False-colour scanning electron micrograph (SEM) of aggregrates of human blood platelets. Platelets (thrombocytes) are small, non-nucleated cells formed in the bone marrow & are present in circulating blood in large numbers. Their individual shape is round or oval (2-3 micrometres in diameter). Platelets function in 2 ways. Firstly, they help plug small defects in the walls of small blood vessels: secondly, they contribute towards the process of clotting & also release serotonin, which reduces blood flow by constricting the damaged vessels. Magnification: X 4,400 at 35mm size.