June 11 (Bloomberg) -- Those gray hairs on top of older noggins may be a sign of stress from exposure to chemicals, radiation and the ravages of aging, Japanese researchers said. The silver strands come with a benefit: protection from cancer.

The fur of mice dosed with radiation or given various chemicals turned gray prematurely because stem cells in their hair follicles also matured too early, the scientists reported today in the journal Cell. This caused them to lose their ongoing ability to make new melanin, the pigment that gives color to hair and skin.

The stem cells of the irradiated mice stopped copying themselves. This may be a way of preventing cells whose DNA has been damaged by toxins from becoming cancerous, said David Fisher, chief of the department of dermatology at Harvard Medical School-affiliated Massachusetts General Hospital.

“Graying may actually be a safety mechanism, that’s a cool twist,” Fisher said in a June 9 telephone interview. “They’ve shown that this mechanism is actually removing damaged stem cells. The good news is if you do find yourself graying, you’re probably better off not having those cells persist.”

‘Far-Reaching’ Consequences

The findings, reported today in the journal Cell, have “far-reaching” consequences and may suggest that early maturation and differentiation in other groups of stem cells help prevent cancer, Fisher said. Fisher wasn’t involved in the current findings, though he worked with the lead author of the study, Emi Nishimura, at Harvard before she left for Kanazawa University in Kanazawa, Japan.

Normally, stem cells keep making copies of themselves while also differentiating into other cell types, Fisher said. The cells of the irradiated mice matured and stopped replicating, eventually leaving the mice with no way to create pigment in their fur.

Nishimura had previously discovered the stem cells within hair follicles and showed that their depletion during aging causes hair to turn gray. For this study, she and her colleagues exposed mice to radiation and drugs used in chemotherapy, then monitored changes in the color of their fur as well as the status of their stem cells.

By looking at the hair follicles under microscopes, they saw when the stem cells turned into other cell types and linked the change to the graying hair. A similar mechanism may operate in people, she said.

Challenge to Theories

The findings challenge existing theories about how the body tries to protect itself when it suffers genetic damage from radiation or other toxins, Nishimura said yesterday in a telephone interview.

“People have speculated that cells die by apoptosis” -- a scientific term for cell suicide -- “if their DNA is damaged,” Nishimura said. This would prevent damaged cells from growing uncontrollably as tumors, she said.

Her findings suggest the body has another way to protect itself, Nishimura said.

“Probably the tissue is trying to get rid of risky stem cell populations which have a lot of DNA damage,” she said.

People are constantly exposed to a range of toxic agents that can damage their DNA from household and industrial chemicals, ultraviolet radiation from the sun, X-rays and low levels of radiation from high-altitude flights on airplanes, Nishimura said, and these can have a cumulative effect on the body.

Nishimura’s study “implies that age-related graying could be a result of accumulated DNA damage,” Fisher said. It will take further experiments to prove the theory and to demonstrate that what’s true of stem cells in hair follicles is true of other types of stem cells.

Nishimura is planning other experiments to investigate these possibilities, she said.

To contact the reporter on this story: Rob Waters in San Francisco at rwaters5@bloomberg.net.

Last Updated: June 11, 2009 12:00 EDT