Scientists have restored hearing to deaf gerbils using human embryonic stem cells in an advance that could eventually help people with an intractable form of deafness caused by nerve damage.

The procedure needs further animal research to assess safety and long-term effectiveness but researchers said on Wednesday the experiment was an important proof of concept, marking a further advance in the growing field of regenerative medicine.

Marcelo Rivolta from Britain's University of Sheffield, who led the research, said the first patients could receive cell therapy for hearing loss in clinical trials in "a few years".

After treating 18 gerbils with complete deafness in one ear, his team reported in the journal Nature that stem cells produced an average 46 percent recovery in hearing function, as measured by electrical signals in the animals' brains.

"If this was a human patient, it would mean going from being so deaf as to be unable to hear a lorry or truck on the street to being able to maintain a conversation," Rivolta told reporters.

"What we have shown here is functional recovery using human stem cells, which is unique."

Gerbils were selected for the test because their hearing range is similar to that of humans, while mice - the usual choice for laboratory tests - hear at higher frequencies.

The animals were deafened using a drug to destroy their auditory nerves before receiving an injection of around 50,000 human embryonic stem cells, which had previously been treated with growth factors to coax them into becoming ear cells.

The response among the gerbils varied, depending on how well the new cells were integrated into the cochlea, the spiral-shaped cavity in the inner ear.

Deafness is caused primarily by loss of sensory hair cells in the ear and auditory nerves. Since these cells are created only in the womb, there is no way to repair them once they have been damaged, resulting in permanent hearing loss.

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Deaf gerbils hear again with human stem cells