Fig. 6. The lLMC is missing in Hoxc10^-/-/Hoxd10^-/- double-mutant embryos, leading to defects in axon projection and inappropriate innervation. (A) Cross-sections of ventral spinal cord segments L1-L4 of control and double-mutant embryos stained for Isl1 to identify MMC and mLMC neurons and for Lim1 to identify lLMC neurons (dashed outlines). Note that Lim1⁺ lLMC neurons are nearly eliminated in double-mutant embryos. (B) The number of Isl1⁺ motoneurons counted unilaterally from L3 to L5 in control and Hoxc10^-/-/Hoxd10^-/- mutant embryos (n=7 for each genotype). There was no significant difference between control and mutant embryos at E13.0-13.5; P>0.2 for each segment, t-test. (C) Cross-sections through lumbar segment L3 stained for Hb9 for motoneurons. Note that the number of Hb9⁺ cells was markedly reduced in Hoxc10^-/-/Hoxd10^-/- mutants. (D) DiI injected into spinal cord segments L3 and L4 labeled axons in the hindlimb. In control embryos, both the tibial (ventral view, arrow) and peroneal (dorsal, arrowhead) nerves were labeled. In double-mutant embryos, the ventral tibial nerve was present, but the peroneal nerve was missing. Some cutaneous axons were also labeled in dorsal views. (E) Cross-sections through ventral spinal cord segment L2 of control, and segment L3 of Hoxc10^-/-/Hoxd10^-/- mutant embryos. In control embryos, tetramethylrhodamine dextran injected into the quadriceps muscle retrogradely labeled a subgroup of Lim1⁺ lLMC neurons (arrow, yellow neurons) in L2, but did not label Isl1⁺ mLMC neurons. By contrast, in double-mutant embryos, injection of dextran into the quadriceps retrogradely labeled Isl1⁺ mLMC neurons in L3 (arrow). Scale bars: 100 µm in A,C; 50 µm in E.