Fig. 6. A model for periodic somitogenesis in the mouse. Schematic representation of the temporal and spatial changes in the expression patterns and relationships among Mesp2 (pink), Tbx6 (green), NICD (blue) and FGF signaling (orange) during a single cycle of somitogenesis. The FGF signal is provided from the PSM with a posterior-to-anterior gradient. The expected threshold in the activity defines the determination front that corresponds to the posterior limit of the Mesp2 expression domain. In phase III, Notch activity reaches the anterior PSM, where Mesp2 transcription has been initiated in the cells with Tbx6 expression and lacking negative effectors such as FGF and Wnt signaling. In phase I, Mesp2 protein accumulates and suppresses NICD by activating Lfng, and also suppresses Tbx6 protein by promoting its rapid degradation via the ubiquitin-proteasome pathway. In phase II, when the next wave of Notch activity has just reached the anterior PSM region, the three signals (NICD, Mesp2 and Tbx6) show complete segregation, thus establishing a boundary between NICD and Mesp2 that demarcates the segmental border, and a boundary between Mesp2 and Tbx6 that demarcates the next Mesp2 anterior limit and, thus, the next segmental border.