Human gastruloids: an in vitro system for the study of human gastrulation

小鼠和人类胚胎干细胞（ESCs）可以在体外诱导形成组织和器官，这一发现提高了人们的期望，即培养中引导分化的相关方案将为成功开发合理的再生医学提供基础。然而，现在还为时尚早。目前，最成功的实验是在小鼠胚胎干细胞上进行的，因为它们允许在一个很好理解和标准化的系统中进行广泛的实验。此外，虽然有一些方案可以在体外产生类似器官的结构（称为类器官），但这些方案既不健壮，也不够可复制，因此用途不大。其中一个原因是，虽然我们对控制特定细胞类型出现的基因的理解取得了很大进展，但我们对这些基因的表达如何转化为表征组织和器官的三维结构知之甚少。理解这一点需要一个用户友好的模拟胚胎的三维实验系统。在人体组织和器官的情况下，情况甚至更具挑战性，因为在这种情况下，我们对人类胚胎的发育知之甚少，对其细胞和分子基础的了解也微不足道。在这一重要研究领域取得进展也受到明显的伦理问题和与使用人类胚胎相关的法规的限制。ESCs提供了另一种实验系统，但目前对人类ESCs的研究仅限于贴壁培养或使用小鼠和小鼠ESCs作为人类生物学的标准参考，因为它们在潜在的发育过程中表面上具有相似性。然而，这两个物种之间存在显著差异，当详细考虑时，突出需要更好地理解人类胚胎发生的特定细胞和分子特征。此外，依附文化不能反映出在三维结构中出现的复杂性和相互作用。越来越多的其他灵长类动物正在被研究并作为参考，但这并没有消除与此类研究相关的伦理问题。在过去的几年里，我们已经建立了一个基于小鼠ESCs的实验系统，令人惊讶的是，它在很大程度上再现了胚胎发育的早期阶段和模式，最值得注意的是，原肠胚形成的过程。这些结构反映了胚胎，直到大约第10阶段（几乎是发育的一半）。这些类器官的一个重要特征，我们称之为“类原肠”，因为它们具有模仿原肠形成的能力，是在没有胚胎外组织的情况下发育的，而胚胎外组织是调节胚胎和母体之间相互作用的。第二个特征是它们没有头部和大脑，只由颈部以下的结构组成。在这里，我们建议在这些观察和我们已经获得的初步结果的基础上建立一个类似的人类ESCs系统。这将使我们能够前所未有地了解人类发育早期阶段的基本机制，特别是组织和器官的原始建立所依据的细胞和分子过程。此外，缺乏大脑和头部，重要的是，这些类器官无法与母亲相互作用，这说明了许多适用于胚胎的伦理考虑。

The discovery that mouse and human Embryonic Stem Cells (ESCs) can be coaxed in vitro to form tissues and organs has raised expectations that the associated protocols for guided differentiation in culture will provide a basis for the successful development of a rational Regenerative Medicine. However it is early days. At the moment the most successful experiments are being done with mouse ESCs because they allow a wide range of experiments in a well understood and standardized system. Also, while there are protocols that can generate organ like structures -called organoids- in vitro, the protocols are neither robust nor reproducible enough to be if much use. A reason for this is that while we have made great advances in our understanding of the genes that control the emergence of specific cell types, we have little understanding of how the expression of these genes is translated into the three dimensional structures that characterize tissues and organs. Understanding this requires a user friendly three dimensional experimental system that mimics the embryo. The situation is even more challenging in the case of human tissues and organs since, in this case, we have very little insight on the development of human embryos and negligible knowledge of their cellular and molecular underpinning. Making progress in this important area of research is also limited by obvious ethical issues and regulations associated with the use of human embryos. ESCs provide an alternative experimental system but current studies with human ESCs restrict themselves to adherent cultures or use the mouse and mouse ESCs as a standard reference for human biology because of its superficial similarities in the underlying developmental processes. However, there are significant differences between the two species that, when considered in detail, highlight the need for a better understanding of the specific cellular and molecular features underlying human embryogenesis. Furthermore, adherent cultures do not reflect the complexities and interactions that emerge in three dimensional structures. Increasingly, other primates are being studied and used as referenced but this does not remove the ethical issues that are associated with this kind of study.Over the last few years we have established an experimental system based on mouse ESCs which, surprisingly, reproduces to a very high degree the early stages of embryonic development and patterning, most notably, the process of gastrulation. The structures mirror the embryo until, approximately, stage 10 (almost half way through its development). An important feature of these organoids, that we call 'gastruloids' because of their ability to imitate gastrulation, is that they develop in the absence of the extraembryonic tissues that mediate the interactions between the embryo and the mother. A second feature is that they lack a head and brain and consist,only, of structures from the neck down. Here we propose to build on these observations and preliminary results that we have obtained to establish a similar system for human ESCs. This will allow an unprecedented access to the mechanisms underlying the early stages of human development and, in particular, the cellular and molecular processes that underlie the establishment of primordial for tissues and organs. Furthermore, the lack of a brain and a head and importantly, the inability of these organoids to interact with the mother account for many of the ethical considerations that apply to embryos.