Biochemical mechanisms regulating human hair growth.

The human hair follicle cycles in active growth and resting phases controlled by a complex network of biochemical processes, yet to be fully understood. It is well known that hair follicles on scalp respond to androgens by a shortening of the anagen growth phase causing hairs to regress to a finer, thinner texture. The target tissue androgens, testosterone, and dihydrotestosterone can circulate systemically to skin or can be formed locally in hair follicles and sebaceous glands by specific enzymes in the steroid cascade. Kinetic constants have been evaluated for several enzymes which mediate dihydrotestosterone formation, including 5a-reductase, and the cytochrome P-450 aromatase enzyme in isolated human hair follicles and sebaceous glands from scalp of men and women with androgenetic alopecia. The levels of these enzymes differed between men and women, and from frontal versus occipital sites within the same patient, indicating that similar steroid mechanisms may be taking place in men and women, but the amount or level of enzymes vary, perhaps explaining why men have more severe patterns of hair loss than women. Knowing the differences between men and women with androgenetic alopecia could shape more effective treatment options in the future.