On the Guidance of Regenerating Axons from a Half-Retina in the Adult Goldfish

We have examined the guidance of regenerating optic axons from half-retinal fragments in the goldfish. Axons regenerating from a dorsal, ventral, or nasal half-retina showed no bias to grow through the retinotopically appropriate parts of the optic tract. By contrast, axons from a temporal half-retina did show a bias, albeit a small one, to grow through the correct part of the tract. In addition, we found evidence that axons from the temporal quadrant of dorsal or ventral half-retinas showed a similar bias. In the optic tectum, axons regenerating from a dorsal, ventral, or temporal half-retina showed strong biases to invade the retinotopically appropriate portion of the synaptic field. Of these three, temporal axons showed the sharpest restriction in their field of innervation. Nasal fibers were unique in that they invaded the entire tectum with little evidence of a preference for their appropriate field. Our results indicate that there is minimal guidance offered to regenerating optic axons in the optic tract. However, once dorsal, ventral, or temporal axons reach the tectum they appear to encounter strong organizing forces which constrain them to preferentially invade their retinotopically appropriate areas. Nasal axons appear to be exempt from these forces. In both the optic tract and tectum, axons from a temporal half-retina are the most effectively guided and constrained.