Figure 8.

Pre-crossing commissural axons become sensitive to Slit2 by premature RabGDI expression. Explants containing young mock-transfected commissural neurons readily extend axons, which correspond to pre-crossing commissural axons, when cultured in a 3D collagen gel (A). The addition of recombinant Slit2 does not affect axon growth from these explants (B). A slight, but not significant reduction in axon growth was observed when explants were taken from embryos expressing RabGDI prematurely at HH21/22 (see Material and Methods and text for details; C,E). However, a strong reduction in axon growth was observed when RabGDI-expressing ‘pre-crossing’ commissural axons were cultured in the presence of Slit2 (D,E). Axons were visualized with anti-neurofilament staining. For quantification of axon growth 20–25 explants per condition were used. Axon length of mock-transfected neurons was set to 1.0 (E). Axon length from mock-transfected neurons was not different in the presence of Slit2 (0.95 ± 0.12; p = 0.765). In the absence of Slit2, commissural neurons expressing RabGDI extended axons that were slightly, but not significantly shorter than mock-transfected neurons (0.76 ± 0.11; p = 0.121). In the presence of Slit2, axon length was significantly shorter when neurons expressed RabGDI prematurely (0.49 ± 0.08; p = 0.0028 compared to mock-transfected neurons in the presence of Slit2 and p = 0.048 compared to RabGDI-expressing axons in the absence of Slit).