A Hair Follicle is a Complex Structure, Still Comparatively Poorly Understood
Why hasn't the research and development community achieved greater progress towards solving the problem of hair loss with age, given the sizable interest in achieving this goal? One possible answer is that a hair follicle is a very complicated structure that undertakes a shifting pattern of behaviors over time. It remains comparatively poorly understood as to why the various forms of hair loss occur, the fine details of the important mechanisms in each case. Efforts to intervene are challenging given the lack of a firm set of target mechanisms.
With stem cells that can be activated and silenced cyclically, hair follicle experiences multiple rounds of growth phase (anagen), regression phase (catagen), and resting phase (telogen) during lifespan. Hair cycling is initiated by cyclic renewal or physiological cyclic regeneration of stem cells. However, tissues and organs undergo structural and functional declines in the aging process, with physiological and pathological changes regulated by intrinsic and extrinsic factors that dictate the cell fate. As one of the important appendages of the skin, the hair follicle is a complex mini-organ with visible signs, such as decreased regenerative ability that leads to alopecia, and hair graying due to less melanin production by melanocyte stem cells during aging.
Hair regenerative ability is gradually decreased because hair follicle stem cells enter a long quiescent state, or differentiate into other skin epithelial lineages, or escape from the hair follicle niche during aging. These features promote hair follicle to become a widely used model for studying regeneration. As over 30% of the population all over the world suffer from partial or complete hair loss, particularly most people undergo alopecia during aging, understanding the mechanism by which hair follicle changes during aging is of great interest in regenerative biology and is essential for regenerative medicine.