For the first time ever, tiny hairs in the ear which detect sounds have been regenerated to reverse deafness for the first time, say US researchers in the journal Neuron.
An injection of a drug led to the creation of new hairs in tests on mice. Normal hearing was not restored, rather the mice went from hearing nothing to detecting sounds such as a door slamming or traffic. Experts said it was "tremendously exciting" but warned treating humans was still a distant prospect.
So, what's the science behind hearing? To hear anything sound waves have to be converted into electrical signals which the brain will understand. The first step in the process takes place deep inside the inner ear where vibrations move tiny hairs and the movement creates an electrical signal. Most hearing problems are as a result of damage to these hairs.
The study, by Massachusetts Eye and Ear and Harvard Medical School, looked at mice which were completely deaf and had virtually no hairs remaining in their ears according to the BBC reporter James Gallagher.
A drug was used to target cells which normally support the individual hairs and it changed the destiny of those cells by altering which genes were being used in the cells to transform them into hair cells.
One of the researchers, Dr Albert Edge, said: "It hasn't been possible to regenerate hair cells in adult mammals before, this is very exciting. It shows for the first time that it's possible."
Brain scans showed that some sounds could be heard.
Dr Edge added: "There was a slight improvement, but not a huge improvement.
"They can detect a loud noise in a low frequency, something like a door slamming or traffic - but this is definitely not normal hearing.
He said: "It's a really promising development, but it is one which needs to be treated with considerable caution in terms of a human therapy.
"There's been a lot of false starts - hair cell regeneration was originally demonstrated in the 1980s and everyone thought it would just be a matter of years."
He said it was an exciting first step, but there was still a huge challenge ahead to develop a useable treatment.
Dr Ralph Holme, head of biomedical research at the charity Action on Hearing Loss, said: "The idea that a drug could be used to 'trick' the cochlea into producing new hair cells to improve hearing is tremendously exciting and offers real hope to the millions of people seeking a cure for their hearing loss.
"But, it is important to remember that this research is still at a very early stage and that only a partial recovery in hearing was observed. Another study being worked on with deaf gerbils and stem cells has proven to be successful. UK researchers say they have taken a huge step forward in treating deafness after stem cells were used to restore hearing in animals for the first time. We are all eagerly awaiting a time when the results will have a more clear impact on humans.