A molecular clock involved in somite segmentation.

Somites are transient embryonic structures that are formed from the unsegmented presomitic mesoderm (PSM) in a highly regulated process called somitogenesis. Somite, formation can be considered as the result of several sequential processes: generation of a basic metameric pattern, specification of the antero-posterior identity of each somite, and, finally, formation of the somitic border. Evidence for the existence of a molecular clock or oscillator linked to somitogenesis has been provided by the discovery of the rhythmic and dynamic expression in the PSM of c-hairy1 and lunatic fringe, two genes potentially related to the Notch signaling pathway. These oscillating expression patterns suggest that an important role of the molecular clock could reside in the temporal control of periodic Notch activation, ultimately resulting in the regular array of the somites. We discuss both the importance of the Notch signaling pathway in the molecular events of somitogenesis and its relationship with the molecular clock, and, finally, in that context we review a number of other genes known to play a role in somitogenesis.