A US study has raised hopes that one day researchers will be able to 'restore' specialised hair cells in the ears of humans suffering from deafness.
Loss of tiny hairs in the inner ear leads to permanent hearing damage
Tiny hairs in the ear detect vibrations which the brain translates into sound, but once lost they cannot be replaced.
However, a mouse study published in Nature has shown that certain cells taken from the inner ear can in fact differentiate into sensory hair cells.
Around one in 24 people worldwide suffer from disabling hearing loss.
In birds, hair cells which are damaged are replaced by new cells that grow from other neighbouring cells, restoring hearing within weeks.
These nearby 'supporting cells' are present in mammals but do not replace hair cells once they are lost.
A team at the House Ear Institute in California developed a way of making these cells glow green in mice so they could separate out them from other types of cell.
They then tested whether they would divide and differentiate in petri dish environment designed to mimic the ear.
Cells from the ears of newborn mice were able to divide and differentiate into hair cells.
In two-week old mice the cells had lost the ability to divide but were still able to differentiate.
The researchers discovered a gene called p27 was switched on in the majority of cells by 14 days which halted the cell cycle and stopped them dividing.
They said the gene could be a potential target for turning on cell division, replenishing cells and reversing hearing loss.
Lead researcher Dr Neil Segil, said: "And so it breaks down into two problems now - one is how you control the cell division and one is how you control the transdifferentiation."
Professor Andy Forge, professor of auditory cell biology at University College London said: "This research tells us that the supporting cells in mammals have not totally lost the ability to undergo cell division.
"That at times when normally in the body the cells cannot be induced to divide, this is not because that capacity is lost as the cells reach maturity; the capacity is retained but normally it is suppressed.
"This suggests that it might be possible to find ways switch it on again and it may provide clues to ways in which hair cells regenerate in the inner ear of mammals, including of course humans."
However, he added that he would like to see the work repeated with adult cells.
Professor Mark Lutman, professor of audiology at the Institute of Sound and Vibration Research at the University of Southampton said: "The only similar cells have the been made to regenerate in mammals are vestibular hair cells so it's not the first time mammalian hair cells have been made to regenerate but it's the first time in the ear.
"In terms if therapeutic applications, that's definitely going to be a long way ahead - 10 or 20 years.
"The issue is going to be the control of reproliferation and these mechanisms are not fully understood yet."