This might shock some people but technology has come so far that it can allow hearing loss to be reversed, in theory. Scientists know that after a fish loses its hearing due to physical trauma or violent noise, the sensory hair cells in its ears can auto-regenerate. In fact, this ear repair is almost universal in vertebrates, species that have spinal cords. Birds and frogs share it. However, only in mammals, hair cell death is 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.

Our hearing arises from a tiny field of swaying cilia deep in the skull. Four rows of hair cells grow in the cinnamon roll-shaped cochlea of the inner ear, which is filled with fluid. Sound vibrations cause them to bend, opening pores that activate electrical signals headed to the brain. We are born with 15,000 hair cells in each ear, but unlike skin or other cell types, they do not re-generate 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 loud noise. A loud sound can permanently bend or physically damage a fragile hair cell, rendering it ineffective.

Each hair cell responds best to a particular frequency of sound—they are arranged in order of frequency along the cochlea—so scientists can pinpoint the effect of these new cells on hearing. Dutch company Audion Therapeutics is working on a proof of concept for regeneration of human ear hair cells.