Long-term degeneration renders central tracts refractory to penetration by regenerating optic fibers.

We have examined time-dependent changes in the ability of degenerating central pathways in the goldfish to be penetrated by regenerative axons. We have found that when a tract has degenerated for 2--5 weeks it is readily penetrated by regenerating optic fibers. However, tracts which degenerated for any longer than 6 weeks, before being exposed to the regenerating fibers, were only sparsely penetrated by them. We conclude that over a period of no less than 6 weeks, degenerating central tracts in the goldfish change their character and become relatively refractory to penetration by regenerating axons.