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Fig. 3. Overview of mesoderm development in Drosophila. Typical
time-lapse sequence of an embryo with three to four segments of photoactivated
mesoderm showing cells undergoing two divisions as they spread out into a
monolayer (see Movie 1 in the supplementary material). (A) At 15
minutes post-gastrulation, the mesoderm shows a medial seam (arrowhead),
indicating that the invaginated mesoderm is still arranged as an epithelial
tube. (B) By 33 minutes, the EMT has occurred and the cells undergo a
synchronous division, as evidenced by the appearance of mitotic spindle poles
(arrowheads) and spindles (arrows). (C) At 1:03, the cells have
re-entered interphase and are migrating out over the ectoderm. (D) At
1:19, the second division is in progress (arrows indicate spindles). During
this division the mesoderm rapidly extends laterally. (E,F) By
2:15 the monolayer has been achieved and the embryo (E) shows a similar
segmentally repeated striped pattern to a control embryo (F)
(twist::CD2) (arrowheads). The segmentally repeated variation in
brightness is due to the changing thickness of the epidermal/neural tissue,
which attenuates the signal from the photoactivated mesoderm (data not shown).
(G,H) z-series reconstruction at the end of time-lapse
acquisition (G), showing that the mesoderm has adopted the typical monolayer
arrangement seen in fixed twist::CD2 control embryos (H). Scale bars:
20 µm.
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