Figure 3

Signaling of each somite to the adjacent neural tube. (A-L) Neural tube and somites were dissected at each somite level as shown in the scheme and cultured for 24 hours. Embryos used as donors were at the 3 (A-C), 4 (D-G) or 5 (H-L) somite stage (3S, 4S, 5S) as indicated above the column. Level 1–5 indicates the origin of explants at each somite level. The expression of Hoxb4 is graded with posterior regions displaying the strongest expression. Note that somite level 1 neural tube does not exhibit any Hoxb4 expression (A, D, H). The number of cases is shown in Additional file 3. (M-R) Expression pattern of RALDH2 in chick embryos at the 4 to 9 somite stages (4S to 9S). Somite numbers are indicated in each figure to identify the level. (S, T) Neural tube explants either with (S) or without (T) flanking somites at the level of 1 to 5 somites, taken at the 5 somite stage, cultured for 24 hours and assayed for Hoxb3 expression. Up-regulation of Hoxb3 requires the presence of flanking somites, in the same manner as that of Hoxb4 (S, n = 5/5; T, n = 6/6; compare with Fig. 2I, O). (U) Neural tube and somites were dissected at the level of somite 1 at the 5 somite stage and cultured for 24 hours to assay for Hoxb3 expression. Up-regulation of Hoxb3 is seen (n = 6/6, compare with Fig. 3H where Hoxb4 is negative). Scale bars; 200 μm.