From conception to the child.

Abstract

An attempt to present all the data that account for the progressive transformation of a fertilized zygote into a child at birth is obviously a herculean work. In this review, I focus on four aspects of central nervous system development in which our understanding has considerably been enhanced during the last century. The first part concerns the phenomenon of neural induction, a process devoted to the acquisition of neural identity. It was classically considered that active molecules were secreted by the organizer (the dorsal lip of the blastopore in amphibians and the node in amniotes) and that these molecules induced the neural induction. Nowadays, the model is much more complex, based upon both positive and negative regulatory mechanisms. The positive inductor is BMP4, and this induces the epidermal induction. The organizer secretes negative signals that can bind BMP4 and prevent its action. The second part is devoted to the morphogenetic movements that allow formation of the neural tube. Classically, primary and secondary neurulations are opposed. It is now well established that the two modes of neurulation obey the same basic rules. The problem of segmentation of the central nervous system during embryogenesis could be easily studied for the rhombencephalon. This structure is formed by repetitive subunits called rhombomeres. Each rhombomere corresponds to a compartment separated from the others. In addition, each rhombomere expresses specific genes, and the invalidation of some of them leads to the absence of these rhombomeres. However, the situation is much more complex with regions specified very early during development and acting on adjacent regions to induce a specific pattern. The last part of my paper concerns the development of the cortex. During the last 50 years, all the classic concepts have been challenged. The fixed law of radial migration has been extensively discussed, and it is now admitted that other kinds of migration do take place during corticogenesis. These results force us to reconsider the interpretations of some cortical malformations. It seems reasonable to adopt descriptive terms for a malformative syndrome instead of terms based on putative mechanisms.