After a songbird loses its hearing due to physical trauma or loud noise, the sensory hair cells in its inner ear regenerate naturally. The healed bird can use its restored hearing to decode complex songs from other birds. In fact, this ear repair is almost universal in vertebrates. Fish and frogs share it. Only for mammals is hair cell death irreparable.
More than a third of seniors suffer from at least moderate hearing loss, and while hearing aids have improved over the years, no drug currently exists on the market to recover their lost hearing. In the lower vertebrates like fish and birds, those new hair cells arise from the supporting cells of the cochlea’s lining. Now, for the first time, scientists are developing methods that could do the same for humans.
We owe our hearing to a tiny field of swaying cilia deep in the skull. Four rows of hair cells sprout in the snail-shaped cochlea of the inner ear, which is filled with fluid. Sound vibrations cause them to bend, opening pores that activate electrical signals bound for the brain. We are born with 15,000 hair cells in each ear, but unlike skin or other cell types, they do not turn over or replenish themselves. Loss of these hair cells over time accounts for much of the age-related hearing loss around the world, as well as that caused by too much loud noise. A loud sound can permanently bend or physically prune a fragile hair cell, rendering it ineffective.