Towards an in vitro model of human hypoblast

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It is widely known that the vast majority of pregnancies initiated following assisted conception programmes fail very early, at around the time of implantation. Although many of these failures can be attributed to incompatibility with the mother's uterus, around one third are caused by defects in the developing embryo. At the time of implantation, the embryo must consist of three tissues: the trophectoderm that will make the first connection with the uterus and give rise to the placenta; the hypoblast that is essential for specifying the anterior and posterior of the foetus and forming the yolk sac; and the epiblast that produces all the tissues of the foetus. In order for normal development to ensue, each early embryonic lineage must be appropriately and proportionately represented. Based on our observations, we hypothesise that failure to specify enough cells of either epiblast or hypoblast, or both, is likely to be a major problem for generating a viable pregnancy. However, the process by which appropriate allocation of these lineages is regulated is poorly understood. It is difficult to attain statistical power to answer questions about this process from embryos. Similarly, it is not possible to correlate apportionment of the lineages with eventual successful uterine implantation. Therefore, in order to understand how early embryonic lineages are allocated, it is essential to have model artificial embryos constructed from cell lines representing the early embryonic lineages. There are validated cell lines representing the epiblast and trophoblast, but not the hypoblast. We have used specially formulated supportive gels and a culture regime that can capture hypoblast cells from mouse embryos and keep them in an early embryonic state as they expand into cell lines. In this study, we will optimise the mechanical and chemical conditions specifically to generate self-renewing hypoblast cell lines from human embryos. Armed with cell lines representing all three embryonic lineages, we will use purpose-built 3D hydrogels to construct artificial embryos. With our model artificial embryos, in combination with the new endometrial organoids developed by our collaborator Margherita Turco, we can quantifiably test predictions concerning, for example, the number of cells of each lineage needed to initiate normal development, including implantation. Our study will provide valuable information on the requirements for specific factors for expanding the human hypoblast population that may enable improvement of culture regimes for assisted conception programmes.