Embryonic disc primordia
The question about the origin of imaginal structures was first approached through transplantation experiments and clonal analyses during the 1960s. Results from these studies indicated that imaginal precursors are set aside during the extended germband stage. At this time, the precursors of the leg and wing discs are not yet separated. Imaginal discs are comprised of separate cell populations that obey strict lineage restrictions. One cell will only give rise to progeny of either the anterior or the posterior part of the disc, called "compartment".

These obersevations have to date been confirmed for the thoracic imaginal discs through experiments using molecular markers. Imaginal discs arise as a cluster of Distalless (Dll) expressing cells on the parasegmental boundary, at the ends of the ventromedial wingless (wg) stripe. wg is required in a narrow time window for the onset of Dll expression which is then maintained in an autoregulatory loop. At stage 11, decapentaplegic (dpp) is expressed in the dorsalmost cells of the cluster. dpp contributes to the specfication of cells within the presumptive disc. It is responsible for the expression of vestigal in the presumptive wing disc cells, which then separate from the leg disc.



Media list
Imaginal disc primordia..........additional information

Genes discussed
Gene
Gene product - Domains
Function
Links
decapentaplegic (dpp)
ligand - TGF-ß homolog
Dpp establishes the dorsal / ventral pattern and defines compartment boundaries
Distal-less (Dll)
transcription factor - homeodomain
defines the termini of appendages and regulates appendage growth
engrailed (en)
transcription factor - homeodomain
segment polarity gene - involved in compartment identity and boundary formation
escargot (esg) transcription factor - zinc finger transcriptional repressor, regulates cell cycle Interactive Fly
vestigial (vg)
presumptive transcription factor - novel
the activation of several genes in the wing field requires Vg/Sd function, for example activation of Serum response factor intervein promoter
wingless (wg)
ligand - wnt family
segment polarity gene, intercellular signaling protein