Platelet derived growth factor is released in vessels from:
Endothelial cells
Fibroblasts
Macrophages
Alpha granules
Platelet activation:
  The adherent platelets get activated, undergo a shape change and degranulate. The granules in the platelets can be

Alpha  granules  having  P-selectin,  fibrinogen,  fibronectin,  factors  V  and  VIII, platelet  factor  4,  platelet-derived  growth  factor,  and  transforming  growth factor-β.
Delta  granules  or  delta  bodies  having  ADP,  ATP,  ionized  calcium,  histamine, serotonin, and epinephrine.