Fig. 1

Conserved reliance of sensory afferent axon (SA) extension on pioneer motor efferent axons (MEs). (A) Peripheral axons visualized in 72hpf zebrafish larva. (B-E) Time-lapse sequence of SA, labeled using -8.4neurog1::GFP transgene (yellow), extending peripherally along preformed CaP ME, labeled by NBT::dsRed transgene (magenta). Arrowheads indicate SA growth cone. (F-H) Delayed and aberrant SA extension in the absence of MEs upon islet1E2/E3 morpholino injection. (I) Longitudinal analysis of SA and ME extension and impacts of ME removal (ne7 segments, e3 embryos per stage and condition). (J) Peripheral axons visualized in E4 chick embryo. (K-N) SAs, labeled using the Isl1^DRG::mGFP transgene (yellow), extending along preformed MEs, labeled by co-transfected Hb9^MN::mCherry. Arrowheads indicate distalmost SA growth cone. Asterisks indicate additional Isl1^DRG::mGFP activity in non-DRG neural crest cells. Dotted line indicates trunk/limb boundary. (O-Q) Delayed and aberrant SA extension (visualized using anti-Tuj1 antibody) in the absence of MEs upon Hb9^MN::Cre/PGKneolox2DTA co-transfection. (R) Longitudinal analysis of relative SA and ME extension, and impact of ME removal (ne7 sections, e3 embryos per stage and condition). (S) Peripheral axons visualized in E10.5 mouse embryo. (T-W) SAs, labeled using the Brn3a^tau:lacZ allele, extending along preformed MEs, labeled using the Hb9^MN::GFP transgene. (X-Z) Delayed and aberrant SA extension in the absence of MEs in Olig2^Cre;Rosa26^lxstopDTA embryos. (AA) Longitudinal analysis of relative SA and ME extension, and impact of ME removal (ne8 sections, e4 embryos per stage and condition). hpf, hours post-fertilization; DRG, dorsal root ganglion; MN, motor neurons. Error bars indicate standard error of the mean (s.e.m.). Scale bars: 10 µm in E,H for B-H; 50 µm in K-Q,T-Z.