Watch thy neighbor: Proprotein convertases activate TGFß-related signals across tissue boundaries

Fig. 1. Axis specification and germ layer formation in the gastrulating mouse embryo. Between embryonic day E3.5 and E6.0, expansion of polar trophectoderm and the inner cell mass gives rise to extraembryonic ectoderm (grey) and the epiblast (blue), respectively. Primitive endoderm at the distal tip differentiates and is displaced to the prospective anterior side to become anterior visceral endoderm (AVE, bright green), and to inhibit posterior cell fates in adjacent epiblast. At the opposite pole, epiblast cells ingress into the primitive streak (white arrows) to give rise to mesoderm and the definitive endoderm germ layer. Towards late gastrulation (E7.5), anterior mesoderm will form the node and the notochord (red line), whereas definitive endoderm displaces the visceral endoderm to the extraembryonic region, and eventually folds up into the primitive gut tube (brown arrows) underneath the headfold (blue arrow). ISREC Institut Suisse de Recherche Expérimentale sur le Cancer