During development, the balance between neural stem cell self-renewal and differentiation is carefully controlled to ensure that the correct number of neurons is produced to build functional neural networks. In the Drosophila optic lobe, as in the mammalian cerebral cortex, neuroepithelial (NE) cells initially divide symmetrically to expand the stem cell pool, before switching to asymmetric division to generate neurons. Andrea Brand and colleagues now report that Notch regulates this important cell fate transition (see p. ). By comparing the transcriptomes of microdissected NE cells and neuroblasts, the researchers show that Notch signalling pathway members are preferentially expressed in NE cells. Notch mutant cells are extruded from the neuroepithelium and undergo premature neurogenesis, they report. Furthermore, a wave of proneural gene expression transiently represses Notch activity in NE cells to enable the transition from symmetrically dividing NE cell to asymmetrically dividing neuroblast. This progression resembles that seen in the vertebrate cerebral cortex, leading the researchers to propose that neurogenesis regulation could be conserved between these two systems.
