Illustration of active and inactive blood platelets. Platelets, or thrombocytes, are small, non-nucleated cells formed in the bone marrow. They occur in the blood in large numbers. In an inactive state (right) they are round or oval discs. Activated platelets (centre) have dendritic processes, or pseudopodia, and are star-shaped. When activated they clump together (left). Platelets help to plug defects in the walls of small blood vessels and are involved in blood clotting and in the release of serotonin, which reduces blood flow by constricting damaged vessels.