Pathway choice by regenerating optic fibers following tectal lobectomy in the goldfish: inferences from the study of gliosis in tectal efferent bundles.

Cell counts in various fiber bundles of the goldfish brain have demonstrated a profound, but transient gliosis of tectal (and pretectal) efferent pathways following removal of a tectal lobe. In the majority of cases, the pathways which underwent gliosis were also those which were penetrated by regenerating optic fibers which had been sectioned by the tectal surgery. However, the dorsal trunk of the horizontal commissure on the intact side of the brain showed only a minimal gliotic response but was consistently innervated by the optic fibers. Conversely, the ansate commissure and the crossed tectobulbar tract invariably demonstrated a marked gliotic response but only rarely received more than minimal innervation by the regenerating fibers. These observations are discussed with regard to the modifications which they demand of the hypothesis that degenerating axon bundles in the goldfish brain are in some way attractive to regenerating axons.