In fact, somites not only give rise to the axial skeleton and skeletal musculature, but also impose the segmented organization of the peripheral nervous system ( Keynes and Stern, 1988). Somites are formed in pairs from the anterior-most portion of the presomitic mesoderm (PSM), on each side of the neural tube, and they are the first morphological manifestation of the characteristic segmented structure of the adult vertebrate axial skeleton. One such process is somitogenesis, which is characterized by the periodic formation of metameric structures, the somites, along the anterior-to-posterior (A-P) axis of the early embryonic body. Vertebrate embryo development comprises several processes that are highly regulated in time. A comprehensive concept of the Embryo Clock is presented 2 The somitogenesis Embryo Clock.Embryo Clock periodicity is tissue-specific within the same organism.The vertebrate Embryo Clock oscillates with species-specific periodicity.An outstanding question, however, remains unanswered: what sets the different EC paces observed in different organisms and tissues? This review aims to summarize the available knowledge regarding the pace of the EC, its regulation and experimental manipulation and to expose new questions that might help shed light on what is still to unveil.

The EC is now known to be present in all vertebrate organisms studied and this mechanism was also described in limb development and stem cell differentiation. The discovery of a molecular Embryo Clock (EC) underlying the periodicity of somite formation shed light on the importance of gene expression dynamics for pattern formation. Pairs of somites flanking the neural tube are formed periodically during early development, and the molecular mechanisms in temporal control of this early patterning event have been thoroughly studied. Vertebrate embryo somitogenesis is the earliest morphological manifestation of the characteristic patterned structure of the adult axial skeleton.

3Champalimaud Research Program, Champalimaud Center for the Unknown, Lisbon, Portugal.

2Faculdade de Medicina e Ciências Biomédicas (FMCB), Universidade do Algarve, Campus de Gambelas, Faro, Portugal.1ABC-RI, Algarve Biomedical Center Research Institute, Faro, Portugal.

Clock beat