Figure 5

Commissural axons in the embryonic chicken spinal cord express all three Robos . We used in situ probes specific for Robo1 (A,E,I), Robo2 (B,F,J), and Robo3 (C,G,K) to assess their expression pattern in comparison to Axonin-1/TAG-1 (D,H,L) at HH19 (A-D), HH23 (E-H), and HH26 (I-L). Robo1 mRNA could be detected in commissural neurons as soon as they start to extend axons (arrow in A), and continued to be expressed in these neurons throughout the time window of their axonal navigation across the floor plate and into the longitudinal axis (arrows in E,I). Robo2 was expressed throughout the neural tube except for the floor plate at HH19 (B). Weak expression in a subset of commissural axons was found at HH23 (arrow in F). By HH26, Robo2 mRNA was barely detectable in dorsal commissural neurons (arrow in J). At all stages, Robo3 mRNA was found in dorsolateral commissural neurons (arrows in C,G,K). Axonin-1/TAG-1 expression indicates the position of dorsal commissural neurons (D,H,L). Bars: 33 μm in A-D, 50 μm in E-H, 100 μm in I-L. Dorsal is to the top in all panels.