J. Cosmet. Sci., 62, 121–125 (March/April 2011) 121 Graying of the human hair follicle EVA M. J. PETERS, DOMINIK IMFELD, and REMO GRÄUB, University-Medicine Charité, CharitéCenter 12 (CC12) for Internal Medicine and Dermatology, Psychoneuroimmunology, Charité Platz 1, 10117 Berlin, Germany (E.M.J.P), Justus-Liebig University Giessen, Department of Psychosomatic Medicine, Psychoneuroimmunology, Ludwigstrasse 78, 35392 Giessen, Germany (E.M.J.P), and DSM Nutritional Products Ltd, Personal Care, P.O. Box 2676, 4002 Basel, Switzerland (D.I., R.G). Synopsis Quality of life in our society depends crucially on healthy aging, a hallmark of which is the graying hair fol- licle. During anagen melanocyte precursors migrate to the hair bulb to form the pigmentary unit where they mature and synthesize melanin. Melanin is transferred to the hair shaft forming keratinocytes giving the hair its colour. Graying is the process in which distinct mechanisms lead to deterioration of the hair follicle melano- cyte population. We briefl y review the hair graying process and state that the aging hair follicle is a valid model for tissue specifi c aging and a promising target to test therapeutic intervention. INTRODUCTION TO HAIR GRAYING Social acceptance in our society depends crucially on visible perception of a person’s health status. It is therefore most people’s intention to manipulate the visual signs of aging as soon as they arise. This is the reason why we dye our hair when it becomes gray (canities). The market turn over rate of hair colorants is estimated at around 12–13 billion Euros per annum. Chemical hair colorants are easy to use and effectively covering gray hair. However, their drawbacks are short lasting, damaging to hair and not able to restore the natural color of hair. Because hair graying is tightly linked with aging we hypothesize that if we understand graying, we would also understand aging and vice versa. For these reasons it has been a focus of academic and corporate research to understand the biological mechanisms of hair graying, and the ability to slow down the graying process or restore the natural pigmentation of hair. Nevertheless there are hardly any treatment concepts available to date that act satisfyingly against hair graying. Establishment of the pigmentary-unit and hair follicle pigmentation is tightly linked to the hair follicle’s growth cycle (1,2). Recruitment of melanoblasts occurs during early anagen. At the top of anagen, in full growth phase, the pigmentary-unit is formed: mel- anocytes mature atop the dermal papilla tightly anchored to the basement membrane separating the epithelial from the mesenchymal compartment of the hair follicle, pigment

JOURNAL OF COSMETIC SCIENCE 122 is produced and in close collaboration with hair follicle keratinocytes it is transferred to the hair shaft. During late anagen, the pigmentary-unit starts to disassemble leading to resolution of the pigmentary unit during catagen (Figure 1), a process which repeats itself many times during a life-time and is characterized by the gradual reduction of melano- cytes active in the pigmentary-unit. In humans we distinguish between premature and senile canities. Senile canitites is believed to occur because of exhaustion of the regenera- tive capacity of hair pigmentation as well as through programmed events during aging. Premature canities can be viewed as caused by environmental factors, infl ammation or psycho-emotional stress. In terms of hair-graying it is well established that oxidative stress is a trigger of melano- cyte apoptosis in the hair follicle bulb, however not in the outer root sheath (3). It is noteworthy that oxidative stress in the hair follicle causes apoptosis selectively in melano- cytes (3). Bcl-2, an anti-oxidative stress protein is required for maintenance of hair follicle melanocytes at the tip of the hair bulb (3) and lack of Bcl-2 leads to disappearance of Figure 1. Scheme of the establishment and cessation of the pigmentary unit during the hair growth cycle.