Growth factors are signaling proteins that bind receptors on a cell surface and instruct the cell to divide, differentiate, or stay active. In the hair follicle several of them regulate the timing of the growth cycle, and two from the fibroblast growth factor family, FGF-7 and FGF-2, are the focus of this program.
FGF-7, also called keratinocyte growth factor, is released by the dermal papilla and acts on the keratinocytes that build the hair shaft. Its signaling is associated with entry into and maintenance of the growth phase. FGF-2, basic fibroblast growth factor, is involved more broadly in cell proliferation and in the formation of the small blood vessels that supply the follicle.
Both are proteins the body already produces. The premise of growth-factor signaling is not to impose an external effect but to supply the follicle with the same class of molecule it uses to regulate its own cycle. This is what separates the approach from actives that work by irritation, a contrast documented in Note 04.
A practical constraint follows at once. Growth factors are large, fragile proteins, and applying them to the surface of the skin is not the same as getting them to the place where the follicle's signaling occurs. That delivery problem is the subject of the next note.