Scientists studying the inner ears of birds have found a clue that could lead to the possible development of a therapy to restore hearing to people who have a deafness caused by exposure to loud noises.
In articles published Friday in Science, two groups of researchers report that both adult quails and young chickens are able to regrow cells in the inner ear after their hearing has been damaged by acoustic trauma.The studies suggest that there may be a hormone or protein secreted by damaged sensory cells in the ear could cause the growth of new, replacement cells.
Edward W. Rubel of the University of Washington, co-author of a study using adult quails, said the finding is "truly new" and could eventually be important to some of the 22 million Americans, among them President Reagan, who suffer from hearing loss.
"We have not found a cure for deafness, but we have found something in the ear of birds that we haven't thought possible," Rubel said.
Douglas A. Cotanche, an assistant professor at the Boston University School of Medicine and co-author of a study using young chickens, said researchers are not yet able to show that the regrowth of the cells critical to hearing also occurs in mammals or humans.
But Cotanche said the findings do offer a theoretical possibility of one day correcting a type of deafness in humans.
Both of the bird studies centered on an inner ear organ called the cochlea, a key part of the hearing process. The researchers discovered that when sensory cells within the bird cochlea are destroyed by loud noise, some chemical process causes the growth of new sensory cells, apparently restoring a key link in the hearing process.
In the hearing process, sound waves cause the ear drum to vibrate. This vibration is transmitted through bony parts of the middle ear to the inner ear where it creates a pressure wave in the fluid-filled cochlea. Microscopic hairs lining the cochlea act as sensors to detect this pressure wave and cause nerve signals to be sent to the brain, which interprets the signals as sound.
Intense levels of noise can cause the cochlea hair cells to be killed, thus lowering the hearing acuity. If enough hair cells are lost, deafness can result.
Some people, such as Reagan, can compensate for the hearing loss by wearing an electronic aid that amplifies sound.
Reagan's hearing problems began early in his movie career. When he was co-starring in a crime story film, a pistol was fired just inches from his ear. He later said that the acoustic trauma was so great that he almost collapsed. The injury severely damaged the hearing in one ear, but Reagan's doctor said he wears two aids, as do many people, to achieve a hearing balance.
Even hearing aids, however, are ineffective if enough of the hair cells have been killed.
"These are the most important cells for hearing and cells that we can't replace surgically," Rubel said.
But the findings in the studies on quails and chickens raise the possibility of finding a way to cause the body to stimulate regrowth of these hearing cells.
"If we can find the protein or the genes that get turned on to do this (ause hair cell regrowth in birds) then we may be able to artificially stimulate them in a mammal, but that's still far down the road," Cotanche said.
For their independent studies, both Rubel and Cotanche injected two groups of laboratory birds with a tracer chemical that marks genetic and cellular activity. One group of birds in each study was then subjected to traumatic levels of sound. Control groups of birds were not exposed to the noise.
Later the birds were killed and their hearing organs analyzed. The scientists, by checking for the presence of the tracer chemical, were able to show that the traumatic sound caused the spontaneous growth of replacement hairs cells in the cochleae of the birds subjected to the noise. Birds in the control groups showed no such new cell growth.
Cotanche was co-author on the chicken study with Jeffrey T. Corwin of the University of Hawaii. Rubel's co-author in his study was Brenda M. Ryals of the Veterans Administration Medical Center in Richmond, Va.