FGF signaling controls somite boundary position and regulates segmentationclock control of spatiotemporal Hox gene activation

Vertebrate segmentation requires a molecular oscillator, the segmentation c lock, acting in presomitic mesoderm (PSM) cells to set the pace at which se gmental boundaries are laid down. However, the signals that position each b oundary remain unclear. Here, we report that FGF8 which is expressed in the posterior PSM, generates a moving wavefront at which level both segment bo undary position and axial identity become determined. Furthermore, by manip ulating boundary position in the chick embryo, we show that Hox gene expres sion is maintained in the appropriately numbered somite rather than at an a bsolute axial position. These results implicate FGF8 in ensuring tight coor dination of the segmentation process and spatiotemporal Hox gene activation .