Making stripes

Somites, the precursors of vertebrae, striated muscle, and dermis, are laid down in a timed sequence from anterior to posterior. Now, Alexander Aulehla, Bernhard Herrmann (Max-Planck- Institut für Immunbiologie, Freiburg, Germany), and colleagues report that a gradient and two dueling molecular clocks, all driven directly or indirectly by Wnt3a, combine to create the striped pattern of somites.The gradient and clock ideas have been proposed before. But, says Herrmann, “what was completely unclear was how the gradient and clock are coupled.” That link is now provided by a single protein, Wnt3a, which is connected to both processes.

Wnt3a came into the picture when the group discovered Wnt-dependent Axin2 expression in the tail bud and presomitic mesoderm (PSM). Axin2 expression was cyclic, but oscillated out of phase with the known oscillation of...