The sensory organs of the inner ear that detect sound and head position are similar across vertebrates such as mammals, fish, and birds. The potential to regenerate these organs, however, is not as widespread.
The sensory hair cells of the inner ear are particularly fragile and are vulnerable to death caused by ototoxic drugs, excess noise, and age-related degeneration. While mammals can regenerate hair cells at perinatal stages, this ability declines rapidly after birth, and by adulthood, regeneration is limited in the vestibular (balance) system and completely lost in the auditory system. As a result, hair cell death can lead to permanent hearing and balance impairment in humans.
In contrast, many other vertebrates, including fish, amphibians, and birds, can regenerate functional hair cells throughout life.
Zebrafish are a commonly used animal model in the lab. They have a remarkable ability to regenerate many organs, including the heart, liver, kidney, fin, retina, and central nervous system, and they are commonly used to study hair cell development, death, and regeneration.
Zebrafish have hair cells in an external sensory system called the lateral line, which is used to detect changes in water flow for behaviors such as schooling and predator evasion. As a result, much of our current understanding of zebrafish hair cell function and regeneration comes from studies of the lateral line.
These diagrams show the crista, a zebrafish inner ear organ, in cross-section. The top images show typical hair cell (HC) development. The middle images show HC development after ablation that kills the HCs. Supporting cell (SC) division increases. HCs are regenerated by direct transdifferentiation of SCs. The initial burst of SC division is temporally uncoupled from HC replacement, which occurs gradually. The organ continues to grow during regeneration, and HC number and patterning are restored to normal by approximately two weeks after ablation. Credit: Beaulieu, Marielle O.; Thomas, Eric D.; Raible, David W./Development
The zebrafish inner ear is a promising model system for studying hair cell regeneration because it shares many similarities of structure and function with the inner ear of mammals. But unlike their lateral line hair cells, zebrafish inner ear hair cells have been relatively understudied.
In the inner ear, hair cells are surrounded by supporting cells that play many important roles during the life and death of hair cells, such as becoming new hair cells. Scientists, including those who are members of the Hearing Restoration Project (HRP), have found the mechanism by which hair cells are regenerated differs by model system, with a critical point of difference being whether precursor cells divide before giving rise to new hair cells.
For example, in the lateral line system, nascent hair cells are added in pairs from supporting cells that divide equally. In mature mammalian vestibular organs, hair cells are added by direct transdifferentiation (transformation) of supporting cells, without division.
In the auditory organ of birds a dual mechanism occurs, where hair cells are regenerated in an initial wave of direct transdifferentiation followed by a later wave of cell division.
In zebrafish, both methods of hair cell regeneration (direct transdifferentiation and cell division) have been observed. Recent studies support the idea of direct transdifferentiation because zebrafish inner ears do not show a clear population of dividing supporting cells, as is seen in the lateral line. Instead, there is a transition state where cells share aspects of both hair cells and supporting cells during regeneration.
In a study published in Development in July 2024, HRP member David Raible, Ph.D., and team aimed to better understand the zebrafish inner ear by quantitatively examining the increase in the number of hair cells during both development and the regeneration process of the larval zebrafish inner ear.
The team used a genetic method to kill the hair cells and observed their gradual regeneration over two weeks. Supporting cells divided in response to hair cell death, expanding the number of possible precursor cells that can turn into hair cells.
In parallel, new hair cells arose from direct transdifferentiation of precursor pool cells temporally uncoupled from supporting cell division. This means that new hair cells were formed directly from a group of precursor cells, without these cells needing to divide first. This process happened at a different time than when the supporting cells were dividing.
In other words, hair cells in the zebrafish inner ear regrow by changing supporting cells into hair cells. This occurs after a brief period when these supporting cells multiply to increase the number of potential new hair cells.
These findings reveal a previously unrecognized mechanism of hair cell regeneration with implications for how hair cells may be encouraged to regenerate in the mammalian inner ear. It also further establishes the zebrafish inner ear as a model for hair cell regeneration that parallels processes that are functional for a limited period in mammals.
This is adapted from the paper in the journal Development. Hearing Restoration Project member David Raible, Ph.D., is the Virginia Merrill Bloedel Chair in Basic Hearing Science and a professor in the department of otolaryngology–head and neck surgery at the University of Washington.
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