Three Streams for the Mechanism of Hair Graying

Abstract

Hair graying is an obvious sign of human aging. Although graying has been investigated extensively, the mechanism remains unclear. Here, we reviewed previous studies on the mechanism of graying and seek to offer some new insights. The traditional view is that hair graying is caused by exhaustion of the pigmentary potential of the melanocytes of hair bulbs. Melanocyte dysfunction may be attributable to the effects of toxic reactive oxygen species on melanocyte nuclei and mitochondria. A recent study suggests that bulge melanocyte stem cells (MSCs) are the key cells in play. Graying may be caused by defective MSC self-maintenance, not by any deficiency in bulbar melanocytes. Our previous study suggested that graying may be principally attributable to active hair growth. Active hair growth may produce oxidative or genotoxic stress in hair bulge. These internal stress may cause eventually depletion of MSC in the hair follicles. Taken together, hair graying may be caused by MSC depletion by genotoxic stress in the hair bulge. Hair graying may also be sometimes caused by dysfunction of the melanocytes by oxidative stress in the hair bulb. In addition, hair graying may be attributable to MSC depletion by active hair growth.

Histopathology of aging of the hair follicle

“…The free radicals are thought to damage melanocytic stem cells, which would be the cause of hair graying, rather than damage to differentiated melanocytes . Supporting the hypothesis of a depletion of melanocyte stem cells migrating into the hair bulb, white hairs are devoid of SOX10, Pax3, or MiTF‐M in the bulbs …”

“…Short hairs usually remain pigmented even when most of the long hairs are already depigmented (Figure B). This supports the theory that active hair growth is related to hair graying, probably by oxidative products of the hair growth process.…”

“…Additionally, other mechanisms might contribute to hair graying such as reduced tyrosinase activity in the bulbar melanocytes, or any abnormal transfer of melanosomes from the melanocytes of the bulb to the keratinocytes of the cortex . This has been linked to active hair growth which produces oxidative or genotoxic stress in the hair bulge and eventually depletion of stem cells in the bulge . However, an alternative or complementary mechanism might be the dysfunction of melanocytes from the bulge due to oxidative stress .…”

“…This has been linked to active hair growth which produces oxidative or genotoxic stress in the hair bulge and eventually depletion of stem cells in the bulge . However, an alternative or complementary mechanism might be the dysfunction of melanocytes from the bulge due to oxidative stress . Additionally, a mitochondrial theory of graying has also been proposed, as mitochondria are targets of stress products …”

“…There is another paradoxical fact of gray hair, at least in animal models: it grows faster and thicker than the pigmented hair, although there is no evidence of such a phenomenon in humans . It has been hypothesized that the transfer of melanosomes plays a role in regulation (and limitation) of the hair growth …”