The U.Va. scientists found that three essential elements for development in the mouse inner ear appear between day 16 and day 17 of gestation, roughly equivalent to the late second trimester or early third trimester in the human fetus. The finding is important for ongoing research on regeneration of sensory hair cells in the human inner ear, say researchers, writing in the October edition of Nature Neuroscience, found online at www.nature.com/neuro.
"We were surprised that development of hair cells in the inner ear takes place so rapidly," said researcher Jeffrey R. Holt, an assistant professor of neuroscience and otolaryngology at U.Va. "Suddenly, the hair cells began working. To eventually discover how to regenerate hair cells in the human ear, we have to know when and how the original hair cells develop. That's why this research is so central to our knowledge."
The next challenge for scientists is to discover the molecular "switches" that turn on inner ear hair cells. "Scientists at U.Va. and elsewhere are working to test stem cells to see if they can develop into hair cells," Holt said. "If we can find the molecular process, we can potentially turn another cell type into an inner ear hair cell." The researchers said they are now assembling a list of processes these important cells go through to develop correctly. "We know the sequence," said study co-author Gwénaëlle Géléoc, an assistant professor of neuroscience and otolaryngology. "Now we can look at different cell types and see if they are on the right track to produce hair cells."
In the lab, Holt and Géléoc found that hair cell transduction in mice, or the response to movement of hair bundles associated with hearing, begins to function over a 24 hours period, starting at embryonic day 16.
Interestingly, all three essential elements develop simultaneously: membrane-bound transduction channels, like trap doors, are formed to carry calcium and potassium which together create an electrical charge sending hearing and balance signals to the brain; microscopic tip-links are formed that operate under tension to open the trap door channel; finally, tiny adaptation motors are formed that regulate sensitivity and allow sounds that range from a faint whisper to a booming cannon to be heard.
The authors of the study said their work may mean a better understanding of congenital hearing and balance deficits in humans.