Here's what we know about hearing loss: while it is not reversible, it is treatable. So the focus of hearing health professionals and advocates has always been on prevention (avoiding or reducing harmful environmental noises) and treatment (getting fitted with hearing aids).
Now, scientists say there is a way to restore hearing by re-growing inner ear hair cells. This breakthrough could lead to significant advances to cure deafness and hearing loss in the future.
Before we examine the implications of this promising development, let's look at what is going on deep inside our ears and what role those miniscule hairs play in our hearing.
Hearing Loss Cures - It's All in the Hair
Unlike the hair that grows on our heads, the hair in our inner ears is not visible to a naked eye, but it has a very important function in our ability to hear. Located in a portion of the ear's cochlea called the organ of Corti, the sensitive hairs are small sensory cells that process and transmit sound to the brain.
We are born with 30,000 hair cells in each ear. Over the course of our lives, these hair cells get damaged and die – for example, because of the aging process, toxic drugs, or prolonged exposure to loud environmental noises- causing varying degrees of irreversible hearing loss. In fact, according to the National Institutes for Health (NIH), more than 90 percent of hearing loss is due to destruction of hair cells or auditory nerve cells.
Unfortunately, humans are unable to spontaneously regenerate these damaged or dying cells. And that's where the new breakthroughs offer glimmers of hope to approximately 33 million Americans currently suffering from hearing loss, and an estimated 500 million worldwide.
|Hair cells located in the organ of corti, in the cochlea of the inner ear. Photo courtesy SPL / Photo Researchers, Inc|
Hearing Loss Cure Research - A Controversial Issue
Scientists agree that the key to reversing hearing loss - as well as a wide range of diseases that are currently deemed incurable -lies in human stem cells derived from embryos that have been fertilized in vitro and used for research purposes with consent of the donors.
In this country, this issue remains controversial, even though President Obama lifted the longstanding ban on federal funding for embryonic stem cell research.
How can these cells be used to cure deafness and hearing loss? British scientists discovered a way to turn stem cells into ones that act like hair cells, which could be surgically inserted into the ear to restore hearing.
That is just one of several studies carried out in various countries that found that stem cells are instrumental in curing deafness.
Hearing Loss Genetics - Of Mice and Men
The latest evidence comes from the Stanford University School of Medicine, where a team of researchers started out with the idea that if they could create hair cells in the laboratory from stem cells, better treatments for deafness and hearing loss would follow.
Stefan Heller, PhD, professor of otolaryngology at the Stanford University School of Medicine and a leader in stem cell-based research on the inner ear, found a way to develop mouse cells that look and act just like the animal's inner-ear hair cells.
Eventually, scientists will need human hair cells to experiment on so they can perfect their formula for hair regeneration in the inner ear. However, in these early stages of experimentation, mice are adequate models. In fact, further tests show that mice cells responded to vibrations in a similar way to hair cells in the (human) inner ear.
"We made hair-cell-like cells in a petri dish," said Kazuo Oshima, MD, PhD, a research instructor at Stanford who works in Heller's lab. "This is an important step toward development of future therapies."
What does this all mean for millions of people with hearing loss around the world? "Our study offers a protocol to generate millions of functional hair cells from a renewable source," Heller said of the study's findings. "We can now generate these cells and don't have to go through dozens of mice for a single experiment. This allows us to do molecular studies with much higher efficiency."
Cure Hearing Loss with Gene Therapy?
Other studies, conducted over the years by the NIH, also suggest that hair cells could be, contrary to earlier assumptions, regenerated. For example, in the 2007 study called "Hair Cell Regeneration and Hearing Loss," the NIH scientists destroyed hair cells in the inner ear of mice, causing deafness. However, the mice's supporting cells spontaneously "migrated to the hair cell region and began growing hair bundles on their surfaces," the researchers reported. Still, hearing was not restored, but a later study offered more hope.
An experiment was performed using guinea pigs and gene therapy developed in previous studies. Again, the supporting cells migrated, but this time partially restored the hearing, indicating that gene therapy could improve hearing in formerly deaf animals.
This means that possibly not one, but a combination of therapies, could be used in the future to restore hearing.
Admittedly, it is too early to draw any conclusions and it is fair to say that we are still years away from hearing regeneration. However, we are a step closer than we were in the past, and in that sense, this is a positive development.