Mechanisms for complexity in the brain: generating the insect central complex.

The central complex of the insect brain is a remarkably miniaturized but highly complex multimodal information-processing network. Recent work on central complex development in Drosophila and grasshopper reveals that the cells comprising its complex circuitry are generated by a surprisingly small number of primary progenitors. Of these, four identified neural stem cells generate a large number of neurons through a novel mode of neurogenesis that involves self-renewing intermediate progenitor cells. Interestingly, a comparable mode of amplification of proliferation also operates in the developing mammalian cortex; this could be a general strategy for increasing brain size and complexity. Although this type of proliferation generates a large number of progeny, it is also prone to dysregulation, resulting in brain tumors. Thus, furthering our knowledge of the development of the central complex is likely to be valuable not only for understanding brain complexity but could also have important implications for identifying developmental pathways that go awry during tumor formation.