Development of the central nervous system in mammals

In humans, the nervous system is induced during the third gestational week by molecular signals coming from the mesoderm, which modulate the temporal and spatial expression of specific genes in the cells of the dorsal ectoder m. The induced neural plate closes to form the neural tube where the cells actively proliferate in the germinal zone. The neuroblasts which have compl eted their last division migrate along the fibers of the radial glia to whi ch they adhere, and this movement is essential to establish the normal cere bral organization. The regional identity of the developing brain is governed by the expression of homeobox genes, and the main central structures are clearly delineated by the end of the fifth week. The cerebral cortex begins to form on the sev enth week, and the early specification of the cortical areas, which is unde r genetic control, would be modulated later on by environmental influences. When the neurons have reached their final position, they extend an axon, us ing surface molecules or diffusible molecules present along its pathway as cues to reach the appropriate target and form a synapse, and this process i s a critical step for the establishment of neuronal relationships. The matu ration and stabilization of neural networks is characterized by the apoptic death of roughly 50% of the neurons, due to insufficient neurotrophic supp ort, and by the remodeling of the initial synaptic connections in the survi ving neurons. These regressive events occur late in development and depend on both the interactions with the environment and the resulting neuronal ac tivity. (C) 2001 Editions scientifiques et medicales Elsevier SAS.