Growth factors such as fibroblast-growth factor (FGF) or transforming-growth factor-β act during vertebrate embryogenesis to control cell growth, differentiation and morphogenesis. Cell movements and rearrangements continually occur during the complex reorganization of the developing embryo — for example convergent extension in a Xenopus laevis embryo leads to polarization of the mesoderm and intercalation of the mesodermal cells to elongate the embryo along the anterior–posterior axis. The FGF pathway is probably involved in this process, together with other pathways such as Wnt signalling, as blocking FGF signalling leads to a truncated anterior–posterior axis in the developing embryo.

ENRIQUE AMAYA

In another model organism, Drosophila melanogaster, Sprouty, a membrane-anchored general inhibitor of receptor tyrosine kinases, inhibits the FGF-signalling pathway. This is an interesting connection as FGF signalling also acts to increase Sprouty expression. Nutt and colleagues have now cloned a Xenopus Sprouty homologue, Xsprouty2 (Xspry2) (Genes Dev., 15, 1152–1166; 2001) which is expressed in embryos in a similar pattern to FGF. From overexpressing an Xsprouty2 construct, it appears that Xsprouty2 is also involved in axis formation during development, as overexpression of Xsprouty2 in Xenopus embryos leads to a shortened anterior–posterior axis (see picture). In particular, overexpression of Xsprouty2 inhibits convergent extension. Nutt and co-workers found that overexpression of Xsprouty2 inhibits the release of calcium from intracellular stores in a way that depends on the FGF receptor.

The role of calcium in convergent extension is poorly understood, but Wallingford et al (Curr. Biol., 11, 652–661; 2001) have recently shown that these morphogenetic changes are accompanied by marked alterations in calcium dynamics. As cells undergo convergent extension in Xenopus explants, there are waves of calcium mobilization from internal stores. These waves lead to contraction of the embryo and are essential for development, as depletion of the calcium stores inhibits convergent extension.

Both papers offer insight into how cell intercalation and movement might be involved in development. Calcium waves could be a common mechanism by which organisms control large cell movements. Although we do not yet fully understand how these movements are regulated, the cloning of Xsprouty2 and the link between this and calcium release indicates that thoughts on the process are extending as well as converging.