The induction of an anomalous ipsilateral retinotectal projection in Xenopus laevis

The conditions necessary for extensive regeneration of fibres from one optic nerve to both tecta in Xenopus have been investigated. The effects of various types and positions of nerve lesion on the distribution of regenerated projections were examined by labelling the regenerated projections with either horseradish peroxidase or tritiated proline. The only types of nerve lesion which consistently gave rise to extensive regeneration to both the contralateral and the ipsilateral tectum were those close to the chiasma, liable to have caused damage to the nerve entry point. However, all other types of lesion studied (near the eye; near the skull; crush or cut) frequently led to regeneration of a very few fibres to the ipsilateral tectum. These fibres gained access to the ipsilateral tectum by various routes: in some cases via the optic tract; but more commonly either across the posterior commissure or by a complex pathway following the oculomotor nerve root. Over time periods of between one and seven months, the distribution of the regenerated fibres following each type of lesion showed little change. We conclude that the regeneration of bilateral retinotectal projections in Xenopus is caused by tissue damage to the region of the chiasma resulting in misrouting of fibres.