Compared with a wild-type wing (A), loss of Dpp function results in reduced growth and loss of pattern elements (B). Ectopic expression of Dpp in a clone of cells results in pattern duplications associated with massive extra growth (C). The region of Dpp expression in (A) and (C) is indicated by the green color. (Zecca et al. 1995; pictures courtesy of B. Müller and K. Basler.)

Mosaic Drosophila wings with compartment-specific manipulations of dAkt function display striking size defects but normal patterning.
(A) Selective reduction of dAkt function in the posterior compartment by means of FLP-mediated mitotic recombination in posterior cells (using engrailed–Gal4 to drive the expression of UAS–Flp) results in a small P compartment largely consisting of dAkt³ mutant cells. The smaller compartment size is due to fewer and smaller cells.
(B) Wild-type wing for comparison.
(C) Expression of dAkt in posterior cells (engrailed–Gal4 UAS–dAkt) of wings with reduced dAkt function (dAkt³) restores the size of the P compartment, whereas the A compartment remains small. The red lines mark the anterior–posterior compartment boundary. Note that similar results in the wing disc have been obtained by Teleman and Cohen (2000).

A schematic representation of a growing (left) and a mature (right) wing disc is shown at the top. Corresponding cross-sections through the wing blade region are depicted below. The wing disc originates from the infolding of the embryonic ectoderm and consists of pseudostratified epithelial cells containing a basal–lateral side (yellow) and an apical side (red). The apical surface faces the disc lumen that is formed by the epithelium and the overlaying peripodial membrane (black), consisting of squamous epithelial cells. The morphogen and growth factor Dpp (yellow) is secreted basal–laterally by the Dpp-producing cells located anterior to the anterior–posterior compartment boundary (line through centre of wing disc). The Dpp concentration gradient from the anterior–posterior boundary to the periphery provides the anterior–posterior patterning cues. In addition, Dpp is also secreted apically into the disc lumen where is can diffuse freely. The model proposes that luminal Dpp acts as a growth-promoting factor stimulating disc growth in young discs. As the disc grows, a hypothetical growth inhibitor (blue dots) is also secreted apically and antagonizes the growth promoting activity of Dpp. Once the concentration of the inhibitor has reached a certain threshold, proliferation of wing imaginal disc cells ceases.