Fig. 5. Commissural axons are mispolarized in BmprIb loss-of-function mutant mice. (A-C) In transverse sections of the spinal cord taken from hind-limb levels from E11.5 wild-type (A) and Math1::cre; BmprIa^flox/flox (B) embryos, Tag1⁺ neurons are highly polarized, with the overwhelming majority (99.9%±0.1 s.e.m., n=27 sections from 4 embryos) extending axons to the FP. Many commissural axons also extend towards the FP in BmprIb^-/- embryos (C); however, a small number of axons (1.8%±0.4 s.e.m., n=50 sections, 5 embryos) project aberrantly towards the lumen (open arrowhead, C). (D) A significantly higher (P<0.001) percentage of commissural axons are mispolarized medially in BmprIb^-/- embryos compared with their wild-type littermates. (E,F) The Math1::cre line drives expression of Cre recombinase (green) specifically in the pLh2⁺ (red) population of commissural neurons. Scale bar in A: 75 µm for A-C, E,F.