Control of neural stem cell self-renewal and differentiation in Drosophila.

The neural stem cells of Drosophila, called neuroblasts, have the ability to self-renew and at the same time produce many different types of neurons and glial cells. In the central brain and ventral ganglia, neuroblasts are specified and delaminate from the neuroectoderm during embryonic development under the control of proneural and neurogenic genes. In contrast, in the optic lobes, neuroepithelial cells are transformed into neuroblasts postembryonically by a spatial wave of proneural gene expression. Central brain and ventral nerve cord neuroblasts manifest a short embryonic proliferation period followed by a stage of quiescence and then undergo a prolonged postembryonic proliferation period during which most of the differentiated neurons of the adult CNS are generated. While most neuroblasts belong to a type I class that produces neuronal lineages through non-self-renewing ganglion mother cells, a small subset of type II neuroblasts generates exceptionally large neuronal lineages through self-renewing intermediate progenitor cells that have a transit amplifying function. All neuroblasts in the CNS generate their neural progeny through an asymmetric cell division mode in which the interplay of apical complex and basal complex molecules in the mitotically active progenitor results in the segregation of cell fate determinants into the smaller more differentiated daughter cell. Defects in this molecular control of asymmetric cell division in neuroblasts can result in brain tumor formation. Proliferating neuroblast lineages in the developing CNS utilize transcription factor cascades as a generic mechanism for temporal patterning and birth order-dependent determination of differential neural cell fate. This contributes to the generation of a remarkable diversity of cell types in the developing CNS from a surprisingly small set of neural stem cell-like precursors.