The coordinated growth of the conceptus is sustained by reciprocal signalling between the epiblast and the extraembryonic ectoderm (ExE). In mice, FGF4 produced by the epiblast promotes Cdx2 expression in the trophoblast stem cell (TSC) niche located in the ExE. Cdx2, Eomes and EIf-5 expression in the ExE constitute an auto-regulatory circuit that maintains the mouse TSC gene network. Cells in the ExE, in turn secrete BMP4, which is a critical mesoderm determinant. The ExE however, is absent in non-rodent embryos, raising the question whether the cross-talk between embryonic and extraembryonic domains is conserved in mammals. Pig embryos represent "mammotypical" embryos in that the flat epiblast is surrounded by trophectoderm (TE). The pig TE undergoes a remarkable elongation, however it is not known whether the epiblast regulates this process and whether a TSC niche supports this remarkable growth.