Getting under the skin of hair aging: the impact of the hair follicle environment.

Like the skin, our hair shows striking changes with age, producing hairs with altered diameter, lustre and texture. The biology of hair aging has focused predominately on various aspects of the hair cycle, follicle size and the fibre produced, but surprisingly the impact of the aging scalp dermal environment on the hair follicle and fibre has been generally overlooked. Hair loss affects both sexes with incidence increasing with age. In men, male pattern-balding (androgenetic alopecia) is driven by androgens and follows a specific pattern of frontotemporal and vertex regression. Women also experience female pattern hair loss (FPHL), presenting as more general, diffuse hair thinning. Hair thinning in women is commonly associated with the menopause, corresponding with other age-related changes in skin. The rapidly growing hair follicle undergoes continued renewal throughout the life span of an individual, where it is exposed to a substantial number of extrinsic and intrinsic stressors. As the hair follicle sits deep within the dermis with its bulb residing in the hypodermis, detrimental age-related changes in the surrounding scalp skin may likely disrupt the hair follicle machinery. The impacts of these changes are unknown, but evidence suggests that scalp skin aging and hair follicle aging go hand-in-hand. Herein, we summarize the evidence that the age-related changes observed in sun-exposed human skin also occur in scalp skin and that these changes are likely to play a contributing role in the aging hair phenotype.