Conditional neuronal-specificity-switch of Met signalling to PI3K and Src is sufficient for pectoralis minor motor pool survival in vivo . (A,B) Correlation between MN numbers in spinal cords with the amount of limb muscle in Met signalling mutants (see Figure 2A,B). Analysis of MN pool survival in vivo after the onset of muscle dependency by Pea3 (A) or Ret (B) ISH on spinal cords from E15.5 WT, metd/d, met2P/2P, and met2S/2S embryos (n = 3 spinal cords for each probe and each genotype). Limb muscle depletion in metd/d embryos causes the death of all limb-innervating MNs, hence the complete loss of Pea3+ neurons and Ret-positive LMC but not axial columns. (C, D) Analysis of MN survival in Nestin-Cre (referred to as Nes) mediated neuronal-specificity-switch mutants, which display unaltered amounts of muscle. (C) High magnification views of ChAt ISH staining of the ventral horn of spinal cord cross-sections in Nes-Metflox/+, Nes-Metd/flox, Nes-Met2P/flox, and Nes-Met2S/flox neonates (P2; n = 6 for each group of animals) at C8-T1 level. In this region, MN clusters are organized in two groups (dashed lines). The more ventral cluster (red dashed lines) exhibits less MNs in the Nes-Metd/flox mutant, in contrast to Nes-Met2P/flox and Nes-Met2S/flox mutants, which are comparable to controls. (D) Quantification of MN numbers throughout the brachial region of Nes-Metd/flox, Nes-Met2P/flox, and Nes-Met2S/flox P2 mutants, showing percentages of MN loss per section. Positions of the C8, T1, and T2 DRGs are indicated by black lines. In the C8-T1 region, where percentages of MNs loss in Nes-Metd/flox mutants are highest, Nes-Met2P/flox and Nes-Met2S/flox mutants show non-significant MN loss, and are significantly different from Nes-metd/flox mutants. Student’s t test: *P < 0.05, **P < 0.01, ***P < 0.001. Error bars represent standard error of the mean.