A Post-Surgical Retinal Progenitor Cell Niche is the Primary Source of Embryonic Eye Regrowth in Xenopus laevis

Xenopus laevis has recently emerged as a vital model for studying functional eye regrowth in pre-metamorphic tadpoles. Following eye removal surgery, tailbud embryos have been shown to regenerate a functionally complete eye within a 3-5 day period. While current studies have primarily focused on the signaling mechanisms required for this rapid regeneration, less is known about the specific stem cell populations and modes of regeneration employed by the embryo. In both the adult and tadpole, eye tissue regeneration can be facilitated through a combination of a pre-existing stem cell niche and the transdifferentiation of cells surrounding retinal or lens injuries, depending on the extent of the tissue removal. Notably, in the Xenopus eye regrowth assay, surgeries typically leave behind approximately 15% of the ocular tissue, indicating a post-surgical stem cell niche with potential for regeneration. In this study, we explored the hypothesis that a residual retinal progenitor cell (RPC) niche is critical for the rapid eye regrowth observed in Xenopus tadpoles. By utilizing a photoconvertible protein, EosFP, which changes permanently from green to red fluorescence, we selectively marked retinal progenitor cells (RPCs) in the presumptive eye area with red fluorescence. We then carefully preserved a small population of these red-labeled RPCs within the post-surgical wound. This progenitor cell niche, comprising not only the red-labeled RPCs but also the surrounding cells, creates a unique signaling environment. This specialized microenvironment is crucial, as it may provide specific signals that dictate the developmental outcomes of the RPCs, effectively controlling their fate. Observations made throughout the regrowth process revealed that the eye predominantly regrew from this red-labeled RPC niche within three days, with all retinal layers comprising red-labeled cells. The regrown lens was observed to be composed of a mix of both cells outside the RPC lineage and RPC progeny. Of interest, we observed cells of the closing optic fissure and ventral retina incorporate progeny from cells outside the labeled RPC lineage. These findings support the notion that the primary mode of regeneration in pre-metamorphic Xenopus eye regrowth involves the use of a pre-existing stem cell niche, and may also involve transdifferentiation, thus providing new insights into the mechanisms of embryonic eye regrowth in Xenopus laevis.