Activity and retinoic acid drive hair cell spatial patterning in the zebrafish utricle

The zebrafish vestibular otolith organs, like those of other vertebrate species, are organized into central (striolar) and peripheral (extrastriolar) zones that drive different vestibular circuits. How and when these hair cell patterns develop in the zebrafish is unknown. We determined that early-developing hair cells (36 hours) mostly develop directly as striolar or extrastriolar. We observed complementary patterns of RA degrading and synthesizing enzymes that colocalize with striolar and extrastriolar zones, respectively, suggesting evolutionarily conserved molecular signaling. RA treatment during development increased the relative proportion of extrastriolar and intermediate-type hair cells, indicating that enriched RA reduces striolar development. However, in fish with mechanotransduction dysfunction from a cdh23 mutation, normal RA patterning is insufficient to finalize the fate of early-born hair cells, which remain transcriptomically unresolved. RA treatment further exacerbates abnormal patterning. Therefore, we conclude that hair cell fate, and thus normal zonal patterning, depends on both hair cell activity and the RA gradient.