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Fig. 2. A molecular map of midline cells during germband elongation.
(A) Engrailed is expressed in two phases at the midline. Embryos
expressing tau-GFP driven by engrailed-GAL4 were stained with
anti-GFP (GFP, brown) and anti-Engrailed (En, blue). Midline cells that
express Engrailed from gastrulation (early Engrailed) can be stained with
anti-GFP and anti-Engrailed (dark brown cells). Midline cells that initiate
Engrailed expression at stage 10 (late Engrailed) can be stained with
anti-Engrailed but not yet with anti-GFP (blue cells, arrow). (B)
Gooseberry-distal (Gsb-D, dark purple) expression coincides with midline cells
expressing early Engrailed (dark brown, arrow), but not late Engrailed (light
brown). (C) At stage 10, midline cells located directly posterior to
the Engrailed domain (brown) express Lethal of scute (black, arrow) but not
Engrailed. (D) About one hour later, all Lethal of scute-positive cells
also express Engrailed (dark brown, arrow). The two midline cells comprising
the early phase of Engrailed expression (arrowhead) continue to express
Engrailed only (light brown). (E) Lethal of scute-positive midline
cells express the Hedgehog receptor Patched. Expression of
ß-galactosidase (red) from the patched promoter
(ptc-lacZ) is very high in Lethal of scute (green)-positive midline
cells (yellow cells, arrow). (F) Patched (Ptc) expression (purple) is
upregulated in the anterior midline cells, labelled by the early expression of
Odd (brown). Due to repression of Patched by Engrailed, the early Engrailed
domain is visible as a white stripe. (G) Using Engrailed as a
reference, the anteroposterior position of midline subsets (see
Fig. 1) is superimposed on the
expression map. Segmentation of the ectoderm is shown by Engrailed and
Hedgehog expression. Engrailed and Odd have two distinct phases of midline
expression. Parasegmental (arrowhead) and segmental borders (arrow) are
indicated. Because MP1 interneurons and midline glia have a very similar
anteroposterior position, it is also possible that MP1 interneurons are the
most anterior cells in every segment. Ventral views, anterior to the left;
brackets outline the midline.
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