Fig. 1. Extension of the dorsal branch (DB) is coupled to dorsal closure of the epidermis. (A-I) Images of time-lapse recordings. White arrowheads indicate the fusion point of the DB (A-C,I,J) or dorsal epidermis (D-F). Green arrowheads indicate the terminal branch. Anterior is leftwards (A-F). (A-C) Dorsal view of DB8 and DB9 labeled with btl>>gfp-moe (UAS-gfp-moe driven by btl-gal4) taken from Movie 1. (D-F) Simultaneous labeling of the trachea and the P compartment of the epidermis with gfp-moe, expressed by btl-gal4 and en-gal4. Tracheal cells are mostly hidden beneath the en stripe. Fusion cells are indicated by red dots. (G-I) Transverse optical sections obtained by y-z scanning of a btl>>gfp-TT-ras embryo. This combination of insertions caused fortuitous GFP expression in most epidermal cells. Trachea (green), epidermis (blue) and amnioserosa (red) were identified by their location and cell shape and were pseudo-colored. The contact point of the epidermis and the DB tip is indicated by an arrow. Dorsal is rightwards. (A,D,G) Time=0 minutes (early stage 15: the dorsal epidermis meets at the midline). The tip of the DB adheres to epidermal cells located two or three cells behind the leading edge (G), and follows dorsal closure prior to 0 minutes. (B,E,H) Time=60 minutes: contact of fusion cells. Fusion cells from opposing sides contact each other with numerous filopodia (B, white arrowhead). The terminal branch has begun ventral extension (B, green arrowhead). (C,F,I) Time=130 minutes (early stage 16): completion of branch fusion. Terminal branch extension is still in progress. (J) Dorsal (upper) and transverse (lower) views of a DB fusion point. The terminal branch follows the curved shape of the epidermis (green arrowheads), whereas the stalk does not associate closely with the epidermis (open arrowhead). (K) Schematic drawing of the DB. T, terminal cell; F, fusion cell.