Brainy but not too brainy: starting and stopping neuroblast divisions in Drosophila.

Drosophila neuroblasts are similar to mammalian neural stem cells in that they self-renew and have the potential to generate many different types of neurons and glia. They have already proved useful for uncovering asymmetric division components and now look set to provide insights into how stem cell divisions are initiated and terminated during neural development. In particular, some of the humoral factors and short-range 'niche' signals that modulate neuroblast activity during postembryonic development have been identified. In addition, recent studies have begun to reveal how the total number of cells generated by a single neuroblast is regulated by spatial and temporal cues from Hox proteins and a transcription-factor series linked to cell cycle progression.