Somitogenesis is thought to be controlled by a segmentation clock, which consists of molecular oscillators in the Wnt3a, Fgf8 and Notch pathways. Using conditional alleles of Ctnnb1 (␤-catenin), we show that the canonical Wnt3a/␤-catenin pathway is necessary for molecular oscillations in all three signaling pathways but does not function as an integral component of the oscillator. Small, irregular somites persist in abnormally posterior locations in the absence of ␤-catenin and cycling clock gene expression. Conversely, Notch pathway genes continue to oscillate in the presence of stabilized ␤-catenin but boundary formation is delayed and anteriorized. Together, these results suggest that the Wnt3a/␤-catenin pathway is permissive but not instructive for oscillating clock genes and that it controls the anterior-posterior positioning of boundary formation in the presomitic mesoderm (PSM). The Wnt3a/␤-catenin pathway does so by regulating the activation of the segment boundary determination genes Mesp2 and Ripply2 in the PSM through the activation of the Notch ligand Dll1 and the mesodermal transcription factors T and Tbx6. Spatial restriction of Ripply2 to the anterior PSM is ensured by the Wnt3a/␤-catenin-mediated repression of Ripply2 in posterior PSM. Thus, Wnt3a regulates somitogenesis by activating a network of interacting target genes that promote mesodermal fates, activate the segmentation clock, and position boundary determination genes in the anterior PSM.

The results of this study revealed developmental stage-specific cell populations of the developing brain and craniofacial region that are vulnerable to ethanol-induced apoptosis and provide new insight relative to the genesis of alcohol-related birth defects.