Some people have hairy ears, but everyone has so-called hair cells in their ears. The key to hearing, these cells sport hairlike projections named stereocilia, which bend in response to sound waves. Scientists have now discovered that stereocilia are in a state of continuous regeneration. Each projection seems to replace its molecular components every 48 hours.
The researchers, who report their work in the Aug. 22 Nature, suggest that their finding may explain why it takes a day or so for normal hearing to return after exposure to abnormally loud sounds, such as music at a rock concert.
“Every 2 days, the whole stereocilia seems to renew its backbone. That time-course is comparable to the time-course of recovery from temporary hearing loss,” says coauthor Bechara Kachar of the National Institute on Deafness and Other Communication Disorders in Bethesda, Md.
To probe how stereocilia arise, Kachar and his colleagues kept the excised ears of newborn mice alive in laboratory dishes. They also created a gene that encodes a fluorescent version of the protein actin, a component of stereocilia. When they added this gene to the lab-grown hair cells, the researchers observed glowing actin being added to the tips of stereocilia. The stereocilia eventually incorporated the protein throughout their whole length. That suggests that newly added actin gradually moves from the tip to the base. “It’s like a molecular treadmill,” says Kachar.
The investigators suggest that some age-related deafness in people may stem from problems with stereocilia regeneration.