RESUMO
A previous study reported that adult mice irradiated at the 16th embryonic day present a severe neuronal number reduction in the dorsal lateral geniculate thalamic nucleus. In the present study, we investigated the time course of the effects of prenatal irradiation on this thalamic nucleus. One day after irradiation, a great number of pyknotic figures were seen mainly in the cerebral proliferative zones. In the geniculate nucleus, only scattered pyknotic figures were identified. On the first week after birth, the geniculate nucleus presented frequent pyknotic figures. From five days after birth onwards, a severe shrinkage of the occipital cortex and a great reduction in the geniculate nucleus neuronal number were found. On the second week after birth this neuronal number reduction reached as high as 75%. At each postnatal analyzed age, severe volumetric geniculate nucleus shrinkage was combined to non-significant neuronal density variations. The presence of few pyknotic figures in the geniculate nucleus one day after irradiation and its delayed neuronal loss indicate an indirect effect of irradiation. We suggest that the effect upon the geniculate nucleus is secondary to the damage of the occipital cortex. A possible interpretation for thalamic neuronal loss is that geniculate neurons fail to establish cortical arbors after major target loss. In this case, the loss of trophic support should also be considered.
Assuntos
Diferenciação Celular/fisiologia , Raios gama , Corpos Geniculados/anormalidades , Neurônios/metabolismo , Degeneração Retrógrada/fisiopatologia , Córtex Visual/anormalidades , Vias Visuais/anormalidades , Envelhecimento/fisiologia , Envelhecimento/efeitos da radiação , Animais , Apoptose/fisiologia , Apoptose/efeitos da radiação , Contagem de Células , Diferenciação Celular/efeitos da radiação , Divisão Celular/fisiologia , Divisão Celular/efeitos da radiação , Núcleo Celular/metabolismo , Núcleo Celular/patologia , Núcleo Celular/efeitos da radiação , Tamanho Celular/fisiologia , Tamanho Celular/efeitos da radiação , Feminino , Corpos Geniculados/patologia , Corpos Geniculados/efeitos da radiação , Masculino , Camundongos , Neocórtex/anormalidades , Neocórtex/patologia , Neocórtex/efeitos da radiação , Neurônios/patologia , Neurônios/efeitos da radiação , Degeneração Retrógrada/etiologia , Degeneração Retrógrada/patologia , Tálamo/anormalidades , Tálamo/patologia , Tálamo/efeitos da radiação , Córtex Visual/patologia , Córtex Visual/efeitos da radiação , Vias Visuais/patologia , Vias Visuais/efeitos da radiaçãoRESUMO
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.