Scientists have successfully grown auditory hair cells in the lab - raising the prospect of new treatments for deafness and age-related hearing loss.
Sensory cells cannot be naturally regenerated
Healthy hair cells are vital to the ability to hear, but they tend to be lost as we get older, and can be destroyed by infection and certain medications.
Scientists discovered in the 1980s that birds can spontaneously regenerate damaged hair cells - but mammals do not have the same ability.
Now a team at the University of Michigan have used gene therapy to grow new cells in adult guinea pigs.
They did this by inserting a gene called Math1 into non-sensory epithelial cells lining the inner ear.
Once the gene was in place, the cells began to generate new sensory hair cells.
The scientists have yet to determine whether these new cells are fully functional, but they did find evidence that they had stimulated growth of auditory nerve fibres towards them.
In a normal ear, vibrations from sound waves striking the eardrum are transferred to fluid inside a snail-shaped bony organ called the cochlea.
When cochlear fluid moves, it stimulates movement in thousands of tiny projections on hair cells lining the inside of the cochlea.
Moving hair cells initiate electrical signals, which are picked up by auditory nerve fibres and carried to an area of the brain called the auditory cortex.
Nerve fibre (green strand) growing towards hair cells (red stains)
If hair cells are damaged or missing, electrical signals are not generated and hearing is impaired.
The scientists injected a virus containing the Math1 gene into the inner ear of 14 adult guinea pigs.
Electron microscope analysis carried out 30-60 days after the procedure showed that new hair cells had begun to grow in areas where hair cells are typically absent.
Lead researcher Professor Yehoash Raphael said: "The inner ear is an ideal target for gene therapy, because it is closed - not sealed, but nicely isolated.
"As long as the amount you inoculate is small, the spread to other organs is minimal."
At present, the injection required to deliver the new gene to cells does cause a small amount of tissue damage.
But the scientists hope to perfect the technique, and believe that it would cause less damage to the human ear, which is considerably larger than that of a guinea pig.
The next stage of the research will be to test the procedure in aging animals and in animals that are completely deaf.
Dr Raphael said: "This is just the beginning.
"It is really just a proof of the principle to show that, with proper gene therapy, these non-sensory cells have the competence to become hair cells."
Dr Ralph Holme, of the Royal National Institute for the Deaf, said: "The prospect of regenerating damaged or lost hair cells is very exciting.
"It could offer a new approach to improving the hearing of deaf and hard of hearing people in the future.
"Whilst it has great potential, there are still major hurdles to overcome before fully working hair cells can be re-grown in humans.
"Professor Raphael's research marks a significant step towards this goal.
"Indeed, RNID recognises the great possibilities of this technology, and we are also funding research into hair cell regeneration."
The research is published in Journal of Neuroscience.