Towards an in vitro model of human hypoblast

众所周知，绝大多数在辅助受孕方案后开始的妊娠很早就失败了，大约在着床时。虽然许多失败可以归因于与母亲的子宫不相容，但大约三分之一是由发育中的胚胎缺陷引起的。在植入时，胚胎必须由三个组织组成：滋养外胚层，它将与子宫第一次连接并产生胎盘；下胚：对确定胎儿的前后和形成卵黄囊至关重要的下胚；以及产生胎儿所有组织的外胚层。为了正常的发育，每个早期胚胎谱系必须得到适当和比例的代表。根据我们的观察，我们假设不能指定足够的外胚层细胞或下胚层细胞，或两者兼而有之，可能是产生可存活妊娠的主要问题。然而，调节这些血统的适当分配的过程却知之甚少。很难从胚胎中获得关于这一过程的统计能力。同样，这是不可能的关联分配谱系与最终成功的子宫植入。因此，为了了解早期胚胎谱系是如何分配的，有必要用代表早期胚胎谱系的细胞系构建模型人工胚胎。有证实的细胞系代表外胚层和滋养层，但没有代表下胚层。我们使用了特殊配方的支持凝胶和培养制度，可以从小鼠胚胎中捕获下胚细胞，并使它们在扩展成细胞系时保持在早期胚胎状态。在这项研究中，我们将优化机械和化学条件，专门从人类胚胎中产生自我更新的下胚细胞系。有了代表所有三种胚胎谱系的细胞系，我们将使用专门建造的3D水凝胶来构建人工胚胎。通过我们的人工胚胎模型，结合我们的合作者Margherita Turco开发的新的子宫内膜类器官，我们可以定量地测试预测，例如，启动正常发育（包括植入）所需的每个谱系的细胞数量。我们的研究将提供有价值的信息，说明扩大人类低胚层人口所需的特定因素，这可能有助于改善辅助受孕计划的培养制度。

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.