Timing Embryo Segmentation: Dynamics and Regulatory Mechanisms of the Vertebrate Segmentation Clock
Figure 3
Cell-cell coupling in the somitogenesis clock. (a) Representation of the intercellular coupling achieved by Delta/Notch communication and the feedback loop underlying the generation of lfng oscillations in the chick embryo. The Delta ligand binds Notch receptor on adjacent cells activating the Notch signaling cascade. Notch intracellular domain (NCID) is released and translocated into the nucleus, where it activates the transcription of hairy1, hairy2, and lfng (black arrows). Lfng protein modifies Notch making it less sensitive to activation by Delta (red arrows). This effect is transient due to the short life of lfng. Oscillations are thus generated by alternation between activation of lfng expression and repression of Notch by lfng [101]. (b) Molecular clock synchronization through intercellular coupling occurs during somitogenesis: cyclic expression in the individual cell is synchronized by cell-cell coupling mediated by the Notch signaling pathway (black spiral). This intrinsic mechanism ensures robust oscillations (blue spiral) and rapid recovery following external perturbations [102]. S0 represents the forming somite and SI and SII represent the two most recent formed somites.
