Platelets

Platelets are irregularly-shaped blood cells without nucleus. A platelet measure between 2 and 3 µm (micrometers) in diameter. The average lifespan of a platelet is 12 days. They are produced by the fragmentation of precursor megakaryocytes. Platelets take part in hemostasis as they cause the blood to coagulate by forming clots to stop bleeding. When blood from a wound flows out, the platelets gather at the site of the damaged tissue to stop the bleeding. Calcium, vitamin K, and a protein called fibrinogen help the platelets form a clot.

When the number of platelets is too high, blood clots called thrombosis can form. This obstructs blood vessels and result in a stroke. An increase in the number of platelets is called thrombocytosis. A low number of platelets is called thrombocytopenia. Platelets release growth factors, which include platelet-derived growth factor (PDGF), a potent chemotactic agent, and a transforming growth factor-ß, that stimulates the deposition of extracellular matrix. Both of these growth factors have been shown to play a significant role in the repair and regeneration of connective tissues.