Abstract: Mammalian embryos remodel dramatically upon implantation. In the case of the mouse embryo, the implanting blastocyst transforms into an elongated cylindrical structure, the egg cylinder. Due to the in-utero development of mammalian embryos the mechanisms controlling the change of the embryo shape upon implantation remained unexplored. In this study, using live imaging of ex vivo cultured mouse embryos, we have revealed the mechanisms behind the fundamental morphogenetic transformation of the mouse embryo at the time of its implantation.
Our work reveals that cross-talk between the embryonic epiblast and the extraembryonic trophectoderm orchestrates the transformation of the blastocyst to the egg cylinder. Specifically, signalling emanating from the embryonic epiblast provides positional information that leads to differential fate acquisition and formation of a tissue boundary within the trophectoderm. This tissue boundary is necessary for the expansion of the trophectoderm abutting the epiblast into a multi-layered tissue. Subsequent folding of the multi-layered trophectoderm results in the spreading of the second extra-embryonic tissue, primitive endoderm, to envelop both the embryonic epiblast and the trophectoderm. In conclusion, upon proper signalling from the embryonic epiblast, the behaviour of the extra-embryonic ectoderm shapes the implanted mouse embryo.
It is well established that extra-embryonic tissues are necessary for embryo implantation, nutrition and patterning. Our study uncovers that in addition to the above roles the extra-embryonic tissues and specifically the extraembryonic trophectoderm impact on mouse embryo shape acquisition. Mice and higher mammalian species like human and non-human primates have similar embryo shape up to implantation when their trophectoderm behaviour differs1-4. Future studies in higher mammalian species will reveal if the dependence of post-implantation shape acquisition on trophectoderm morphogenesis is a conserved mechanisms in mammals.
Reference: Morphogenesis of extra-embryonic tissues directs the remodelling of the mouse embryo at implantation, Neophytos Christodoulou, Antonia Weberling, Douglas Strathdee, Kurt I. Anderson, Paul Timpson, M , Nature Communications volume 10, Article number: 3557