The spatial and temporal periodicity of somite formation is controlled by the segmentation clock, in which numerous cells cyclically express hairy-related transcriptional repressors with a posterior-to-anterior phase delay, creating 'traveling waves' of her1 expression. In zebrafish, the first traveling wave buds off from the synchronous oscillation zone in the blastoderm margin. Here we show that the emergence of a traveling wave coincides with the anterior expansion of Fgf signaling and that transplanted Fgf8b-soaked beads induce ectopic traveling waves. We thus propose that as development proceeds, the activity of Fgf signaling gradually expands anteriorly, starting from the margin, so that cells initiate her1 oscillation with a posterior-to-anterior phase delay. Furthermore, we suggest that Fgf has an essential role in establishing the period gradient that is required for the her1 spatial oscillation pattern at the emergence of the traveling wave.
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