Scientists have managed to manufacture key ear cells in the laboratory - raising hopes of treatments for age-related deafness.
Hair cells grown by the Harvard team
So-called hair cells help pick up sound vibrations and turn them into nerve signals - their destruction over the years is a common cause of deafness.
A Harvard University team has found a way of producing a supply of these from embryonic cells.
It could eventually help produce drugs to improve hearing in many patients.
Four out of five people aged over 65 have some kind of hearing loss, and this is caused, in many cases, by loss of hair cells inside the ear.
People exposed to very loud noise on a frequent basis can lose their hearing for the same reason at a much younger age.
Until relatively recently it was thought little could be done to help them, as the body cannot repair or replace any of the 15,000 hair cells in each ear.
Grown to order
The Harvard team carried out experiments on stem cells from a mouse embryo.
Stem cells are essentially the master cells of the developing foetus - and, given the right conditions and stimulation, be transformed to form any type of cell in the body.
Scientists are just beginning to discover how they can take an embryonic stem cell and manipulate it to produce various different cell types.
In this case, the stem cells were exposed to chemicals known to be involved in the development of the inner ear in the foetus.
The scientists found hair cells were produced - and when they had been inserted into the developing ears of chicken embryos, they developed in an apparently correct way, producing body chemicals consistent with normal hair cells.
Their study, published in the journal Proceedings of the National Academy of Sciences, is a significant step forward towards treatments for many age or damage-related hearing problems.
Hope for drug
But a British expert warned it could still be many years before a treatment emerged.
Professor Matthew Holley, from the University of Sheffield, told BBC News Online: "I'd say that if you are 65 years old with hearing damage today, it won't arrive in time for you.
"However, children born now with these problems might reasonably expect this research to make a difference for them at some point."
He said the idea of physically transplanting new hair cells into the ear was fraught with difficulty.
Each tiny cell would have to be placed precisely in the right spot on protein bands stretched across the inner ear to restore a decent range of hearing, he said - a few microns error could make the whole exercise worthless.
But he said understanding how cells could be transformed into hair cells might potentially allow the development of drugs which could perform the same trick - but on adult ear cells instead.
He said: "These would already be in the right place.
"This research is helping us understand the mechanics involving in producing these cells."
He added: "This is now theoretically possible - we are moving from a situation where there are impossible obstacles to overcome to one of solving practical problems, albeit large ones."