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Fig. 1. Cells round up and disperse swiftly in th⁵ mutant embryos. (A-C) Stage 10 embryos stained for Engrailed. In wild-type (A), the germ-band is fully extended and the Engrailed-expressing cells are arranged in segmental stripes in the ectoderm. In hypomorphic th⁴ mutants (B), germ-band extension fails halfway, and the Engrailed-expressing cells in the dorsal-most part of the ectoderm are found dispersed. In th⁵-null mutants (C), germ-band extension fails completely, and all Engrailed-expressing cells are dispersed. The pale regions correspond to fragments of the yolk cell emerging at the surface of the embryo. (D-I) Stage 8 embryos showing distribution of Even-skipped (Eve), Twist (Twi) and Forkhead (Fkh) to mark the ectoderm, mesoderm and endoderm, respectively. In wild-type embryos (D), Eve-expressing cells are arranged into pair-rule stripes. These cells are found completely dispersed in th⁵ embryos (E). (F) Ventral view showing Twist-expressing cells at the ventral midline and underneath, where the mesoderm has invaginated. In th⁵ embryos (G), the mesoderm invagination collapses and the Twist-expressing cells reappear at the surface of the embryo. Fkh-expressing cells mark the anterior (arrow) and posterior midgut rudiment in wild type. Note that at stage 8, the posterior Fkh domain has already invaginated (arrowhead) (H). In th⁵ embryos, the Fkh-expressing cells reappear at the surface of the embryo (I). (J-M) Kinetics of cell dispersal in live embryos, observed either by labelling nuclei using HistoneYFP (J,K, see supplementary material Movie 1), or in unlabelled embryos filmed under Nomarski optics (L,M, see supplementary material Movie 2). Times are indicated from the end of cellularisation (abbreviated as C). In both movies, no defects are detected until the beginning of germ-band extension, at which point the morphogenetic folds of the embryos start to regress (C+21-minute frame in K and C+18-minute frame in M), then the embryos contract abruptly and the cells round up and disperse in a few minutes (C+26-minute frame in K and C+25-minute frame in M). Note that in K, the nuclei within the field of view at C+26 minutes are entirely different from those within the field of view at C+21 minutes, showing that the nuclei have dispersed extensively. In J and K, anterior, posterior, ventral and dorsal sides are labelled, and the arrow indicates the posterior transverse furrow. In L and M, the arrow marks the cephalic furrow, and the arrowhead indicates the posterior midgut invagination.

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