If you've ever been warned about listening to too much loud music, then you may have heard that the sound-processing cells in our inner ear are killed by loud noises … and they don't regenerate. Well, it's true. Known as hair cells, they detect sound waves and convert them into nerve signals. We start out with only about 15,000 of them in each cochlea, and once any of them are gone, they're gone for good. There may now be hope for restoring that lost hearing, however, as scientists have reported a new method of regrowing hair cells in substantial numbers.
Along with loud sounds, hair cells are also destroyed by certain medications, or just die off as we age. In the case of animals such as birds and amphibians, however, those cells do grow back. Inspired by that fact, a team of researchers from Brigham & Women's Hospital, MIT and Massachusetts Eye & Ear set out to see if the same could be done with human hair cells.
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Using previous research on regenerating intestinal cells as a jumping-off point, they placed immature cochlear progenitor cells from mice in a lab dish – progenitor cells are like stem cells, in that they can convert into other types of cells. The researchers then added a drug "cocktail," which caused those cells to rapidly multiply. Once a sufficient number of those progenitor cells were grown, they were then stimulated with additional drugs to differentiate into mature hair cells.
When the procedure was attempted on an extracted mouse cochlea, the second step wasn't needed, as the progenitors were naturally signalled to differentiate. The process (which was also successfully tried using human cells) produced about 60 times more hair cells than the existing next-best technique, in which progenitor cells were prompted to differentiate, but a sizeable population of them wasn't grown first.
The scientists now believe that treatment for hearing loss could be as simple as administering an injection into the ear. To that end, they have formed a spinoff company to commercialize the technology, and hope to begin clinical trials within 18 months.
A paper on the research was recently published in the journal Cell Reports.
"Hearing loss is a real problem as people get older," says MIT's Prof. Robert Langer, one of the senior authors. "It's very much of an unmet need, and this is an entirely new approach."