Figure 9

Mechanism of commissural axon growth and guidance after midline crossing. Schematic drawings of the commissural axon trajectory under various experimental conditions. Dorsally located commissural interneurons are depicted in red, whereas ipsilaterally projecting interneurons are shown in green. Wnt (dark green) and Shh (light red) gradients are indicated. Expression sites for MDGA2 are highlighted by the plus signs. Areas of MDGA2 knockdown are indicated by the yellow striped regions. (A) Under control conditions commissural axons turn rostrally to grow along the longitudinal axis towards the brain. (B) In embryos lacking rostrocaudal activity gradients of the guidance cues Wnt4 or Shh, commissural axons either stall or turn randomly either rostrally or caudally. (C,D) Ipsilateral knockdown of MDGA2 prevents commissural axonal growth along the longitudinal axis (C), a phenotype also seen after contralateral knockdown of MDGA2 (D). FP, floor-plate.