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upposed hair-loss cures abound  everything from magnetic resonance imaging to enzyme blockers are marketed to men and women who lose their hair as they age. But the underlying mechanism behind hair thinning has never been sussed out.

Now, two new studies point a finger at the scalp’s constantly-renewing stem cells. Mutations in these cells appear to make hair follicles shrink, causing regular hair to be replaced by thin wisps. Understanding the genes at work in this process may allow scientists a way to prevent such hair loss.

Researchers reached that conclusion after studying hair loss in mice and in humans. Hiroyuki Matsumura, assistant professor at Tokyo Medical and Dental University, and colleagues focused on hair follicle stem cells, self-renewing cells within the hair follicle. Over time, they found, the stem cells accumulated genetic errors that led them to not be able to rejuvenate any longer. This in turn caused hair follicles to shrink.

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However, one key gene could reverse this hair loss. When mice were genetically modified to make extra of the Col17A1 protein, their hair follicles didn’t shrink and they didn’t lose nearly as much hair.

Researchers then analyzed skin samples from the scalps of women aged 22 to 70 years old. They found that older women’s hair follicles were, on average, more shrunken, and that this was correlated with genetic mutations in the follicle stem cells. The findings were published Thursday in Science.

Once stem cells go, there’s no getting them back, said Elaine Fuchs, a professor of cell biology and development at the Rockefeller University in New York, who was not involved with the study.

“Once the hair growth cycle goes, it goes,” she said. “Once hair loss is triggered, it’s a self-propelling event.”

But a related study offers a way to possibly intervene to save stem cells before they lose their growth cycles.

Researchers at the University of Colorado studied how hair follicle stem cells go dormant. They report in a companion paper in Science that a protein called Foxc1 keeps them dormant; when this gene is deleted, the follicles stay permanently “on.”

“We discovered that these cells ‘know’ when to stop,” said lead author Rui Yi, associate professor at the University of Colorado.

That may present a treatment opportunity. “If we can interfere with that factor, or take that factor away from the stem cell, we can shorten the dormant stage and instead induce the cells to continue dividing and stimulate hair growth,” Yi said.

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