Platelet

A platelet, or thrombocyte, is a tiny piece of cytoplasm wrapped in cell membrane that circulates in the blood. Platelets are not complete cells and have no nucleus. They are pinched off from megakaryocytes, very large cells in the red bone marrow, each of which sheds thousands of platelets. Their production is driven by the hormone thrombopoietin, made mainly in the liver and kidneys. Platelets are only 2 to 4 micrometers across and last about ten days, and about a third of them are held in reserve in the spleen.

Platelets are central to hemostasis, the process that stops blood loss from a damaged vessel. At an injury they stick to exposed collagen in the vessel wall and to each other, and they release chemicals, such as ADP, that call in more platelets, building a platelet plug. Clotting factors are activated on their surfaces, and the platelets become trapped in the mesh of fibrin that turns the plug into a firm clot, which they then tighten. They also release growth factors that help repair the tissue.

Too few platelets, thrombocytopenia, can cause easy bruising and prolonged bleeding, while too many, thrombocytosis, can lead to unwanted clots. Drugs such as aspirin reduce platelet clumping and are used in the treatment of heart attacks and strokes.