Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways

Studies using neonatal surgical lesions to reduce the target area of the re tina have supported the idea that developing axons show only a limited spec ificity in their targeting. This: investigation tested whether retinogenicu late axons adjust for partial target depletion by repositioning of axons. W e used adult Su iss mice exposed to gamma rays at the time when layer IV ce lls are generated in the ventricular zone (16 days of gestation). Nissl-sta ined brain sections were used for histological analyses in thalamus and cor tex. Retinal ganglion cells were backfilled from the optic tract with horse radish peroxidase. Intraocular injections of horseradish peroxidase were us ed to study the retinal projections. In the posterior cortex there was a ne arly complete absence of layer IV. The irradiated animals showed a 75% redu ction of the dorsal lateral geniculate: nucleus. The ventral division, supe rior colliculus, and other visually related nuclei were not affected. The l oss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%,). Therefore, t he shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, p attern 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 visu al cortex. The absence of aberrant retinal projections suggests that reposi tioning of axons is not the first mechanism employed by retinal axons to ma tch connections in numerically disparate populations. (C) 2001 ISDN. Publis hed by Elsevier Science Ltd. All rights reserved.