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- "The barrier to Wingless at the posterior edge of the engrailed
domain corresponds to the prospective seg- ment boundary. The
finding that boundaries between rhombomeres in the chick embryo
act as barriers to inductive signals (Martinez et al., 1995) suggests
that barriers to signals could be a universal item in the embryo’s
toolbox. Note that the Drosophila segment boundary allows the
passage of Hedgehog and the EGFR ligand Spitz, since their respective
target genes are activated across the boundary (Ingham, 1993;
O’Keefe et al., 1997; Szuts et al., 1997). The segment boundary
might therefore act as a filter for signaling molecules. How Wingless
is prevented from crossing the segment border is unknown, but,
as discussed above, it might be preferentially trapped or degraded
there." (Sanson,
1999)
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