Figure 4. Two proposed models for the role of Wnt signaling in determination of neural versus non-neural ectodermal fates.

A. In the first model, in the absence of Wnt signaling BMP expression promotes epidermal fate while repressing neural specification through the inhibition of FGF signaling (upper left panel). Active Wnt signaling promotes ectodermal induction through activation of FGF signaling as well as direct and indirect (through the induction of BMP antagonists [α-BMPs]) repression of BMP signaling (upper right panel). B. In an alternative model, absence of Wnt signaling allows for active FGF signaling to induce neural cell fates (lower left panel). Active Wnt signaling attenuates FGF signaling thereby providing for BMP signals to promote epidermal cell identities (lower right panel).