Platelets

Platelets, also called thrombocytes, are the third and smallest formed element of blood, normally present at 1,50,000 to 3,50,000 per cubic millimetre. Unlike red and white cells, platelets are not whole cells at all - they are cell fragments pinched off from much larger cells in the bone marrow called megakaryocytes, and like mature red cells they have no nucleus. Their entire biological role is built around stopping blood loss: when a vessel is injured, platelets are the first responders, releasing substances that trigger the clotting cascade and forming the initial platelet plug that seals small breaks in the vessel wall (described in detail in the next section on coagulation). Because of this, a fall in platelet numbers is medically significant - it direct …