Reconstituting Mouse Embryogenesis Ex Utero in Natural and Stem Cell Embryos

After implantation, mammalian embryos initiate the process of gastrulation, in which pluripotent epiblast transits from a symmetrical ball of cells into an advanced embryo with a defined body plan. Yet, the small size and intrauterine confinement of the developing embryos limits the study of post-implantation embryogenesis, due to the inability to observe and manipulate living embryos at these stages. We have devised efficient platforms for ex utero culture of early post-implantation mouse embryos, enabling proper development of embryos between pre-gastrulation (embryonic day E5.5) until the hindlimb formation stage (E11). These culture systems allow the introduction of a variety of embryonic perturbations and micro-manipulations, which can be followed ex utero in living embryos. Further, by adapting the ex utero culture for growing stem cell-embryo models, we established mouse stem cell-derived embryos (sEmbryos) capable of completing gastrulation and early organogenesis when cultured ex utero. These stem-cell-derived embryos resemble E8.5 natural embryos and contain both embryonic and extraembryonic compartments originated from naïve pluripotent stem cells as the sole stem cell source. The ability to continuously culture natural and stem-cell mouse embryos ex utero for long-term opens new possibilities for understanding mammalian embryogenesis by allowing experimentation in living embryos during stages that have remained hidden inside the maternal uterus, and paves the way for implementing analogous culture methods in non-human primates and human embryos.