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的发现，提高了人们的期望，即相关的诱导分化培养方案将为理性再生医学的成功发展提供基础。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易于理解和标准化的系统中进行广泛的实验。此外，尽管有一些方案可以在体外产生类似器官的结构--称为类有机物质--但这些方案既不够健壮，也不够可重复性，如果有很多用途的话。其中一个原因是，尽管我们在了解控制特定细胞类型出现的基因方面取得了很大进展，但我们对这些基因的表达如何转化为组织和器官特有的三维结构知之甚少。要理解这一点，需要一个用户友好的模拟胚胎的三维实验系统。在人类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知识也微不足道。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。胚胎干细胞提供了一种替代的实验系统，但目前对人类胚胎干细胞的研究仅限于贴壁培养，或者使用小鼠和小鼠胚胎干细胞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相似。然而，当仔细考虑时，这两个物种之间存在着显著的差异，这突显了更好地了解人类胚胎发生的特定细胞和分子特征的必要性。此外，附着性文化没有反映出三维结构中出现的复杂性和相互作用。在过去的几年里，我们建立了一个基于小鼠胚胎干细胞的实验系统，令人惊讶的是，它在非常高的程度上再现了胚胎发育和模式形成的早期阶段，最显著的是原肠形成过程。这些结构反映了胚胎，直到大约第10阶段(几乎发育到一半)。这些器官的一个重要特征是，它们在没有胚外组织的情况下发育，而胚胎和母亲之间的相互作用是在没有胚外组织的情况下发生的。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，为人类ESCs建立一个类似的系统。这将使人们能够史无前例地了解人类发育早期阶段的潜在机制，特别是建立组织和器官原始的细胞和分子过程。此外，缺乏大脑和头部，以及更重要的是，这些有机化合物无法与母亲互动，这是适用于胚胎的许多伦理考虑因素。

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.