The pattern of neuroblast formation, mitotic domains and proneural gene expression during early brain development in Drosophila.

In the Drosophila embryo, studies on CNS development have so far mainly focused on the relatively simply structured ventral nerve cord. In the trunk, proneural genes become expressed in small cell clusters at specific positions of the ventral neuroectoderm. A lateral inhibition process mediated by the neurogenic genes ensures that only one cell within each proneural cluster delaminates as a neural stem cell (neuroblast). Thus, a fixed number of neuroblasts is formed, according to a stereotypical spatiotemporal and segmentally repeated pattern, each subsequently generating a specific cell lineage. Owing to higher complexity and hidden segmental organisation, the mechanisms underlying the development of the brain are much less understood. In order to pave the way towards gaining deeper insight into these mechanisms, we have undertaken a comprehensive survey of early brain development until embryonic stage 11, when all brain neuroblasts have formed. We describe the complete spatiotemporal pattern of formation of about 100 brain neuroblasts on either side building the trito-, deuto- and protocerebrum. Using 4D-microscopy, we have uncovered various modes of neuroblast formation that are related to specific mitotic domains of the procephalic neuroectoderm. Furthermore, a detailed description is provided of the dynamic expression patterns of proneural genes (achaete, scute, lethal of scute, atonal) in the procephalic neuroectoderm and the individual neuroblasts. Finally, we present direct evidence that, in contrast to the trunk, adjacent cells within specific domains of the procephalic neuroectoderm develop as neuroblasts, indicating that mechanisms controlling neuroblast formation differ between head and trunk.