Postnatal changes in the uncrossed retinal projection of pigmented and albino Syrian hamsters and the effects of monocular enucleation.

Anterograde and retrograde tracing techniques have been used to study the uncrossed retinal projection in neonatal pigmented and albino Syrian hamsters. The total number of retinal ganglion cells projecting ipsilaterally peaks at postnatal days 2-4 (P2-P4) and declines to adult values by P12. The change in cell numbers has a similar time course in albino and pigmented animals. Although the population of uncrossed cells in the temporal retina of albino hamsters is always less than that in pigmented hamsters, no difference between the colour phases was found for the population of uncrossed cells in nasal retina. Differential cell death also contributes to the adult albino decussation pattern in hamsters: The relative loss of cells from temporal retina in albinos (72%) is greater than that in pigmented animals (56%). The additional loss in albinos does not appear to depend on binocular interactions: The same proportion (30%) of uncrossed cells is "rescued" from death by neonatal monocular enucleation in both colour phases. Flat-mount preparations showing the distribution of uncrossed fibres reveal that a distinct focus of terminals emerges in rostral superior colliculus, which is topographically appropriate for a binocular mapping, at the peak of uncrossed ganglion cell numbers (P4). Comparison of uncrossed terminal distributions and ganglion cell death reveals considerable refinement of the terminals prior to the main phase of cell death. Monocular enucleations performed some time after birth have a greater effect on uncrossed terminal distributions than on cell death. These observations suggest that independent mechanisms may be involved in the regulation of terminal distributions and of cell numbers in the developing uncrossed retinal pathways.