What are you synching about? Emerging complexity of Notch signaling in the segmentation clock

The Segmentation clock is a population of cellular genetic oscillators, located in the posterior of the elongating vertebrate embryo, that governs the rhythmic and sequential segmentation of the body axis into somites. Somites are blocks of cells that give rise to the segmented anatomy of the adult, including the backbone, muscles and skin. Malfunction of the segmentation clock results in malformations of these structures, a condition termed congenital scoliosis in the clinic. In all vertebrates, the oscillating cells of the segmentation clock are coordinated in a wave pattern, such that each new wave corresponds to a new segment. Maintenance of this wave pattern is important for precise segmentation and requires the local synchronization of the cellular oscillators. Existing models of the segmentation clock have explored the role of the Delta-Notch intercellular signaling pathway primarily as a coupling mechanism between neighboring autonomous oscillators. Recent work challenges several aspects of this simplification, suggesting that the mechanism of synchronization is more complex and may differ between species, and that Notch signaling may do more than synchronize cells. Here, we first examine evidence and models concerning the role of Notch signaling in driving, maintaining and synchronizing the mouse clock, highlighting results emerging from ex vivo culture systems of mouse segmentation clock cells. We then compare this to synchronization in the zebrafish, where accumulating evidence suggests that Notch signaling impacts the amplitude of the oscillating signal, and discuss whether the amplitude itself is meaningful for segmentation. Finally, we review work showing that multiple Delta ligands are active in segmentation, and consider how an interplay between these ligands could confer effective Notch functions in the segmentation clock. These lines of enquiry suggest that synchronization and Notch signaling are more complex than previously described, and reveal exciting new avenues for investigation into the coordination and precision of patterning the early embryo.