>> british researchers gave the animals shots of stem cells taken from human embryos. the bbc's fergus walsh shows us how it works. >> reporter: these nerve cells under the microscope are what researchers hope one day will reverse deafness. they were created from human embryonic stem cells which have the ability to turn into any tissue. the scientists are trying to tackle a form of deafness that affects one in ten people. with profound hearing loss. in the condition, nerve cells in the cochlea, inner ear are damaged preventing sound from traveling along the auditory nerve to the brain. like cutting a telephone wire. in the lab, they grew stem cells, smaller than a pinhead and grew them into healthy replacement nerve cells and injected these into 18 deaf gerbils, considered a good animal model for human hearing. tests showed on average, about 45% of their hearing was restored.