Fig. 4. A caudorostral wave of RALDH2 conveys anteroposterior (AP) information to the cardiac field. (A) HH6 chick RALDH2 in situ hybridization. The anterior limit of chick Raldh2 expression lies near the anterior tip of Hensen's node. (B) HH6 chick GATA4 in situ hybridization. The posterior limit of chick GATA4 lies near the anterior tip of Hensen's node (star). (C) HH6 double chick RALDH2/chick GATA4 in situ hybridization. The domains of chick Raldh2 (red) and chick Gata4 (blue) converged in the lateral mesoderm at the level of Hensen's node contacting posterior cardiac precursors with mesoderm producing RALDH2. (D) HH8 chick RALDH2 expression starts at the lateral mesoderm from the level of the transition between the last formed somite and the unsegmented paraxial mesoderm, almost up to the anterior intestinal portal (AIP) (star). (E) HH8 chick GATA4 expression is in the lateral mesoderm from the level of somite 2 almost up to the AIP. (F,G) Chick Raldh2 and chick Gata4 expression in sections respectively cut from embryos in D and E at the level indicated by the upper bar. (H,I) Chick Raldh2 and chick Gata4 expression in sections respectively cut from embryos in D and E at the level indicated by the lower bar. Below the AIP chick Raldh2 is expressed in splanchnic and somatic mesoderm (F). Chick Gata4 is expressed in splanchnic mesoderm and endoderm (G). At the level of somite 2 chick Raldh2 is expressed in mesoderm (H). Chick Gata4 is expressed in mesoderm (I). Isotopic (35 S) in situ hybridization for RALDH2 (J) and GATA4 (K) in consecutive sections indicate that these genes are co-expressed in the splanchnic mesoderm. Ant Limit, anterior limit; e, endoderm; m, mesoderm; Post Limit, posterior limit; SOM, somatic mesoderm; SPM, splanchnic mesoderm.