Fig. 3. In situ hybridization studies for the Cdk5 mutant hindbrain. Sections were probed with either peripherin (A–D, G–J) or Phox2b (E, F, K, L); these sections were from either Cdk5+/+ (A, C, E, G, I, K) or Cdk5−/− (B, D, F, H, J, L) mouse embryos of age E18.5 (A, B), E14.5 (C–F), and E16.5 (G–L). The sections shown are either coronal (A–F) or sagittal (G–L) sections. The ectopic neuronal mass was identified as VIIth motor nuclei because it stained positive for Phox2b as well as peripherin (arrows inB and H). I andJ are higher magnifications of G andH, as indicated with arrows, respectively. The arrow in A indicates the VIIth motor nucleus in Cdk5+/+ mice. E, F, and K, L are serial sections of C, D and I, J, respectively, hybridized with Phox2b probe. In Cdk5−/− mice (N, O) at E18.5, the motor nuclei of the Vth (arrows in O), the Xth (arrowheads in M andN), and the XIIth (arrow inM and N) nuclei are comparable with Cdk5+/+ mice (M), as shown by the expression pattern of peripherin. However, segregation of the Xth (arrowheads) and XIIth (arrow) nuclei was less clear in Cdk5−/− mice (N). Although the VIIth nucleus (arrows) appeared normal in p35−/− mice (P), an elongated shape of the VIIth nucleus (arrows) was observed in p35−/−Cdk5+/− mice (Q) at E18.5, as visualized by peripherin expression. The arrowhead in P indicates motor nuclei of the Vth nerve in p35−/− mice. Scale bars: B, 400 μm; C, 80 μm; N, 100 μm.WT, Wild type; KO, knock-out.