Stem Cells Return Lost Hearing

Journal Stem Cells, May 5, 2009

Researchers at the University of Sheffield have announced that they have differentiated fetal stem cells into the sensory hair cells of the ear, which could then be surgically inserted into the ear to restore lost hearing, at least theoretically.

According to Dr. Marcelo Rivolta, who led the study, "It will take several years before we are in a position to start doing human trials." He and his colleagues caution that the research is still in the preclinical, animal model phase, and has not yet advanced to a level where it could be performed safely and effectively in humans.

In the past, whenever auditory hair cells were damaged, permanent hearing loss would result since it was not possible to repair the damaged cells. Now, however, stem cells could be differentiated into auditory cells, thereby restoring lost hearing.

According to Dr. Ralph Holme, director of biomedical research at the Royal National Institute for Deaf People, which helped fund the research, "This research is incredibly promising and opens up exciting possibilities by bringing us closer to restoring hearing in the future."

Although the scientists used fetal stem cells, they are also investigating the ability of embryonic and adult stem cells to differentiate into auditory hair cells. Fetal stem cells have a number of inherent problems and risks, not the least of which is the formation of tumors. (Please see the related news article on this website, entitled "Fetal Stem Cell Therapy Could Prove Fatal", dated February 17, 2009). Presumably this is also the reason why iPS (induced pluripotent stem) cells were not differentiated into auditory neurons, which iPS cells are certainly capable of doing, but scientists have not yet been able to create a completely safe iPS cell which is free of cancer-causing genes and which also lacks the strong tendency to form teratomas (a type of tumor with cells and tissue from all 3 germ layers).

The auditory hair cells of the cochlea, or cochlear hair cells, function as the sensory receptors of both the auditory system and the vestibular system, not only in humans but in all vertebrates. In the past, damage to any part of the cochlear apparatus would often result in decreased hearing sensitivity, which was always considered to be irreversible. These fine hair cells are "bundled" in two sections, known as outer and inner hair cells, each of which performs a specialized function. The outer hair cells provide a purely mechanical amplification of sound, in a similar way as an acoustic preamplifier. The inner hair cells act as a more complex transducer, converting the mechanical vibrations into electrical signals that travel via the auditory nerve to both the auditory brainstem and the auditory cortex of the brain. Although the normal human hearing range is considered to be from 20 Hz to 20,000 Hz, some degree of sensorineural hearing loss is normal with advanced age.

Since all aspects of the auditory system were originally formed from stem cells, then at least in principle it should be possible to regenerate all aspects of the auditory system, given the right type of stem cell. At least conceptually, it should be possible to regenerate not only the auditory sensory receptors of the cochlear hair cells but also auditory neurons as well, regardless of where, exactly, the anatomical damage is located.