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Fig. 3. Expression of ß-Spectrin in the nervous system. Confocal views of dissected nervous systems of wild-type stage-16 embryos stained for expression of ß-Spectrin (white), axonal membranes (anti-HRP, green) or the SemaIIbMyc marker (red). (A) In wild-type embryos, ß-Spectrin expression is found in neuronal cell bodies and specific axonal fascicles (arrowheads). The arrow denotes increased levels of ß-Spectrin at the CNS midline. (B) In homozygous-mutant gcm embryos, lateral glial cells are absent. ß-Spectrin can still be detected at specific fascicles (arrowheads) and at the CNS midline (arrow). (C,C') The SemaIIbMyc marker is expressed by only one neuron per hemineuromere. This neuron is positioned at the lateral margin of the neuropil. SemaIIbMyc-positive axons project across the midline in the anterior commissure and then make a sharp turn to follow a specific path in the longitudinal connective. (D,D') In hemizygous-mutant kus^S012 embryos, the SemaIIbMyc-positive neurons appear normally specified, but show irregular positions in the nerve cord, often being displaced towards the CNS midline. The SemaIIbMyc-positive fascicles are found closer to the CNS midline (double-headed arrow indicating distance in C and D) and the fascicle morphology appears changed. Often, ectopic projections are found that may correspond to enlarged growth cones (arrows). In addition, the precision in axonal pathfinding is lost; however, we did not observe ectopic crosses of the CNS midline.

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