Separating the Fgf signaling requirements of clock oscillation from wavefront activity during somitogenesis

Lewandoski M1, Kageyama R2 and Anderson M1

  1. National Cancer Institute, NIH, USA.
  2. Kyoto University, Kyoto, Japan.

The vertebrate axial skeleton is comprised of segmented vertebrae, differing in size and shape, but with a pattern nearly invariant between individuals. Vertebrae form from somites, which segment from the posterior presomitic mesoderm (PSM). Somite segmentation is controlled by two classical activities, known as the "clock" and "wavefront". Wavefront activity maintains the posterior PSM in an undifferentiated state; we previously showed that Fgf4 and Fgf8 each encode this activity. The clock is governed by oscillating Notch signals that segment new somites at the anterior wavefront edge. Loss of only Fgf4 in the PSM causes misshaped vertebrae specifically in cervical and thoracic regions. We have characterized these mutants with Hairpin Chain Reaction mRNA in situ hybridization, allowing for direct quantification of up to four different mRNA sequences per embryo. We found that wavefront gene expression is unaffected in Fgf4 mutants; however, clock gene oscillations are highly abnormal. Hes7, a transcriptional repressor of Notch signaling, is reduced in Fgf4 mutant PSM, but only during the developmental window when future malformed vertebrae are generated. Due to the central role of Hes7 in regulating clock oscillation, we hypothesize that this reduction is the cause of Fgf4 segmentation defects. We also find that defects are more extreme when one copy of Fgf8 is additionally removed and are rescued when Fgf8 is overexpressed. We conclude that correct clock oscillations require an FGF signal that is genetically separable from the FGF wavefront requirement. The FGF signal appears to be permissive and provides a robustness factor for somite segmentation, buffering the system to changes in Hes7 expression dynamics.