Ultrastructure of slit² mutant CBs. (A–F) Transverse ultra-thin sections of wild-type (A) and slit² mutant embryos (B–F) at late stage 17. (C–F) Ultra-thin sections at different levels along the AP axis, with C the most anterior and F the most posterior. (C′–F′) High magnifications of C–F showing the extending cell–cell contact between the CBs of the opposite rows (yellow arrows), compared with the wild type (A). (B–F) In slit² mutant embryos, CBs have a much more rounded shape than in wild-type embryos (A) and the lumen is not formed. (C′ and D′) Note that in slit² mutants, the contact between CBs often shows interruptions where some vacuolelike structures are observed (blue arrows). L, lumen; CB, cardioblast; N, nucleus; yellow arrows, cell–cell contact; black arrowheads, hemi-adherens junctions; red arrowheads, adherens junction; asterisks, basement membrane. Bars, 1 μm.