Fig. 7. Model of regulation of cell differentiation and restriction of boundary spreading. The diagrams illustrate the similar regulatory hierarchy of gene regulation and intercellular signalling in (A) the anterior compartment of the Drosophila wing disc and (B) the zebrafish hindbrain. In both systems, localised expression of wg/wnt1 is induced by fringe-mediated modulation of Notch activity at boundaries. In the hindbrain, there is in addition fringe-independent expression of Wnt1 in the roof plate. Wg/Wnt1 acts on non-boundary cells to upregulate proneural gene expression (as-c/ash) and thus induce or enable a neural fate. In the wing disc, proneural expression only occurs in neighbouring cells, whereas in the zebrafish hindbrain Wnt1 acts at longer range to promote neurogenesis throughout the segments. Proneural genes upregulate Delta expression, which in turn activates Notch in boundary cells. In addition, proneural gene products and/or Delta cell autonomously suppress boundary cell formation, thus preventing spreading of boundaries. Because in the zebrafish, Notch activation is not sufficient to induce boundary cell marker expression (Cheng et al., 2004), another factor (X) is proposed to be required for boundary cell specification.