Cells of the body perform specific functions. But stem cells are cells which are undifferentiated but have the power to perform as any cell as required by the body. This characteristic of stem cells has also shown some promises to treat several health conditions and diseases. This, therefore, has made Audiologists to question whether these stem cells could be used for hearing loss treatment. It is still an area which is under research.
How Do We Hear?
The human ear can be subdivided into three main parts - the outer ear, the middle ear and the inner ear. Sound waves travel from the outer ear to the middle ear in which the small ear bones, known as the ossicles, vibrate on receiving the sound stimuli. This vibration is sent to the inner ear and passes through the cochlea and stimulates the hair cells present in it. Hair cells are the main components for which we can hear. These are small sensory cells present in bundles inside the cochlea of the inner ear which convert the sound vibrations into electrical impulses. These impulses are sent to the brain via the auditory nerve and then can a person ‘hear’.
When these microscopic hair cells and/or the auditory nerve are damaged - by disease, injury or due to ageing - a person suffers from severe to profound sensorineural hearing loss. Although fish, amphibians and birds have the ability to regenerate damaged hair cells, mammals don’t have this ability to regenerate hair cells naturally on their own. Although wearing hearing aids or going for other hearing loss treatment methods such as cochlear implantation, would help you to hear, but otherwise, your hair cells don’t regenerate on their own.
What was the Scenario Yesterday?
According to studies, about 90% of the permanent sensorineural hearing loss is due to damages in either the hair cells or the auditory nerve cells. Scientists previously believed that the hair cells in mammals once damaged could not be restored.
But some studies suggested that when a hair cell develops, it inhibits the neighbouring cells from becoming hair cells. Instead, they become supporting cells, which made scientists wonder whether these supporting cells have the ability to turn into hair cells when the neighbouring hair cells are injured or damaged.
It was very necessary for the researchers to find an answer to this. Hence, a research study was carried out by a group of NIH-supported scientists to test the destroyed hair cells in the inner ear of a mouse. A time-lapse photography set-up showed that the neighbouring supporting cells of the damaged hair cells gradually migrated to the hair cell region and grew to be hair bundles on their surfaces. Although the procedure couldn’t restore the hearing ability of the mouse, this experiment definitely opened up windows to new possibilities of whether damaged hair cells could be restored, thus restoring or at least improving hearing loss. This would then be the greatest research in the history of the development and the innovations made in the hearing loss treatment procedures to date.
What New Researches Say?
Hearing loss has become an alarming problem as it affects people of all ages from birth to adulthood. This compelling need to address the problem, together with the previously done researches, has urged scientists to invent methods and the possibilities for hair cell regeneration with the help of stem cells.
The previous research studies have proved that the supporting cells have the ability to become hair cells under certain conditions as and when required such as when the hair cells of the inner ear have been damaged or severely injured. NIH-supported scientists have identified a gene in mice called Rb1 which inhibits the growth of new hair cells in the early stages of their development. An experiment was done with mature hair cells growing in culture dishes in which it is proved that if this gene is deleted, these hair cells were able to regenerate.
However, the hearing ability would not be restored if the nerve endings were not properly reconnected. The scientists were later able to find out a way to successfully reestablish connections between these newly formed hair cells and the nerve cells in an organised way. This can bring permanent solutions in the field of hearing loss treatment.
Further Researches to be Made in the Future
Although experiments proved that it is possible to reconnect the hair cells with the nerve cells, yet more discoveries and inventions are yet to be made which would include the following -
The hair cells generated from the stem cells should be mature enough to mimic the environment that the naturally developing hair cells would experience.
How these newly generated hair cells would send appropriate signals to the hearing centre of the brain needs to be explored.
New methods yet to be developed to protect the hair cells from damage after trauma or disease, thereby enhancing the survival of these hair cells. For this, a better understanding of the cellular and the molecular mechanisms of the body need to be studied.
Researches are going on to use stem cells to regenerate hair cells and restore the lost hearing ability of a person. If this becomes a success, then in the future it might be even possible to eradicate hearing loss.