Fig. 3. An Ush-mediated signal from the AS induces elongation and D-V microtubule realignement of epidermal cells.

(A–D) Stage 14 embryos stained for F-actin. (E-G) Early stage 15 (E) and stage 14 (F,G) stained for α-tubulin. (A′–D′) Magnification of boxed region made by camera lucida tracing. (A) Wild type embryo. Epidermal cells are elongated in the D-V direction. (A′) DME cell are colored green, lateral epidermal cells blue. (B) ush² embryos. Epidermal cells fail to elongate. (B′) DME and second row of epidermal cells are often stretched in the A-P direction while the bulk of epidermal cells tend to be isotropic. (C) Expression of Ush in epidermal stripes of ush² mutants does not affect the shape of epidermal cells. (C′) Magnified region has been selected to cover a boundary between Ush-expressing and mutant epidermal cells and shows that the two first rows of epidermal cells still stretch in the A-P direction. (D) Expression of Ush in the amnioserosa of ush² mutants rescues the elongation of epidermal cells. (D′) DME and lateral epidermal cells clearly elongate in the D-V direction. (E) Epidermal cell elongation is associated with formation of apical microtubule bundle that are oriented in the D-V direction, as shown here in wild type embryos. Right panel: magnification of the boxed region in (E). (F) Microtubules fail to form bundles and to orient in ush² mutant embryos, Note their disorganized pattern in the corresponding magnification. (G) Restoring Ush in the amnioserosa of ush2 mutant embryos elicits correct bundling and D-V alignment of microtubules in the epidermis. All embryos are oriented with anterior to the left except embryo in G whose anterior end faces top left. Camera lucida drawings are 2× magnifications and the tubulin images are 2.5× magnifications of the corresponding boxed regions. All magnifications are as in Fig. 2 except for the insets in E, F and G, which are taken from the corresponding panels.