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Fig. 5. Ttk is involved in the formation of terminal and fusion branches. (A) Stage 15 embryo labelled to highlight Esg-positive cells (arrows). (B) Two adjacent dorsal branches (DBs), the tip cells (probably fusion cells) of which make contact. The lumen (highlighted in white) does not penetrate the contacting cells. (C,D) Contralateral (C) or adjacent (D) DBs, the tip cells of which make contact. An E-cad-rich structure (in greyscale in insets) forms at the contact point (arrows). (E-F') Details of DBs and lateral trunks (LTs). The lumen marker 2A12 (in greyscale in E',F') reveals the presence of abundant luminal vesicles at the tips of branches in the presumptive fusion (arrowhead) and terminal (arrows) cells in wild type (E'). ttk mutants (F,F') show a decreased density of luminal dots. (G,H) DBs showing DSRF-positive cells and terminal branches (arrow in G). Notice the absence of terminal branches (arrow in H) in spite of DSRF expression in ttk mutants (H). (I,J) Fragments of LTs of ttk mutants at stage 16. Terminal branch formation is rescued when ttk is expressed in tracheal cells (arrows in J), but not when the Btl pathway is constitutively activated (arrows in I). All panels except I and J show projections of confocal sections of laterally viewed embryos.

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