Synchronised cycling gene oscillations in presomitic mesoderm cells require cell-cell contact.

Segmentation of the vertebrate body axis is initiated early in development with the sequential formation of somites. Somitogenesis is temporally regulated by a molecular oscillator, the segmentation clock, which acts within presomitic mesoderm (PSM) cells to drive periodic expression of the cyclic genes. We have investigated the kinetics of the progression of cycling gene expression along the PSM. Here we show that c-hairy1 and c-hairy2 mRNA expression traverses the PSM in an entirely progressive manner and that both these genes and c-Lfng maintain a similar anterior limit of expression during each cycle. However, some differences are seen regarding both the onset of a new oscillation of these genes and the duration of their expression in the caudal PSM. We also investigated whether oscillating cyclic gene expression in the PSM is entirely cell autonomous. We find that while small PSM explants are still able to maintain their oscillation schedule, once they are dissociated, PSM cells are no longer able to maintain synchronous oscillations. The results imply that cell communication or a community effect is essential for the normal pattern of cyclic gene expression in these cells.

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