Fig. 6. Removal of PSA prevents commissural axons in the hindbrain from crossing the midline. Schematic drawings illustrate the projection pattern of commissural axons in the hindbrain in cross section (A) and in a ventral view (B). Cell bodies of commissural interneurones occupy a dorsolateral position in each hindbrain segment. Their axons extend ventrally, cross the medial longitudinal fascicle (mlf) and the floorplate (FP) and terminate in contralateral mid-dorsal positions in the same hindbrain segment (A). Axons of reticulospinal neurones (B, dark green) cross the floorplate in a curved pathway and project into the contralateral mlf. Axons of commissural interneurones (B, light green) cross the mlf in a straight pathway and defasciculate upon contact with PSA-positive floorplate cells (yellow). (C,G) Hindbrain of zebrafish embryos at 36 hpf in an overlay of interference contrast and DiI-fluorescence. To analyse the growth pattern of commissural axons in more detail, DiI was inserted close to the cell bodies of commissural interneurones (asterisks) rostral to the otocyst (oto). (C-F) In buffer-injected control embryos, all DiI-labelled axons cross the midline (dotted line) and terminate on the contralateral side. (G-K) In endo N-injected embryos, either all (H,I) or subsets (K) of DiI-labelled axons stop at the midline. (C-K) Dorsal views, rostral at the top. Scale bar: 50 µm.