The vertebrate segmentation clock: the tip of the iceberg

The vertebrate segmentation clock was identified 10 years ago as a molecular oscillator associated with the rhythmic production of embryonic somites. Since then, three major signaling pathways - Notch, FGF, and Writ - have been shown to be activated periodically during segmentation and proposed to constitute the clockwork of the system. However, recent results from zebrafish embryonic studies demonstrate that Notch signaling is involved in the coupling of oscillations among cells rather than in the pacemaker of the oscillator. Furthermore, genetic analyses in mouse indicate that Writ and FGF play only a permissive role in the control of the oscillations. Therefore, the nature of the segmentation clock pacemaker still remains elusive.