Clock driven waves of Tbx6 expression prefigure somite boundaries

The segmented body plan of vertebrates is established during embryogenesis by periodic and sequential formation of multi-cellular structures called somites. Somitogenesis is an example of patterning by a biological oscillator, the segmentation clock, which manifests as traveling waves of oscillating Hes/Her gene expression, reiterating during the formation of each1-3. How these waves are converted into the striped Mesp gene expression pattern that prefigures morphological somite boundaries4-8 remains unclear. Here, we image this conversion in real-time at single-cell resolution in zebrafish, using light-sheet microscopy of a novel reporter of Tbx6, a key activator of Mesp expression. We observe cellular oscillations and kinematic waves of Tbx6 expression that are driven by Hes/Her genes. Tbx6 waves arrest precisely in boundary cells that eventually express Mesp, thereby prefiguring the Mesp pattern, whereas Hes/Her waves do not. Although Hes/Her oscillations began before somitogenesis9-11, the first Tbx6 wave defines the boundary cells of the anterior-most somite, forming the head-trunk interface. Our findings imply that Tbx6 acts as a genetic clutch, converting Her/Hes pacemaker waves into Mesp stripes. We propose that this clock design shields the pacemaker from external perturbations, allowing flexible and robust patterning, making it of interest for organoids and tissue-engineering.