Akiyama, R., Masuda, M., Tsuge, S., Bessho, Y., and Matsui, T. (2014) An anterior limit of FGF/Erk signal activity marks the earliest future somite boundary in zebrafish. Development (Cambridge, England). 141(5):1104-1109.
Vertebrate segments called somites are generated by periodic segmentation of the anterior extremity of the presomitic mesoderm
(PSM). During somite segmentation in zebrafish, mesp-b determines a future somite boundary at position B-2 within the PSM. Heat-shock experiments, however, suggest that an earlier
future somite boundary exists at B-5, but the molecular signature of this boundary remains unidentified. Here, we characterized
fibroblast growth factor (FGF) signal activity within the PSM, and demonstrated that an anterior limit of downstream Erk activity
corresponds to the future B-5 somite boundary. Moreover, the segmentation clock is required for a stepwise posterior shift
of the Erk activity boundary during each segmentation. Our results provide the first molecular evidence of the future somite
boundary at B-5, and we propose that clock-dependent cyclic inhibition of the FGF/Erk signal is a key mechanism in the generation
of perfect repetitive structures in zebrafish development.