Fig. 3. Formation of a dorsal branch. Dorsal branch development from embryonic stage 12 to stage 15. Stills from a time-lapse movie showing an embryo expressing GFP::Actin in the tracheal system (see Movie 2 in the supplementary material). (A) A few tracheal cells at the tip of the tracheal bud (asterisk) form filopodial extensions under the control of FGF signalling. Tracheal cells in the dorsal branch stalk are initially paired (arrows), connecting the dorsal branch tip cells to the tracheal sac (dotted circle), and enclosing the dorsal branch lumen (arrowhead). (B) Gradually, as the dorsal branch elongates, the cells of the dorsal branch stalk adopt an end-to-end configuration (arrows). (C) In this image, stalk cells have fully intercalated (arrows). The terminal cell (TC) and the fusion cell (FC) continue to form filopodial extensions. Scale bars: 10 µm.