A novel treatment using human embryonic stem cells has successfully restored some hearing to previously deaf gerbils, according to a study published this week in the journal Nature.
Hearing loss is generally caused by the interruption of two different types of cells: The loss of hair cells in the ear, which transform vibrations into electrical signals, and loss of the auditory nerve, which transmits the signals detected by the hair cells to the brainstem. While cochlear implants have proven effective in restoring hearing in cases of hair cell damage, no such treatment has existed for the roughly 10% cases in which the auditory nerve itself is damaged.
The new strategy, designed by Marcelo Rivolta and his team at the University of Sheffield, uses techniques the group has recently developed to coax human embryonic stem cells to differentiate into what are called "otic progenitor cells" -- cells that have the possibility to develop further into either hair cells or auditory nerve cells. The progenitor cells are then transplanted into the ears of gerbils with damaged auditory nerves, and allowed to differentiate further. Gerbils were used in the experiment because they hear a similar range of sounds as humans do.
At that point, the researchers held their breath, hoping that the cells would integrate themselves with the existing infrastructure and take their place in the chain of sensory signaling between the hair cells and the brainstem. In nearly all cases, the scientists could clearly see under the microscope that the new cells had targeted the right spots, reconnecting the hair cells to the brainstem.
But the ultimate test is hearing itself. To test this, the researchers used a standard approach called auditory-evoked responses, which are detected in the brainstem and provide a clear verdict of whether or not sound is being successfully transmitted to the brain.
Control animals with their auditory nerves knocked out did not recover during the experiment -- in order for a sound to register an auditory-evoked response in the brainstem, the control animals basically had to be at a rock concert, requiring a 76-decibel sound. But in the treated animals, that number dropped to 50 decibels on average, and in some animals approached the levels of animals whose hearing was never damaged at all. The strength of the effect was akin to suddenly being able to hear someone talking while previously not being able to hear them yell.
The researchers hope that their method will spark a new interest in using stem cells to treat hearing loss in people, though much work needs to be done before that is a real possibility. Hurdles include developing a surgical technique to access the appropriate part of the ear in people, and ensuring that the treatment sticks over long periods of time.
Nevertheless, the scientists are optimistic that the approach can be directly translated to humans with hearing loss, finally allowing people who cannot benefit from a cochlear implant to hear again.