Fgf4 is critical for maintaining Hes7 levels and Notch oscillations in the somite segmentation clock
During vertebrate development, the presomitic mesoderm (PSM) is periodically segmented into somites, which will form the segmented vertebral column and associated muscle, connective tissue, and dermis. The periodicity of somitogenesis is regulated by a segmentation clock of oscillating Notch activity. Here, we examined mouse mutants lacking only Fgf4 or Fgf8, which we previously demonstrated act redundantly to prevent PSM differentiation. Fgf8 is not required for somitogenesis, but Fgf4 mutants display a range of vertebral defects. We analyzed Fgf4 mutants by quantifying mRNAs fluorescently labeled by hybridization chain reaction within Imaris-based volumetric tissue subsets. These data indicate that FGF4 controls Notch pathway oscillations through the transcriptional repressor, HES7. This hypothesis is supported by demonstrating a genetic synergy between Hes7 and Fgf4, but not with Fgf8. Thus, Fgf4 is an essential Notch oscillation regulator and potentially important in a spectrum of human Segmentation Defects of the Vertebrae caused by defective Notch oscillations.