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，尽管植入子宫之前的早期阶段在不同物种之间似乎非常相似，但植入的方法可能会有很大的差异。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，内部细胞团(ICM)，它将分离成外胚层，胎儿的创始人，以及内层将形成卵黄囊的次母细胞。ICM被一个扩张的空腔(囊腔)移位到一侧，该空腔将这一阶段定义为“胚泡”。整个结构被一层保护性的“透明带”包围，使其能够沿着输卵管进入子宫。关于不同哺乳动物的囊胚形成细节的细微差异已有报道，但随后的植入过程可能会有巨大的差异。从透明带孵化出来的小鼠胚胎被包裹在子宫分泌的蜕膜组织中，这种组织持续到组织指定的早期阶段，直到怀孕前三个月之后才与母体组织直接接触。相比之下，人类胚泡通过覆盖在ICM上的滋养外胚层的快速入侵直接植入子宫壁。这种入侵对于确保胚胎在子宫内的安全以及建立与母亲的联系以交换营养和胎儿发育所需的废物至关重要。在我们的HFEA许可证下对人胚泡的研究中，我们注意到覆盖在ICM上的滋养外胚层(称为“极”滋养外胚层)随着胚胎成熟准备着床而变得几层厚。极地滋养外胚层扩张的机制尚未被研究，因此我们发展了外周细胞的连续标记方法，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基础ICM的细胞募集和转化来实现的，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，导致异常发育，因为我们在知情同意的情况下观察到来自多个诊所的体外受精治疗遗留下来的大约1/3的胚胎中滋养外胚层过度生长，而ICM的衍生品被捐赠给我们的项目。我们推测，人类滋养外胚层的异常过度生长可能是快速附着和侵入子宫的进化需要的结果，而小鼠的情况并非如此。此外，在人类妊娠早期出现的一些已知问题，如异位植入、植入后早期失败或形成完全由滋养外胚层组织组成的葡萄胎，可能是人类植入所需的滋养外胚层快速扩张所致的异常下游后果。这些故障很少发生在小鼠胚胎的植入过程中。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动物胚胎如何为植入做准备的理解，它还将提供一个离散和易处理的系统，用于研究单个上皮如何形成多层细胞，这可能与身体中的其他系统共享特征。胚泡发育阶段可以用干细胞系来模拟，这些干细胞系可以被诱导组装成非常类似于滋养外胚层、上胚层和下胚层的组织。我们将利用我们对指定每个谱系所需的指导性信号的知识来建立滋养外胚层过度生长的模型，从而仔细研究它发生的机制，并确定可能抑制它的培养基补充剂。