Tissue-intrinsic Wnt signals antagonize Nodal-driven AVE differentiation
The anterior-posterior axis of the mammalian embryo is laid down by the anterior visceral endoderm (AVE), an extraembryonic signaling center that is specified within the visceral endoderm. Current models posit that AVE differentiation is promoted globally by epiblast-derived Nodal signals, and spatially restricted by a BMP gradient established by the extraembryonic ectoderm. Here, we report spatially restricted AVE differentiation in bilayered embryo-like aggregates made from mouse embryonic stem cells that lack an extraembryonic ectoderm. Notably, clusters of AVE cells also form in pure visceral endoderm cultures upon activation of Nodal signaling, indicating that tissue-intrinsic factors restrict AVE differentiation. We identify Wnt signaling as a tissue-intrinsic factor that antagonizes AVE-inducing Nodal signals. Together, our results suggest that interactions between epiblast and visceral endoderm alone enable local AVE differentiation in the absence of graded BMP signals. This may be a flexible solution for axis patterning in a wide range of embryo geometries.