We first summarize wing development during metamorphosis of Drosophila and identify four critical steps in the conversion of a folded single layered wing disc to a flat bilayered wing. Each step occurs twice, once during the 12 hour prepupal period and again during the 84 hour pupal period. (1) Apposition in which basal surfaces of dorsal and ventral epithelia come close together. (2) Adhesion in which basal junctions form between the apposed basal surfaces. (3) Expansion in which wing area increases as a result of cells flattening. (4) Separation in which dorsal and ventral epithelia are separated by a bulky extracellular matrix but remain connected by slender cytoplasmic processes containing the microtubules and microfilaments of the transalar cytoskeleton. Disc ultrastructure is correlated with the distribution of the beta chain of integrin, laminin A, and filamentous actin for each key stage of pupal development. Integrin and laminin exhibit a mutually exclusive distribution from the adhesion stage onwards. Integrin is present on the basal surface of intervein cells but not on vein cells whereas laminin A is absent from the basal surfaces of intervein cells but is present on vein cells. We conclude that laminin is not a ligand for integrin in this context. During apposition and adhesion stages integrin is broadly distributed over the basal and lateral surfaces of intervein cells but subsequently becomes localized to small basal foci. These foci correspond to basal contact zones between transalar processes. The distribution of filamentous actin is dynamic, changing from an apical distribution during hair morphogenesis to a basal distribution as the transalar cytoskeleton develops. Basal adherens-type junctions are first evident during the adhesion stage and become closely associated with the transalar cytoskeleton during the separation stage. Thus, basal junction formation occurs in two discrete steps; intercellular connections are established first and junction/cytoskeletal connections are formed about 20 hours later. These observations provide a basis for future investigations of integrin mediated adhesion in vivo.

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