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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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Summary
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
Development