Overview: Androgenetic Alopecia
Androgenetic alopecia (AGA) is the most common type of hair loss in men and women, affecting an estimated 50 million men and 30 million women in the US.1
- It is characterized by varying degrees of hair thinning that occurs as terminal (mature) hairs are gradually “miniaturized” to vellus (fine/wispy) hairs.2
- AGA generally presents in a characteristic pattern and varies by age and race.
- It is most prevalent in Caucasians, with approximately half of men developing hair loss by age 50 and one third of women developing some form of loss as age increases.2-4
Healthy hair follicles follow a cycle, progressing through the anagen (growth) phase followed by the catagen (regression) phase, the telogen (quiescent) phase, and then back into an anagen phase.5 In AGA, this cycle is disrupted and hair follicles miniaturize and stop growing. There are many molecular pathways involved in this process of hair follicle formation and maturation, including6,7:
- Sonic hedgehog (SHH)
- Bone morphogenetic protein (BMP)
In particular, activation of the Wnt signaling pathway has been shown to be required for the initiation of hair follicle development8, and epithelial β-catenin signaling is required for maintenance of proliferation in the anagen phase.9
- NIH Genetics Home Reference. https//ghr.nlm.nih.gov/condition/androgenetic-alopecia#statistics. Accessed Oct. 4, 2016.
- Sinclair R. Male pattern androgenetic alopecia. BMJ. 1998;317(7162):865-9.
- Sinclair R, Patel M, Dawson TL Jr, et al. Hair loss in women: medical and cosmetic approaches to increase scalp hair fullness. Br J Dermatol. 2011;165(suppl 3):12-8.
- Norwood OT. Incidence of female androgenetic alopecia (female pattern alopecia). Dermatol Surg. 2001;27(1):53-4.
- Hardy MH. The secret life of the hair follicle. Trends Genet. 1992;8;55-61.
- Paus R & Cotsarelis G. The Biology of Hair Follicles. NEJM. 1999;341(7):491–97.
- Krause K & Foitzik K. Biology of the Hair Follicle: The Basics. Semin Cutan Med Surg. 2006;25(1):2–10.
- Andl T, Reddy ST, Gaddapara T, Millar SE. WNT signals are required for the initiation of hair follicle development. Dev Cell. 2002;2(5):643-53.
- Choi YS, Zhang Y, Xu M, et al. Distinct functions for the Wnt/β-catenin in hair follicle stem cell proliferation and survival and interfollicular epidermal homeostasis. Cell Stem Cell. 2013;13(6):720-33.