The segmentation clock, which controls the periodic formation of somites along the vertebrate body axis, involves the oscillating expression of clock genes in presomitic mesoderm (PSM) cells. Oscillations are synchronised between cells, giving rise to a sweeping wave of gene expression throughout the PSM. This cyclic wave of gene expression is known to slow as it moves anteriorly, but the causes and implications of this slowing have remained unclear. Here, Sharon Amacher and co-workers investigate segmentation clock dynamics in zebrafish embryos (p. ). Using a her1:her1-venus reporter to visualise clock gene oscillations in real-time, the authors show that the periodicity of oscillations slows as PSM cells become displaced anteriorly. This slowing gives rise to a situation in which cells that are one somite apart are actually in opposite phases of the clock. Thus, they report, a one-segment periodicity is observed in the posterior of the embryo, whereas a two-segment spatial periodicity is seen in the anterior. The researchers further demonstrate that neighbouring cells oscillate synchronously in both the posterior and anterior PSM until they are incorporated into somites. Based on these findings, the authors propose an updated model of the segmentation clock.
