Embryonic Development of the Drosophila Brain:
Formation of Commissural and Descending Pathways

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Stavros Therianos1, Sandra Leuzinger1, Frank Hirth1, Corey S. Goodman2 and Heinrich Reichert1

1Laboratory of Neurobiology, Institute of Zoology, University of Basel, CH-4051 Basel, Switzerland
2Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA

Originally published in: Therianos et al., 1995, Development 121, 3849-3860

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The establishment of initial axonal pathways in the embryonic brain of Drosophila melanogaster was investigated at the cellular and molecular level using antibody probes, enhancer detector strains and axonal pathfinding mutants. During embryogenesis, two bilaterally symmetrical cephalic neurogenic regions form, which are initially separated from each other and from the ventral nerve cord. The brain commissure that interconnects the two brain hemispheres is pioneered by axons that project towards the midline in close association with an interhemispheric cellular bridge. The descending longitudinal pathways that interconnect the brain to the ventral nervecord are prefigured by a chain of longitudinal glial cells and a cellular bridge between brain and subesophageal ganglion; pioneering descending and ascending neurons grow in close association with these structures. The formation of the embryonic commissural and longitudinal pathways is dependent on cells of the CNS midline. Mutations in the commissureles gene, which affects growth cone guidance towards the midline, result in a marked reduction of the brain commissure. Mutations in the single-minded gene and in other spitz 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 remarkable similarities as well as distinct differences in the embryonic development of the brain and the segmental ganglia, and forms the basis for a comprehensive genetic and molecular genetic dissection of axonal pathfinding processes in the developing brain.

Figure 1
Early embryonic brain development. prospero immunoreactivity in cephalic neurogenic regions.

Figure 2
Commissure formation. Frontal views through the brain hemispheres (laser confocal microscopy).

Figure 3
Formation of circumesophageal axon pathways.

Figure 4
Es ablishment of a primary axon scaffold.

Figure 6
Commissure differentiation is perturbed in commissureless mutants

Figure 7
Paired circumesophageal connectives fail to form in single-minded mutants.

© Reichert Lab 1996

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