Regeneration of the human segmentation clock in somitoids in vitro.

Each vertebrate species appears to have a unique timing mechanism for forming somites along the vertebral column, and the process in human remains poorly understood at the molecular level due to technical and ethical limitations. Here, we report the reconstitution of human segmentation clock by direct reprogramming. We first reprogrammed human urine epithelial cells to a presomitic mesoderm (PSM) state capable of long‐term self‐renewal and formation of somitoids with an anterior‐to‐posterior axis. By inserting the RNA reporter Pepper into HES7 and MESP2 loci of these iPSM cells, we show that both transcripts oscillate in the resulting somitoids at ~5 h/cycle. GFP‐tagged endogenous HES7 protein moves along the anterior‐to‐posterior axis during somitoid formation. The geo‐sequencing analysis further confirmed anterior‐to‐posterior polarity and revealed the localized expression of WNT, BMP, FGF, and RA signaling molecules and HOXA‐D family members. Our study demonstrates the direct reconstitution of human segmentation clock from somatic cells, which may allow future dissection of the mechanism and components of such a clock and aid regenerative medicine. Synopsis: Spatial patterning and temporal rhythmicity of gene expression define human embryogenesis during primitive streak stages but have been notoriously difficult to explore. This resource establishes a reprogramming culture system to mimic spatio‐temporal dynamics during somitogenesis and body axis formation, offering new insights into the signaling pathways involved. Direct reprogramming of human urine epithelial cells into presomitic mesoderm (PSM) cells recapitulates somitogenic differentiation and tissue segmentation.The Nodal pathway is required for urine‐induced PSM (UiPSM) self‐organization into somitoids and long‐term culture.UiPSM‐derived somitoids show oscillation of HES7 and MESP2 transcripts.UiPSM‐derived somitoids establish anterior–posterior polarity and regional expression of WNT, BMP, FGF, and RA signaling pathway components. [ABSTRACT FROM AUTHOR]