A Conserved Mechanism in Eye Optical Development: Lens Nucleus Centralization in Xenopus laevis

Developing eye optics, determined by the lens and cornea, must coordinate with the axial length of growing eyes to focus light onto the retina to form an image. It was found that zebrafish (Danio rerio) lens nuclei are initially anteriorly localized in optical axes in larvae, then centralize at older stages. An anteriorly placed lens nucleus would increase lens power, thereby likely enabling a functional optical system in larvae, where eye axial length is short. To assess if alike mechanisms occur in other aquatic animals, we studied the clawed frog, Xenopus laevis, a fully aquatic species similarly relying on vision for survival at stages where eyes are small. We found the Xenopus tadpole lens nucleus also shifted from an anterior to a central location in the optical axis during the prometamorphosis period. Similarly, in eyes regenerated after embryonic ablation, tadpole lens nuclei are anteriorly localized then centralize before metamorphosis, recapitulating the same pattern as control developing eyes. Moreover, lens nuclei localization in optical axes in developing and regenerated Xenopus eyes show close correlations to axial eye length. Close correlation of these two parameters suggests lens nuclei centralization is required for a functional optical system by coordinating the focal length. Our findings suggest a conserved evolutionary mechanism for eye optical development in at least two aquatic species. Understanding key mechanisms regulating crosstalk between eye optics and eye axial length will aid in discovering mechanisms of optical development and future therapies to prevent or delay formation of refractive error when these two properties mismatch.