Humans have about 15,000 inner ear-hair cells, each one picking up sound vibrations, converting them to electric signals and sending them to the brain for processing.

Over time, loud noise, medications and old age combine to kill these cells — and their microscopic hairs called stereocilia — which leads to hearing loss. Unlike other animals, however, humans and mammals can't regrow them. But a group of scientists based in Boston say they've figured out a way to switch on the body's cellular factories and possibly reverse hearing loss.

"The biology is there, we just need to awaken it," said Jeffrey Karp, associate professor at Brigham and Women's Hospital, Harvard Medical School and an author on the new study appearing Tuesday in the journal Cell Reports. "For some reason there are brakes that we need to release for a short period of time to allow new hair cells to be produced."

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Karp and colleagues were able to regrow the hair cells by activating a stem cell in the cochlea called Lgr5 with a small molecule drug treatment. A similar stem cell is found in the human intestine and allows the body to regrow the exterior lining of the organ every five days.

The team also obtained a human cochlea from a patient who suffered from cancer and were able to regrow hair cells with their drug treatment.

"We don't want to provide false hope, but we are highly encouraged by this work. And our ability to produce bona fide functional hair cells is very compelling," Karp said.

The next step is taking the experimental data and starting a human clinical trial. Karp and Robert Langer of MIT are co-founders in a small startup firm, Frequency Therapeutics, that's working toward a phase I trial in the next 18 months, according to Karp.

A possible drug treatment for hearing loss could help the 360 million people worldwide who suffer from the condition.

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"Their proposal is very novel and essentially by activating these supporting cells, a natural process will take over and a certain percentage would become hair cells capable of playing a role in the encoding of sound," said Nicolas Reed, an instructor in otolaryngology at the Johns Hopkins School of Medicine. "I don't see any obvious negative indications right now."

Hearing loss can lead to big problems as we age, including the onset of Alzheimer's disease, falls and social isolation, according to Larry Medwetsky, chairman of our Department of Hearing, Speech and Language Sciences at Gallaudet University.

"It is not a minor matter," Medwetsky said. "Hearing loss can affect you mentally and emotionally. If you can prevent or remediate it than you can also you can restore quality of life and avoid some of these issues."

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