Patterns of Proliferation in the Pacific Hagfish ( Eptatretus stoutii ) Retina Suggest an Ancient Origin for Adult Neurogenesis

Comparative morphology has revealed fascinating structural and organizational differences in the eyes of cyclostomes (jawless vertebrates) compared to gnathostomes (jawed vertebrates). This work has helped us to better understand the evolutionary origin of the vertebrate retina, but certain questions remain unresolved. Here we complement that approach by examining retinal development. We sought to characterize post-larval ("adult") retinogenesis in a representative cyclostome, the Pacific hagfish (Eptatretus stoutii). Knowledge of hagfish neurodevelopment is limited by poor access to embryos, but evidence suggests continued retinal neurogenesis occurs late into their ontogeny. A brief pulse of EdU was used to detect proliferating cells, if any, in the retina. This was followed by in situ hybridization to test whether homologs of gnathostome genes might have conserved expression patterns that direct retinogenesis in a jawless vertebrate. EdU+ cells were detected at the retinal periphery of the hagfish (a region reminiscent of the ciliary marginal zone of gnathostomes) and within the central retina. In addition, hagfish eyes were found to express several key genes required for vertebrate retinal neurogenesis. At least two of these genes, OtxA and Rx (retinal homeobox), were enriched within the proliferative retinal periphery. These findings support adult retinogenesis as an ancient and deeply conserved trait within the vertebrate lineage.