NEW YORK (Bloomberg) – Those grey 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 grey prematurely because stem cells in their hair follicles also matured too early, the scientists reported last week in the journal Cell. This caused them to lose their ongoing ability to make new melanin, the pigment that gives colour 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.

"Greying may actually be a safety mechanism, that's a cool twist," Fisher said in a telephone interview. "They've shown that this mechanism is actually removing damaged stem cells. The good news is if you do find yourself greying, you're probably better off not having those cells persist."

The findings, reported 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 grey. For this study, she and her colleagues exposed mice to radiation and drugs used in chemotherapy, then monitored changes in the colour 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 greying hair. A similar mechanism may operate in people, she said.

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.