Deciphering the mechanisms facilitating rapid uterine invasion of implanting human embryos

我们对哺乳动物发育的大部分了解都是通过研究小鼠胚胎获得的。然而，尽管在植入子宫之前的早期阶段在物种之间似乎非常相似，但植入方法可能差异很大。受精后，所有的哺乳动物胚胎都经历了几轮细胞分裂，形成一个球形结构。这包括外部的滋养外胚层，将分离成上胚层（胎儿的创始者）和将形成内部卵黄囊的下胚层的“内细胞团”（ICM）。ICM被一个扩张的腔（“囊胚腔”）移到一侧，这一阶段被定义为“囊胚”。整个结构被一层保护性的透明膜所包围，使其能够沿着输卵管到达子宫。不同哺乳动物的胚泡形成的细节有细微的差异，但随后的植入过程可能差异很大。从透明卵孵化后，小鼠胚胎被包裹在子宫分泌的蜕膜组织中，这种蜕膜组织在组织特化的早期阶段一直存在，并且它们直到第一个三个月后才与母体组织直接接触。相反，人类胚泡通过覆盖ICM的滋养外胚层的快速侵入直接植入子宫壁。这种入侵对于将胚胎固定在子宫内并与母亲建立联系以交换营养和废物以促进胎儿发育至关重要。在我们根据HFEA许可证对人类囊胚进行的研究中，我们注意到，随着胚胎成熟准备植入，ICM上的滋养外胚层（称为“极性”滋养外胚层）变得几层厚。极性滋养外胚层扩张的机制尚未研究，因此我们开发了用于外部细胞的顺序标记的方法，以确定人类滋养外胚层的快速扩张是通过滋养外胚层细胞的复制还是通过从底层ICM募集和转化细胞来发生，以满足植入滋养外胚层组织的需求。我们怀疑人类滋养外胚层的扩张容易失控，导致发育异常，因为我们观察到滋养外胚层过度生长是以ICM衍生物为代价的，大约1/3的胚胎是从多个诊所的IVF治疗中遗留下来的，这些胚胎在知情同意的情况下捐赠给我们的项目。我们假设，这种异常的过度生长的人滋养外胚层可能是一个进化的需要，快速附着和入侵到子宫的结果，这是不是在小鼠的情况。此外，在人类妊娠早期出现的一些已知问题，如异位着床、早期着床后失败或完全由滋养外胚层组织组成的葡萄胎的形成，可能是人类着床所需的快速滋养外胚层扩张引起的异常下游后果。这些故障很少发生在小鼠胚胎植入过程中。我们将使用各种分子分析来研究滋养外胚层的形成和生长。该项目的成果不仅将进一步加深我们对一些非啮齿类哺乳动物胚胎如何准备植入的理解，还将提供一个离散且易于处理的系统，用于研究如何从单个上皮形成多层，这可能与体内其他系统共享特征。胚泡发育阶段可以使用干细胞系建模，所述干细胞系可以被诱导组装成与滋养外胚层、上胚层和下胚层非常相似的组织。我们将使用我们的知识所需的指示性信号，以指定每个谱系建立模型的滋养外胚层过度生长，从而仔细检查其发生的机制，并确定补充剂的培养基，可能会抑制它。

Most of what we understand about mammalian development has been garnered by studying mouse embryos. However, although the early stages, prior to implantation in the uterus, appear to be quite similar between species, the method of implantation can vary enormously. Following fertilisation, all mammalian embryos undergo several rounds of cell division to form a spherical structure. This comprises trophectoderm on the outside, an 'inner cell mass' (ICM) that will segregate into epiblast, the founder of the foetus, and hypoblast that will form the yolk sac on the inside. The ICM is displaced to one side by an expanding cavity (the 'blastocoel'), that defines this stage as the 'blastocyst'. The whole structure is surrounded by a protective 'zona pellucida' to allow it to travel along the oviduct to the uterus. Subtle differences in the details of how blastocysts of different mammals form have been reported, but the subsequent process of implantation can vary enormously. After hatching from the zona pellucida, mouse embryos become encased in decidual tissue secreted by the uterus which persists throughout the early stages of tissue specification and they do not make direct contact with maternal tissue until after the first trimester. In contrast, human blastocysts implant directly into the uterine wall via rapid invasion by the trophectoderm that overlies the ICM. This invasion is essential to secure the embryo within the womb and establish the connection to the mother for exchange of nutrients and waste for development of the foetus. During our studies with human blastocysts under our HFEA licence we have noticed that the trophectoderm overlying the ICM (known as the 'polar' trophectoderm) becomes several layers thick as the embryos mature in preparation for implantation. The mechanism by which polar trophectoderm expands has not been studied, so we have developed methods for sequential labelling of the outside cells to determine whether the rapid expansion of the human trophectoderm occurs by replication of trophectoderm cells or by recruitment and conversion of cells from the underlying ICM, to satisfy the demand for implanting trophectoderm tissue. We suspect that expansion of the human trophectoderm is prone to become out of control, leading to abnormal development, since we observe trophectoderm overgrowth at the expense of derivatives of the ICM in around 1/3 of embryos left over from IVF treatment from multiple clinics, donated to our project with informed consent. We hypothesise that this aberrant overgrowth of the human trophectoderm may be a consequence of the evolutionary need for rapid attachment and invasion into the uterus, which is not the case in the mouse. Furthermore, some of the known problems arising during early human pregnancies, such as ectopic implantation, early post-implantation failure, or formation of a hydatidiform mole composed entirely of trophectoderm tissue, may be an abnormal downstream consequence attributable to the rapid trophectoderm expansion required for human implantation. These malfunctions rarely, if ever, occur during implantation of mouse embryos. We will use various molecular analyses to investigate trophectoderm formation and growth. Not only will the output from this project further our understanding of how embryos from some non-rodent mammals prepare for implantation, it will also provide a discrete and tractable system with which to investigate how multiple layers can form from a single epithelium, which may share features with other systems in the body. The blastocyst stage of development can be modelled using stem cell lines that can be induced to assemble into tissues closely resembling trophectoderm, epiblast and hypoblast. We will use our knowledge of the instructive signals required to specify each lineage to build models of trophectoderm overgrowth and thereby scrutinise the mechanisms by which it occurs and identify supplements for the culture medium that may restrain it.