Dr Marcelo Rivolta, who led the research team at the University of Sheffield, said: 'What we have shown here is functional recovery using human stem cells, which is unique'. This is the first time that functional recovery has been seen using stem cells in the ear, marking an important proof of concept.
In response to sound, auditory nerve cells send signals from the ears to the brain. Of the ten million deaf people in the UK around 15 percent have a condition called auditory neuropathy, where there is a break down in these neural connections. Restoring auditory nerve cells using stem cells may therefore have the potential to help those with this condition.
Scientists transformed human embryonic stem cells into early stage auditory nerve cells using a variety of chemicals. These nerve cells were then implanted in the inner ears of gerbils who had been given a drug to destroy their auditory nerves and make them deaf, mimicking the condition of auditory neuropathy. Gerbils were chosen, instead of the more commonly used mice, as they hear sound in a similar range of frequencies as humans.
Ten weeks after treatment the animals' hearing was tested. On average, the animals who received the treatment regained 46 percent of their hearing, compared to no recovery seen in the animals that did not receive the treatment. The improvement was variable with some animals showing no improvement and others regaining up to 90 percent of their hearing, dependent upon the number of nerve cells that were successfully implanted into inner ear.
Dr Rivolta put the recovery into context, saying: 'If this were a human patient, it would mean going from being so deaf that you wouldn't be able to hear a lorry on the street to a point where you can maintain a conversation. It's not a full restoration and the restoration is very variable but, on average, that is the kind of recovery we see'.
Technical challenges need to be overcome if the technology is to be successful in humans. Professor Dave Moore, director of the Medical Research Council's Institute of Hearing Research in Nottingham, told the BBC: 'The biggest issue is actually getting into the part of the inner ear where they'll do some good. It's extremely tiny and very difficult to get to and that will be a really formidable undertaking'.
Dr Ralph Holme, head of biomedical research for the charity Action on Hearing Loss who partly funded the study, said: 'The research is tremendously encouraging and gives us real hope that it will be possible to fix the actual cause of some types of hearing loss in the future'. Reported in Reuters, Dr Rivolta suggested the therapy could reach clinical trials in humans in a few years time.