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 spatial hair cell patterns develop in the zebrafish is unknown. We determined that early-developing hair cells (<36 h) expressed both striolar and extrastriolar transcriptomic markers. After 36 h, these hair cells become specified as extrastriolar hair cells. Later-developing hair cells (>36 h) mostly develop directly as striolar or extrastriolar. We observed complementary patterns of retinoic acid (RA) degrading and synthesizing enzymes that colocalize with striolar and extrastriolar hair cells, respectively, indicating evolutionarily conserved molecular signaling. RA treatment during development increased the proportion of extrastriolar and intermediate-type hair cells, indicating that increased RA reduces striolar development. However, in fish with mechanotransduction dysfunction from a cadherin 23 mutation, normal RA patterning is insufficient to finalize the fate of early-born hair cells, which remain transcriptomically unresolved. RA treatment further exacerbates this abnormal patterning. We conclude that hair cell fate, and thus normal zonal patterning, depends on both hair cell activity and the RA gradient.