Pathogenesis of the Neuronal Migration Disorder, with Special Reference to the Animal Model of Prenatal Exposure to Low-Dose Ionizing Radiation

Abstract

Cortical malformations such as cortical dysplasia and heterotopia constitute the underlying pathology of epilepsy and mental retardation. It is thus important to elucidate the pathogenesis of these migration disorders from the neuropathological viewpoint based upon animal experiments. In this review, I describe the experiment in which low-dose prenatal X- or γ-irradiation was performed at the mid-gestational period of mice or rats. Low-dose irradiation as low as 150 mGy induced decelerated migration of cortical neurons during the embryonic period together with a changed pattern of cell adhesion molecule, N-CAM. In addition, the effect of radiation remained at least up until 3-week postnatal as disorganized neuronal allocation with respect to the birthdate. With time of further maturation in the neocortex, however, the architecture, in terms of the pattern of distribution of the labeled neurons, returned closely to that found in non-irradiated control animals. Considering the fact that the number of labeled cells per unit cortical area decreased considerably from 3-week to 8-week postnatal, it is conceivable that apoptotic cell death might have occurred in aberrantly placed neurons.
Recent progress of molecular genetical approach to human hereditary neurodevelopmental diseases is briefly reviewed, since it greatly contributes to our understanding on the pathogenesis of neuronal migration disorders.