Growth and myelination of goldfish optic nerve fibers after retina regeneration and nerve crush.

Axonal regeneration in the optic nerve and tectum of the goldfish was studied both after retina regeneration and nerve crush. The retina regeneration was evoked by ouabain-induced damage of at least the ganglion cells and cells of the inner nuclear layer. The necrotic retinal neurons are substituted by mitotic processes in the outer nuclear layer and the marginal growth zone at the ora serrata. The axons of these newly developed retina ganglion cells grow through the degenerating, but mechanically undamaged, optic nerve into the tectum, establishing there synaptic contacts already 16 days after the intraocular ouabain-injection. The fibers were myelinated at first in the tectum, later on in the optic nerve. Thus, the myelination process proceeds in retrograde direction. About 60--80 days after injection the myelination has become nearly normalized. On the contra-lateral side of the same animal, the optic nerve was crushed near the eye-bulb. The axons of the original retina ganglion cells grow out into the degenerating optic nerve and tectum. They also find synaptic contacts and are myelinated in retrograde direction, but to a much lesser extent than the axons of the regenerated retina ganglion cells. An axonal factor is discussed, which would influence the oligodendroglial myelination activity. The effectiveness of this factor is probably dependent on the neuronal age and suggested to be triggered by the establishment of synaptic contacts.