Fig. 3. Axon guidance defects in piranha mutants arise during embryonic development and affect all motor axon pathways. (A-D) Micrographs of motor axons in three consecutive hemisegments in dissected wild-type (A,C) and piranha^D427/piranha^K788 mutant (B,D) embryos at stage 17 stained with anti-Fas II antibodies. (A) In the dorsal region of a wild-type embryo, the ISN has reached its terminal branch point at muscle pair 1/9 in all three hemisegments, and all three branch points are well developed (arrowheads 1-3). (B) The ISN fails to reach its final branch point properly (arrow) in piranha mutants, and most branch points appear underdeveloped (arrowheads 1-3). (C) Lateral and ventral body wall region of a wild-type embryo. The SNa bifurcates into a dorsal branch (arrow) and posterior branch. The ISNb (arrowheads) forms a clearly visible projection innervating muscles 12, 13, 6 and 7. (D) In piranha mutants, a bifurcated dorsal branch of the SNa is often observed (arrowheads). The ISNb fails to innervate muscles 12, 13, 6 and 7. In the left segment, the ISNb remains fused to the ISN and branches off at muscle 4 (arrow). In the middle and right segments, a split bypass occurs, with the ISNb temporarily detaching from the ISN (right) or fusing to different nerve tracts (middle).