Model for mesoderm induction and organizer formation by the interaction of β-catenin and TGF-β proteins. (A) At late blastula stages, Vg1 and VegT are found in the vegetal hemisphere, while β-catenin is located in the dorsal region. (B) β-catenin acts synergistically with Veg1 and VegT to activate the Xenopus Nodal-related (Xnr) genes. This creates a gradient of Xnr proteins across the endoderm, highest in the dorsal region. (C) The mesoderm is specified by the gradient of Xnr proteins. Mesodermal regions with little or no Xnr proteins have high levels of BMP-4 and Xwnt-8; they become ventral mesoderm. Those having intermediate concentrations of Xnrs become lateral mesoderm. Where there is a high concentration of Xnrs, the goosecoid gene and other dorsal mesodermal genes are activated, and the mesodermal tissue becomes the organizer. (These results may explain the activity concentration experiments mentioned in Chapter 3.) (After Agius et al. 2000.)