Figure 7

The temporal expression of RabGDI is consistent with its role as a regulator of Robo1 expression on commissural axons. RabGDI is not expressed by dorsolateral commissural neurons when their axons grow toward the floor plate between HH19 (not shown) and HH21 (open arrowhead in A) or when they have reached the floor plate at HH22 (open arrowhead in B). RabGDI is detectable only after commissural axons have entered the floor-plate area at HH23 (arrowhead, C). The adjacent section to the one shown in (C) was hybridized with the corresponding sense probe as a control (insert in C). At HH22, only very low levels of RabGDI protein were detectable in cells lining the central canal (arrow in D) and in motor axons (arrowhead in D). At HH24, RabGDI protein was localized almost exclusively to post-crossing axons (E,F; compare levels in post-crossing axons (arrow in F) with pre-crossing commissural axons (open arrow in F)). Following premature expression of RabGDI, many commissural axons failed to enter the floor plate (arrow in G), consistent with premature expression of Robo1 on commissural growth cones. Bar 100 μm in A-E and G.