Hearing is Believing: Cells that Enable Hearing after Birth

The sense of hearing is a critical part of how we experience life and the world around us. It is so important, in fact, that the ears are fully formed and functional when we are born. Despite its significance, hearing is often underappreciated until it is lost. Hearing loss affects more than 10% of all people worldwide. Whether it is due to age, exposure to loud noises, or genetic mutations, hearing loss occurs when hair cells, the receptors in the ears that respond to sound, become damaged and die. One of the biggest challenges in counteracting hearing loss is that hair cells cannot repair or regenerate themselves, so the damage is often permanent.

Currently, people with hearing loss and deafness can use a number of devices to improve their hearing abilities. Hearing aids amplify sounds that come into the ear, and cochlear implants stimulate the nerve that sends auditory information to the brain. While these devices are useful, it would be even better to devise new ways to treat or even prevent hearing loss and deafness. Researchers at the Stanford University School of Medicine have discovered certain cells in the inner ear that develop into sensory hair cells. Knowing how these cells differentiate into hair cells, could lead to the innovation of new ways to treat the underlying cause of deafness: the loss of hair cells in the inner ear.

The inner ear is a highly specialized structure for gathering and transmitting vibrations from the air. The auditory compartment, or cochlea, is a small, spiral-shaped cavity where hair cells sense the vibrations created by sound. The supporting cells that surround hair cells are also important for transmitting this auditory information to the brain. Although the inner ear is fully developed at birth, recent research has suggested that certain cells in the cochlea, called progenitor cells, retain their ability to divide even after birth. In the current study, the research team worked to identify this group of cells and understand the mechanisms that control their proliferative capacity, or their capacity to divide into new cells.

To do this, the researchers used a special type of mouse that allowed them to track the activation of the Wnt pathway in different cells. This cell-signaling pathway is involved in many developmental functions and controls the renewal and proliferation of stem cells. The researchers identified a class of cells, called tympanic border cells, which actively divide during a phase of cochlear maturation right after birth. These cells divide vigorously in isolated cochleas when the Wnt pathway is activated, but they stop dividing when the pathway is inhibited. In cell culture, these tympanic border cells specialize into hair cells and support cells.

Altogether, these results hint at the potential for targeting tympanic border cells in new therapeutic strategies for hearing loss. If scientists cannot revive damaged hair cells, perhaps they can make the tympanic border cells create new ones! Of course, there’s a lot more to be understood about these cells before this can become a reality. How well do these cells regenerate in models of hearing loss? Under what conditions are they most proliferative? Still, this study offers a new insight into how cells develop in the inner ear even after birth, and offers new ideas for how to stimulate the regeneration of sensory hair cells in the cochlea.

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References:

Oishi N. & Schacht J. (2011). Emerging treatments for noise-induced hearing loss, Expert Opinion on Emerging Drugs, 16 (2) 235-245. DOI: 10.1517/14728214.2011.552427

Jan T.A., Chai R., Sayyid Z.N., van Amerongen R., Xia A., Wang T., Sinkkonen S.T., Zeng Y.A., Levin J.R., Heller S. & Nusse R. & (2013). Tympanic border cells are Wnt-responsive and can act as progenitors for postnatal mouse cochlear cells, Development, 140 (6) 1196-1206. DOI:10.1242/dev.087528

Images adapted from Radius Images/Corbis, Wikimedia Commons, and made by Kate Jones.