A minute disc-shaped cell fragment, 2–3 µm in diameter, occurring in mammalian blood. Platelets are formed as fragments of larger cells (megakaryocytes) found in red bone marrow; they have no nucleus. They play an important role in blood clotting and release thromboxane A₂, serotonin and other chemicals, which cause a chain of events leading to the formation of a plug at the site of the damage, thus preventing further blood loss. There are typically 250 000 to 400 000 platelets per cubic millimetre in mammalian blood.