The number of tracheal dorsal trunk and dorsal branch cells is not greater than wild type in the mutant embryos. At stage 16, the apical circumference of cells in the dorsal trunk is highly irregular. In particular, the cells positioned at the outer edge of each curve become excessively elongated compared to the cobblestone-shaped cells that line the lumen of wild-type embryos. Cross sections show that the morphology of the septate junctions and the basal part of the dorsal trunk cells is normal in mutant embryos, but the apical cell domain appears strikingly overgrown and distorted. These defects are first seen in early stage 16, and the apical surface continues to enlarge, so that it folds over neighbouring cells, resulting in several layers of cuticle deposition. The apical adherence junctions are often abnormal in mutant tracheal cells; these are often misplaced, laying parallel rather than perpendicular to the lumen and occasionally appear less electron dense than normal.