Control of cell number in the developing visual system. III. Effects of visual cortex ablation.

The effect of unilateral deletion of the visual cortex on early cell death and eventual cell number in various structures of the visual system was examined. At minimum, this manipulation potentially provides excess retinal afference to the superior colliculi, partially denervates the superior colliculi, reduces normal retinal terminal area and opens up potential target space for the retina and superior colliculus in those areas where they share terminal space with the visual cortex. All layers of the superior colliculus, bilaterally, showed an initial decrease in the rate of cell death relative to normal followed by an increase in cell death rates. No change in the number or distribution of cells in the retinal ganglion cell layer resulted despite a substantial loss of retinal terminal area, and a substantial alteration of the pattern of retinal central termination. These results are interpreted as evidence for two stages in normally occurring cell death, a first in which axons compete to colonize any available terminal space, and a second in which axon-to-target specificity must be matched. These results also provide evidence that the amount of target required for neuron survival is clearly variable.

Temporal retina is preferentially represented in the early retinotectal projection in the hamster.

Retrograde transport of horseradish peroxidase (HRP) after complete transection of the brachium of the superior colliculus on the day of birth in hamsters revealed preferential labelling of the temporal retina. Cytochrome oxidase staining of the retina showed similar preferential temporal labelling. A discrete lack of label of the extreme temporal periphery of the retina contralateral to the HRP placement and a complementary label of ipsilateral temporal periphery were also observed.