Fig. 5. Genetic control of cell intercalation during branch outgrowth. A four-step model of cell intercalation in a dorsal branch stalk. See also text for details. (A-D) Four representative micrographs, visualizing the AJs using Cat::GFP expression in the tracheal system, are shown in parallel with (E) four sketches representing the same steps. (F) The proteins involved in the genetic control of this process. (A-E) To better understand the `topology' of the cells, the edges of two neighbour cells were colored in red and green, respectively. (A) Pairing: tracheal cells are in a side-by-side arrangement along the branch lumen. (B) Reaching around the lumen: individual cells establish contact with themselves and start to form the first autocellular AJs (arrowhead). (C) Zipping up: autocellular AJs extend as the two cells, which were initially paired, change their respective positions. (D) Termination: in order not to lose all intercellular AJs (and thus the adhesion between neighbouring cells), the transformation of intercellular AJs into autocellular AJs stops, with small intercellular AJ loops connecting adjacent cells to each other. Adapted, with permission, from Ribeiro et al. (Ribeiro et al., 2004). (F) The genetic control of these intercalation steps is beginning to be characterized. Proteins involved in this process are indicated with blue boxes.