A Mechanical Model of Early Somite Segmentation

The clock-and-wavefront model (CW) hypothesizes that the formation of body segments (somites) in vertebrate embryos results from the interplay of molecular oscillations with a wave traveling along the body axis. Our experiments, however, suggest that somites self-organize mechanically during a wave of epithelialization of the dorsal pre-somitic mesoderm that precedes somite segmentation in chicken embryos. Signs of somite formation first appear in the dorsal pre-somitic mesoderm, as a series of arched segments of epithelial monolayer separated by clefts. The clefts seem to correspond to the position of the eventual inter-somite boundaries. We develop a corresponding mechanical model of epithelial segmentation in early somitogenesis in which a wave of apical constriction leads to increasing tension and periodic failure of adhesion junctions within the dorsal epithelial layer of the PSM, forming clefts which determine the eventual somite boundaries. Computer simulations of this model can produce spatially periodic segments whose size depends on the speed of the contraction wave (W) and the rate of increase of apical contractility (Λ). We found that two values of the ratio Λ/W determines whether this mechanism produces spatially and temporally regular or irregular segments, and whether segment sizes increases with the wave speed (scaling) as in the clock-and-wavefront model. Based on this, we discuss the limitations of a purely mechanical model of somite segmentation and the role that biomechanics play along with CW during somitogenesis.