Regionalised signalling within the extraembryonic ectoderm regulates anterior visceral endoderm positioning in the mouse embryo

The development of the anterior–posterior (AP) axis in the mammalian embryo is controlled by interactions between embryonic and extraembryonic tissues. It is well established that one of these extraembryonic tissues, the anterior visceral endoderm (AVE), can repress posterior cell fate and that signalling from the other, the extraembryonic ectoderm (ExE), is required for posterior patterning. Here, we show that signals from the prospective posterior ExE repress AVE gene expression and affect the distribution of the AVE cells. Surgical ablation of the prospective posterior, but not the anterior, extraembryonic region at 5.5 days of development (E5.5) perturbs the characteristic distal-to-anterior distribution of AVE cells and leads to a dramatic expansion of the AVE domain. Time-lapse imaging studies show that this increase is due to the ectopic expression of an AVE marker, which results in a symmetrical positioning of the AVE. Surgical ablation of this same ExE region after the distal-to-anterior migration has already commenced, at E5.75, does not affect the localisation of the AVE, indicating that this effect takes place within a short time window. Conversely, transplanting the prospective posterior, but not the anterior, extraembryonic region onto isolated E5.5 embryonic explants drastically reduces the AVE domain. Further, transplantation experiments demonstrate that the signalling regulating AVE gene expression originates from the posterior ExE, rather than its surrounding VE. Together, our results show that signals emanating from the future posterior ExE within a temporal window both restrict the AVE domain and promote its specific positioning. This indicates for the first time that the ExE is already regionalised a day before the onset of gastrulation in order to correctly set the orientation of the AP axis of the mouse embryo. We propose a reciprocal function of the posterior ExE and the AVE in establishing a balance between the antagonistic activities of these two tissues, essential for AP patterning