Noncyclic Notch activity in the presomitic mesoderm demonstrates uncoupling of somite compartmentalization and boundary formation.

To test the significance of cyclic Notch activity for somite formation in mice, we analyzed embryos expressing activated Notch (NICD) throughout the presomitic mesoderm (PSM). Embryos expressing NICD formed up to 18 somites. Expression in the PSM of Hes7, Lfng, and Spry2 was no longer cyclic, whereas Axin2 was expressed dynamically. NICD expression led to caudalization of somites, and loss of Notch activity to their rostralization. Thus, segmentation and anterior-posterior somite patterning can be uncoupled, differential Notch signaling is not required to form segment borders, and Notch is unlikely to be the pacemaker of the segmentation clock.