False-colour scanning electron micrograph (SEM) of unactivated blood platelets (yellow) and two red blood cells (pink). Platelets (thrombocytes) are small, non-nucleated cells formed in the bone marrow; they are present in circulating blood in large numbers. Unactivated platelets are round or oval in shape and 2-3 micrometres in diameter. Activated platelets have dendritic processes or pseudopodia. Platelets function in two ways. First, they help plug small defects in the walls of small blood vessels. Secondly, they are involved in clotting and the release of serotonin, a compound which reduces blood flow by constricting the damaged vessels. Magnification: x3150 at 6x4.5cm size.

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