Compartments can be simply defined as separate, different, adjacent cell populations, which upon juxtaposition, create a lineage boundary.
[2] Subdivisions are established by morphogen gradients and maintained by local cell-cell interactions, providing functional units with domains of different regulatory genes, which give rise to distinct fates.
[6] In addition, this separation creates a region of specialized cells close to the boundary,[7] which serves as a signaling center for the patterning, polarizing and proliferation[8] of the entire disc.
Compartment boundaries establish these organizing centers [5][7] by providing the source of morphogens [9] that are responsible for the positional information required for development and regeneration.
[8][11] Differences in growth rates and patterns in each compartment, maintain the two lineages separated [12] and each control the precise size of the imaginal discs.
[12] Eventually these selector genes become fixed in either an expressed or unexpressed state and are stably inherited to the descendants,[5][8] specifying the identity of the compartment and preventing these genetically different cell populations from intermixing.
[14] The difference in selector gene activity not only establishes two compartments, but also leads to the formation of a boundary between these two that serves as a source of morphogen gradients.
[2] The founder cells, found at the border between parasegments 4 and 5 of embryo, are already determined at the early blastoderm stage and defined into the two populations they will generate by stripes of the engrailed gene.
[18][19][20] The presence of engrailed in the posterior cells leads to the secretion of the short-range inducer Hh [8] which can cross over to the anterior compartment to activate the long-range morphogen, Dpp.
[6] Optomotor-blind (omb) is involved in the transcriptional response of Dpp, which is only required in the anterior cells to interpret Hh signaling for boundary formation and maintenance.
[15][22] Loss of engrailed function in posterior cells, results in anterior transformation, where Hh expression is decreased and dpp, ci and patched (ptc) is increased, resulting in the formation of a new A/P boundary, suggesting that en positively regulates hh, while negatively regulating ci, ptc and dpp.
Clones mutant for the Smoothened (smo), the gene responsible for transducing Hh signaling, retain anterior-like features, but move into the posterior compartment without any changes in the expression engrailed or invected.