Until 1987, scientists thought that only amphibians and certain fish could regenerate the tiny sensory cells in the inner ear that are responsible for hearing and balance. In those animals, the tiny "hair cells," as they're called, can be regenerated throughout life in response to injury from loud noise or toxic substances.
Birds and mammals, on the other hand, were thought to be incapable of regenerating these vital structures during their adult lives. They were believed to have their full adult complement of hair cells at birth, and any loss of hair cells thereafter led to irreversible impairment.
In 1987, however, the finding by UW researcher Edwin W. Rubel that birds can regenerate hair cells, restoring lost hearing and balance, spurred a new wave of intense research activity, with the ultimate hope of finding a way to do the same in humans.
Deafness, notes Rubel, is the second most common handicap in the United States. And hearing loss resulting from hair cell damage brought about by noise, infection, toxins, or age-related causes is the most common form of hearing loss today. Rubel is a professor of otolaryngology, physiology and biophysics, neurological surgery, and psychology with the Virginia Merrill Bloedel Hearing Research Center at the UW.
Balance disorders also are widespread; 30% of all Americans experience episodes of dizziness by the age of 65.1 Consequently, methods to replace lost hair cells, either through transplantation or regeneration, are eagerly sought.
Two serendipitous discoveries led to the realization that birds could regenerate hair cells. Rubel and coworkers had set out to study not regeneration, but rather the onset, rate, and mechanism of hair cell degeneration in baby chicks after they were given the antibiotic gentamycin. They measured the hair cell counts in chicks treated with gentamycin compared to age-matched controls, from 1 day to 3 weeks after treatment. When gentamycin-treated chicks were allowed to live 2 weeks longer than that, the researchers found that hair cells were actually regenerating.
In an independent study elsewhere at about the same time, a researcher studied chicks after acoustic trauma and found that hair cells were growing back several days after the exposure to noise.
Recent studies by Rubel and colleagues with a variety of animals suggest that partial hair cell regeneration in mammals may be possible. Rubel finds evidence that cells in a particular region responsible for balance in the ear may be capable of renewed cell division following damage to the sensory tissue. Furthermore, it may be possible to enhance this regeneration process by administering growth hormones.
UW researchers Hiroshi Yamashita and Elizabeth Oesterle tested several growth factors on cultured balance-sensory tissue from mice and found they could stimulate the proliferation of cells. But much more research will be needed before a treatment for hearing loss in humans is realized. The first step will be to develop methods of culturing the hearing-sensory tissue in the laboratory. Then, if that hurdle can be overcome, the next challenge will be to try to stimulate the growth of those hearing cells.