ABSTRACT

The vertebral column is characterized by the periodic arrangement of vertebrae along the anterior-posterior axis. This segmental or metameric organization is established early in embryogenesis when pairs of embryonic segments called 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. As it is not feasible to observe the human segmentation clock in vivo, we have established an in vitro system based on the differentiation of pluripotent stem cells towards PSM fate by dual Wnt activation and BMP inhibition (Chal et al. 2015, 2016). We first verified that in vitro derived cells faithfully recapitulate the segmentation clock by differentiating mouse embryonic stem cells harboring a Hes7 fluorescent reporter and confirming that the resulting PSM-like cells oscillate with normal period (i.e. 2.5 hours). Then, we deployed the same strategy for human induced pluripotent stem cells and observed oscillations of the HES7 cyclic gene with a 5-hour period. Using this system, we found that the human segmentation clock exhibits Wnt, Notch and YAP-dependent oscillations, similar to its mouse counterpart. We also demonstrate that FGF signaling controls the phase and period of the oscillator. This contrasts with classical segmentation models such as the "Clock and Wavefront" where FGF merely implements a signaling threshold specifying where oscillations stop. Overall, our work identifying the human segmentation clock represents an important breakthrough for human developmental biology.