Platelets, or thrombocytes are small, regularly-shaped clear cell fragments (i.e. cells that do not have a nucleus containing DNA), 2–3um in diameter, which are derived from fragmentation of precursor megakaryocytes. The average lifespan of a platelet is normally just 5 to 9 days. Platelets play a fundamental role in hemostasis and are a natural source of growth factors. They circulate in the blood of mammals and are involved in hemostasis, leading to the formation of blood clots.