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Prenatal exposure to ionizing irradiation has been shown to be an effective method to eliminate selectively certain neuronal population. This investigation studied the effects on the ganglion cell layer of the retinae of adult mice exposed to a gamma source (total dose=3 Gy) at 16 days gestation. There was a significant reduction in the total number of neurons (displaced amacrine+ganglion cells) in the ganglion cell layer (33%) that was mainly caused by a pronounced loss (59%) of displaced amacrine cells. The diameters of the surviving retinal ganglion cells were consistently larger than those of the controls. Prenatal irradiation is the first experimental approach that partially eliminates displaced amacrine cells. It is suggested that the morphogenesis of retinal ganglion cells may be affected by displaced amacrine cells.

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.

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