Figure 6.

Post-crossing commissural axons are guided by higher Wnt activity in the rostral compared with the caudal floor plate. A, Explants of post-crossing commissural neurons were cultured with floor-plate explants (fp) taken from either rostral (R) or caudal (Ca) lumbosacral levels. Three explants containing commissural neurons and two floor-plate explants dissected from a single open-book preparation were cultured as depicted. Neuronal explants were randomly assigned to one of three conditions (a, b, or c). For quantification, neurite growth obtained under conditions a and b were normalized to the control condition c from the same embryo (set as 1.0, n = 19). B, C, Both the presence of a rostral (1.85 ± 0.18, n = 19) (B) and a caudal (1.28 ± 0.13, n = 19) (C) floor-plate explant had a significant effect on post-crossing commissural axon growth compared with the control condition (supplemental Table S4, available at www.jneurosci.org as supplemental material). D, However, the growth-promoting effect of rostral floor-plate explants was significantly stronger compared with caudal floor-plate explants. E–G, The growth-promoting effect of the floor-plate explants could be blocked by addition of Sfrp1 (1 μg/ml; rostral floor-plate explants, 0.96 ± 0.14, n = 7; caudal floor-plate explants, 1.04 ± 0.12, n = 7; control set as 1.0, n = 7). For statistical analysis the two-tailed Student's t test was used. Values are given ±SEM. ***p < 0.001 for rostral floor-plate explants compared with control; *p < 0.05 for caudal floor-plate explants compared with control and for rostral versus caudal floor-plate explants. Scale bar, 200 μm.