Résumé
Neurons and macroglial cells of the vertebrate central nervous system (CNS) arise from neuroepithelial precursor cells that proliferate rapidly in the ventricular and subventricular zones and present similar morphological characteristics. In most CNS regions, neurons develop first, together with a subpopulation of immature glial cells, the radial glia. The other glial cell types, astrocytes and oligodendrocytes, develop later on according to spatio-temporal schemes specific for each central structure. This sequence of differentiation events raises several key questions regarding the lineage relationships of the various CNS cell types and the mechanisms by which these lineages segregate and differentiate. One important aspect of these questions is to define when and how precursor cells become irreversibly committed to a specific differentiation pathway. Neuroepithelial cells initially could be all endowed with equivalent differentiation capabilities. The specification of these multipotential precursor cells toward a defined phenotype could occur progressively, resulting, as development proceeds, in the formation of discrete families of determined progenitors. Alternatively, they could be restricted in their developmental fate only when they undergo their final round of division, just before differentiating. In either of these two alternatives, the environment should play an important role in specifying the ultimate cell phenotype. In another possible scheme, neuroepithelial progenitors could be already segregated into subpopuladons with differing potentialities at early stages in nervous system ontogeny. In this case, environmental cues would be less critical than intrinsic developmental programs in regulating phenotypic choices.