Fig. 5. Epithelial depolarisation in th⁵ embryos. (A-D) Staining for Baz in WT and th⁵ mutant embryos. (A) In WT, Baz localises at the cell apex and is enriched at the dorsoventral membranes of intercalating cells in the embryo trunk (head is in the top-left corner). In th⁵ embryos immediately after cell rounding (B), Baz `fibres' are still intact, although their relative position at the cell cortex is altered, presumably because cells are changing from a elongated columnar shape to a rounded shape. (C) Close-up of th⁵ embryo undergoing cell dispersal. In the trunk region (right-part of the panel), Baz staining has now lost its fibrous appearance and is found at the cortex of the round cells, with a slight enrichment on one side, presumably reflecting the previous position of the apical domain. In this embryo, the head region (left-part of the panel) still exhibits epithelial characteristics, with Baz localised at the cell apices. (D) th⁵ embryo at the cell fragmentation stage. Baz staining is completely delocalised and is found in the cytoplasm of fragmenting cells as well as in smaller nuclear cell fragments. (E,F) Stills from time-lapse movies of WT and th⁵ mutant embryos labelled with DEcadGFP. No initial difference in DEcadGFP localisation can be detected between WT (E) and th⁵ embryos (F) after cephalic furrow formation (frames C+0-10 minutes). Then the apposing cells in the cephalic furrow become misaligned in th⁵ embryos compared with WT (C+16 minutes). Whereas germ-band extension proceeds in WT (C+16-64 minutes), the th⁵ embryo contracts, DEcadGFP appears to move apically (arrowhead in C+24-minute frame), the cell apices contract (arrowhead in C+48-minute frame) and then DEcadGFP is delocalised in dots (C+64 minutes). (G-I) Dlg staining of WT and th⁵ mutant embryos. (G) In WT, Dlg labels the lateral membrane. In th⁵ embryos undergoing cell dispersal (H), the cells that have already rounded up have lost all Dlg localisation, whereas Dlg staining is normal in the cells that still retain epithelial characteristics. In older th⁵ mutant embryos (I), Dlg localisation is lost in all cells. (J) Schema showing the localisation of Baz (apical membrane), DE-cadherin (adherens junctions) and Dlg (lateral membrane) in the early epidermis of Drosophila (see also Tepass et al., 2001). Note that in the early embryos looked at in this study the cells are either still connected to the yolk cell basally, by a cytoplasmic bridge or have just separated from the yolk cell through a cytokinesis-like process.