The role of apoptosis in shaping the tracheal system in the Drosophila embryo

The tubular network of the tracheal system in the Drosophila embryo is created from a set of epithelial placodes by cell migration, rearrangements, fusions and shape changes. A designated number of cells is initially allocated to each branch of the system. We show here that the final cell number in the dorsal branches is not only determined by early patterning events and subsequent cell rearrangements but also by elimination of cells from the developing branch. Extruded cells die and are engulfed by macrophages. Our results suggest that the pattern of cell extrusion and death is not hard-wired, but is determined by environmental cues.