Loss of hearing due to old age impacts a great many people – often its cause is to be found in the depletion of sensory receptors known as hair cells. We are born with roughly 15,500 of them, but they don’t regenerate over time. Loud noises, infections, toxins, and the natural aging process harm them and we can end up losing our hearing.

However, scientists might now have found a way to regenerate them. In the European Journal of Neuroscience, researchers showed that they can regrow hair cells in the cochlea of mice. This important breakthrough was possible by studying how birds regrow these cells. All vertebrates can do it, but mammals are not capable of such a feat.

“It’s funny, but mammals are the oddballs in the animal kingdom when it comes to cochlear regeneration,” co-author author Dr Jingyuan Zhang, from the University of Rochester, said in a statement. “We’re the only vertebrates that can’t do it.”

The project was led by Dr Patricia White, who in 2012 discovered how the generation of hair cells is fostered by nearby supporting cells. In birds, a family of receptors called the epidermal growth factor (EGF) activate the supporting cells. In mammals, this signal is blocked for some reason, so the team had to come up with a way to temporarily switch it on.

They used three methods that all focused on a specific receptor called ERBB2, which is found in the support cells inside the cochlea. One experiment saw the use of two drugs known to activate the receptor. A second experiment used genetically engineered mice that produced ERBB2. The third one used a specially designed virus that targeted the ERBB2 receptors.

The team showed that activating these specific receptors led to the desired effect. ERBB2 stimulated the support cells and, as they began to multiply, neighboring stem cells turned into new sensory hair cells. On top of that, the hair cells naturally integrated with the nerve cells. This is a critical step without which regaining hearing is not possible.

“The process of repairing hearing is a complex problem and requires a series of cellular events,” Dr White explained. “You have to regenerate sensory hair cells and these cells have to function properly and connect with the necessary network of neurons. This research demonstrates a signaling pathway that can be activated by different methods and could represent a new approach to cochlear regeneration and, ultimately, restoration of hearing.”