Figure 3.

Schematic diagrams of the temporal development of neural progenitors in the early stages of CNS formation. (A) Early neuroepithelial progenitors of the ventricular zone are columnar cells self-renewing by symmetric divisions. These cells can generate some neurons. (B) As neurogenesis proceeds, neuroepithelial cells are transformed into radial glial cells which ultimately will give rise to neurons and glial cells. Radial glia cells can undergo either symmetric divisions, generating two progenitors or asymmetric divisions, producing a neural progenitor and a neuron. Also illustrated is the interkinetic nuclear migration of the nuclei during the cell cycle in the VZ. The nucleus of a single neuroepithelial cell moves during the G₁ phase, from the ventricular surface to the border of the VZ where it enters S phase. During G₂, the nucleus moves down to the ventricular surface where it enter mitosis (M phase). Interkinetic nuclear migration in radial glial cells is confined to the VZ portion, does not extend to the border of the MZ. (C) In the cerebral cortex a second proliferative zone, the subventricular zone (SVZ), appears adjacent to the VZ; the postmitotic neurons and glia arise from both the ventricular and the subventricular zones. In the SVZ interkinetic nuclear migration does not occur, mitotic cells are found throughout the SVZ. Abbreviations: CNS, central nervous system; IZ, intermediate zone; MZ, marginal zone; SVZ, subventricular zone; VZ, ventricular zone.