Met signalling is required for hindlimb nerve patterning prior to the onset of muscle-dependency for MN survival. (A) Lumbar spinal nerves and plexuses at hindlimb levels based on a lateral view of an E11.5 WT embryo. The sciatic plexus (from which the tibial (ventral) and peroneal (dorsal) nerves emerge) receives axons mostly from L4 and L5 spinal segments, while the femoral plexus (anterior) collects axons mostly from L2 and L3 spinal segments. (B) ISH with Met and Ret probes on E12.5 WT spinal cords. The LMC pool corresponding to peroneal MNs is indicated in yellow, spanning from mid L3 to L6. (C) Met expression was followed by Salmon-Gal staining in spinal cords from E12.5 metLacZ/+, metLacZ/d, metLacZ/2P, and metLacZ/2P embryos. The position of peroneal MNs is indicated in yellow. (D) Quantification of the size of Met-expressing MN pools along the orange line at L5 level. Each plot represents the average signal distribution of the indicated number of spinal cord sides (metLacZ/+: n=24; metLacZ/d: n=12; metLacZ/2P: n=8; metLacZ/2P: n=10. (E) Ret expression was followed by ISH in lumbar spinal cords from E12.5 WT, metd/d, met2P/2P, met2S/2S embryos. (F) The size of the Ret-expressing MN pools was quantified along the orange line at L5 level. Each plot represents the average signal distribution of the indicated number of spinal cord sides (met+/+: n=12; metd/d: n=10; met2P/2P: n=8; met2S/2P: n=6). (G, H) Quantifications and statistical analyses of the sum of signal intensity corresponding to measurements of metLacZ expression (G) or Ret ISH staining (H) based on intensity plots in (D) and (F), respectively. The numbers of samples are as indicated in (D) and (F). At E12.5 the size of the MN population is not significantly altered by the reduced muscle mass in metd/d, met2P/2P, and met2S/2P embryos, indicating that this stage precedes the phase of MN death.