Canthus Form and Function in Dorsal Closure in <italic>Drosophila</italic> Embryogenesis
Dorsal closure in <italic>Drosophila</italic> embryos provides an excellent model system for the analysis of the coordinated cell shape changes and biomechanical processes that drive morphogenesis. During closure, the dorsal surface of the embryo displays an eye–shaped structure consisting of amnioserosa flanked by sheets of lateral epidermis. The canthi are found at the corners of the eye–shaped dorsal opening and are the focus of this dissertation. A synthesis of the four biomechanical processes that contribute to dorsal closure occurs in each canthus. Apical constrictions of amnioserosa cells and contractile actomyosin cables provide forces that favor closure. The two opposing sheets of lateral epidermis that flank the amnioserosa come together in the canthi where they are zipped together. Zipping at the canthi ensures the formation of a continuous epithelium and serves to maintain the curvature of the actomyosin cable necessary to resolve force in a dorsal–ward direction. This dissertation first describes the formation of the canthi, particularly interesting due to the radically different tissue organization for germ band retraction, the preceding stage of development. After canthus ontogeny, I describe dorsal closure stage canthi in three morphologically and mechanically distinct zones. I interrogate each zone by both confocal fluorescent microscopy and laser microsurgery to achieve a thorough visual and mechanical description. Finally, I describe the results of completely removing both canthi — the lateral epidermis leading edges straighten out to become parallel or nearly parallel fronts that move at native or nearly native rates and closure completes at the dorsal midline. Closure, again, proves to be robust and resilient — it can proceed without zipping or much if any leading edge curvature that in control embryos resolves purse string contraction into dorsal–ward forces. In total, the canthus proves to be an excellent source for many avenues of investigation with many more questions left to answer.
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