Activated platelets. Coloured scanning electron micrograph of activated blood platelets on the surface of damaged endothelium. Platelets (thrombocytes) are small, non-nucleated cells formed in the bone marrow, and 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. Magnification: x3,800 at 6x7cm size. Magnification: x13,000 at 8x10inch size.