A quantitative study of the reinnervation of the goldfish optic tectum following optic nerve crush

Stereological and quantitative morphometric methods were used to study changes in the stratum fibrosum et griseum superficialis (SFGS), the major retinal target, in optic tectum of goldfish, during regeneration of the optic nerve. Orthograde transport of HRP by optic axons was used to characterize the retinal projection in SFGS. Profiles of HRP-labeled optic terminals contained rounded vesicles, contacted small dendrites, and were distributed uniformly throughout the area of SFGS sampled; labeling density estimates indicate that at least 37% of the total terminal population in SFGS is retinal in origin. Partial denervation of the tectum by optic nerve crush is accompanied by a loss of 40% of the total terminal population in SFGS and by a marked decrease of SFGS thickness. Entry of massive numbers of regenerating optic axons into SFGS begins about 3 weeks postoperatively, about the time that some visual function recovers and produces a marked increase in SFGS thickness which persists for several months postoperatively. The area occupied by regenerating axons and the number of terminals in the tectum only approach preoperative levels 3 months postoperatively. The recovery of normal synaptic number is therefore delayed several months beyond the time of entry of regenerative axonal sprouts. The results indicate that return of synaptic number to normal is temporally associated with the reduction of the excess number of regenerating optic axons and that both these processes are prolonged.