Fig. 1. Inter-rhombomeric boundaries and segmental neuronal differentiation are affected in spg mutants. (A-C) Lateral views of the hindbrain of live wild-type (A), spg^m216 (B) and spg^m793 (C) embryos at the 22-somite stage. Red arrows indicate the positions of rhombomere boundaries and black arrowheads mark the size of the otic vesicle (ov). (D-F) Dorsal views of the hindbrain of wild-type (D), spg^m216 (E), and spg^m793 (F) embryos at 1 dpf that were assayed for pax6.1 expression by WISH. (G-I) Lateral views of wild-type (G), spg^m216 (H), and spg^m793 (I) embryos at 1 dpf that were assayed for lim1 expression by WISH. (J-L) Confocal images of wild-type (J), spg^m216 (K) and spg^m793 (L) embryos at 5 dpf in which hindbrain reticulospinal neurons were back-filled with lysinated rhodamine-dextran. Stacks of confocal images were merged to produce a composite image in which the signal from each layer was depth-coded in color. A color-coded depth scale (in µm) is shown below each figure (dorsal-most blue to ventral-most red). In the spg^m216 mutant embryo shown (K), reticulospinal neurons in r1 and r6 are not seen while the large Mauthner neurons in r4 are duplicated. In spg^m793 mutant embryos (L), fully-differentiated primary reticulospinal neurons are not observed. Anterior is to the left in all panels. 1-7, rhombomeres 1-7; g, ganglia of the posterior lateral line neurons; m, nuclei of the medial longitudinal fascicles; tg, tegmentum; v, vestibular nuclei.