Histogenetic Abnormalities of the Brain Caused by Environmental Factors : Sensitive Period for Microcephaly

Abstract

Histogenesis of the cerebral cortex takes place according to the following steps: proliferation of matrix cells in the ventricular zone; differentiation of matrix cells into young neurons after completion of final division; migration of young neurons to the outer zone of the brain mantle to form the cortical plate; and growth and maturation of cortical neurons. Each step can be affected by environmental factors directly or indirectly, and any disturbance in these steps can cause histogenetic abnormalities. Thus, the sensitive period for histogenetic abnormalities covers the entire fetal period. According to experimental studies, the sensitive period for microcephaly by environmental factors was found to be the stage in which the young neurons began to differentiate from the proliferating matrix cells. In order to analyze the high sensitivity of the developing brain in the above stage to external agents, the author examined the radiosensitivity of matrix cells in the different cell cycle phases in the telencephalon of day 13 mouse fetuses. High radiosensitivity of G_1 phase matrix cells or young neurons just after the final division may contribute to the high sensitivity in this stage.