Hair Cell Regeneration Research
Hair cells in the inner ear are essential for hearing and balance. Without them we would be deaf or disoriented. If hair cells are damaged in any way, we suffer permanent hearing loss or balance degeneration. New research, however, is showing that there may be a way to regenerate damaged hair cells.
What are hair cells? They are called hair cells because under a microscope they look as though they have a tiny hair protruding from them. These hair-like structures are called stereocilia, and they bend in response to sound. There are two types – auditory and vestibular. Auditory hair cells are located in the cochlea of the inner ear, and are responsible for detecting sounds. Movements of the hairs are translated into electrical signals which are transmitted to the brain and interpreted as sound.
There are also hair cells responsible for our sense of balance. These are located in a different part of the inner ear - the vestibular organs - and are called vestibular hair cells. While damaged auditory hair cells can be compensated for with the use of hearing aids, there is no replacement or cure for damaged vestibular hair cells.
Hair cells do not function in isolation, but need to be connected to the auditory centers of the brain through nerve fibers. The challenge of hair cell replacement is not only to replace damaged hair cells, but to somehow get them to reconnect to the nerves so they can send information to the brain.
Sounds louder than 90 decibels will cause temporary damage to the hair cells. The hair cells become flattened resulting in short-term hearing loss. Usually though, they will regain their shape within two days. Long-term exposure to loud sounds, however, will result in permanent hearing loss.
Other things that can damage hair cells include ototoxic drugs (drugs which damage the hearing), disease, and aging. Once human hair cells have died, they cannot be regenerated. In other species however, hair cells are constantly regenerated to restore hearing loss. Researchers are now working to see if this process can be somehow applied to human hair cells, thereby restoring lost hearing.
Hair Cells in Birds
In the mid 1980s, scientists discovered that adult birds can regenerate hair cells. In researching this phenomenon, it was discovered that birds have the ability to recover their hearing when the hair cells have been damaged by loud noises or drugs. The recovered hearing seems to be equivalent to the hearing ability the bird had before the damage occurred.
Besides birds, amphibians such as frogs can also regenerate hair cells. Mammals, however, do not have this ability. Researchers are trying to discover the mechanism behind hair cell regeneration and to see if it can be applied to humans.
There seems to be two major factors in this process – the regeneration of the hair cell, and the reconnection of the hair cell to the nerve cells. It appears that when the hair cell is produced it secretes molecules called “trophic factors” which attract nerve fibers. When the hair cells are connected to the nerve cells, hearing is restored.
The major obstacle in duplicating this process in humans is to generate new hair cells. This has to be done through a process of cell division. Recent experiments using Guinea pigs, mice, and rats have succeeded in promoting cell division within the inner ear using growth-promoting molecules. So far, similar molecules for human use have not been found, but at least the possibility of hair cell regeneration in mammals has been confirmed.
Transplants and gene therapy
Another approach that is being investigated is to transplant hair cell precursors into the inner ear. Precursors are cells from which other cells are formed. Unfortunately, hair cell precursors have not yet been identified, but embryonic research is being done to identify which cells in the embryo develop into hair cells.
Gene therapy is also being researched as a way to stimulate hair cell growth. Already, the genes responsible for stimulating precursor cells into hair cells had been identified. This method for re-generating hair cells within humans is being actively investigated with steady progress.
At this time, nothing can be done to replace hair cells in humans. Damaged hair cells must be compensated for with the use of hearing aids or cochlear implants. There is also no current therapy for damaged vestibular (balance) hair cells. While someone might think that they should hold off treatment because “the cure is just around the corner”, this really is not the case. In children, this philosophy is especially damaging because children have a relatively short period of time for explosive language development, and waiting for better treatments to come along may result in that window closing, and the child not receiving any meaningful benefits even when the better treatment is finally commercially available.
Research over the past twenty years, however, has shown amazing potential. Despite limited funding, scientists have learned much about the mechanisms involved in hair cell regeneration and its possible application to human hearing loss. Perhaps within twenty years we will have a viable method to replace human hair cells and restore lost hearing.
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