A mechanical model of early somite segmentation

Somitogenesis is often described using the clock-and-wavefront (CW) model, which does not explain how molecular signaling rearranges the pre-somitic mesoderm (PSM) cells into somites. Our scanning electron microscopy analysis of chicken embryos reveals a caudally-progressing epithelialization front in the dorsal PSM that precedes somite formation. Signs of apical constriction and tissue segmentation appear in this layer 3-4 somite lengths caudal to the last-formed somite. We propose a mechanical instability model in which a steady increase of apical contractility leads to periodic failure of adhesion junctions within the dorsal PSM and positions the future inter-somite boundaries. This model produces spatially periodic segments whose size depends on the speed of the activation front of contraction (F), and the buildup rate of contractility (Lambda). The Lambda/F ratio determines whether this mechanism produces spatially and temporally regular or irregular segments, and whether segment size increases with the front speed.

Automated summary

The process of somitogenesis in chicken embryos involves a caudally-progressing epithelialization front in the dorsal pre-somitic mesoderm, preceding somite formation. A mechanical instability model proposes that apical contractility increases lead to periodic failure of adhesion junctions within the dorsal pre-somitic mesoderm, determining the future inter-somite boundaries. The spatially periodic segments produced by this model depend on the speed of contraction activation front and the rate of contractility buildup, and whether these segments are regular or irregular is determined by the Lambda/F ratio.