Stem cell scientists have created inner ear sensory cells in the laboratory, paving the way to a cure for permanent deafness.
A leading charity has described the research as “really exciting” and said it had the potential to benefit millions.
The work holds out the prospect of regenerating the sensitive hair cells that turn sound vibrations into nerve signals.
Humans are born with 30,000 hair cells in each ear.
When the cells are lost or damaged — possibly due to excessively loud noise — it can lead to permanent hearing loss or tinnitus (ringing in the ears).
Damage to hair cells may also affect balance, causing symptoms of vertigo and dizziness.
Regenerating the sensory hair cells of the inner ear has been the holy grail of deafness research.
The new breakthrough is the culmination of 10 years' work by scientists in California.
A team led by Professor Stefan Heller, from Stanford University School of Medicine, has succeeded in programming mouse stem cells to develop into immature hair cells.
Viewed under an electron microscope, they were seen to have bundled structures reminiscent of the hair-like tufts of “stereocilia” that give the cells their name.
“They really looked like they were more or less taken out of the ear,” said Prof Heller. Most importantly, tests showed that the cells responded to being moved the way hair cells do, by converting mechanical signals into electrical ones.
Experts hope the cells, which could be made in large numbers from multiplying stem cells, will provide an invaluable research tool for studying the molecular basis of hearing and deafness.
Further down the line, they may also help scientists find a way of coaxing a patient's hair cells to renew themselves.
The research is already being taken forward by scientists supported by the Royal National Institute for Deaf people (RNID).
Dr Ralph Holme, head of biomedical research at the charity, said: “The possibility that stem cells could one day be used to restore hearing is really exciting and could benefit millions.”
US expert David Corey, Professor of Neurobiology at Harvard University in Boston, said: “This gives us real hope that there might be some kind of therapy for regenerating hair cells.
“It could take a decade or more, but it's a possibility.”