The vertebrate body plan is organized in a segmented fashion, best illustrated by the repetition of the vertebrae. The first signs of segmentation arise during early embryogenesis when somites bud off in a rhythmic fashion from the anterior part of the presomitic mesoderm (PSM). The periodic formation of somites is proposed to be controlled by a molecular oscillator-the segmentation clock-acting in the PSM. The signals of the segmentation clock are converted into the repetitive serie of somites by a traveling front of maturation-the determination front-formed by a molecular gradient regressing in concert with axis elongation. In order to gain insights in the mechanisms controlling somite number, I compared the regulation of somitogenesis in a new model species exhibiting a large number of somites-the corn snake- with mouse, chicken and zebrafish. I first cloned the genes involved in corn snake's somitogenesis and analyzed their expression patterns by in situ hybridization. The results showed that the genes associated with the determination front had conserved expression patterns. Unexpectedly, lunatic fringe, a gene associated with the segmentation clock, exhibited several stripes of dynamic expression in the PSM. A comparative study based on a mathematical model suggested that this pattern reflected an accelerated pace of the clock relative to the axis growth rate. In conclusion, our studies propose that the relationship between clock and axis growth is an important factor explaining the difference in somite numbers between the corn snake and the other species.
