Hair Cell Regeneration—Looking Beyond the Hype

© November 2004 by Neil Bauman, Ph.D.

Question: Hair cell regeneration has been in the news for some time now. Will it soon restore hearing for the millions of hard of hearing people? I'd love to know your thoughts about this.—J. S.

Answer: Investigation into hair cell regeneration has come a long ways from the late 1970s when researchers first discovered that sharks could produce hair cells throughout their lives.¹ However, it was not until 1986-87, when researchers discovered that birds could naturally regenerate hair cells to restore damaged hearing,² that scientists got excited and began to think, "If it works in birds, maybe we can make it work it humans too!"

Since then, research on hair cell regeneration has accelerated. In the past 3 or 4 years, researchers have made remarkable strides towards one day being able to regenerate hair cells in people with hearing loss.

Unfortunately, every time there is another discovery or breakthrough in this research, the media hype seems to indicate that hair cell regeneration is just around the corner—that in just a few years hearing loss is going to be a thing of the past. Hard of hearing people are getting their hopes up—thinking that in a few years, they will get their hearing back. Is this really going to happen, or are their hopes going to be dashed once again? Let's look at the facts.

First, hair cell regeneration will not help all people with hearing loss. For example, hair cell regeneration won't help deaf people who lost their hearing before they acquired speech. This is because brains wire for sound during the first 6 years of life. If a person doesn't hear any sounds during this time, their brains never develop the necessary auditory capability to understand speech. Thus, even if their ears could grow new hair cells, these hair cells would be useless to them because their brains wouldn't know how to process these new sound signals. (However, if hair cells were regenerated in deaf children in the very first few years of their lives, the results could be fantastic.)

Furthermore, hair cell regeneration won't help people with conductive losses such as are caused by middle ear infections or otosclerosis, nor will it help people with auditory nerve conditions such as acoustic neuromas. Also, hair cell regeneration will not help people with hearing loss if their hearing loss is caused by the absence of certain genes that result in hearing loss even though adequate numbers of hair cells are present. That's the bad news.

The good news is that the majority of people with hearing loss have a sensorineural type of hearing loss that maybenefit from hair cell regeneration.

Second, hair cell regeneration is still a long ways off—several decades at least. It is not just around the corner. As of 2004, the most realistic time frame is still 20 or more years in the future. Dr. Rubel, perhaps the leading researcher in the world today on hair cell regeneration, says, "My most hopeful prediction is 20 years, and that's being very optimistic."³ He further states, "Over 15 years of studies on hair cell regeneration in the inner ears of birds has taught us that a quick and easy cure for sensorineural hearing loss is unrealistic."⁴ He adds, "It will be a long time until we have anything near a perfect cure for hearing loss."⁵

Third, once hair cell regeneration is possible, the public has been lead to believe that treating hard of hearing people will result in them having normal hearing once more. However, if you carefully read the reports as they come out, you begin to realize that researchers are not talking about hard of hearing people receiving normal hearing through hair cell regeneration. They are talking about "growing enough hair cells where hearing aids could be used more effectively and provide much more acoustic information"⁶ than would otherwise be possible.

In fact, Dr. Rubel expects that hair cell regeneration, far from leading to the demise of hearing aids, will actually make them even more common and useful. He explains, "Hair cell regeneration will, if anything, increase the population of people who could benefit from hearing aids."⁷

This is because a normal human ear has between 16,000 and 30,000 hair cells, yet hair cell regeneration researchers are talking about only being able to grow a few hundred hair cells⁸—not the thousands upon thousands needed for normal hearing. Obviously, regenerating a few hundred hair cells is a drop in the bucket and will in no way restore hearing to normal—better hearing, yes, but not normal hearing.

Did you know that even in the animals that God designed to naturally regenerate hair cells, hearing does not return to normal? For example, researchers used loud noise and antibiotics to produce a 70 dB loss in chickens. When these chickens regenerated hair cells to replace the damaged ones, their hearing returned, but not to normal. They had a permanent 23 dB hearing loss.⁹ Most studies on birds have reported mild permanent hearing losses and mild to moderate tuning (discrimination) impairments.¹⁰

Thus, even though much hearing returns, the regenerated hair cells are not as "good" as the originals. Before and after photomicrographs of hair cells are revealing. Before noise damage, the hair cells are symmetrical and beautifully ordered. Later, pictures taken of regenerated hair cells show them as irregularly shaped and the stereocilia (the "hairs" of the hair cells) look like they were all hashed together—not beautifully arranged like before.¹¹

These are some of the facts. Therefore, at the present time (2004), hair cell regeneration is nowhere near ready to be applied to humans. However, if research into hair cell regeneration continues at the present pace, hair cell regeneration in humans will very likely become a reality sometime in the next few decades. Even so, it won't be a cure for hearing loss. Rather, it will be another aid to better hearing, just like hearing aids are today.

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¹ Corwin, J. T. 1981. Postembryonic Production and Aging of the Inner Ear Hair Cells in Sharks. J. Comp. Neurol. 1981: 201:541-53.

² Rubel, Edwin. 2004. Promising Research on Hair Cell Regeneration. The Hearing Review. http://www.hearingreview.com/Articles.ASP?articleid=H0410F01. p. 1.

³ Rubel, Edwin. 2004. HR Interviews Edwin Rubel. The Hearing Review. http://www.hearingreview.com/Articles.ASP?articleid-H0410F01. p. 6.

⁴ Bermingham-McDonogh, Olivia. 2003. Hair Cell Regeneration: Wing Our Way Towards a Sound Future. Current Opinion in Neurobiology. 13:119-126. http://www.current-opinion.com. p. 124.

⁵ Rubel, Edwin. 2004. HR Interviews Edwin Rubel. The Hearing Review. http://www.hearingreview.com/Articles.ASP?articleid-H0410F01. p. 6.

⁶ Ibid. p. 3.

⁷ Ibid. p. 6.

⁸ Ibid. p. 6.

⁹ Dooling, Ryals. ~1997. Recovery of Hearing Following Hair Cell Regeneration. http://www.bsos.umd.edu/psyc/dooling/HCL.html. p. 2.

¹⁰ Bermingham-McDonogh, Olivia. 2003. Hair Cell Regeneration: Wing Our Way Towards a Sound Future. Current Opinion in Neurobiology. 13:119-126. http://www.current-opinion.com. p. 121.

¹¹ Rubel, Edwin. 2004. Promising Research on Hair Cell Regeneration. The Hearing Review. http://www.hearingreview.com/Articles.ASP?articleid-H0410F01. p. 1.