Regeneration

Hair cells in the organ of Corti are "born" during the first three months of fetal development. They resemble neurons because they must last a lifetime. Unlike the cells that make up our liver, heart and kidney, cochlear hair cells and neurons do not replace themselves. One reason that both hair cells and neurons do not replace themselves may lie in the number of contacts they have with other neurons. During development, neurons and hair cells make contact with many other neurons with which they communicate. The process that controls the formation of contacts appears to be both genetic and competitive. Many contacts are made, correct connections that benefit the brain survive and inappropriate connections disappear. If a new hair cell were to come into existence in an adult ear it would have to be rewired to the myriad of fibers that connect it with the brain. The precise organization of the organ of Corti poses yet another obstacle for a regenerating hair cell. The cells that might change into hair cells are located near the central axis of the cochlea, if one of these could be made to change into an outer hair cell it would have to migrate outwards past the inner hair cells, through all the supporting cells to find its correct place in the precise rows of outer hair cells. Even though outer hair cells change length when electrically stimulated, they cannot move in a manner that would allow them to make their way through the intervening supporting cells.

Several years ago it was reported that bird ears recover function after hair cell damage and this was followed by a report that new hair cells can appear in human balance organs. These reports triggered considerable optimism that regeneration might be possible in the adult organ of Corti, but so far there is no evidence for cochlear hair cell regeneration. The problem of hair cell regeneration is comparable to that of neural regeneration. The severe spinal cord injuries of Former Governor Wallace and the actor Christopher Reeve have focused a great deal of attention on research into neural regeneration. One benefit of the research was that when Mr. Reeve sustained his injuries he was quickly treated with drugs that reduced the amount of neural damage. He is now regaining some sensation that might not have been possible 10 years ago. A similar strategy would be to investigate the use of similar drugs to protect hair cells when they are subjected to potentially damaging situations. Just as research continues on neural regeneration, research should continue on hair cell regeneration and perhaps someday we will be able to coax new hair cells to grow. Even if we are successful in achieving regrowth, considerable work will have to take place in learning how to promote correct rewiring of the hair cells with the central nervous system. The process could be more challenging than learning a foreign language as an adult.

Next chapter: Central Hearing System - Where Sound Is Reconstructed

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