Disruption of optic fibre growth following eye rotation in Xenopus laevis embryos.

Eye rotation is an experimental paradigm used to study axial specification of the amphibian retina and its connections to the tectum. Jacobson reported that the naso-temporal and dorso-ventral axes are determined sequentially and independently between Nieuwkoop-Faber (NF) stages 28 and 32 in Xenopus. Others claim however, that both retinal axes are determined much earlier and not independently in Xenopus and Rana. We now find that in Xenopus this operation can disturb the exit of optic fibres from the retina with the severity of disruption depending on the time of surgery. Eyes rotated 180 degrees at NF stage 26-27, before any fibres develop, form aberrant retinal fascicle patterns in which the first optic fibres to differentiate (pioneer fibres), failing to exit from the eye, are deflected into circling instead of radial trajectories. Optic fibres appearing later follow these early misguided axons, creating circular bundles. Eyes rotated later (NF 32-34), after optic fibres differentiate, develop normal but inverted patterns because radial fascicles in the retina at the time of operation accurately guide all newly arising fibres to the optic nerve head. This stage-dependent sensitivity of retinal fascicle development must be considered when interpreting the results of rotation experiments.