Post-metamorphic retinal growth in Xenopus

The postmetamorphic growth of the retina in Xenopus was studied using 3H-thymidine ( 3HT ) autoradiography and quantitative morphometric assays. 3HT was administered to tadpoles at stages 58, 62 and 66 and the animals sacrificed between 3 weeks and 12 months after metamorphosis. Reconstructions were made from serial sections and the position of labelled cell groups in the retina were established. On the reconstructed retina, regions formed up to stage 58, between stages 58 and 66 and after metamorphosis were measured. The area of the dorsal, ventral, temporal and nasal retinal halves was also determined from stage 58 through to adult. The entire retinal area increased 10-fold from stage 58 to 12 months after metamorphosis, the fastest growing region being the retinal periphery due to continuous cell addition at the ciliary margin. Concommitant with the retinal area growth, the number of ganglion cells increased from 20,000 to 85,000 over the time of investigation. Asymmetric cell addition to the ciliary margin from stage 58 onwards resulted in a predominantly crescentic retinal growth along the nasoventral ciliary margin. Consequently, the optic nerve head became displaced away from the geometric centre of the eye into the dorso-temporal retinal quadrant. These results suggest that besides a sustained cell production exclusively at the ciliary margin, a passive area expansion contributes to the overall retinal growth from the metamorphic climax to adulthood. It is also apparent that the steady increase of the number of retinal ganglion cells and optic fibers necessitates a continuous remodelling of the retinotectal connections throughout the lifespan of the animal.