You and a chicken are at a loud rock concert. Both of you get hearing damage from standing too close to the speakers. Within a month the chicken regains its hearing, but yours is never quite the same. Why?
This is one of a few questions a group of University professors are trying to answer in their laboratory study of sensory receptor cells.
Unlike that chicken, the nerve cells responsible for mammals' (i.e. human) hearing and balance are unable to recover after injury (i.e. from a loud rock concert). Non-mammals like the chicken can gradually recover from hearing damage.
Loud sounds, infections and drug treatments can damage auditory hair cells. These hair cells, which are located in the inner ear, are not made of the same hair we have on our heads. Rather, the cells are named this way because their surfaces are covered with giant microvilli that resemble strands of hair.
When a sound wave enters the ear, the "hairs" tilt because of vibrations. The inner ear cell senses this movement and then generates an electric signal that informs us of the sound.
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But if these hair cells die, this could cause permanent hearing loss, since they cannot be regenerated, said Jeffrey Corwin, otolaryngology professor at the University Medical School.
"If auditory hair cells in mammals are damaged, it is thought that they can't regenerate," which can result in a pretty significant loss of hearing, Psychology Prof. David Hill said. "In contrast, the taste and olfactory [smell] systems are continually being renewed even without injury."
In an attempt to discover why auditory hair cells are unique in their inability to regenerate, all five professors will lead three sensory cell research projects. Their work is funded by a $2.8 million, five-year program project grant that was received from the National Institute on Deafness and Other Communication Disorders in May 1999.
The first project, led by Biology Prof. Robert Grainger, Psychology Prof. Peter Brunjes and Corwin, studies human embryo development and focuses on how human ears and noses grow.
The second project examines regeneration of auditory hair cells and is led by Mark Warchol, a former University professor who is currently a professor at Washington University's Central Institute for the Deaf in St. Louis.
The final project, which is being headed by Hill, will attempt to examine the regeneration of taste receptor cells.
The scientists hope to find parallels between the way sensory cells for taste, and the way those for hearing.
All sensory cells have important interactions with the environment and send signals back to the brain, Grainger said. Such similarities have led to the suggestion that they all have the same genetic basis. So if scientists can trace the development of one, they may be able to trace the development of another cell and figure out a way to repair sensory hair cells.
"By examining early embryonic development," he said, we hope to "see if they are formed by processes that are related."
An understanding of the normal processes of development for such sensory cells ultimately would benefit efforts to regenerate auditory hair cells.
"If you understand how sensory cells generate, you will be able to better understand how they are able to repair," Grainger said.
Grainger is studying several genetic mutations on frog embryos, which are large and relatively easy to examine, in an attempt to understand the normal processes of sensory development.
In one such mutation, the two nostrils of the frog are fused together, to help the group study how the nose of a non-mammal develops. In another genetic mutation, the frog's ears develop improperly, causing the embryos to swim in circles, since hearing is essential to balance, he said.
While Grainger is examining the early development of cells, Warchol is focusing his research on the ability of auditory cells to repair after injury.
One concentration of his research is on the impact immune cells can have on the regeneration of sensory cells.
There is evidence that in other parts of the nervous system immune cells can play a crucial role in nerve regeneration, like after spinal injuries, Warchol said.
It is known that numerous immune cells are present around sensory cells, and immune cells become more abundant and change shape after injury to the sensory cells, he said. His research will study how the presence of such immune cells affects the regeneration of sensory cells.
Specifically, Warchol is examining the various ways birds are able to regenerate hair cells and how these mechanisms differ from those used by mammals.
Hill's research also is exploring sensory cell regeneration and the immune system - this time in the taste system.
In the second aspect of his research, he is continuing to study how newly formed receptors respond to different taste stimuli. He discovered that if you initially restrict the cell environment exposure to sodium, the taste cells will no longer respond appropriately to sodium, Hill said.
Through these three research projects and their collaboration, the scientists are trying to understand the basic biology of sensory cell systems, he said.
"This information could later be used by clinicians and then applied to the community as a whole," he added.
One tenth of the general American population suffers from significant hearing loss. 80 percent of these people had loss that originated from damage to sensory hair cells in the ear. The remaining 20 percent were born deaf.
Hill noted that those people with damaged sensory hair cells have always been accustomed to communicating and hearing auditory dialogue. So whereas people born deaf have their own special language, those with damaged sensory hair cells are at a particular disadvantage because they will have to alter their lifestyle to be able to communicate with the outside world.
The mission for these three interdisciplinary projects is to provide the information necessary so that someday doctors can stimulate the growth of sensory hair cells in mammals and help improve the quality of life for affected individuals, Corwin said.
Maybe someday humans and chickens will be able to enjoy another beautifully rendered rock concert together, without any threat of losing their hearing permanently.
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