False-colour scanning electron micrograph of activated blood platelets on the surface of damaged endothelium. Platelets, or thrombocytes, are small, non-nucleated cells formed in the bone marrow. They occur in the blood in large numbers. In an unactivated state they are shaped like round or oval wafers 2-3 micrometres in diameter (see photo P256/012); activated platelets have dendritic processes, or pseudopodia. Platelets function in 2 ways: they help plug defects in the walls of small blood vessels; & they are involved in blood clotting & in the release of serotonin, which reduces blood flow by constricting damaged vessels. Magnification: x3600 at 6x4.5cm size.