The wing disc
The wing disc develops as a sack-like structure that will give rise to the adult wing and surrounding boy wall tissue, the notum of the fly. The wing imaginal discs are the largest imaginal discs in a mature 3rd instar larva. In a sagittal section, distinct structures are visible: The disc epithelium will give rise to the ectodermal imaginal structures such as wing with hinge, scutum, scutellum and notum. The adepithelial cells will give rise to parts of the imaginal muscles of the thorax. The ectodermal peripodial epithelium contracts during wing disc eversion and also contributes to the mesothorax of the adult fly.

During metamorphosis, the wing disc will evert.





The mature wing is a flattened pouch consisting of two opposing epithelial sheets, connected by the wing margin. Morphologically, the wing can be subdivided into different cells that are surrounded by veins.


Lineage analyses have uncovered boundaries of cell lineage restriction that divide the wing disc into an anterior and a posterior "compartment". The progeny of a cell born in one compartment will never give rise to cells of the other compartment, but are restricted to one side of a morphologically invisible boundary. Cells of the posterior compartment of the wing disc inherit engrailed expression from the embryo. The anterior-posterior compartment boundary of the wing corresponds to the parasegmental boundary of the embryo.

Cells of the posterior compartment express hedgehog, which signals at short range to neighboring cells. Only cells of the anterior compartment can respond to the hedgehog signal by expressing decapentaplegic (dpp), resulting in a narrow stripe of expression along the anterior-posterior compartment boundary. Organizing centres have been identified in the developing appendages of Drosophila by producing genetic mosaics that either delete or ectopically express patterning genes in clusters of cells. This can lead to the appearence of an ectopic organizing centre, which will exert an influence on the surrounding tissue, thereby leading to pattern duplications.

A further lineage restriction later divides the wing disc into a dorsal and a ventral compartment, creating a new boundary that similarly acts as an organizing centre. The dorsal compartment is specified by the localized expression of apterous. Signaling of dorsal cells via Serrate to ventral cells leads to the expression of wingless, which acts as the organizing molecule of the dorso-ventral axis.



Media list
Wing disc, virtual microscope, sagittal sections
Wing disc eversion
Wing disc map
Organizing centres within the wing disc..........additional information

Genes discussed
Gene
Gene product - Domains
Function
Links
apterous (ap) transcription factor - homeodomain; LIM domain involved in boundary formation in the developing wing Interactive Fly
decapentaplegic (dpp)
ligand - TGF-ß homolog
Dpp establishes the dorsal / ventral pattern and defines compartment boundaries
engrailed (en)
transcription factor - homeodomain
segment polarity gene - involved in compartment identity and boundary formation
hedgehog (hh)
Hedgehog N-terminal signaling domain and C-terminal autoprocessing domain
segment polarity gene intercellular signaling protein
Serrate (Ser)
transmembrane - EGF homolog - ligand for Notch
involved in the induction, through Notch, of the wing margin