Fig. 1. Sequential transitions in neuroblast gene expression generate layered sublineage expression domains. During each temporal gene expression window, asymmetric NB divisions give rise to GMCs that are marked by the continued presence of the temporal factor that is expressed in the NB during its birth. These transcription factors are also detected in nascent postmitotic neurons and glia. Cells that express Hb are positioned on the inner basal surface of the developing ganglion, and are pushed deeper into the developing neuromere upon the birth of subsequent lineages. As a consequence of these transitions in NB gene expression during lineage development, layered transcription factor expression domains are formed throughout the developing CNS. The temporal factors may act as competence factors, determining the ability of the NB to generate progeny with distinct differentiative states.

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