Figure 2

Sensory axon defects in nrg LOF mutants. (a) Summary of sensory axon stalling positions (indicated by red lines) and percentage of neurons that stalled at those positions in nrg LOF mutant embryos (nrg¹⁴/Y and nrg¹⁷/Y data pooled). Data on the green line are for all lateral cluster neurons combined (n = 108), on the blue line for all dorsal cluster neurons combined (n = 7), excluding dbd, and on the purple line for v'ch1 alone (n = 3). (b-i) Axon morphology of sensory neurons in nrg¹⁴/Y embryos as revealed by single neuron DiI labeling. (b-g) DIC images of photoconverted preparations; (h, i) confocal z-projections. Embryos in (b, d, e) were co-immunostained with mAb1D4 to reveal motor axon projections in the periphery, while the embryo in (i) was co-stained with mAb22C10 to reveal all other sensory axons. (b) An lch5-5 stall in the ISN, before its junction with the SN. The arrow shows the position of the lch5-5 growth cone. (c) An lch5-5 stall in the ISN at a similar position to (b). (d) An lch5-5 stall in the ISN nerve root within the CNS. The arrow indicates the position of the stall. (e) An lch5-3 stall just before entry into the ISN. The axon has grown internally along the spiracular branch (SB). (f) An lch5-5 axon has stalled at the edge of the LC in the CNS (arrow). (g) The adjacent lch5-4 in the same hemisegment as the lch5-5 neuron in (f) has stalled a short distance from the cell body (arrow). (f, g) also show the disruption in the alignment of the lch5 cell bodies often seen in the nrg mutant. (h) dbd shows a normal, bifurcating axon projection within the CNS in the nrg mutant. (i) The axon of lch5-1 (red) lies separate to the dorsal cluster axons (arrow, green, 0.5 μm separation) within the ISN in this stage 16 nrg mutant embryo. (j, k) nrg¹⁴/Y embryos immuno-stained with mAb22C10. (j) Segment A3 in a stage 15 embryo showing a normal distribution of sensory nerves. This montage shows the initial segment of the dorsal cluster axon fascicle (d) and the lateral cluster fascicle to the point where it turns internally to join the ISN (l). The axon of v'ch1 (arrow) follows a normal course to the v' cluster (v'). (k) Segments A3-5 in a stage 13 embryo. lch5-1 has projected a short axon in all three segments (arrow), while the soma of lch5-2 is only weakly stained and has yet to form an axon (arrowhead). Scale bar = 10 μm in all figures.