Stem Cell Factor

Stem Cell Factor (SCF) is a signalling protein (a cytokine) made naturally in the body. It is also called KIT ligand, steel factor and mast cell growth factor. SCF works by binding to a receptor on the cell surface called c-KIT (also known as CD117). This signal matters for blood-forming cells, mast cells and pigment-producing cells (melanocytes) (Ashman 1999).

In cosmetics, a lab-made (recombinant) version of human SCF is sold under the ingredient name sh-Polypeptide-4. The "sh-" prefix stands for "synthetic human".

What is known about SCF in the hair follicle

Most of what is known about SCF in hair concerns hair colour, not hair growth.

• In mice, SCF/c-KIT signalling is needed for hair colour. Blocking c-KIT with an antibody during a hair cycle produced grey or white hairs, which grew back fully pigmented in the next cycle. Mice that made extra SCF in the follicle had more pigment cells (Botchkareva et al. 2001). This was a pigmentation study in mice, not a hair-growth study.
• In human scalp, one study compared balding and non-balding follicles. c-KIT levels and pigment cells were the same in balding and non-balding follicles. Dermal papilla cells grown in the lab from balding scalp released less SCF than cells from non-balding scalp. The authors linked this to the paler colour of thinning hairs (Randall et al. 2008). Whether low SCF contributes to follicle shrinkage in people has not been shown.

Evidence for topical sh-Polypeptide-4

Based on lab and mouse studies, some researchers have proposed that adding SCF could support hair growth or colour. This has not been tested in controlled human trials. SCF is a large protein, and it is not known whether it reaches the follicle when applied to the scalp.

Where SCF appears in hair products, it is usually one ingredient in a mix of growth factors and peptides, so any effect cannot be attributed to SCF itself. Injected growth-factor or stem-cell-derived products are not FDA-approved for hair loss. FDA has approved no stem-cell product for hair loss (the only approved stem-cell products are cord-blood products for blood disorders) and no exosome product for any use. It warns that such products may be illegally marketed and have not been shown to be safe or effective (FDA consumer alert, updated 2024).

Safety

When recombinant human SCF was injected under the skin in a small trial of 10 patients with advanced breast cancer, every injection site developed a hive-like (wheal-and-flare) reaction caused by mast-cell activation. Mast cells also increased at skin sites away from the injections, and 5 of the 10 patients developed lasting dark patches at the injection sites (Costa et al. 1996). Cosmetic topical products are not tested to this standard, and their long-term safety is unknown. People with a mast-cell disorder, and anyone offered injected or microneedled SCF products, should talk to a doctor first.

Summary

SCF has a clear role in hair colour in lab and mouse studies. No controlled human trial has shown that topical sh-Polypeptide-4 improves hair loss, and it is unclear whether it penetrates the scalp.

References

Ashman, L. K. (1999). The biology of stem cell factor and its receptor C-kit. International Journal of Biochemistry & Cell Biology, 31(10), 1037–1051. https://pubmed.ncbi.nlm.nih.gov/10582338/

Botchkareva, N. V., Khlgatian, M., Longley, B. J., Botchkarev, V. A., & Gilchrest, B. A. (2001). SCF/c-kit signaling is required for cyclic regeneration of the hair pigmentation unit. FASEB Journal, 15(3), 645–658. https://pubmed.ncbi.nlm.nih.gov/11259383/

Costa, J. J., Demetri, G. D., Harrist, T. J., Dvorak, A. M., Hayes, D. F., Merica, E. A., Menchaca, D. M., Gringeri, A. J., Schwartz, L. B., & Galli, S. J. (1996). Recombinant human stem cell factor (kit ligand) promotes human mast cell and melanocyte hyperplasia and functional activation in vivo. Journal of Experimental Medicine, 183(6), 2681–2686. https://pubmed.ncbi.nlm.nih.gov/8676090/

Randall, V. A., Jenner, T. J., Hibberts, N. A., De Oliveira, I. O., & Vafaee, T. (2008). Stem cell factor/c-Kit signalling in normal and androgenetic alopecia hair follicles. Journal of Endocrinology, 197(1), 11–23. https://pubmed.ncbi.nlm.nih.gov/18372228/

U.S. Food and Drug Administration. (2020, updated 2024). Consumer alert on regenerative medicine products including stem cells and exosomes. https://www.fda.gov/vaccines-blood-biologics/consumers-biologics/consumer-alert-regenerative-medicine-products-including-stem-cells-and-exosomes