured scanning electon micrograph (SEM) of human erythrocytes (red blood cells) and a platelet aggregate (cream). Platelets are fragments of white blood cells that under normal circumstances are small and biconcave in form. However, if there is a break in the surface of a blood vessel the platelets come into contact with molecules they are not used to and become activated. They become amorphous in form, with long projections (pseudopodia) that help them adhere to other cells and each other, forming a clot. Magnification: x3000 when printed at 10 centimetres wide.
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