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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.
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