Figure 5

The dorsal side of the neural tube precedes the ventral side in up-regulating Hoxb4 expression. (A-D) A series of chick neural tube from 10 (A), 12 (B), 15 (C) and 17 (D) somite stage embryos, stained for Hoxb4. The neural tube is flat-mounted by opening it from the dorsal side which is now located laterally. Arrowheads indicate the rhombomere 6/7 boundary (r6/7). At the 10 somite stage, when the anterior-most expression has not reached r6/7 (see also Fig. 1E), the up-regulation is restricted to the dorsal-most edge of the neural tube (A). At the 12 somite stage, a distinct D-V difference is seen (B). By the 15 somite stage, this D-V difference becomes less apparent in accordance with the expression becoming stronger, although the most-ventral region of the neural tube does not express Hoxb4 (C). (D) is at the 17 somite stage when the expression is mostly homogeneous except in the floor plate region. Scale bar; 100 μm. (E-G) Transverse sections of a 12 somite chick embryo stained for Hoxb4, at the level of the 1^st somite (E), 5^th somite (F) and 10^th somite (G). Dorsally localized expression is seen in the neural tube at all axial levels. Scale bar; 50 μm.