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Stress which damages DNA is referred to as genotoxic stress. In this case, when specialized cells, called melanocyte stem cells, (MSCs) are damaged, it ultimately results in a malfunction of those cells which express hair color. Genotoxic stress can deplete the MSCs in hair follicles that make the pigment-producing melanocytes. However, when exposed to the stress, the MSCs actually differentiate into mature melanocytes, as opposed to dying off. This means that if the genotoxic stress can be limited, graying may actually be halted.
Virtually all cells are exposed to some form of genotoxic stress, every day. However, cells are built to handle the stress and repair damaged DNA, and even work to prevent the damage from being passed on to daughter cells. However, once stem cells are irreparably damaged, they must be eliminated so that the quality of the stem cells pools can be maintained. Stresses on stem cell pools and genome maintenance failures are also thought responsible for the decline of tissue renewal capacity and the accelerated appearance of aging-related characteristics.
“We found that excessive genotoxic stress triggers differentiation of melanocyte stem cells,” said Emi Nishimura of Tokyo Medical and Dental University, who led the research team. Nishimura's team had previously traced graying hair to the gradual dying off of the stem cells that manufacture a continuous supply of new melanocytes, which are responsible for hair's youthful color. The study found that those specialized stem cells are not only lost, they also differentiate into fully committed pigment cells, and in the wrong place.
The study supports the idea that genome instability is a major factor in aging. It also supports the “stem cell aging hypothesis”, which states that DNA damage to stem cells can be attributed to many of the conditions that come with age.
“In this study, we discovered that hair graying, the most obvious aging phenotype, can be caused by the genomic damage response through stem cell differentiation, which suggests that physiological hair graying can be triggered by the accumulation of unavoidable DNA damage and DNA-damage response associated with aging through MSC differentiation,” the researchers wrote.
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