Hearing is an essential function that enables people to take part in daily communication, appreciate music and nature, and enjoy their life.
In the U.S., about 35 million Americans suffer from hearing impairments, with 2 million people completely deaf, according to the National Organization for Hearing Research Foundation. Developing an effective therapy for curing hearing loss is therefore a dream for both patients and ear surgeons.
Samuel Gubbels, an assistant professor at the Department of Surgery and the Waisman Center at the University of Wisconsin-Madison, recently reported that his lab has been working to regenerate a specific kind of ear cell that takes the major responsibility for the human’s hearing loss, called a hair cell.
Hair cells are the tiny, neural receptor cells in the human’s inner ear. They are components of a delicate inner ear organ shaped like a snail shell, called a cochlea. The cochlea plays a crucial role in the human’s hearing: When sound waves travel from the outer ear to the cochlea, hair cells would receive the waves and change them into electrical impulses that are then transferred by auditory nerves to the human brain.
“They are called hair cells, because on the top of the cells, there are little projections, which look like a bush of hairs on the cells,” Gubbels said.
According to him, each person has thousands of hair cells in the inner ear. The more the hair cells are lost, the worse a person’s hearing would be.
“Most of the time, about 80 percent of the patient cases, when the human loses hearing, it is because the hair cells have been lost in the inner ear,” Gubbels said.
Unlike certain species—like some birds, fish or amphibians — who can regenerate their hair cells when lost, once the human’s hair cells are damaged or dead, whether it is due to noises, drugs or aging, the hearing would be lost permanently.
Thus, if hair cells could be regenerated by scientists and transplanted into the human’s inner ears, there may be chances that hearing impairment could be cured at its root, according to Gubbels.
Current medical treatments for hearing loss patients include using hearing aids and cochlea implants. Although those therapies have worked well for thousands of patients, the help is still limited and has several disadvantages.
For example, hearing aids are sound-amplification devices, which can only stimulate the hair cells that are still alive. Since different hair cells are sensitive to different sound frequencies, even with the help of the hearing aid, patients still have difficulty hearing certain sounds.
Cochlea implants, small but complex electronic devices that function by directly simulating the auditory nerve, work well by bypassing the hair cells. However, the surgery implanting the devices is expensive and has certain risks such as infection. Patients also are required to wear the electronic device for their entire lifetime. Moreover, according to Gubbels, the sounds that the patients hear through the cochlea implants are not natural sounds and are more like electronic or synthesized sounds.
In America, hearing loss due to hair cell damage or damage to the auditory nerve has affected the ability to communicate for more than 22 million people, according to the NOHR foundation. Thus, if scientists can find a solution that triggers the hair cell’s regeneration, then replacing dead hair cells with new ones would be a much more effective therapy for treating hearing impairment.
Although in recent years, some scientists have been able to regenerate certain human organs like neural tissues, or even some sorts of ear cells, in the laboratory, hair cell regeneration is still a pioneer research and full of challenges.
“We don’t know how to do it for hair cells, there is no route map for how we do this. It is a lot of trial and error,” Gubbels said.
But ongoing trial and error has led Gubbels’ lab on the way. So far, the group has found a method that can turn stem cells into a specific sort of inner ear cells, called the progenitor. The progenitor cells are “parent” cells of hair cells, which have the potential to develop into the hair cells and other ear cells.
Previous researchers have reported that hair cells are hard to root in the places where they should be. Progenitors are a good solution to this problem because of their ability to take the root in the inner ear where the cells are needed.
Gubbels’ lab is now working to investigate how the progenitor cells can be grown into hair cells in the lab culture. According to the NOHR foundation, a breakthrough finding reported by several labs in 1993 has shown the existence of progenitors in mammalian ears, such as the mouse. Although there is no evidence to show whether a human will still retain progenitors after birth, investigating how the progenitors can be evolved into hair cells is an essential step to a more effective hearing loss therapy.
Gubbels’ team now is also drafting a research paper on its findings and hopes to be published soon.
As every pioneer scientific research does, hair cell regeneration therapy could be a lengthy year and a difficult task, but Gubbels is optimistic about the future.
“I do know when I see a baby right now who hasn’t any hearing whatsoever, probably in her lifetime, [she] will have regenerate therapy available for her.”