Towards an in vitro model of human hypoblast

众所周知，绝大多数在辅助受孕计划后开始的怀孕都在植入时很早就失败了。虽然这些失败中的许多可归因于与母亲子宫的不相容，但约三分之一是由发育中的胚胎缺陷引起的。在着床时，胚胎必须由三种组织组成：滋养外胚层，它将与子宫进行第一次连接并产生胎盘;下胚层，它对指定胎儿的前部和后部并形成卵黄囊至关重要;上胚层，它产生胎儿的所有组织。为了保证正常的发育，每个早期胚胎谱系都必须有适当的比例代表。根据我们的观察，我们假设，未能指定足够的细胞的上胚层或下胚层，或两者，可能是一个主要的问题，产生一个可行的怀孕。然而，对这些谱系的适当分配的调节过程知之甚少。很难获得统计能力来回答关于胚胎这一过程的问题。同样，也不可能将谱系的分配与最终成功的子宫着床联系起来。因此，为了了解早期胚胎谱系是如何分配的，有必要从代表早期胚胎谱系的细胞系构建模型人工胚胎。存在代表上胚层和滋养层但不代表下胚层的经验证的细胞系。我们使用了特殊配方的支持凝胶和培养体系，可以从小鼠胚胎中捕获下胚层细胞，并在它们扩展成细胞系时将它们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层细胞系。拥有代表所有三种胚胎谱系的细胞系，我们将使用特制的3D水凝胶来构建人工胚胎。通过我们的人工胚胎模型，结合我们的合作者马尔盖丽塔图尔科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，启动正常发育所需的每个谱系的细胞数量，包括植入。我们的研究将提供有价值的信息，具体因素的要求，扩大人类下胚层人口，可能使辅助受孕计划的文化制度的改善。

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.