Gray hair caused by stress

That's because DNA is “under constant attack” by damaging agents, such as chemicals, ultraviolet light and ionizing radiation, according to study lead author Emi Nishimura of Tokyo Medical and Dental University.

A single mammal cell can encounter up to 100,000 events a day that damage DNA, Nishimura added in a statement. The stem cells within hair follicles responsible for color are the most impacted by this irreparable DNA weathering.

Stem cells are cells in the body that can reproduce indefinitely and that have the potential to “mature” into other, more specialized cells. The stem cells in hair follicles mature into melanocytes, or cells that produce the pigment melanin.

In younger people, the hair's stem cells maintain a balance between those that reproduce and those that turn into pigment cells, so that pigment is constantly being added to growing hair. But as a person ages, too many of the stem cells mature until the pool of pigment cells gets totally drained and hair grows gray.

Scientists have been unsure what exactly spurs the stem cells to change. According to Nishimura, the answer may be accumulated DNA damage.

Forcing the cells to mature may be the body's “more sophisticated way” of purging the damaged stem cells without killing them off, she said.

Unavoidable Damage

The study focused on graying because it is a typical sign of aging in mammals, the authors wrote.

The researchers put laboratory mice through whole-body x-rays and chemical injections. When the team examined the mice's hair follicles, they found that the stem cells showed permanent damage. These mice then regrew hair with no pigment.

The research supports the idea that instability in genes may be a major factor in aging, the authors say. It also lends credence to the theory that damage to stem cells may be the main driver of aging.

The DNA damage observed in the study is mainly “unavoidable,” the authors write.

Linzhao Cheng, a member of the Johns Hopkins Institute of Cell Engineering, agreed that it's tough to avoid stem cell damage — especially for people who spend a lot of time outdoors, which exposes them to ultraviolet radiation from the sun.

However, the study helps scientists understand graying, Cheng said by e-mail, which may lead to new chemicals that can prevent the hair's stem cells from switching roles.

“We may soon have anti-graying creams for aging populations,” he said.

The study appeared in the journal Cell.

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