Modeling Human Paraxial Mesoderm Development with Pluripotent Stem Cells.

Paraxial mesoderm in the early embryo is segmented into epithelial blocks called somites that establish the metameric organization of the vertebrate body plan. Somites are sequentially formed from head to tail in a rhythmic manner controlled by an oscillating gene regulatory network known as the segmentation clock. We know very little about this important process during human development due to limited access to human embryos and ethical concerns. To bypass these difficulties, model systems derived from human pluripotent stem cells have been established. Here, we detail three protocols modeling different aspects of human paraxial mesoderm development in vitro: a 2D cell monolayer system recapitulating dynamics of the human segmentation clock, a 3D organoid system called "somitoid" supporting the simultaneous formation of somite-like structures, and another organoid system called "segmentoid" reconstituting in vivo-like hallmarks of somitogenesis. Together, these complementary model systems provide an excellent platform to decode somitogenesis and advance human developmental biology.