SUMMARY After implantation, the mouse embryo undergoes gastrulation and forms mesoderm and endoderm. Mesoderm participates in embryonic and extra-embryonic tissues including the amnion, yolk sac, chorion and allantois, the umbilical cord precursor. Extra-embryonic mesoderm is rich in intermediate filaments. Two-photon live imaging of keratin 8-eYFP knock-in embryos allowed recording nucleation and elongation of keratin filaments, which formed apical cables coordinated across multiple cells in amnion, allantois, and blood islands. Embryos lacking all keratins displayed a deflated exocoelomic cavity, a narrow thick amnion, and a short allantois, indicating a hitherto unknown role for keratin filaments in post-implantation extra-embryonic membranes morphogenesis. Single-cell RNA sequencing of mesoderm cells, microdissected amnion, chorion, and allantois provided an interactive atlas of transcriptomes with germ layer and regional information. Keratin 8 high mesenchymal cells in contact with the exocoelom shared a cytoskeleton and adhesion expression profile that might explain the adaptation of extra-embryonic structures to the increasing mechanical pressure. GRAPHICAL ABSTRACT

Keratin dynamics govern the establishment of the maternal-fetal interface