Two populations of hair cells, type I and type II, are necessary for the sense of balance in many vertebrates. A new study of healthy adult mice showed not only a steady renewal of type II cells, but also an accelerated renewal among them when spurred by an injury.
“This gives us a new understanding of the degree of plasticity in the balance organs of mammals,” said Jennifer Stone, research professor of otolaryngology-head and neck surgery at the UW School of Medicine. She was corresponding author of the study, published March 6 in the journal eLife.
Hair cells in the cochlea and balance organs of the inner ear are stimulated by sound waves and head movements, respectively. In birds and fish, the vestibular (balance) hair cells renew naturally – akin to the process of human skin replacing itself. In mammals, though, evidence has been lacking to indicate such turnover of vestibular hair cells. Scientists have thought that they could only degenerate over time with age and injury.
To discern whether renewal was occurring in the vestibular hair cells of adult mice, the researchers used fluorescent marker to label the supporting cells, which surround hair cells. Over time, the type II hair cells adopted the fluorescent marker, while type I hair cells did not. This finding indicated that supporting cells had converted into type II hair cells.
The study’s implications, Stone said, involve balance – and maybe hearing, too.
“This discovery in the vestibular system gives us the opportunity to understand the mechanisms by which vestibular hair cells are generated, so we can apply that knowledge to promote hair-cell regeneration in balance organs and also in the cochlea,” where similar hair cells process sound waves but are known not to regenerate or renew.
Type I and II hair cells are intermingled in the inner ear – picture gray hairs among black hairs on one’s head – and have distinct molecular, morphological and physiological properties. Stone wondered aloud why it would be advantageous for mammals to develop an ability to replace one hair cell subtype but not the other, even though both types communicate through the same neuron.
“We think type IIs might play a critical role in the balance system such that they are not disposable,” she said. “Type II hair cells might have a purpose that we haven’t tested for yet.”
The function of inner-ear hair cells and their properties of renewal are important considerations for the aging populace: Humans’ balance and hearing both diminish with age, and falls among seniors are a major threat to health and well-being. As well, learning how to trigger hair-cell replacement in humans might someday help to reverse symptoms caused by drugs that have ototoxic properties, such as medications for some forms meningitis and cancer.