The brain is made up of many different types of neuronal and glial cells, but how their parental neural stem cells generate such diversity during development is largely unknown. Several lines of evidence point to a time element in the development pattern of neural stem cells, and two papers in this issue of Nature use Drosophila melanogaster models to demonstrate a role for temporal progression under the control of regulatory cascades. Omer Ali Bayraktar and Chris Doe (using the fly larval brain) and Claude Desplan and colleagues (using the fly visual system) show that neuronal progenitors change over time as they produce successive waves of regulatory proteins, thus increasing both the size and the diversity of their neuronal and glial progeny. As similar neuronal progenitors — and homologous regulatory proteins — have been identified in developing mammalian brains, it is likely that such temporal patterning also contributes to the neuronal complexity of the human neocortex.
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