About 37 million Americans have some level of hearing problem, and science hasn't come up with a perfect solution to restore this valuable sense.
Help may be on the way, at least in theory. A team of researchers reports in the journal Nature that they have used embryonic stem cells to restore some hearing in impaired gerbils. But more investigation is necessary before the technology can move to humans.
Everyone has two main sensory cell types associated with hearing: the hair cell and the auditory neuron. Hair cells take sound and make it into an electrical signal. Neurons pick up the signal and transfer it to the brain, so you know what you're hearing.
Most deafness is caused by a problem in one or both of these cells, said Marcelo Rivolta, senior author of the study and stem cell biologist at the University of Sheffield, United Kingdom. The cells are usually created during the embryonic stage of development.
"Adult mammals have lost the ability to replace these cells," Rivolta said. In other words, if these cells are damaged, you can't naturally grow them back.
Cochlear implants are electronic devices designed for people with hearing loss, but they don't work well in people with auditory nerve damage, Rivolta said.
How they did it
Researchers used a drug to chemically damage the auditory nerve in gerbils, creating a condition that would be called auditory neuropathy.
To see if hearing could be brought back, researchers used human embryonic stem cells, and applied biological factors to them that the human body would naturally use in its development. This coaxed them into becoming otic progenitor cells, which can differentiate into cells that function as hair cells and auditory neurons.
The scientists injected the progenitor cells into the gerbils to see if they would restore hearing function. Since you can't ask a gerbil what it hears, they used a technique called auditory brain stem response to determine if the animal was receiving auditory input. If the brain is hearing sound, it produces an electrical wave that the surface electrodes pick up.
Before the treatment, a human with the gerbils' level of impairment would only be able to hear loud sounds, such as a truck driving by, Rivolta said.
After the stem cell therapy, the gerbils' hearing was, on average, restored to the level that a person would be able to engage in conversation in a busy environment, he said. Scientists saw a range of success in the gerbils: some showed almost complete recovery, while others had a weaker response.
Eighteen of the gerbils received cell transplants, while eight did not. Those eight did not show any signs of recovery.
There's still a long way to go before this stem cell therapy is given routinely to humans. Even before it gets to clinical trials, the method must be tested further in animals to see whether the hearing recovery is permanent. Rivolta and colleagues only have data for 10 weeks of observing the animals; they would like to see what happens a year after the therapy.
This is "a proof of concept, a validation that stem cells can be used to repair the damaged ear," Rivolta said.