Aberrant setting of CB membrane domains in slit² mutant embryos. (A–F) Merged Z views of CBs from slit² mutant embryos stained with Arm (red), Dg (green), and How (blue) antibodies at different stages of embryogenesis (from stages 13–14 in A to stage 16 in F). A′–F′ show Arm and How, whereas A″–F″ show Dg and How staining. CBs keep their initial rounded shape during all stages of cardiac tube formation. Initially, Arm is correctly localized at the future J domains (arrowheads), but is not completely excluded from the Dg-positive domain as it is in the wild type (A′–C′, compared with Fig. 4, E′ and F′). When CBs join dorsally, Arm shows an extended area of expression compared with the wild type (Fig. 5, E and E′ [arrowheads], compare with Fig. 4, G–H and G′–H′). Finally, in a few cases a lumen can be detected, Arm is localized all along the extended dorsal J domain (Fig. 5 F, compare arrowheads with Fig. 4 J). (G) Schematic representation of CB shape dynamics and membrane domains in slit² mutant embryos (compare with Fig. 4 K). Step 1 is similar to that of the wild type except that the Arm-positive domain is larger in the mutant, and, as a consequence, Dg and Arm partially colocalize. (2) In the mutant, CBs do not change their shape, do not form a leading edge (3), and make contact by an enlarged Arm-positive J domain. Finally, CBs are either not able to form a lumen (4) or, rarely (5), they show an ectopic small lumen formed ventrally. Bars, 4 μm.