Stem Cell Research Reveals Improved Hearing Possible

Healthy Hearing Staff Writer

Editor: Here's a status report on recent efforts to regrow hair cells from stem cells. Thanks to bhNEWS for this article, which originally appeared in Healthy Hearing.

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What Is a Stem Cell?

Stem cells are remarkable. They occur naturally in the human body. By themselves, they have no "definition," or identity, i.e. they aren't blood cells, nerve cells, brain cells or any other kind of cell - yet. That's what makes stem cells so remarkable. They can develop into a number of different kinds of cells, replacing cells that have been damaged or are, somehow, defective.

Stem cells also "self-replicate," meaning they can clone themselves, making more stem cells. In fact, adult stem cells, which exist in mature body tissue and organs, continue to renew their numbers throughout an individual's life, replacing damaged cells on an "as needed" basis.

The Process of Hearing

Hearing occurs when sound waves, traveling through the air, are captured by the outer ear and directed into the ear canal. This is where you find the tympanic membrane (ear drum) and the three smallest bones in the human body (stirrup, hammer and anvil). This is also where you'll find the cochlea, a snail-shaped organ that contains delicate, hair-like nerve fibers that translate sound waves into electrical impulses that are in turn transmitted to the brain.

These delicate, hair-like nerve fibers can be damaged or impaired due to age, disease, exposure to loud noise and other medical and non-medical conditions. Hearing loss, most often permanent in nature, is the result of the damage to these nerves.

Hearing and Stem Cell Research

A group of researchers from the Indiana University School of Medicine, led by Dr. Eri Hashino, Ph.D., have been able to transform stem cells, taken from the bone marrow of lab mice, into cells that have numerous characteristics of sensory nerve cells, called neurons. Neurons transmit sound from the outside world to the brain where the sound is processed and interpreted.

These laboratory results suggest that it may be possible to re-grow and replace damaged neurons with stem cells extracted from bone marrow. "We were interested in [bone] marrow cells because of their potential for use in autologous cell-based therapy," Dr. Hashino stated. Autologous cell-based therapy is simply using stem cells from a patient as part of that patient's treatment. No outside stem cells are used. Even better, the bone marrow cells are easily collected and can be kept alive in the laboratory until needed.

In the laboratory, Dr. Hashino's team was able to manipulate marrow cells from mice using chemicals known to encourage stem cells to grow into rudimentary neurons. When these primitive neurons were mixed with molecules found in the ear during embryonic development, the primitive neurons took on additional characteristics of healthy neurons found in the human ear.

Dr. Hashino and her colleagues are now beginning a series of experiments to determine if bone marrow cell transplants can be used to stimulate the growth of nerve cells that are often missing from patients who experience profound hearing loss, sometimes from birth.

Recovered Hearing

Additional research studies, this time conducted in Tokyo, have shown "that bone marrow stem cells injected into a damaged ear can speed hearing recovery after partial hearing loss," according to a press release from the American Journal of Pathology. (http://ajp.amjpathol.org/)

Some regeneration (re-growth) of cochlear fibrocytes is possible after these cells have been damaged. However, recovery from hearing loss has never been complete. Partial recovery of hearing is possible over a period of weeks and months but patients who experience trauma-induced hearing loss tend to permanently lose their ability to hear high frequency sounds.

In lab experiments on rats, a team of researchers led by Dr. Tatsuo Matsunaga of the National Tokyo Medical Center found that rats who received transplanted stem cell therapy recovered much faster from hearing loss, in particular, high frequency hearing loss. In fact, lab rats that received stem cell therapy displayed a 23% increase in improved hearing when compared to rats that didn't receive the stem cell injections.

The Future Sounds Good

The research conducted at the IU School of Medicine was undertaken to help people who experience severe or profound hearing loss, often evident at birth, or in senior citizens. People with profound hearing loss hear very little, if at all. In some cases, the cause of this hearing loss is the absence or impairment of the neurons found in the cochlea.

The evidence developed by the IU researchers indicates great promise for those who experience severe or complete hearing loss.

The researchers in Japan focused more on routine hearing loss caused by noise trauma, certain diseases and the aging process, for example. Again, the results of using adult stem cells, from the patient's own bone marrow, showed genuine promise for the future.

A great deal of additional laboratory research must still be conducted but the signs are there. Stem cells, with their ability to develop into a variety of different kinds of cells, offer hope for those with hearing loss. Someday, a stem cell injection may give hearing to those who have never experienced it, and improve hearing for those who have experienced hearing loss - whatever the cause.

Research on the practical use of a patient's stem cells to restore or improve hearing is just beginning. Animal studies indicate the potential to regenerate the nerves that convert sound waves into electrical impulses that are subsequently delivered to the brain for processing.

However, this is just the beginning of the journey. Researchers around the world are working to improve the procedures to implant stem cells into the inner ear.