EMBRYONIC-DEVELOPMENT OF THE DROSOPHILA BRAIN - FORMATION OF COMMISSURAL AND DESCENDING PATHWAYS

The establishment of initial axonal pathways in the embryonic brain of Drosophila melanogaster was investigated at the cellular and molecula r level using antibody probes, enhancer detector strains and axonal pa thfinding mutants. During embryogenesis, two bilaterally symmetrical c ephalic neurogenic regions form, which are initially separated from ea ch other and from the ventral nerve cord. The brain commissure that in terconnects the two brain hemispheres is pioneered by axons that proje ct towards the midline in close association with an interhemispheric c ellular bridge. The descending longitudinal pathways that interconnect the brain to the ventral nerve cord are prefigured by a chain of long itudinal glial cells and a cellular bridge between brain and subesopha geal ganglion; pioneering descending and ascending neurons grow in clo se association with these structures. The formation of the embryonic c ommissural and longitudinal pathways is dependent on cells of the CNS midline. Mutations in the commissureless gene, which affects growth ca ne guidance towards the midline, result in a marked reduction of the b rain commissure. Mutations in the single-minded gene and in other spit z group genes, which affect the differentiation of CNS midline cells, result in the absence or aberrant projection of longitudinal pathways. The analysis of axon pathway formation presented here reveals remarka ble similarities as well as distinct differences in the embryonic deve lopment of the brain and the segmental ganglia, and forms the basis fo r a comprehensive genetic and molecular genetic dissection of axonal p athfinding processes in the developing brain.