The researchers made adult guinea pigs grow new sound-sensing cells, called hair cells, in the spiral-shaped chamber called the cochlea.
Some 30 million Americans have significant hearing loss, and scientists say most of these cases -- perhaps more than 90 percent -- are due to lost or damaged hair cells. The cells can be damaged by aging, infection, loud noise, genetic conditions and exposure to certain medicines.
People normally develop about 16,000 hair cells in the cochlea of each ear, but they can't replace lost or damaged ones. The cells are critical to hearing because, using their hair-like projections, they convert sound waves into nerve impulses that go to the brain.
The new work is an early advance toward developing a therapy that might help restore hearing, said researcher Yehoash Raphael of the University of Michigan Medical School. He and colleagues present the results in Sunday's issue of the Journal of Neuroscience.
Edwin Rubel, who studies hair-cell regeneration at the University of Washington, called the results "a very, very important step. ... I wish I had done this study."
Raphael emphasized that the work is at an early stage and far from testing in humans. The researchers don't yet know how long the newborn hair cells can survive or even whether they can function. The scientists have just begun work to see if they can restore hearing to deaf guinea pigs.
For the reported study, Raphael and colleagues worked with a gene called "Math1," which must be active for a fetus to develop the initial supply of hair cells. In a surgical procedure, they squirted a solution containing Math1 genes into the cochleas of adult guinea pigs. The genes had been placed inside viruses, which acted like shuttles to get the genes into the animals' cells.
One and two months later, the researchers examined the cochleas of 14 treated animals. All showed immature hair cells, usually between 25 and 50. Apparently, the treatment had transformed some non-sensory cells into hair cells, Raphael said.
Many of the immature cells were outside the region where hair cells normally grow, so those clearly resulted from the treatment, Raphael said. Despite their odd location, it's possible that at least some of them might be able to function, he said.
Other immature-looking cells were mingled in with the animals' original hair cells, and it's not clear whether they were new, or whether they were simply original cells recovering from the trauma of the surgery. Raphael speculates they were new cells that resulted from the treatment.
The researchers were encouraged to see nerve fibers growing toward some of the immature cells. That indicates the nervous system might be able to hook up with the new cells and transmit their signals to the brain, Raphael said.
Without such connections, the new hair cells would be useless. But the observed response of the nerve fibers to the new cells "is strong circumstantial evidence there will be an interconnection between the two," said hair-cell expert Dr. A.J. Hudspeth of The Rockefeller University and the Howard Hughes Medical Institute.