Hearing in mammals is dependent on the correct development of the spiral-shaped structure called the cochlea, which lies within the inner ear. The cochlea contains two major cell types responsible for hearing; hair cells that detect sound and convert it into electrical impulses and spiral ganglion neurons that transmit these impulses to the brain. Damage to the cochlea, which can lead to sensorineural hearing loss, is usually associated with the degeneration and irreversible loss of these cell types.
In order to better comprehend the processes underlying deafness and drive new therapeutic strategies, a greater understanding of human cochlear development is needed. Research published in a recent study in Neural Development by Heiko Locher and colleagues from Leiden University Medical Center, the Netherlands, investigates the onset of hair cell differentiation and innervation in the human fetal ear at various stages of development between 10-20 weeks.
In particular, they looked at the spatial and temporal dynamics of hair cell differentiation by examining the expression of three members of the SOX family of transcription factors, which are known to be involved with cell fate decisions: SOX2, SOX9, and SOX10. In humans these SOX genes are likely to play an important role in cochlear development, as mutations in all three genes have been shown to cause sensorineural hearing loss.
Through staining and immunohistochemistry of human embryonic and fetal cochleae, Locher and colleagues find that SOX2 and SOX9 exhibit a similar expression profile as in mice, with SOX2 defining the early prosensory domain of the fetal human cochlear duct. Their findings also support the notion that SOX2 and SOX9/SOX10 may have different roles in determining the fate of hair cells versus supporting cells.
With much of the current knowledge on cochlear development arising from animal studies, these results bring us closer to understanding the timing of some of the essential steps during human ear development, as well as identifying some of the key molecular players.