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A better understanding of hearing loss
3/22/13

(EN)image-confocale"The hair cells and the spiral ganglion neurons don't regenerate after they are destroyed. Cochlear implants, which are the only means we currently have to restore partial hearing in some cases of hearing loss, only work if at least some of the connections between cells are present," indicates the scientist. These very specific connections are made during embryonic development. But there are two types of hair cells and spiral ganglion neurons and they cannot come together at random.  The inner hair cells connect to type I neurons while the outer hair cells connect to type II neurons. "The inner hair cells are those that transmit sound while their outer counterparts serve more as frequency adapters," explains Brigitte Malgrange. The most important connection for auditory function is therefore the one between inner hair cells and type I spiral ganglion neurons.

When the hair cell-neuron pairs are formed

Jean Defourny has focused on these connections as part of his PhD thesis in the Developmental Neurobiology research unit. "The two types of spiral ganglion neurons emit projections that extend towards the specific hair cells that they must connect to," adds Brigitte Malgrange.  "We are interested in the proteins that allow each type of neuron to find the right hair cell."

In the mid-90s, a Swedish research team identified proteins secreted by the hair cells that attracted neuron projections. "It's what we call chemoattraction. Back then the scientists thought that each type of hair cell produced a different neurotrophin. But during the 2000s, we realized that it wasn't so simple because we could replace one neurotrophin with another without affecting the neuron-hair cell connections at all," continues Brigitte Malgrange.

In light of this information the researchers from Liège thought that there might also be molecules designed to repel neuron projections. For in order to reach the inner hair cells, the neurites have to pass through the outer hair cells. "Our theory was that the outer hair cells might be sending some kind of message that would repel the type I neuron projections and ensure they reach their targets," explains Brigitte Malgrange.

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