The vertebrate segmentation clock

Vertebrate somitogenesis has been shown to be associated with a molecular o scillator, the segmentation clock, whose periodicity matches that of the pr ocess of somitogenesis. The existence of such a clock in presomitic mesoder m (PSM) cells was originally proposed in theoretical models such as the 'cl ock and wavefront'. Molecular evidence for the existence of this clock in v ertebrates has been obtained on the basis of the periodic expression of sev eral genes, most of which are related to the Notch signalling pathway. Thes e genes are expressed in a dynamic sequence which appears as a wave sweepin g caudo-rostrally along the whole PSM once during each somite formation. No tch-pathway mouse and fish mutants lose the dynamic expression of the cycli ng genes, indicating that Notch signalling is required for their periodic e xpression, or is required to coordinate the oscillations between PSM cells. Therefore Notch signalling is either part of the mechanism of the oscillat or itself or acts as a cofactor required for cycling gene expression. A fur ther potentially important role for the segmentation clock is to periodical ly activate Notch signalling in the rostral presomitic mesoderm, thereby ge nerating the periodic formation of somite boundaries.