Tectal paths of regenerated optic axons in the goldfish: Evidence from retrograde labelling with horseradish peroxidase

Summary

To compare the distributions of normal and regenerated optic axons in the goldfish tectum, small groups of axons crossing the rostromedial tectum were cut and filled with horseradish peroxidase which subsequently revealed the retinal locations of their somata.

In normal fish, the peroxidase-filled ganglion cells were virtually confined to a narrow arc spanning the ventronasal quadrant of the retina. In fish with regenerated visual projections (50–736 days after optic nerve transection, optic nerve crush or deflection of optic axons to the ipsilateral tectum) the filled cells were distributed across the full extent of the retina from centre to periphery and were less rigidly confined within appropriate quadrants. The absence of any detectable arc of filled cells in the ventronasal quadrant after regeneration showed that few, if any, of the regenerated axons followed their original paths across the tectum. Quantitative analysis of local cell distributions indicated that axons were re-routed independently rather than in groups. Nevertheless, axons consistently displayed a crude bias towards appropriate tectal regions, even in ipsilateral tecta where the relative positions of these regions are inverted.

These results imply that regenerating optic axons are widely scattered by the effects of surgery. They may subsequently show preferences for appropriate central paths but with a resolution too low to define much more than the orientation of the retino-tectal map. Since there is both anatomical and electrophysiological evidence that regenerated optic terminal arborizations eventually adopt a precise retinotopic arrangement, this arrangement must chiefly reflect ordering mechanisms which act in the final stages of axon growth or synapsis.