Embryonic retinal gene expression in sonic-you mutant zebrafish.

Hedgehog (Hh) signaling is required for proper eye development in vertebrates; known roles for Hh in the zebrafish include regulation of eye morphogenesis, ganglion cell neurogenesis, and photoreceptor differentiation. To gain insight into the mechanisms by which Hh signaling influences these developmental events, we have examined proliferation, cell death, and expression patterns of several retinal genes in the eyes of embryonic zebrafish lacking the sonic hedgehog gene. We find that features of the eye phenotype of the sonic-you (syu) mutant are consistent with multiple roles for the Hh signal during retinal development. Most interestingly, half of the mutant retinas failed to initiate cell differentiation and, instead, retained a neuroepithelial appearance. In the other half of the mutants, retinal cell differentiation was initiated, but not fully propagated. We also find that Hh signaling is important for retinal cell proliferation and retinal cell survival; together, these functions provide an explanation for progressive microphthalmia in the syu-/- mutant.