Fig. 7. Model of the signalling interactions that position the pre-placodal region in the cranial ectoderm. (A) The section through an embryo viewed from anterior to posterior represents signalling within the ectoderm on the left and mesoderm-derived signals on the right. The pre-placodal region (dark blue) lies next to the neural crest (red) at the border of the anterior neural plate and is induced by FGF together with BMP and WNT antagonists derived from the underlying mesoderm (light blue). WNT signals from the lateral and posterior mesoderm (pink, right) cooperate with WNT from the trunk ectoderm (pink, left) to limit the PPR. BMP signals from the lateral ectoderm (grey) also prevent expansion of the PPR into more lateral regions. WNT proteins in the neural folds promote neural crest formation (red, left), but inhibit PPR gene expression. The neural plate expresses BMP and WNT inhibitors, which may account for its limited PPR inducing ability. (B) Model for neural crest and placode specification at the border of the neural plate; the terminology follows a recent molecular network for neural crest cell specification proposed by Meulemans and Bronner-Fraser (Meulemans and Bronner-Fraser, 2004), which classifies three categories of molecules: secreted inducers (orange), neural plate border specifiers (black) and neural crest specifiers (red). We propose that the Six/Eya/Dach network may act as pre-placode specifier (blue). Pax7 was classified as a neural plate border specifier (Meulemans and Bronner-Fraser, 2004) and is, like Msx1, expressed early; however, so far it is not clear which upstream signalling pathways induce Pax7. Our experiments reveal that at neural fold stages its expression is controlled by WNT. FGF signalling plays a dual role: it promotes border specifiers and later pre-placode specifiers, while levels of WNT and BMP activity control neural crest and placode fates in the border.