(a)–(c) Schematic representation of a mouse embryo illustrating the position of the extraembryonic tissues before and after axial rotation. During the process of axial rotation the embryo becomes enwrapped in its extraembryonic membranes. Extraembryonic mesoderm is shown in red; yellow represents amniotic ectoderm and embryonic ectoderm (embryonic mesoderm is not depicted); green represents trophectoderm-derived extraembryonic lineages; blue shows extraembryonic endoderm. For more detailed description of the extraembryonic membranes, see [2]. (d)–(h) Sagittal sections through BRE: LacZ mouse embryos in the time range between amnion closure (E7.0) and head fold stages (E8.0). X-gal staining (blue) for β-galactosidase detection in BRE : LacZ heterozygous embryos [5] reports dynamic SMAD1/5/8 mediated BMP signaling in the developing amnion. Abbreviations: E: embryonic day; LSEB: late streak-early bud; NP: neural plate; NPLB: neural plate-late bud; EHF: early head fold; LHF: late head fold.
