REGENERATION OF RETINAL AXONS INTO GOLDFISH OPTIC TECTUM

The growth of regenerating retinal axons into the central portion of the optic tectum of adult goldfish [Carassius auratus] was examined with the light microscopy and EM. Optic tracts were cut and, 2 days-5 mo. later, the animals were perfused and the tecta prepared for microscopy. Regenerating axons 1st reached central regions of the tectum 7-10 days postoperatively. Regenerating axons appeared in very large numbers and travelled in fascicles in the stratum opticum (SO) and in the adjacent neuropil the stratum fibrosum et griseum superficiale (SFGS). In the SO, the fascicles were bordered by glial cells and degenerating debris. Within the SFGS the fascicles did not seem to be similarly associated with glial cells and degenerating debris. The youngest regenerating axons were very slender processes, containing microtubules but few or no neurofilaments or dense granular material. By 10-14 days postoperatively, neurofilaments could be seen and large numbers of vesicles with dense cores appeared. The vesicles with dense cores increased in numbers until about 28 days postoperatively and then became quite rare. That vesicles with dense cores were seen in regenerating axons in both SO and SFGS during the period of growth into the tectum but were not seen in axon terminals at any time, suggested that they may be concerned with axon elongation. During the period 1 mo.-5 mo. postoperatively, the regenerating axons gradually increased in diameter but did not reach preoperative sizes, suggesting that the regenerative changes may still be occurring. Remyelination was delayed and proceeded slowly. Many axons remained unmyelinated for as long as 5 mo. postoperatively.