Fig. 3. Expanded apical membrane in grh mutants. (A,B) Electron micrographs of the DT (cross section) in wild-type (A) and grh mutants (B) at early stage 17. The lumen is disorganised in grh mutants, with tongues of cell masses extending from the apical surface to occupy part of the lumen. grh mutant embryos can secrete lumenal and cuticular components, but the characteristic taenidial structure is in places disrupted. (C,F) DT cross section at higher magnification showing a single cell of wild-type (C) and a grh mutant (F) with the membranes outlines in black. (D,G) Drawings of single cells based on EM cross sections in C and F. The cell membrane is shown in black, the apical junctions are marked with red, and the apical membrane is blue. In grh mutants, the amount of cell membrane apical to the AJs is significantly greater than in the wild type and excessive membrane folds over neighbouring cells. (E,H) High magnification images focused on a single AJ. In the wild-type cell (E), the AJs are positioned near the apical surface perpendicular to the lumenal surface. In the grh mutant (H), AJs are generally positioned further away from the lumen and are occasionally smaller. Scale bars in A,B, 1 µm; C,F, 0.5 µm.

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