Ets2-dependent trophoblast signalling is required for gastrulation progression after primitive streak initiation.

Although extraembryonic ectoderm trophoblast signals the embryo for primitive streak initiation, a prerequisite for gastrulation, it is unknown whether it also signals for the progression of gastrulation after primitive streak initiation. Here, using Ets2-/- mice, we show that trophoblast signalling is also required in vivo for primitive streak elongation, completion of intraembryonic mesoderm epithelial-mesenchymal transition and the development of anterior primitive streak derivatives such as the node. We show that Ets2-dependent trophoblast signalling is required for the maintenance of high levels of Nodal and Wnt3 expression in the epiblast and for the induction of Snail expression in the primitive streak, between embryonic day 6.3 and 6.7. Within extraembryonic ectoderm trophoblast, Ets2 maintains the expression of the transcription factors Elf5, Cdx2 and Eomes, and that of the signalling molecule Bmp4. We propose a model that provides a genetic explanation as to how Ets2 in trophoblast mediates the progression of gastrulation within the epiblast.