A 2025 review in Stem Cell Research & Therapy identified Wnt signaling molecules as central drivers that activate and maintain anagen, the hair follicle’s growth phase.
That growth window largely determines hair length: scalp follicles typically stay in anagen for 2 to 7 years, while arm hair lasts weeks and eyelashes only months.
The review says opposing signals including BMP push follicles toward rest, making the balance between Wnt and suppressive pathways the key molecular “timer” for when growth stops.
Genetics strongly shapes that timer, but protein, iron, zinc and B-vitamin deficiencies can shorten anagen, and physiological stress can trigger telogen effluvium with shedding 2 to 3 months later.
Hormones add another layer: a 2022 review described an “androgen paradox” in which androgens shorten scalp-hair anagen but extend facial-hair growth, helping explain male-pattern baldness.