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.