Platelets, or thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are small, regularly shaped clear cell fragments (i.e. cells that do not have a nucleus containing DNA), 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes. The average lifespan of a platelet is normally just 5 to 9 days. Platelets 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. If the number of platelets is too low, excessive bleeding can occur...