Fig. 4. Notch feedback mechanisms operating along the DV boundary and presumptive veins of the wing disc. (A) A model representing asymmetric intercellular communication that refine and maintain the DV boundary cell fate during the third larval instar (Micchelli et al., 1997). The red domain represents Notch-activated boundary cells that express wg and cut, and the blue domains represent the flanking cells where Notch is repressed by DL and SER. The feedback loop represented here restricts and maintains the boundary cell fate through autonomous inhibition of Notch in the flanking cells, and non-autonomous maintenance of Notch activation in the boundary cells. (B) A model representing the feedback loop operating during pupal development that refines EGFR activation and thus vein differentiation to a subset of the vein competent provein cells (de Celis et al., 1997). The blue domain represents a presumptive vein where EGFR is active, and the red domains represent the lateral proveins where Notch is active. DL is required in the presumptive veins to activate Notch in the lateral proveins. Active Notch signalling then represses EGFR signalling, allowing lateral provein cells to differentiate as intervein.

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