Research

In vertebrates, skeletal muscles, the axial skeleton and several other derivatives such as brown adipose tissue and dorsal dermis originate from the embryonic paraxial mesoderm. This tissue forms during gastrulation, generating two bilaterally symmetrical bands of mesenchyme known as presomitic mesoderm or PSM. The PSM condenses at its anterior extremity to periodically form somites in an anterior-posterior sequence. Somites provide a blueprint for the organization of segmented anatomical modules, such as the vertebrae in the spine. This type of segmental organization has long been recognized as an obvious common feature of the body plan of vertebrates and invertebrates. How this segmental pattern is established during embryonic development has been actively studied for over two centuries, yet the underlying cellular and molecular processes remain poorly understood.

Building on our understanding of somite development, we developed a protocol to efficiently differentiate murine and human pluripotent stem cells (PSCs) into PSM in vitro. Using reporter lines expressing fluorescent markers, we demonstrated that the oscillations of the segmentation clock can be reproduced in vitro from human iPS cells differentiated into PSM, thus identifying the human segmentation clock for the first time. These studies highlighted the significant level of conservation of this mechanism in mammals. We then developed three-dimensional (3D) culture systems of human iPS cells that recapitulate somite formation, demonstrating the remarkable self-organizing properties of these structures in vitro. We developed two 3D differentiation systems for human paraxial mesoderm one that recapitulates embryonic elongation in vitro (segmentoids) and the other that allows the production of human paraxial mesoderm capable of segmenting into somites in vitro (somitoids). The human PSM generated in vitro using these protocols is very similar to that of mouse embryos, making it a remarkable model for studying somitogenesis in humans, which would not be feasible with embryonic tissues. Using these in vitro systems, our work challenged the classical view of the antero-posterior compartmentalization of somites, a crucial process for the formation of the vertebral column and the segmentation of the peripheral nervous system. We have shown that this process relies on a cellular sorting mechanism periodically triggered by the segmentation clock, thus ensuring the coordination of somite formation and their anteroposterior compartments.

Metabolism is often considered as a constitutive “maintenance” function in the embryo. However, research from my laboratory has shown that the FGF signaling gradient in the PSM regulates the transcription of glycolytic enzymes, thereby establishing a glycolytic activity gradient in the posterior PSM. We demonstrated that the cells in the posterior PSM, which exhibit high aerobic glycolytic activity, also show an inverted pH gradient compared to normal cells. Consequently, their metabolism closely resembles the Warburg metabolism observed in cancer cells. We showed that glycolytic activity regulates Wnt signaling via the regulation of intracellular pH, promoting the acetylation of β-catenin. This work provides an example of crosstalk between cellular signaling and metabolism regulation in the embryo. Moreover, it demonstrates the remarkable conservation of metabolic strategies between embryonic and cancer cells.

The vertebrate anteroposterior axis forms through elongation of multiple tissues during embryogenesis. This process is based on tissue-autonomous mechanisms of force generation and intertissue mechanical coupling whose failure leads to severe developmental anomalies such as body truncation and spina bifida. Similar to other morphogenetic modules, anteroposterior body extension requires both the rearrangement of existing materials—such as cells and extracellular matrix—and the local addition of new materials, i.e., anisotropic growth, through cell proliferation, cell growth, and matrix deposition. Numerous signaling pathways coordinate body axis formation via regulation of cell behavior during tissue rearrangements and volumetric growth. From a physical perspective, morphogenesis depends on both cell-generated forces and tissue material properties. As the spatiotemporal variation of these mechanical parameters has recently been explored in the context of vertebrate body elongation, the study of this process is likely to shed light on the cross talk between signaling and mechanics during morphogenesis.