Initiation and Formation of Stereocilia during the Development of Mouse Cochlear Hair Cells

Stereocilia are apically located actin-protrusions found on the hair cells of the inner ear. At least three rows of stereocilia are arranged in a graded staircase pattern, which is vital for mechanosensation. Stereocilia form soon after the specification of hair cells. While these steps have been well-characterized in the avian auditory epithelium, the equivalent information in mice is lacking. Using scanning electron microscopy and super-resolution microscopy, we investigate stereocilia formation from hair cell specification stages in the mouse organ of Corti. Even before differentiation, we find that sensory progenitors, which will give rise to both hair cells and support cells, have a dense lawn of microvilli. Hair cell specialisation is first apparent as an enrichment in junctional actin, followed by the relocalisation of kinocilium into an eccentric position and the thickening of hair cell microvilli closest to the kinocilium. To determine actin signatures associated with hair cell development, we use a new analytical method to map cellular actin filament distribution during development. By nomalising relative actin filament density, we obtain insights into cuticular plate development and actin redistribution during the earliest phases of hair cell specialisation.