Somites form transiently in the presomitic mesoderm (PSM) and organize the segmental pattern of vertebrate embryos. FGF and Wnt gradients that decrease caudorostrally control the maturation of the PSM, and also the position in the embryo where the transition to segmental units occurs (the `differentiation wavefront'). But what regulates these gradients? On , Nagano and co-workers reveal that Shisa2, a member of the novel Shisa gene family, regulates FGF and Wnt signals during somitogenesis in Xenopus. The researchers show that Shisa2, like its relative Shisa1,encodes an endoplasmic reticulum protein that inhibits signalling by Wnt and FGF by preventing the maturation and cell-surface expression of their receptors. Knockdown of Shisa2, they report, delays PSM maturation and shifts the differentiation wavefront anteriorly, thus reducing somite numbers. This phenotype can only be rescued by inhibiting both Wnt and FGF signals. Thus,the researchers conclude, Shisa2 plays an essential role in establishing the segmental pattern in Xenopus embryos by individually inhibiting both these signals.
