| During rapid cell proliferation, compartment boundaries play key roles in tissue organization by maintaining adjacent cell populations separate. These restrictions to cell mixing are thought to restrict cells of different fate to the correct tissue location during development and to organize distinct signaling centers necessary for growth and patterning of the whole tissue. Despite boundaries being discovered in Drosophila over thirty years ago, and subsequently discovered in a variety of vertebrate tissues as well, very little is known about the mechanisms that govern their formation. Based on phenotypes garnered from mosaic analyses, a differential affinity mechanism is often invoked to explain boundary formation.;The Notch signaling pathway regulates the formation of the dorsal-ventral compartment boundary in the wing. Several observations indicate that the mechanism by which Notch governs this process is novel in nature and suggests that Notch signaling induces the formation of a boundary "fence", thus maintaining dorsal and ventral cells separate. Work outlined in this thesis shows that the dorsal-ventral boundary is characterized by a distinct accumulation of F-actin and Myosin at the adherens junctions. In addition boundary cells on both sides form a more elongated and rectangular shape along the smooth boundary interface. Experiments that disrupt Notch signaling at the endogenous boundary, as well as that induce ectopic Notch signaling, highlight Notch's requirement and sufficiency for these boundary characteristics, respectively. Mutants that disrupt normal F-actin polymerization or Myosin function give rise to bi-directional and non-autonomous violations along the dorsal-ventral boundary. The nature of these violations are reminiscent of those witnessed upon loss of Notch activation. Importantly, the roles for F-actin and Myosin are dorsal-ventral specific as their disruptions fail to induce violations along the anterior-posterior boundary.;Taken together, the data suggest that the Notch-induced boundary fence is the result of an F-actin/Myosin contractile activity at the dorsal-ventral interface. In addition, the work suggests that dorsal-ventral compartmentalization occurs sequentially. An initial Apterous-dependant mechanism maintains a transient adhesive difference between dorsal and ventral cells. A second Notch-dependant mechanism mediated by F-actin and Myosin then creates a smooth interface by creating a strong boundary restriction. |
