Fig. 2.

Illustration of the process of somite formation from the presomitic mesoderm (PSM) in the chick embryo. a Paired somites are formed in a periodic fashion every 90 minutes. b A molecular clocked linked to segmentation is identified by dynamic and periodic expression of the cyclic genes in the PSM. Top: The expression of Lunatic Fringe mRNA appears as a wave sweeping across the whole PSM once during each somite formation as illustrated by in situ hybridization in this 17-somite-old chick embryo. PSM cells as illustrated by the dot undergo a phase of upregulation of the cycling genes followed by a phase of downregulation of these genes each time a somite forms. Bottom: As shown in this schematic representation of the progression of somitogenesis in the embryo, the cycles of expression of the cyclic genes will last while the cells remain in the PSM, which corresponds approximately to the time to form 12 somites in the chick embryo. These PSM cells undergo 12 oscillations of the expression of the cycling genes. Reproduced with permission from Giampietro et al., 2008.