In vitro characterization of the human segmentation clock

The segmental organization of the vertebral column is established early in embryogenesis when pairs of somites are rhythmically produced by the presomitic mesoderm (PSM). The tempo of somite formation is controlled by a molecular oscillator known as the segmentation clock. While this oscillator has been well-characterized in model organisms, whether a similar oscillator exists in humans remains unknown. Genetic analysis of patients with severe spine segmentation defects have implicated several human orthologs of cyclic genes associated with the mouse segmentation clock, suggesting that this oscillator might be conserved in humans. Here we show that in vitro-derived human as well as mouse PSM cells recapitulate oscillations of the segmentation clock. Human PSM cells oscillate twice slower than mouse cells (5-hours vs. 2.5 hours), but are similarly regulated by FGF, Wnt, Notch and YAP. Single cell RNA-sequencing reveals that mouse and human PSM cells in vitro follow a similar developmental trajectory to mouse PSM in vivo. Furthermore, we demonstrate that FGF signaling controls the phase and period of oscillations, expanding the role of this pathway beyond its classical interpretation in 'Clock and Wavefront' models. Overall, our work identifying the human segmentation clock represents an important breakthrough for human developmental biology. Single-cell mRNA sequencing of mouse E9.5 embryonic cells, mouse embryonic stem cells, and human iPSC in vitro cultures.