Common mechanisms in vertebrate axonal navigation: retinal transplants between distantly related amphibia.

Embryonic eye primordia were transplanted from the anuran, Xenopus laevis, to the urodele, Ambystoma mexicanum, in order to assess whether retinal axons of one species could grow to appropriate central targets in a distantly related species. To trace the early retinal projections, Xenopus primordia were removed, incubated for 20 min in a solution containing [3H]proline, washed, and then transplanted to host axolotl embryos. Genetically eyeless hosts were used in some of the experiments so that the Xenopus optic fibers would not be guided to their targets by the host's own optic nerve projections. Xenopus eyes were usually able to differentiate into small eyes in the axolotl host. Shortly after primary differentiation, the development of these eyes seemed to arrest. Autoradiography on paraffin sections of the central nervous system of the host revealed that in some cases Xenopus retinal ganglion cells were indeed able to send axons to the tectum of the axolotl host. This result suggests that the mechanisms of axonal navigation for this particular central projection are evolutionarily conserved.