Figure 9.

Proposed causes for hair greying during aging. The age-related pigment loss in hair fibres may be caused by the following: (1) depletion of melanotic melanocytes (Mel-Mc) in the hair matrix. Intrinsic (melanogenesis generates ROS) and extrinsic (inner root sheath and pre-cortical keratinocyte terminal differentiation involves ROS generation) sources of ROS may lead to oxidative damage of bulb melanocytes if antioxidant systems start to fail. (2) Exhaustion of the melanocyte stem cell (MelSC) reservoir, and (3) MelSC defective activation or migration during the onset of the anagen phase of the hair growth cycle. The inability to maintain MelSC quiescence due to ageing, oxidative stress, and stress-induced sympathetic nervous activity results in ectopic MelSC differentiation. Differentiated melanocytes are no longer able to replenish the pigmentary unit in the beginning of the hair cycle.