STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Active platelet. Coloured transmission electron micrograph (TEM) of a section through an activated blood platelet. Platelets plug defects in the walls of blood vessels, and are involved in clotting. They also release serotonin which constricts blood vessels. Platelet cells (thrombocytes) are formed in the bone marrow, and circulate in blood in large numbers. In an unactivated state they are round/oval, whereas activated platelets develop pseudopodia or extensions from the cell wall. Platelets contain cytoplasmic organelles such as mitochondria, endoplasmic reticulum and granules. Magnification: x41,500 when printed 10cm wide.
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