There may be hope for the most common type of hearing loss. Researchers at the University of Michigan have succeeded in growing new hair cells in the inner ears of laboratory animals, the first time that such cells have been regenerated in mammals.Hair cells are microscopic reed-like structures within the cochlea of the inner ear. They translate hydraulic pressure waves into nerve impulses, which are carried by auditory nerves into the brain. Mammals are born with a fixed number of hair cells, which do not regrow once they have been damaged by loud noises, infections, or by some drugs. Approximately 90% of the 30 million Americans with hearing loss suffer from hair cell damage.It has long been known that birds and sharks regrow hair cells, but until Wei-Qiang Gao at Genetech's Department of Molecular Oncology in South San Francisco, CA, grew rat hair cells in vitro, no one had successfully done so with mammalian tissue. In an experiment based on studies showing that a gene called Math 1 could trigger immature inner ear cells to become hair cells, Gao infected tissue taken from newborn rats with a virus carrying the Math1 gene. In 2000, in concert with his colleagues, Gao reported that the tissue had grown what appeared to be new hair cells.
Would the technique work on adult animals? Extrapolating from these results, Yehoash Raphael and colleagues at the Kresge Hearing Research Institute in the Department of Otolaryngology at the University of Michigan in Ann Arbor injected a virus carrying the gene into the inner ear fluid of guinea pigs. Weeks later, the scientists extracted tissue slices from the animals' ears and saw what looked like hair cells sprouting within the organ of Corti (the sound-sensing part of the cochlea), in places where they don't usually occur. They also found nerve fibers outside the organ, extending toward the new hair cells—suggesting that the appearance of new hair cells might trigger an integrative response from the auditory system.Of course, it's a long way from initial experiments with rodents to creating reliable therapies for humans. Sometimes, promising results in the laboratory ultimately go nowhere, but Genetech's Gao describes the work done so far as "another major step toward hair cell regeneration in the human ear." The work of Raphael's team was reported in the June 1 issue of the Journal of Neuroscience. A report on Gao's and Raphael's work appeared in the June 7 edition of Science News.
