Fig. 3. Wnt-Fz interaction mediates the rostral growth of post-crossing commissural axons in the vertebrate spinal cord. (A-C) Schematics of the vertebrate embryonic spinal cord in the open book configuration under (A) normal or (B,C) experimental conditions. (A) Commissural growth cones turn rostrally after crossing the midline, attracted by Wnt4 expressed in the floor plate in an anterior-high to posterior-low gradient (graded grey shading). (B) The addition of Wnt4-expressing cells at the caudal end of short spinal cord explants cultured in the open book configuration disrupts the Wnt gradient at the floor plate (solid blue colour) and reorients the axons toward the posterior spinal cord. (C) The addition of cells transfected with the Wnt inhibitor Sfrp2 in the proximity of the floor plate inactivates Wnt function (pale blue), and commissural axons stall at the floor plate or turn randomly. C, caudal; D, dorsal; FP, floor plate; R, rostral; V, ventral.