Research paves the way for cure for deafness
The research, which involves regenerating the sensitive hair cells that turn sound vibrations into nerve signals, was described as "really exciting" and could benefit millions of people.
Humans are born with 30,000 hair cells in each ear.
When the cells are lost or damaged – possibly due to exposure to excessive loud noise or injury – 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, 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 hairlike 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.
"RNID-funded research has shown that human stem cells can also give rise to hairlike cells, an important step forward in developing a clinically relevant therapy.
"We are now supporting research to investigate whether hearing can be restored using these cells in preclinical models of deafness and to find ways of scaling up the production of safe, clinical-grade cells."
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."
The Stanford research, the first to create functional inner-ear cells, is reported in the journal Cell.
Prof Heller's team used both mouse embryonic stem cells and artificially "induced" stem cells made by reprogramming ordinary skin cells.
Embryonic stem cells, removed from early-stage embryos, are "mother" cells with the ability to transform into virtually any kind of tissue in the body. Induced stem cells have similar "pluripotent" properties.
In both cases, the cells were exposed to special cocktails of chemicals that caused them to pass through a range of development phases normally seen in the womb.
"We looked at how the ear develops in an embryo, at the developmental steps, and mimicked these steps in a culture dish," said Prof Heller.
"These cells have a very intriguing structure. They look like they have hair tufts of stereocilia."
Inside the fluid-filled inner ear, hair cells respond to currents set up by the vibrating ear drum via a set of tiny bones.
The movements trigger electrical nerve impulses from the cells that are transmitted to the brain.
A similar property was observed in the lab-manufactured cells.