The establishment of the head to tail axis at early stages of development is a fundamental aspect of vertebrate embryogenesis. In mice, experimental embryology, genetics and expression studies have suggested that the visceral endoderm, an extra-embryonic tissue, plays an important role in anteroposterior axial development. Here we show that absence of Wnt3 in the posterior visceral endoderm leads to delayed formation of the primitive streak and that interplay between anterior and posterior visceral endoderm restricts the position of the primitive streak. Embryos lacking Wnt3 in the visceral endoderm, however, appear normal by E9.5. Our results suggest a model for axial development in which multiple signals are required for anteroposterior axial development in mammals.

The formation of the anteroposterior axis in mice requires a Wnt3-dependent symmetry-breaking event that leads to the formation of the primitive streak and gastrulation. Wnt3 is expressed sequentially in two distinct areas of the mouse embryo before the appearance of the primitive streak; first in the posterior visceral endoderm and soon after in the adjacent posterior epiblast. Hence, although an axial requirement for Wnt3 is well established, its temporal and tissue specific requirements remain an open question. Here, we report the conditional inactivation of Wnt3 in the epiblast of developing mouse embryos. Contrary to previous studies, our data shows that embryos lacking Wnt3 specifically in the epiblast are able to initiate gastrulation and advance to late primitive streak stages but fail to thrive and are resorbed by E9.5. At the molecular level, we provide evidence that Wnt3 regulates its own expression and that of other primitive streak markers via activation of the canonical Wnt signaling pathway.

The formation of the anterior-posterior axis requires a symmetry-breaking event that starts gastrulation. Ultimately, the morphogenetic movements of gastrulation reshape the embryo to its final tri-dimensional form. In mouse embryos, the identity of the molecule that breaks the bilateral symmetry and sets in motion gastrulation remains elusive. The Wnt signaling pathway plays a pivotal role during axial specification and gastrulation in metazoans. Loss-of-function experiments have demonstrated a requirement of Wnt3 for gastrulation in mice. But because Wnt3 is expressed sequentially in two tissues, the visceral endoderm and the epiblast, its tissue specific requirements remain uncertain. Here, we report that embryos lacking Wnt3 specifically in the visceral endoderm do not form a primitive streak, mesoderm, endoderm or any derivatives. Visceral endoderm-specific Wnt3 mutants also lack primordial germ cells. Moreover, we provide data demonstrating that Wnt3 carries out its actions in the epiblast via the canonical Wnt pathway. Together, these data suggest that the posterior visceral endoderm via Wnt3, regulates the development of mouse embryos in a similar fashion to the amphibian Nieuwkoop center. Next, we conditionally ablated Wnt3 locus in the epiblast to investigate whether Wnt3 expression is also required in that tissue. Embryos lacking Wnt3 expression in the epiblast, but retaining its expression in the visceral endoderm, show delayed but not absent gastrulation. We conclude that the expression of Wnt3 in the epiblast is required for maintenance but not initiation of gastrulation in mouse embryos. Furthermore, we used in vitro and in vivo approaches to demonstrate that the Wnt3-mediated activation of the canonical Wnt pathway leads to β-catenin occupancy followed by transcription of key loci, including the Wnt3 locus itself, during gastrulation in mice. Our data indicate the presence of an autoregulatory loop in which Wnt3 controls its own expression and orchestrates the process of gastrulation in the mouse embryo.