The retinas of adult teleost fish can regenerate neurons following injury. The current study provides the first documentation of functional whole retina regeneration in the zebrafish, Danio rerio, following intraocular injection of the cytotoxin, ouabain. Loss and replacement of laminated retinal tissue was monitored by analysis of cell death and cell proliferation, and by analysis of retina-specific gene expression patterns. The spatiotemporal process of retinal ganglion cell (RGC) regeneration was followed through the use of selective markers, and was found to largely recapitulate the spatiotemporal process of embryonic ganglion cell neurogenesis, over a more protracted time frame. However, the re-expression of some ganglion cell markers was not observed. The growth and pathfinding of ganglion cell axons was evaluated by measurement of the optic nerve head (ONH), and the restoration of normal ONH size was found to correspond to the time of recovery of two visually-mediated behaviors. However, some abnormalities were noted, including overproduction of RGCs, and progressive and excessive growth of the ONH at longer recovery times. This model system for whole-retina regeneration has provided an informative view of the regenerative process.

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.