Figure 1: Mechanisms that establish compartment boundaries during development.

(a) Schematic diagram of anterior (A) and posterior (P) compartments in both the Drosophila wing imaginal disc and embryo. Dotted lines indicate compartment boundaries. In both the wing disc and the embryo, P-type identity is specified by the transcription factor Engrailed (En). En-expressing cells secrete Hedgehog (Hh) signal that activates the transcriptional regulator Cubitus interruptus (Ci) and A-type identity. (b) Three contrasting models of compartment formation. Differential adhesion: quantitative differences in cell–cell adhesion cause different cell/tissue types to have different tissue surface tensions (arrows along interface). By analogy to liquids, tissues with different surface tensions are immiscible. Differential contractility: different tissue surface tensions are not due to differential adhesion, but to different levels of cell contractility between cell types. Interfacial actomyosin cable: in contrast to differential adhesion and differential contraction, which result from the mechanical properties of the group, a boundary-specific actomyosin cable results from interactions between different cell types. Increased mechanical tension at the boundary restricts cell movement. Junctions could be required to transmit tension between interfaces along the compartment boundary.