Control of cell number in the developing visual system. II. Effects of partial tectal ablation.

The effects of potential excess innervation on cell survival in the superior colliculus and related structures during the period of normally occurring cell death was examined. A unilateral, partial lesion of the superficial layers of the superior colliculus on the day of birth, which results in a compression of the retinotectal map into the remaining area, was the manipulation used to produce the potential excess innervation. Cell density was reduced in the tectal fragment early in development, consistent with hyperinnervation, but had returned to normal by the end of the period of normally occurring cell death. The overall incidence of cell degeneration in the remaining partial colliculus was not different from the undamaged contralateral colliculus or from normal, though there was evidence of a transitory depression and later elevation of cell loss. Cell loss in the retina contralateral to the lesion was increased in the late part of the period of normal cell loss and there were fewer cells in the retinal ganglion cell layer at maturity. The amount of the cell loss in the retina was small compared to the amount of target removal. These results suggest that the survival of neurons with branching axons does not sensitively reflect target availability.