Fig. 5. Removal of PSA interferes with the development of hindbrain commissures. (A,B) Hindbrain of a buffer-injected control embryo at 32 hpf labelled with antibodies against PSA and Tag-1. (A) PSA is expressed by the motor nuclei of cranial nerves (nV and nVII), by their motor axons leaving the CNS (arrowheads) and on cells of the floorplate (fp). (B) Tag-1 labeling shows the VII cranial nerve and commissural axons crossing the midline (B, arrow). The boxed regions rostral to the otocyst (oto) in A and B represent the position chosen for the analysis of commissural axon growth. (C) In control embryos, PSA is expressed on cells of the floorplate (fp) but not on commissural axons. (D) Tubulin labeling of the same embryo shows commissural axons crossing the medial longitudinal fascicle (mlf) in thick bundles (asterisks). These axons defasciculate (arrowheads in D) upon arrival at the PSA-positive floorplate cells. (E,F) The growth pattern of commissural axons is markedly disturbed in endo N-injected embryos. Fewer axons cross the floorplate (E) or bundles of commissural axons appear to stop at the PSA-negative floorplate cells (arrowheads in F). All images are confocal images, ventral views, rostral at the top. Scale bars: in B, 20 µm in A,B; in F, 5 µm in C-F.