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

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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.