Hair cycle and hair pigmentation: dynamic interactions and changes associated with aging

The document from April 1, 2004, discusses the relationship between hair pigmentation and the hair growth cycle, particularly how it changes with aging. It explains that the hair follicle's ability to produce pigment is optimally coupled with hair growth during the first 10 hair cycles, which typically occurs by the age of 40. After this point, a genetically regulated decline in pigment production leads to the development of gray and white hair. This decline is primarily due to reduced tyrosinase activity in melanocytes within the hair bulb. Other factors contributing to pigment loss include impaired interactions between melanocytes and keratinocytes, and the failure of melanocytes to migrate from a reservoir to the hair bulb. The document also suggests that damage to DNA from reactive oxygen species, along with dysregulation of antioxidant and apoptotic factors, may play a role in the aging of melanocytes. Interestingly, the loss of pigment may also impact keratinocyte behavior, indicating that melanocytes have roles beyond pigment production. The review highlights the potential for reversing hair graying by manipulating hair follicle melanocytes in vitro, as these cells remain present in white hair follicles and can be induced to produce pigment in culture. The possibility of rejuvenating the entire hair follicle is considered a dream, but optimizing its function is seen as clinically relevant and achievable. The document also notes that changes in hair color can affect the visibility of hair when counting hair fibers using optical methods, suggesting that observed changes might not solely be due to hair growth but also to changes in hair color.