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Fig. 5. Wingless and Hedgehog control different phases of Engrailed expression
at the ventral midline. (A) In wild-type, at stage 9, two to four
midline cells per segment express Engrailed (En, dark purple). (B) In
wingless (wg) mutants, Engrailed expression in the ectoderm
and at the ventral midline is lost. (C) In hedgehog
(hh) mutants, Engrailed expression in the midline weakens but
persists. (D) At stage 13, Engrailed-positive midline cells form a
prominent cluster. (E) In wingless mutants, despite the loss
of early Engrailed expression, the late phase of Engrailed expression in the
midline is initiated. (F) In hedgehog mutants, the late phase
of Engrailed expression is never induced. The number of Engrailed-positive
midline cells does not increase after stage 9. (G) In wild-type,
Engrailed continues to be expressed in the midline until stage 17. (H)
At stage 16, wingless mutants show a severe reduction in
Engrailed-expressing midline cells. Most midline cells die. Surviving midline
cells do not integrate into the CNS, resulting in the separation of the two
sides of the CNS. (I) Ectopic expression of Hedgehog (Hh, brown) in all
midline cells in wingless mutants only partially rescues the loss of
Engrailed (black). Midline cells are excluded from the CNS. Cell fragments can
be found in macrophages (arrows). (J) At stage 14, the expression of
tau-GFP (GFP, brown) in all midline cells reveals an elaborate axon pattern.
(K) In wingless mutants, midline cells are not integrated into
the CNS and show no differentiation. Ventral views, anterior to the left;
brackets outline the midline.
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