False-colour scanning electron micrograph (SEM) of a mass of human blood platelets (thrombocytes). Platelets are small, non-nucleated cells formed in the bone marrow, and are present in circulating blood in large numbers. Their shape in an unactivated state is round or oval (2-3 micrometres diameter); activated platelets have dendritic processes, or pseudopodia. Platelets function in 2 ways. Firstly, they help plug defects in the walls of small blood vessels: secondly, they are involved in clotting & also release serotonin, which reduces blood flow by constricting damaged vessels. Magnification: x2,000 at 6x7cm, x1000 at 35mm size.