Oscillations of the Snail Genes in the Presomitic Mesoderm Coordinate Segmental Patterning and Morphogenesis in Vertebrate Somitogenesis

The segmented body plan of vertebrate embryos arises through segmentation of the paraxial mesoderm to form somites. The tight temporal and spatial control underlying this process of somitogenesis is regulated by the segmentation clock and the FGF signaling wavefront. Here, we report the cyclic mRNA expression of Snail1 and Snail2 in the mouse and chick presomitic mesoderm (PSM), respectively. Whereas Snail genes' oscillations are independent of NOTCH signaling, we show that they require WNT and FGF signaling. Overexpressing Snail2 in the chick embryo prevents cyclic Lfng and Meso1 expression in the PSM and disrupts somite formation. Moreover, cells misexpressing Snail2 fail to express Paraxis, remain mesenchymal, and are thereby inhibited from undergoing the epithelialization event that culminates in the formation of the epithelial somite. Thus, Snail genes define a class of cyclic genes that coordinate segmentation and PSM morphogenesis