Platelets are produced in the bone marrow, where they bud off of mature megakaryocytes in response to thrombopoetin. Also known as thrombocytes, platelets play a critical role in hemostasis, the formation of blood clots to prevent excessive blood loss. Platelets become activated and promote coagulation in response to collagen released from damaged endothelial cells that line the surface of blood vessels. By secreting a number of growth factors such as platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF), platelets can contribute to wound healing. Low numbers of platelets (thrombocytopenia) can increase vulnerability to bleeding, whereas high platelet counts (thrombocytosis) can lead to thrombosis-induced heart attack or stroke.