A model of the mouse inner ear. The Y-shaped structures are lying horizontally in this model. The long part of each Y shape appears in light blue. At the top of the Y shapes are outer hair cells (these appear in a variety of colors) and two other branches called Dieters' cells and phalangeal processes (these appear in aqua and blue-purple).
Image credit: Sunil Puria, Ph.D., Harvard Medical School
Hearing is an amazing process, and it’s all thanks to the 15,000 or so tiny hair cells inside our cochlea—the small, snail-shaped organ for hearing in the inner ear. The cells are called hair cells because tiny bundles of stereocilia—which look like hairs under a microscope—sit on top of each hair cell. When sounds are too loud for too long, these bundles are damaged. Damaged hair cells cannot respond to sound, causing noise-induced hearing loss. Since hair cells can’t be repaired or replaced in humans, hearing loss is often permanent. (Watch Noisy Planet’s Journey of Sound video for a detailed explanation of how we hear.)
For years, scientists around the world have been studying how to regrow and replace damaged or destroyed hair cells to restore hearing. Replacing hair cells might not be enough, however, according to researchers from Massachusetts Eye and Ear. Their computer simulations show that, to function properly, new hair cells would also need to be organized in a specific arrangement. The study, which was funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), part of the National Institutes of Health (NIH), was published in 2018 in the Proceedings of the National Academy of Sciences.
We have two types of hair cells in our cochlea: inner hair cells (we have about 3,500 per ear) and outer hair cells (we have about 12,000 per ear). Inner hair cells collect and relay sound information to the brain through the auditory nerve. Outer hair cells, which were the focus of this study, work to amplify sounds, helping us to pick up quiet sounds by making them seem louder. Outer hair cells also help us tell the difference between the pitches of sounds, even when the difference between two pitches is very small. Outer hair cells exist in a Y-shaped formation that is repeated thousands of times across the cochlea.
Using computer simulation in a mouse cochlea, the researchers showed that outer hair cells can do their job only if they are part of this Y-shaped formation. Their findings suggest that the two branches of the Y-shaped structure (made of structures called Dieters’ cells and phalangeal processes of the Dieters’ cells), connected to the outer hair cells, are also critical for the cochlea to be sensitive to quiet sounds. Scientists working on therapies to regenerate outer hair cells will also need to figure out how to restore this formation.
Loud sounds aren’t the only way hair cells can be damaged and cause hearing loss; aging and some life-saving cancer therapies and other medicines can also destroy hair cells in the ear. The good news is that you can take steps to prevent hearing loss from loud sounds, also known as noise-induced hearing loss. Follow these three simple rules:
- Move away from the sound.
- Turn down the volume.
- Wear hearing protectors, such as earplugs or earmuffs.
This work was supported in part by National Institute on Deafness and Other Communication Disorders Grant R01 DC07910.