-Catenin in Anterior-Posterior Axis Formation in Mice

The anterior-posterior axis of the mouse embryo is defined before formation of the primitive streak, and axis specification and subsequent anterior development involves signaling from both embryonic ectoderm and visceral endoderm. T he Wnt signaling pathway is essential for various developmental processes, but a role in anterior-posterior axis formation in the mouse has not been previously established. b -Catenin is a central player in the Wnt pathway and in cadherin-mediated cell adhesion. We generated b -catenin –deficient mouse embryos and observed a defect in anterior-posterior axis formation at embryonic day 5.5, as visualized by the absence of Hex and Hesx1 and the mislocation of cerberus-like and Lim1 expression. Subsequently, no mesoderm and head structures are generated. Intercellular adhesion is maintained since plakoglobin substitutes for b -catenin. Our data demonstrate that b -catenin function is essential in anterior-posterior axis formation in the mouse, and experiments with chimeric embryos show that this function is required in the em-