What role does Notch signalling play in segment boundary formation during somitogenesis? Some studies suggest that segment boundaries form where boundaries of Mesp2 expression coincide with boundaries of on/off Notch expression (where cells with an activated Notch pathway interface with cells in which it is off). However, other studies suggest that segment boundaries form correctly even when there are no Notch on/off boundaries. On p. 1515, Yumiko Saga and co-workers resolve this long-standing controversy by examining somitogenesis in a transgenic mouse that lacks Notch on/off boundaries in the anterior PSM but retains Notch signal oscillation in the posterior PSM (Notch signal oscillation is a component of the ‘segmentation clock’ that controls the timing of somitogenesis). Segmented somites are continuously generated in this mouse, they report, and, surprisingly, rostral-caudal patterning within the somites is normal. Given these results, the researchers propose that the oscillation of Notch activation, but not its boundary, is both required and sufficient to establish the Mesp2 expression pattern needed for normal somitogenesis.
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