Fig. 1. Pan-neuronal misexpression of either Mmp1 or Mmp2 inhibits motor axon defasciculation. In each micrograph, two abdominal hemisegments of stage 17 dissected embryos stained with -FasII to label the motor projections are shown with anterior left and dorsal up. Below each image are schematics diagramming the observed phenotypes with motor axons in brown and muscles represented by gray boxes. In all ISNb schematics, the muscles are drawn as if transparent in order to depict both the ISN and ISNb pathways. However, note that the ISN extends in an external plane whereas the ISNb extends in an internal plane. (A,D) In wild type, ISNb axons defasciculate from the ISN at a choice point proximal to the VLM field (arrowheads), then continue to extend dorsally to innervate muscles 7, 6, 13, 12. (B,C,E,F) In elav>Mmp1 and elav>Mmp2 embryos, ISNb axons appear to defasciculate correctly from the ISN at their first choice point, but fail to defasciculate at their appropriate muscle targets, a phenotype referred to as `stall'. (G,J) In wild type, the SNa branch innervates the lateral musculature and comprises dorsal and posterior branches. The posterior branch extends posteriorly to innervate muscle 8, whereas the dorsal branch makes two stereotyped turns en route to muscles 23 and 24. (H,K) In elav>Mmp1 embryos, the dorsal branch of the SNa stalls (arrowhead) before reaching its final target. (I,L) In elav>Mmp2 embryos, the dorsal (arrowheads) and lateral branches of the SNa stall before reaching their synaptic targets. Scale bar: 15 µm.