3D Human Axial Development In Vitro: using novel human in vitro somitogenesis models to study birth defects with patient-relevant iPS cell lines

脊柱是人体的重要组成部分，由33块单独的骨骼线性排列成一个连续的结构，容纳脊髓，并为肋骨和相关肌肉提供支撑。这种结构揭示了我们身体的节段性，它起源于体发育过程，这是一个发生在早期胚胎的事件，在这个过程中，身体轴的伸长导致一组细胞（中胚层）连续分裂成离散的块，称为体质体。这些小体将产生骨和肌肉衍生物的祖细胞，包括椎骨。椎骨形成的结构异常是早期发育的常见病理，其严重程度跨越一系列，在极端情况下，可能对个体的生活产生巨大影响。因此，我们更好地了解这些疾病的基础至关重要。体细胞发生的过程已经在许多不同的模式生物中进行了研究，包括鱼、鸡和老鼠。遗传分析表明，中胚层的周期性分裂与某些基因在空间和时间上的表达振荡有关。这些基因的突变导致各种分割缺陷，表现为脊柱和胸肌组织的畸变，如脊柱缺陷患者所见。然而，小鼠和人类的发育存在差异，这使得很难推断突变和特定综合征之间的因果机制关系。由于在胚胎分割阶段难以获得胚胎，对人类这些基因的分析受到阻碍。在过去几年中出现的一种解决这一问题的方法是多能干细胞（PSCs），它已被证明在实验室环境中概括了许多发育事件。特别是，有可能使用它们来模拟与小鼠和人类体细胞发生相关的基因表达模式。然而，这些模型缺乏胚胎的空间组织，这种空间组织将中胚层的长度细分为不断按比例划分的区域，并导致体结构的发展。本项目旨在建立一个新的体外研究正常和异常人类体细胞发生的模型系统。这将通过两个具有互补专业知识的小组的联合来实现。日本京都的一个研究团队已经建立了一个体细胞发生模型，该模型从人类psc中重现了振荡和分化为骨骼和肌肉的过程，但以一种无序的方式。位于剑桥的另一个研究小组开创了“类胃原质”，这是一种基于psc的早期哺乳动物发育的三维模型，最近扩展到了人类psc。胃原体在三维中以前后极性概括躯体发生的基本空间和时间方面。该项目将把这两个小组聚集在一起，优化人类类胃原体，以研究体的形成和分化，并测试来自椎体缺陷患者的一些iPSC细胞系。该项目的结果将是建立一个新的平台，用于检测和分析与人类胚胎中胚层分割相关的病理，从而更好地了解这些综合征的原因。

The vertebral column is a key element of the human body, made up of 33 individual bones arranged linearly in a continuous structure that houses the spinal cord and provides support for the ribs and associated muscles. This structure reveals the segmental nature of our body and has its origin in the process of somitogenesis, an event that takes place in the early embryo where elongation of the body axis leads to the sequential partitioning of a group of cells, the mesoderm, into discrete blocks, called somites. These somites will give rise to progenitors for bone and muscle derivatives, including the vertebrae. Structural abnormalities in vertebrae formation are a common pathology of early development, and span a range of severities that, in extreme forms, can have dramatic effects on the life of individuals. It is therefore critically important that we better understanding the basis of these disorders.The process of somitogenesis has been studied in many different model organisms, including fish, chicken and mice. Genetic analysis has revealed that the periodic partitioning of the mesoderm is associated with oscillations in the expression of certain genes in space and time. Mutations in these genes lead to a variety of segmentation defects that manifest as aberrations in the organisation of the vertebral column and thoracic muscles, as seen in patients with vertebral column defects. However, there are differences in the development of mice and humans that make it difficult to infer causal, mechanistic relationships between mutations and specific syndromes.Analysis of these genes in humans is hampered by the difficulty of accessing embryos at the stages of segmentation. One approach to this problem that has emerged over the last few years is Pluripotent Stem Cells (PSCs), which have been shown to recapitulate many developmental events in a laboratory setting. In particular, it is possible to use them to model gene expression patterns associated with somitogenesis in mouse and human. However, these models lack the spatial organization of the embryo that subdivides the length of the mesoderm into constantly proportioned territories and leads to the development of somite structures. This project aims to develop a new model system to study normal and aberrant human somitogenesis in vitro. This will be achieved by bringing together two groups with complementary expertise. One, based in Kyoto, has established a somitogenesis model from human PSCs that recapitulates oscillations and differentiation into bone and muscle, but in a disorganized manner. A second group, based in Cambridge, has pioneered 'gastruloids', a three-dimensional PSC-based model of early mammalian development recently extended to human PSCs. Gastruloids recapitulate the basic spatial and temporal aspects of somitogenesis in 3D with an anteroposterior polarity. The project will bring the two groups together to optimize humans gastruloids for the study of the formation and differentiation of somites, and to test a number of iPSC lines derived from patients with vertebral defects. The outcome of the project will be the establishment of a new platform for the detection and analysis of pathologies associated with the segmentation of the mesoderm in human embryos, leading to a better understanding of the causes of these syndromes.