Axonal PlxnA2 is required for the pathfinding of post-crossing axons. (A) Downregulation of PlxnA2 in the dorsal spinal cord led to aberrant commissural axon guidance at the contralateral floorplate border (arrowheads). (B) Commissural axon guidance was normal (arrows) when dsSema6D was electroporated dorsally. (C) Electroporation of only the dorsal spinal cord was verified by EGFP expression (asterisk). (D) Quantifications of DiI injection sites with aberrant axon pathfinding. **P<0.01; error bars indicate s.e.m. The dsSema6D value is taken from Fig. 1. (E) Staining of transverse sections of HH25 spinal cords localized PlxnA2 immunoreactivity to the pre-crossing segment and commissure (arrowheads). (F) Sema6B was expressed all along dissociated commissural axons and on growth cones (open arrowhead), as was axonin 1 (G,J), a marker for commissural neurons. PlxnA2 (H,K) was also expressed on axons and growth cones, in contrast to PlxnA4 (I) which was found in a punctate pattern along axons (arrowheads). No PlxnA4 was found on growth cones (L). Arrows (F-I) indicate neuronal soma. Scale bars: 100 μm in A-C,E; 50 μm in F-I; 20 μm in J-L.