I did not find much of the effects of growth factors on hair growth. Remember that science is a process in motion. Even if no major mistakes were made, hair follicles in culture may behave differently than in the skin, mostly because it is so hard to mimic the real environment with it "soup" of growth factors, enzymes, etc.
Philpott, Michael P.; Kealey, Terence Effects of EGF on the morphology and patterns of DNA synthesis in isolated human hair follicles. Journal of Investigative Dermatology (1994), 102(2), 186-91
The authors have previously reported that human hair grows at a normal rate in vitro for about10 d. The authors have also reported that, on gross observation, epidermal growth factor appears to induce a catagen-like effect on cultured hair follicles, but the authors have not characterized the details of this. The authors now report that when isolated human hair follicles are maintained in the presence of epidermal growth factor, the rate of hair follicle elongation is significantly stimulated but hair fiber prodn. is inhibited. Light microscopy showed that epidermal growth factor stimulated a thickening and vacuolation of the cells of the lower outer root sheath of the hair follicle and that the matrix cells of the hair follicle underwent an upward migration resulting in the formation of a 'club hair'-like structure that remained connected to the dermal papilla by a thin strand of epithelial cells. [Methyl-3H]thymidine autoradiog. was carried out to investigate the patterns of DNA synthesis and showed that epidermal growth factor inhibited DNA synthesis in the hair follicle matrix cells but dramatically stimulated DNA synthesis in the outer root sheath. The authors conclude from these studies that epidermal growth factor may be inducing an artificial 'catagen-like' effect by stimulating outer root sheath proliferation, which uncouples the normal patterns of proliferation and migration that occur in the anagen hair follicle and that result in an anagen-co-catagen-like transition. Moreover, these results also suggest that, under certain conditions, outer root sheath cells in the hair follicle may be capable of downward migration.
Mak, Kingston K. L.; Chan, Siu Yuen Epidermal growth factor as a biologic switch in hair growth cycle. Journal of Biological Chemistry (2003), 278(28), 26120-26126
The hair growth cycle consists of three stages known as the anagen (growing), catagen (involution), and telogen (resting) phases. This cyclical growth of hair is regulated by a diversity of growth factors. Although normal expression of both epidermal growth factor and its receptor (EGFR) in the outer root sheath is down-regulated with the completion of follicular growth, here we show that continuous expression of epidermal growth factor in hair follicles of transgenic mice arrested follicular development at the final stage of morphogenesis. Data from immunopptn. and immunoblotting showed that epidermal growth factor signals through EGFR/ErbB2 heterodimers in skin. Furthermore, topical application of tyrphostin AG1478 or AG825, specific inhibitors of EGFR and ErbB2, resp., completely inhibited new hair growth in wild type mice but not in transgenic mice. When the transgenic mice were crossed with waved-2 mice, which possess a lower kinase activity of EGFR, the hair phenotype was rescued in the offspring. Taken together, these data suggest that EGFR signaling is indispensable for the initiation of hair growth. On the other hand, continuous expression of epidermal growth factor prevents entry into the catagen (involution) phase. We propose that epidermal growth factor functions as a biol. switch that is turned on and off in hair follicles at the beginning and end of the anagen phase of the hair cycle, guarding the entry to and exit from the anagen phase.