Making stripes

Making stripes omites, the precursors of vertebrae, striated muscle, and dermis, are laid down in a timed sequence from anterior to posterior. 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 S Axin2 (left) and the Notch pathway (right) oscillate out of phase to create somites. Chromosomes in the 'hood t is almost unbearable (at least for scientists) to contemplate a complete lack of order. So, it comes as some relief that chromosomes may be positioned nonrandomly in the nucleus, thus giving rise to more frequent translocations between certain chromosomes. The seeming chaos of mitosis led most researchers to believe that any such order would have to be reestablished after each division. But now and colleagues have found that positioning is maintained through mitosis by a timing mechanism. Congressing chromosomes make a beeline for the metaphase plate, the group found, and thus preserve information about their relative position perpendicular to the spindle axis. But congression erases information about how far the chromosomes had to travel to reach the metaphase plate. Despite this, the group found that marked territories, chromosomes or centromeres reestablished their previous geographies after mitosis. (Conflicting conclusions were recently drawn by Walter et al. in these pages.) Chromosomes reestablished their positions by initiating anaphase separation at different times. Chromosomes that I 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 Notch pathway activity, which is also dependent on Wnt3a activity. Known signaling pathways provide some plausible circuitry. Axin2 produced by Wnt pathway activity should both turn off the Wnt pathway (via a negative feedback loop) and turn on the Notch pathway (by binding to a negative regulator). The instability of Axin2 would later reverse the situation, leading to a continuous cycle. The gradient idea arose because Wnt3a is made in the tail bud but can direct Axin2 expression throughout the PSM. High concentrations of Wnt3a near the tail bud prevent differentiation. But as …