Embryonic retinal ablation and post-metamorphic optic nerve crush: effects upon the pattern of regenerated retinotectal connections.

We examined relationships between healing observed during embryonic Xenopus retinal and optic nerve regeneration and resultant visuotectal pattern formation. Dorsal (D) and nasoventral (NV) 1/3 sized eye fragments were surgically created in stage 32 Xenopus laevis embryos. Gross anatomical healing modes of these fragments were examined 2 days post-surgery (stage 43). Healing was categorized according to the degree of cell movements observed. Animals were reared through metamorphosis and electrophysiologic mapping techniques were employed on those animals whose eyes regenerated. All D 1/3 fragments showed normal (non-duplicated) projections to the tectum; most (80%) of the healing observed showed little cell movements (the remaining 20% showed substantial cell movements, yet failed to show duplicated projections). Most NV 1/3 fragments (73%) formed two mirror image projections to the contralateral midbrain optic tectum (pattern duplication). Most (88%) of the healing observed among these animals showed massive cell movements in the ventral retinal region (the remaining 12% showed moderate cell movements). The remaining NV 1/3 fragments (27%) showed moderate cell displacement and failed to show duplicated projections). These data are compatible with a cell-movement:intercalary cell division hypothesis in which duplication is dependent upon specific positional confrontation and subsequent cell division. In additional studies, in adult animals, the optic nerves of eyes with duplicated projections were crushed and allowed to regenerate for 1 year. Duplicated projections were restored, indicating that developmental and maturational factors are probably not responsible for duplicative pattern formation; rather, information intrinsic to the eye, possibly created during healing interactions and/or fiber ingrowth to the tectum, underlies duplicate innervation of the tectum.