The fate of hair follicle melanocytes during the hair growth cycle

The fate of the follicular pigmentary unit during the hair growth cycle has long been one of the great enigmas of both hair follicle and pigment cell biology. Although melanocytes are distributed in several different compartm ents of the anagen hair follicle, melanogenically active cells are located only in the hair bulb, where they are directly involved in hair shaft pigme ntation. These pigment cells are readily detectable only when they become m elanogenically active during anagen III of the hair growth cycle. Thus, the ir status during hair follicle regression (catagen), when melanogenesis is switched off, until they re-appear again as pigment-producing cells in the anagen III hair follicle, has remained poorly defined. Historically, it has been proposed that hair bulb melanocytes adopt a self-perpetuating, catage n-resistant strategy of de-differentiation during hair follicle regression and re-differentiation upon entry into a new anagen phase; however, this ex planation remains problematic in the absence of evidence for de-differentia tion/re-differentiation plasticity in most nonmalignant cell systems.