Regeneration of optic fibres through the chiasma in Xenopus laevis tadpoles.

The path through the chiasma followed by regenerating optic nerve fibres in Xenopus tadpoles was studied at light- and electron-microscopic levels, and with horseradish peroxidase as a fibre label. Over the period (5-10 days) in which regenerating fibres reach and cross the chiasma, they did not follow residual deep fibres through the chiasma, nor were they associated with the trail of degeneration in the chiasma which represented the remains of the deeper (older) parts of the original projection. The regenerating optic fibres were always seen in close association with the inner surfaces of the ependymoglial endfeet, or in the extracellular spaces that lie close to the endfeet in the most superficial part of the chiasma, where newly-growing fibres from the retinal margin are normally to be found.