Fig. 4. kette mutants stop fusion during formation of electron-dense plaques. (A-C) Transmission electron microscope analyses of kette^J4-48, blow² and mbc mutants confirm that kette^J4-48 and blow² mutants do form muscle precursor cells, while mbc mutants do not. Scale bars: 2 µm. (A) Stage 14 kette^J4-48 mutant embryo; asterisks indicate precursors with two or three nuclei. (B) Stage 12-13 blow² mutant embryo; developing precursors with two nuclei are clearly visible. (C) Stage 13-14 mbc mutant embryo. (D-F) Stage 13 wild-type embryo. (D) A muscle precursor has established contact with fusion-competent myoblasts, while groups of electron dense vesicles start to build the prefusion complex of paired vesicles (arrowheads). Nearby, a prefusion complex has already started to dissolve and will form a electron-dense plaque (arrow). Scale bar: 1.5 µm. (E,F) Detailed view of a group of electron dense vesicles in D. (F) Dissolving prefusion complex and developing electron dense-plaque (arrow) forming within a cloud of vesicles. Scale bar: 500 nm. (G) Developing electron-dense plaque in a stage 15 kette^J4-48 embryo. Remains of the dissolving prefusion complex are still visible (arrow); the length of the plaque is nearly twice that of the wild type plaque described by Doberstein et al. (Doberstein et al., 1997). Scale bar: 150 nm.