Fig. 4. E-cadherin and ß-catenin, but not p120, are required for fusion track assembly. Arrowheads, fusion sites. Anterior, leftwards; dorsal, upwards. (A-C) Stage 14 wild-type embryo. (A) Fusion is complete, and F-actin is concentrated along a continuous apical surface spanning the fusion site. Scale bar in A: 10 µm for A-F. (B) Shot is also concentrated apically. (C) Merge of A and B. F-actin, red; Shot, green. (D-F) Stage 14 shg² mutant embryo. (D) F-actin does not form a track in fusion cells and the tracheal tubes remain blind-ended. (E) Shot-GFP does not form a track in fusion cells. (F) Merge of D and E. F-actin, red; Shot, green. (G-I) Stage 13 wild-type embryo. (G) E-cadherin contacts form between fusion cells. Scale bar in G: 10 µm for G-L. (H) Tracks of Shot form at the new E-cadherin contacts. (I) Merge of G and H. (J-L) Stage 13 arm^YD35; P[arm^S14-C] embryo. (J) New E-cadherin contacts fail to form between fusion cells. K) Tracks of Shot fail to form in fusion cells. L) Merge of J and K. E-cadherin, red; Shot, green. M) A stage 15 p120³⁰⁸ mutant embryo stained with mAb 2A12, which reveals the tracheal lumen (Samakovlis et al., 1996a). Dorsal trunk (arrowheads) and lateral trunk (arrows) fusion appears normal, as does tracheal branching. Scale bar in M: 10 µm.