It turns out that the alpha granules in the BLOOD PLATELETS of PLATELET-RICH PLASMA INJECTION THERAPY CONTAIN GROWTH FACTORS... SUCH AS PDGF, IGF-1, EGF, and TGF-β which begin the healing of wounds by attracting and activating macrophages, fibroblasts, and endothelial cells.
For example:
Platelet-derived growth factor (PDGF)
Insulin-like growth factor-1 (IGF-1)
Epidermal growth factor (EGF)
Transforming growth factor beta (TGF-β)
Fibroblast growth factor 1(FGF1)
Fibroblast growth factor 2(FGF2)
Fibroblast growth factor 3(FGF3)
Fibroblast growth factor 4(FGF4)
Fibroblast growth factor 5(FGF5)
Fibroblast growth factor 6(FGF6)
Fibroblast growth factor 7(FGF7)
Fibroblast growth factor 8(FGF8)
Fibroblast growth factor 9(FGF9)
Fibroblast growth factor 10(FGF10)
Fibroblast growth factor 11(FGF11)
Fibroblast growth factor 12(FGF12)
Fibroblast growth factor 13(FGF13)
Fibroblast growth factor 14(FGF14)
Fibroblast growth factor 15(FGF15)
Fibroblast growth factor 16(FGF16)
Fibroblast growth factor 17(FGF17)
Fibroblast growth factor 18(FGF18)
Fibroblast growth factor 19(FGF19)
Fibroblast growth factor 20(FGF20)
Fibroblast growth factor 21(FGF21)
Fibroblast growth factor 22(FGF22)
Fibroblast growth factor 23(FGF23)
https://en.wikipedia.org/wiki/Growth_factor