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The formation of body segments (somites) in vertebrate embryos is accompanied by molecular oscillations (segmentation clock). Interaction of this oscillator with a wave traveling along the body axis (the clock-and-wavefront model) is generally believed to control somite number, size, and axial identity. Here we show that a clock-and-wavefront mechanism is unnecessary for somite formation. Non-somite mesoderm treated with Noggin generates many somites that form simultaneously, without cyclic expression of Notch-pathway genes, yet have normal size, shape, and fate. These somites have axial identity: The Hox code is fixed independently of somite fate. However, these somites are not subdivided into rostral and caudal halves, which is necessary for neural segmentation. We propose that somites are self-organizing structures whose size and shape is controlled by local cell-cell interactions.
Cell-Cell Interactions in Development
In vertebrate embryos, the number, size, and positional identity of mesodermal segments (somites) located bilaterally along the anteriorposterior axis is widely believed to be controlled by a molecular clock of oscillating gene expression interacting with a traveling wave of signals to determine how many cells make up a somite. Dias et al. (p. 791, published online 9 January; see the Perspective by Kondo) reveal that it is possible to generate somites of normal size, shape, and identity without either a clock or a wavefront. Instead, the findings suggest that somite size and shape are regulated by local cell-cell interactions.
↵* These authors contributed equally to this work.