Oscillatory links of Fgf signaling and Hes7 in the segmentation clock.

Somitogenesis is controlled by the segmentation clock, where the oscillatory expression of cyclic genes such as Hes7 leads to the periodic expression of Mesp2, a master gene for somite formation. Fgf signaling induces the oscillatory expression of Hes7 while Hes7 drives coupled oscillations in Fgf and Notch signaling, which inhibits and activates Mesp2 expression, respectively. Because of different oscillatory dynamics, oscillation in Fgf signaling dissociates from oscillation in Notch signaling in S-1, a prospective somite region, where Notch signaling induces Mesp2 expression when Fgf signaling becomes off. Thus, oscillation in Fgf signaling regulates the timing of Mesp2 expression and the pace of somitogenesis. In addition, Fgf signaling was found to be a primary target for hypoxia, which causes phenotypic variations of heterozygous mutations in Hes7 or Mesp2, suggesting gene-environment interaction through this signaling.