Cardiac lumen formation. Sequence of events extracted from Video 1 . Dorsal XY views (A–D) and transverse Z views (E–I) of CBs at different time points during cardiac tube formation (from stages 13–14 to 16–17 of embryogenesis). (E′–I′) Schematic 3D views of CBs in E–I. Blue dashed lines show the Z section positions. UAS Dmoesin-GFP; 24B-Gal4 flies were used to follow CB shape remodeling. (A, E, and E′) At the onset of migration, CBs have a square shape. (B, F, and F′) Subsequently, during their migration they constrict, adopt a triangular shape, and form a dorsal leading edge. (B, G, and G′) Soon after, CBs adopt a pearlike shape and the cells join at the dorsal leading edge. (C, H, and H′) After joining dorsally, CBs take a crescentlike shape that brings their ventral sides into proximity. (D, I, and I′) Finally, CBs join ventrally to close the tube and form the lumen. At the end of embryogenesis, CBs and the lumen grow concomitantly. The whole process lasts ∼90 min. Black holes correspond to CB nuclei. For all figures of the manuscript, dorsal (ectoderm side) is up and ventral (amnioserosa side) is down. Bars, 4 μm.