Hearing Aid Blog

Hair Cells from Stem Cells

July 11, 2010

FROM THE NATIONAL INSTITUTES OF HEALTH:

Hair cells are the tiny sensory cells located in the cochlea of the inner ear that turn sound vibrations into electrical signals. Each ear shelters fewer than 15,000 of them, and once they are damaged or die, there are no others to take their place. Unlike birds, fish, and some reptiles, humans don’t have the ability to grow new hair cells if some are lost due to disease, drugs, or long-term exposure to noise. So that’s why we have so many people coming into our office to buy hearing aids.

This past May, a group of NIDCD-funded researchers led by Stefan Heller, Ph.D., at Stanford University School of Medicine announced that they had developed a system for making what appear to be functional hair cells from stem cells. Their findings were published in the May 14 issue of  Cell.

This is exciting research because hair cells in the inner ear have a complicated structure. They resemble other kinds of epithelial cells (cells that line the cavities and surfaces of structures in the body) but with a twist. At their tips, hair cells display a spiky bundle of filaments–known as stereocilia–which contain mechanosensitive ion channels that are able to produce electrochemical signals when stimulated by sound vibrations. Building a hair cell from scratch meant not only replicating its complicated architecture, but also endowing it with the ability to respond electrochemically to sound vibrations.

In the lab, Heller and his colleagues began with stem cells from mouse embryos, setting up conditions to mimic what they knew about how hair cells form during fetal development. They tried out various combinations of growth-inducing substances until they found one that made the cells cluster and display hair cell-like characteristics. The key ingredients were chemicals known as fibroblast growth factors (FGF), which were shown in previous studies to be intimately involved in inner ear development.

Eventually their efforts produced groups of cells that looked intriguingly hair cell-like–with recognizable hair cell bundles at their tips–and when stimulated by electrodes, the cells generated electrochemical currents that resembled those made by young hair cells.

With their “ear in a test tube,” Heller and his team also plan to start a series of tests to search for the biochemical basis for the inner ear’s inability to regrow hair cells. Since all the other hair cell-bearing organs in the body are able to replace lost hair cells, it’s not unreasonable to assume that the current structures in the inner ear evolved from structures that once had the capacity for self-repair.

Finding the switch that turns hair cell regeneration on and off could make returning the inner ear’s capacity to grow hair cells as simple as hitting a chemical reset button, although getting there may take some time. “We’re one step further on a journey,” says Heller. “It will take a while until we reach any kind of clinical relevance.”

How Nature Protects the Eardrum

December 28, 2009

The eardrum is a highly sensitive membrane and nature has taken various precautions to protect it from damage:

Cerumen (earwax): The external part of the ear canal contains sweat and sebaceous glands. The glands produce a waxy substance known as cerumen or earwax, which traps bacteria and dust. Cerumen is only produced in the outer third of the ear canal where it can be removed. Never try to clean down inside of the canal yourself. If you have a problem, consult your hearing care professional or physician.

Protective hair: The external part of the ear canal is covered with tiny hairs that act like a curtain to protect the eardrum from dust and dirt. If dust or bacteria do succeed in penetrating the curtain they are trapped by the cerumen.

Ear canal: The eardrum is further protected by bends in the ear canal. This helps prevent objects accidentally entering the ear and damaging the eardrum.

Cerumen and hearing instruments: When you first start wearing a hearing instrument, it often seems like a foreign object in the ear. This feeling disappears after a short familiarization period. All the same, hearing instruments can stimulate the production of cerumen. Some instruments are equipped with a protective cerumen filter to help prevent earwax from entering the instrument. If the outlet of the hearing instrument is blocked by earwax, the volume may be reduced or even cut-off altogether.

Hearing with our skin

December 12, 2009

Hearing with our skin

If your ears can’t hear what someone is saying what about listening with your skin?
According to a study in the journal Nature, sensations on the skin can help people understand speech.
Just like looking at someone’s lips in a noisy place, or if you have a hearing loss, feeling, can also help you hear.
“From our brain’s point of view, we can hear with our eyes,” says Bryan Gick, a professor of phonetics at the University of British Columbia in Vancouver.
“From my point of view, we’re whole-body perceiving machines,” Gick says. “We just take all of the information that comes at us in our environment and merge it into a percept of something that happened in the world.”
Where does the Integration Happen?
David Ostry, a professor of psychology at McGill University in Montreal says these days the big scientific question isn’t whether our brains routinely integrate sensory information, but how.
“It’s up for grabs where within the brain this kind of integration is happening,” he says.
One possibility is areas in the brain that process sensory information, Ostry says. But he notes it’s also possible that integration takes place in the motor cortex, which controls our muscles.
Researchers say what they learn about how other senses influence hearing could help people with hearing loss, as well as people such as commercial airline pilots, who often have to decipher speech in a noisy environment.

If your ears can’t hear what someone is saying what about listening with your skin?

According to a study in the journal Nature, sensations on the skin can help people understand speech.

Just like looking at someone’s lips to help you hear better,  feeling sounds can also help you hear.

“From our brain’s point of view, we can hear with our eyes,” says Bryan Gick, a professor of phonetics at the University of British Columbia in Vancouver.  ”We’re whole-body perceiving machines,” Gick says. “We just take all of the information that comes at us in our environment and merge it into a percept of something that happened in the world.”

Where does the Integration Happen?

There’s no question that our brains routinely integrate sensory information, but how our brains do that, is still unclear.  One possibility is that integration takes place in the motor cortex, which controls our muscles.

Researchers say what they learn about how other senses influence hearing could help people with hearing loss, as well as people such as commercial airline pilots, who often have to decipher speech in a noisy environment.

Noise Exposure and Convertible Cars

November 6, 2009

It is known that noise exposure can cause temporary and/or permanent damage to hearing ability. Repeated and prolonged exposure increases the chance of causing damage. In a study completed by Michael, Opie, & Smith, the noise levels while driving a convertible car were measure to analyze the potential for noise induced hearing loss. In the study they used 7 different convertibles and compared the noise level at different speeds and with the windows up and down. The results suggest that when the windows and roof of the convertible are down, the level of noise is such that it has the potential to cause damage to hearing. Interestingly, at a speed of 50 mph the noise level was higher than at 70 mph. The mean noise level was 85 dB – 90 dB.  The noise level can be exaggerated by road surface and traffic load. The article suggests that hearing protection can be worn, although the potential for damage can be reduced by rolling up the windows.

Who do you really need two hearing aids?

October 10, 2009

With some exceptions, if you have hearing loss in both ears, you should have hearing aids in both ears.  There are three primary reasons for this:
1.  The brain needs input from both sides of the head for balanced hearing.
2.  The brain needs balanced hearing to be able to localize sounds.
3.  The brain needs balanced hearing to hear voices in noise.*
* Difficulty hearing in noise is one of the most common complaints we hear.  A single hearing aid will be of benefit in a quiet listening situation.  However, in a noisy situation such as a restaurant, dining room, work environment, or car, the brain needs input from both ears to pull out a specific speech signal from the background noise.  Hearing aids cannot do this alone! Advanced technology hearing aids often employ a circuit to help reduce noise; however, your brain will not perceive this benefit from only one ear.
Expectations from a hearing aid
A hearing aid CAN:
1.  Make soft sounds louder, thus making them easier to hear.
2.  Allow you to hear in some situations that used to give you trouble, such as conversations.
3.  Help you hear high-pitch sounds better, thus helping you to understand speech better.
4. Help you feel more at ease in social situations by making it easier to hear what is being said.
A hearing aid CANNOT:
1.  Allow you to hear extremely soft sounds.
2.  Cure distortion in your hearing.  Distortion is usually due to a problem in your inner ear.  A hearing aid will mechanically equalize the sounds you hear, but not correct the damaged inner ear.
3.  Allow you to hear well in ALL situations of background noise.  An advanced hearing aid noise setting will enhance speech and reduce background noise; however noise will likely continue to be the most difficult listening environment.  Lip-reading will compliment the sounds you hear in all situations.
4.  Amplify only what you want to hear.

With some exceptions, if you have hearing loss in both ears, you should have hearing aids in both ears.  There are three primary reasons for this:

1.  The brain needs input from both sides of the head for balanced hearing.

2.  The brain needs balanced hearing to be able to localize sounds.

3.  The brain needs balanced hearing to hear voices in noise.*

* Difficulty hearing in noise is one of the most common complaints we hear.  A single hearing aid will be of benefit in a quiet listening situation.  However, in a noisy situation such as a restaurant, dining room, work environment, or car, the brain needs input from both ears to pull out a specific speech signal from the background noise.  Hearing aids cannot do this alone! Advanced technology hearing aids often employ a circuit to help reduce noise; however, your brain will not perceive this benefit from only one ear.

Cheap Hearing Aids

September 13, 2009

Recent research done at Michigan State University indicated that although consumers with hearing loss might think they are saving significantly more by purchasing inexpensive catalog or over-the-counter hearing aids,  they most likely will be disappointed – or could be taking risks.

There is a high cost variability of hearing aids, and because most consumers do not have or have only partial insurance coverage for hearing aids, people often look for inexpensive options.  Low-cost options are typically marketed on the Internet and in mail-order magazines as listening devices – often for bird watchers or deer hunters.

“These low-cost amplifying devices can look tempting to individuals with hearing impairment because of the significant cost differences,” on researcher, Punch said. “But our research found that the low-cost aids generally don’t meet the fitting requirements to help a hearing-impaired person and could potentially damage a person’s hearing.” 

The research is important to consumers, Callaway says. “Aside from being of extremely poor quality, very low-cost hearing aids (those under $100) have the potential to damage your hearing because they send very loud sounds into the ear.

“Based on the research, the best advice for consumers is to talk to an audiologist. Because hearing aids have complex technical features, they need to be fitted and customized to the individual.” 

The study measured how well the electronic features of the devices could compensate for commonly occurring types of hearing loss, employing methods that audiologists use to fit conventional hearing aids – a process audiologists call prescriptive fitting. Specifically, the researchers found that only a few of the aids they studied met the basic fitting requirements, and, for the few that did, that was true only for a specific degree of hearing loss. 

Although the Food and Drug Administration officially regulates hearing aids, those regulations are not enforced for low-cost amplifying devices that are sold through mail order and on the Internet.

Genetic Hunt turns up Deafness

In genetics, as in life, surprising things can turn up in unexpected places. That was certainly the case when an international group of researchers found three mutations responsible for a form of hereditary deafness in a gene that is implicated in cancer. The research is published in the July 10 issue of The American Journal of Human Genetics.

This surprising finding happened when researchers from the National Institute on Deafness and Other Communication Disorders (NIDCD), National Cancer Institute (NCI), Baylor College of Medicine, Houston, All India Medical Institute in India, and Punjab and Islamabad Universities in Pakistan were scanning the DNA of over a thousand families with deaf children, searching for genes that could be responsible for inherited forms of deafness. Pooling their data, the researchers narrowed the region of one such gene, which they called DFNB39, to a stretch of DNA on chromosome 7.

Genes are segments of DNA that contain the codes for making proteins, which are the building blocks of all the tissues and organs in the body. Nearly all of the human genetic diseases known by scientists are caused by changes in the protein-coding gene sequences in DNA.

However, there are other parts of genes that do not specify the protein’s composition. Instead, these “noncoding” sections regulate the expression of a gene. For example, a regulatory section might turn the gene on or off in one tissue, such as the brain, but not another, such as the heart. Or it might control the timing of the gene’s expression, for example by turning it on only during development or leaving it on all the time. Scientists know much less about these noncoding, regulatory sequences than they do about the protein-coding sequences. As a result, fewer human genetic diseases are known to be associated with such sequences.

When the investigators examined the protein-coding sequences of every gene in the DFNB39 region, they came up short. They could find no protein-coding DNA changes in any of the genes. Thus, they concluded the deafness-causing change had to be in a regulatory section. Exploring further, the scientists found three non-coding mutations in the gene that encodes hepatocyte growth factor, or HGF, which is important for the growth and regeneration of liver tissue.

HGF has also been shown to have different and powerful effects in other tissues. Generally, it influences the production and growth of cells, so many scientists study its effects in cancer, fetal development, and wound repair in adults. Until recently, very few mutations had been found in the HGF gene. Those that were found were in the protein-coding parts of the gene and caused cancer-related diseases. There was no reason to expect that mutations in the HGF gene could cause deafness.

So how do mutations in a gene related to cancer cause deafness?

Often, scientists turn to the mouse as a model organism to study the effects of a gene mutation; however, mice with mutations in the coding sequence of the HGF gene die as embryos. Taking another tack, by looking at mice in which the HGF protein was normal, but over-expressed in all tissues, or under-expressed in just a few tissues including the inner ear, the scientists discovered that the mice survived, but were deaf. This led the researchers to wonder if the DFNB39 mutations caused deafness in humans because of over- or under-expression of HGF.

The group also noted that another mechanism of gene regulation, alternate splicing, was at work in the HGF gene, and was producing alternative forms of the protein-called isoforms. The HGF gene produces five isoforms, and potentially a sixth that the team discovered. The NIDCD-led researchers found that one of the three mutations appears to influence the way the HGF gene chooses between some of the isoforms. The other two mutations in the HGF gene occur in a region that may affect the sixth isoform. Subtle differences in these isoforms could be what drive the development of deafness.

The researchers are now working on a “knock-in” mouse model of DFNB39, in which they will make mutations in the mouse gene that are similar to the regulatory mutations they see in deaf humans. They will use the mice to test their ideas about how regulation of this gene plays a role in hereditary deafness.

Contributors to this research include Drs. Julie Schultz and Robert Morell of NIDCD’s Laboratory of Molecular Genetics, along with lab chief Dr. Thomas Friedman, NCI’s Dr. Glenn Merlino, Dr. Suzanne Leal at Baylor College of Medicine, Dr. Sheikh Riazuddin, of Punjab University, and others.

Taken directly from http://www.nidcd.nih.gov.