Fig. 2. Endoderm is required for the induction and maintenance of Nkx2.5 and Mef2c but not Gata4. Endoderm adjacent to the heart region was removed (as shown by the boxed outline) from embryos at stage 5-6 (A). The unoperated side served as a control. Embryos were incubated until stage 8-9 (B) and processed for in situ hybridization for Nkx2.5 (C,D,G), Mef2c (E,H) or Gata4 (F,I). As seen in C-F, both Nkx2.5 and Mef2c expression were downregulated on the experimental side, while Gata4 expression persisted. On sections, it could be determined where the endoderm had been removed, and it could be seen that endoderm removal led to loss of Nkx2.5 and Mef2c expression in the adjacent cardiac mesoderm (G,H), while mesodermal Gata4 expression was not affected (I). The edges of the removed endoderm are marked in G-I. ecto, ectoderm; edge, edge of removed endoderm; endo, endoderm; hn, Hensen’s node; hp, head process; meso, mesoderm; np, neural plate; ps, primitive streak.
