Pathways of Xenopus optic fibres regenerating from normal and compound eyes under various conditions.

We have used Horseradish peroxidase to investigate the pathways taken by Xenopus optic fibres regenerating from normal and electrophysiologically-confirmed compound eyes to the optic tectum. Optic fibres, when sectioned near the chiasma, regenerate up both sides of the diencephalon to both tecta. We have therefore been able, by using animals in which one eye had or had not been removed at early embryonic stages, to look at the behaviour of regenerating axons in three different situations: (1) regeneration to the contralateral tectum, previously innervated by the sectioned fibres; (2) regeneration to a "virgin' ipsilateral tectum, never before innervated by optic fibres; and (3) regeneration to an ipsilateral tectum already innervated by fibres from a normal eye. From the chiasma to the tectodiencephalic junction regenerating fibres behave similarly in all three situations, following roughly the course of the normal optic tract, but running in a rather disorganised way, with frequent crossing over of fibres. However fibres of nasal retinal origin (from an NN eye) spread to occupy a much larger area of the side of the diencephalon than those of temporal origin (from a TT eye). From the tectodiencephalic junction to the tectal termination of the fibres there are differences between the three situations investigated; fibres regenerating to a 'virgin' ipsilateral, or to a denervated contralateral tectum, tend to grow straight onto the tectum, instead of being channelled into lateral or medial brachium as uncut fibres tend to be. There is however, the remains of a brachial organisation, and of differential selection of these brachia by fibres from the different types of compound eye, this being well seen on "virgin' tecta. Fibres regenerating to an ipsilateral innervated tectum behave very differently. As they reach the tectodiencephalic junction they suddenly start to grow in a less disorganised way, and are channelled into well defined brachia. If from a compound eye, these fibres terminate on only that part of the tectum innervated by fibres from the corresponding part of the normal eye. Thus fibres from a VV eye and those from the ventral half of the normal eye all terminate on medial tectum; fibres from an NN eye, and those from the nasal half of the normal eye all terminate in caudal tectum; and temporal fibres from both normal and TT eyes terminate in rostral tectum.