Figure 6

Dorsal precedence of Hoxb4 expression is associated with the dorsalized feature of the neural tube. (A-F) Grafting of the 8^th somite anteriorly to the pre-otic region induces ectopic up-regulation of Hoxb4. In (A-C), the D-V orientation of the somite was kept as normal. In (D-F), the somite graft was taken from the contralateral side and rotated 180°, which reversed the D-V orientation of the graft but kept the medio-lateral orientation the same. (B) and (E) are flat-mounts of neural tube at HH 16, showing the dorsal neural tube at lateral sides. Asterisks show ectopic induction of Hoxb4 by grafted somites. Arrowheads show rhombomeres 6/7 boundary (r6/7). Scale bar; 200 μm. (C) and (F) are transverse sections of similar embryos at HH 15, at the level of rhombomere 4–6. In both cases, strongest induction is observed at the dorsal neural tube (arrows) and graded ventrally, regardless of the orientation of grafted somites (n = 5/5 for each orientation). Scale bar; 100 μm. (G-L) Ablation of the surface ectoderm and dorsal neural tube. In (G-I), the roof plate of the left side of the neural tube was ablated at the 3–4 somite stage from somite level 1–5, which was accompanied by removal of surface ectoderm just covering the neural tube as shown in the scheme (G). Six hours later, the surface ectoderm had regenerated (arrowheads in H, I), and both Msx1 (H, n = 8/8) and Hoxb4 (I, 8/10) expression had been restored (asterisks in H, I). In (J-L), the roof plate and surface ectoderm were similarly ablated but a foil barrier was placed to prevent the ectoderm from regenerating, as illustrated in (J). In these embryos, the surface ectoderm maintained discontinuity with the dorsal neural tube throughout the following incubation (arrows in K, L). These embryos failed to up-regulate Msx1 (K, n = 4/5) and Hoxb4 (L, n = 5/7) at the dorsal neural tube (double arrows in K, L). Scale bar; 100 μm.