Deciphering the mechanisms facilitating rapid uterine invasion of implanting human embryos

破译促进植入人类胚胎快速侵入子宫的机制

• 批准号: BB/Y005120/1
• 负责人: Jennifer Nichols
• 金额: $ 44.46万
• 依托单位: University of Edinburgh
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2024
• 资助国家: 英国
• 起止时间: 2024 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=BB%2FY005120%2F1
• 关键词: Deciphering; mechanisms; facilitating; rapid; uterine

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.

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，尽管不同物种在子宫植入之前的早期阶段似乎非常相似，但植入方法可能差异很大。受精后，所有哺乳动物胚胎都会经历几轮细胞分裂，形成球形结构。这包括外部的滋养外胚层、“内细胞团”（ICM），它将分离成外胚层（胎儿的创建者）和下胚层（将在内部形成卵黄囊）。 ICM 被一个不断扩大的空腔（“囊胚腔”）移至一侧，该阶段将该阶段定义为“囊胚”。整个结构被保护性“透明带”包围，使其能够沿着输卵管到达子宫。据报道，不同哺乳动物的囊胚形成细节存在细微差异，但随后的植入过程可能存在巨大差异。从透明带孵化后，小鼠胚胎被子宫分泌的蜕膜组织包裹，该蜕膜组织持续存在于组织规范的早期阶段，并且直到妊娠前三个月后它们才与母体组织直接接触。相比之下，人类囊胚通过覆盖 ICM 的滋养外胚层快速侵入，直接植入子宫壁。这种入侵对于确保胚胎在子宫内的安全以及与母亲建立联系以交换营养和废物以促进胎儿的发育至关重要。在我们根据 HFEA 许可对人类囊胚进行研究期间，我们注意到，随着胚胎成熟准备植入，覆盖 ICM 的滋养外胚层（称为“极性”滋养外胚层）会变得几层厚。极滋养外胚层扩张的机制尚未被研究，因此我们开发了外部细胞顺序标记的方法，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中招募和转化细胞来实现，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很容易失控，导致发育异常，因为我们观察到，在多个诊所 IVF 治疗留下的约 1/3 胚胎中，滋养外胚层过度生长，以牺牲 ICM 衍生物为代价，这些胚胎是在知情同意下捐赠给我们的项目的。我们假设人类滋养外胚层的这种异常过度生长可能是进化需要快速附着和侵入子宫的结果，而小鼠的情况并非如此。此外，人类早期妊娠期间出现的一些已知问题，例如异位着床、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，可能是由于人类植入所需的滋养外胚层快速扩张而导致的异常下游后果。这些故障在小鼠胚胎植入过程中很少发生（如果有的话）。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的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，用于研究单个上皮如何形成多层，这可能与体内其他系统共享特征。囊胚发育阶段可以使用干细胞系进行建模，干细胞系可以被诱导组装成与滋养外胚层、外胚层和下胚层非常相似的组织。我们将利用我们对指定每个谱系所需的指导信号的知识来建立滋养外胚层过度生长的模型，从而仔细检查其发生的机制并确定可能抑制其发生的培养基补充剂。