Fig. 10

Floor plate-dependent axonal pathfinding is intact in the caudal hindbrain of cyc;ntl double mutants. Dorsal views, with anterior to the left, of the caudal head region of 30-hr embryos, labeled with the zn-12 monoclonal antibody (Trevarrow et al., 1990). This antibody binds the HNK-1 epitope broadly expressed on developing neurons (Metcalfe et al., 1990), including the medial longitudinal fascicles (MLF, arrowheads) which emanate from bilateral neuronal clusters in the midbrain, the nuclei of the MLF (nMLF, asterisks). The MLFs collect additional axons from hindbrain neurons (Metcalfe et al., 1986; Hatta, 1992). The trigeminal ganglion (tg) is approximately at the level of the midbrain – hindbrain boundary. In cyc mutants (A) the normally paired nMLF clusters form a single unpaired midline cluster and the MLF axons are poorly defasciculated, present in or near the midline, and often cross the midline (Hatta, 1992). In contrast, in ntl mutants (B) the nMLF positions and the MLF axonal trajectories are as in wild-type embryos described previously (Hatta, 1992), and the MLF axons are likewise well-fasciculated. In the midbrain and rostral hindbrain, cyc;ntl double mutants (C) exhibit defects similar to cyc mutants. Correct axonal pathfinding and axonal fasiculation, however, recovers in the hindbrain, the level approximately corresponding to the most rostral region where floor plate is present in these mutants. Scale bar, 100 um.