1. How well did a quail's hearing recover after loud noise?
Deep inside the ear, in the cochlea, rows of hair cells convert sound vibrations into signals for the nerves. In mammals, including humans, it is known that these cells do not regenerate if they are damaged. [2]
However, in the 1980s, results in bird ears challenged this common knowledge. Two papers were reported in the American science journal Science in 1988. [2] In one study, quails that suffered acoustic trauma (injury from loud noise) saw the number of lost hair cells return to within 5% of their original count. [2] Observations within 10 days showed markers used during cell division in the nuclei of both hair cells and supporting cells. [2]
It is important to note that this is not a return to "exactly the same" state, but rather "within 5%." Still, this was a major discovery showing that a damaged ear can repair itself by replacing cells. The question then becomes: where do the new hair cells come from? [2]
2. Which cells in a bird's ear make the new hair cells?
Research examining the bird cochlea in detail suggests that the source of new hair cells is the supporting cells located next to them. [1] These supporting cells, which are usually inactive, begin to respond when hair cells are damaged. Their descendants become either hair cells or supporting cells. This is the first pathway. [3]
In the second pathway, supporting cells do not divide but transform directly into hair cells. In both pathways, the estimated time for the numbers to return is 2 to 3 weeks. [3]
In 2025, a group from Kyoto University reported that a receptor (EDNRB2) in chicken regeneration is active only during the short period when progenitor cells are set on the path to becoming either hair cells or supporting cells. [1] It is also important to note that the key to regeneration cannot be narrowed down to just one factor. [1]
Is this mechanism unique to birds?
3. How do fish regrow hair cells in their sensory organs?
The lateral line is a sensory organ on the surface of a fish’s body that detects water flow. It also contains hair cells, and zebrafish hair cells can regenerate after damage. [5]
In this case too, the source is the supporting cells. The supporting cells become precursor cells and produce new hair cells. In experiments with young larvae, regenerated hair cells were able to send signals to nerves within 48 hours. [5] However, this refers to individual new cells. The time for the entire neuromast to return to its original function is separate; in late-stage larvae, it takes more than 120 hours. [5]
In both fish and birds, which are distant relatives, supporting cells are providing replacements. So, have mammals, including humans, lost this ability? [5]
4. Why don't supporting cells in human ears make new hair cells?
The National Institute on Deafness and Other Communication Disorders (NIDCD) in the United States explains that most noise-induced hearing loss is due to damage or death of hair cells. Unlike in birds and amphibians, human hair cells do not return. [6]
Research looking for the reason why they do not return focuses on supporting cells. In the mature mammalian cochlea, supporting cells do not restart division on their own, and they have a lower capacity to produce hair cells compared to newborn mice, according to reports. [4] This means that even after hair cells are lost, the supporting cells remain but stay "asleep." [4]
Review articles discuss the possibility that differences in how genes are activated or how genetic switches are turned on are involved in why they sleep. However, the cause is not confirmed. The evolutionary reason why only mammals gave up regeneration is not discussed in the materials. [4]
So, can dormant supporting cells be woken up with medicine? [4]
5. How close are medicines that wake up supporting cells?
In 2013, a joint study by Keio University and Harvard University tried this in mice. They locally administered a drug that suppresses the signal (Notch signal) that says "do not turn neighboring cells into hair cells" into the inner ears of mice with noise-induced hearing loss. [7] As a result, outer hair cells increased from surrounding supporting cells, and the hearing threshold improved by 3 to 8 decibels, according to reports. [7]
This number represents a small improvement. It cannot be called a completed treatment. [7]
The gene Atoh1, which acts as a switch to create hair cells, was also studied. During development, Atoh1 is necessary to change into a hair cell, and it is said to be sufficient on its own. However, in a mature cochlea, Atoh1 alone has limited effects, and a review states it cannot fully turn supporting cells into hair cells. It is also written that after hearing begins, regeneration can hardly be induced. [8]
What can be said from the research so far?
6. What questions remain about regrowing hair cells in human ears?
A 2025 review by Benkafadar and Heller in Developmental Biology states that birds can fully regenerate hair cells throughout life, and argues that biological recovery might be possible in humans too. [9] This is the authors' outlook, not a promise of a treatment. Research on bird ear regeneration may provide clues for finding ways to restore human hearing. [9]
However, there is no treatment yet in humans to restore lost hair cells. The 3 to 8 decibel improvement seen in the Keio University mouse study is just a beginning. [7]
Questions remain, such as these two. What are the conditions that wake up sleeping supporting cells? Why do mammals not regenerate hair cells? The original review is in English, but Kyoto University’s research news is available in Japanese. From there, you can read what lies ahead in bird ear research. [1]