Figure 3.

Temporal mechanisms of neuronal fate specification. (A) Distinct neuronal subtypes are generated sequentially in an inside-first–outside-last order during neurogenesis, by progenitors located in the cortical ventricular zone (VZ) and subventricular zone (SVZ), resulting in the layered organization of the adult cerebral cortex. The intermediate zone (IZ), subplate (SP), and marginal zone (MZ) are present only transiently in the embryonic cortex. (B) Cortical projection (CP) neurons are generated either directly from radial glial stem cells or indirectly via intermediate basal progenitors (BPs). (C) The transcription factors that specify the identity of the distinct classes of projection neurons forming the different cortical layers (see Molyneaux et al. 2007). (D) Transcription factors that specify cortical neuron identities are involved in cross-repressive interactions whereby a layer-specific determinant represses directly (solid lines) or indirectly (dashed lines) the transcription factors involved in neuronal specification in neighboring layers. Abbreviations: VI, layer VI; V, layer V; IV, layer IV; I/II/III, layers I, II, and III.