Figure 4: Illustration of somite formation from the presomitic mesoderm (PSM) in the chick embryo. Paired somites are formed every 90 minutes in a periodic fashion every 90 minutes shown in (a). (b).A molecular clocked linked to segmentation by dynamic and periodic expression of the cyclic genes in the PSM. Top: Lunatic Fringe mRNA expression 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. During each somite formations, PSM cells illustrated by the dot undergo a phase of upregulation of the cycling genes followed by a phase of down regulation of these genes. 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 Expert Opinion in [9]. Reproduced from Expert Opinion in Expert Opin. Med. Diagn. (2008) 2(10):1107-1121 with permission of Informa UK Ltd.