Fig. 4. Expression of di inhibits adhesion between the two surfaces of the wing disc. (A,B) Longitudinal sections (1 µm) through prepupal wings at 4 hours after puparium formation. (A,A') The wild-type wing disc has already folded and the basal surfaces of the epithelial cells contact cells on the opposite surface via basal projections, except for a small gap at the marginal vein (^*). (B,B') In wing discs expressing di in the whole wing pouch, the cells lack projections and do not come in contact with cells at the opposite surface. In addition, folding does not take place along the wing margin (wm) leading to a mismatch between dorsal (d) and ventral (v) surfaces (arrows). (C,D) Scanning microscopy of late third-instar wing discs reveals that wing margin cells in discs expressing di cannot be morphologically distinguished from the rest.