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

脊柱是人体的关键组成部分，由33块单独的骨骼组成，这些骨骼线性排列在一个连续的结构中，容纳脊髓，为肋骨和相关肌肉提供支持。这种结构揭示了我们身体的节段性本质，其起源于体细胞发生过程，这一事件发生在早期胚胎中，体轴的伸长导致一组细胞，即中胚层，顺序地划分为离散的块，称为体节。这些体节将产生骨骼和肌肉衍生品的祖细胞，包括脊椎。椎骨形成中的结构异常是早期发育的常见病理，在极端情况下，会对个体的生活产生戏剧性的影响。因此，我们更好地了解这些疾病的基础是至关重要的。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、鸡和小鼠。遗传分析表明，中胚层的周期性分割与某些基因在空间和时间上的表达振荡有关。这些基因的突变会导致各种分段缺陷，表现为脊柱和胸肌组织的异常，就像脊柱缺陷的患者所见。然而，老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关系和机制关系。在人类中，这些基因的分析受到在分割阶段获取胚胎的困难。过去几年出现的解决这一问题的一种方法是多能干细胞(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，它们有可能用来模拟与小鼠和人类的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，这种组织将中胚层的长度细分为恒定比例的区域，并导致体节结构的发育。本项目旨在开发一种新的模型系统来研究人类体外正常和异常的体格发生。这将通过将两个具有互补专业知识的小组聚集在一起来实现。一家总部设在京都的公司已经建立了一个人体干细胞的体细胞发生模型，该模型重述了振荡和分化为骨骼和肌肉的过程，但方式混乱。位于剑桥的第二个研究小组开创了“原肠细胞”的先河，这是一种基于PSC的三维早期哺乳动物发育模型，最近扩展到了人类的PSC。胃型肌在3D中以前后两极概括了躯体发生的基本空间和时间方面。该项目将把这两个小组结合在一起，优化人类胃肠，以研究体节的形成和分化，并测试一些来自脊椎缺陷患者的IPSC系。该项目的成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.