Multiple forces contribute to cell sheet morphogenesis for dorsal closure in Drosophila

The molecular and cellular bases of cell shape change and movement during m orphogenesis and wound healing are of intense interest and are only beginni ng to be understood. Here, we investigate the forces responsible for morpho genesis during dorsal closure with three approaches. First, we use real-tim e and time-lapsed laser confocal microscopy to follow actin dynamics and do cument cell shape changes and tissue movements in living, unperturbed embry os. We label cells with a ubiquitously expressed transgene that encodes GFP fused to an autonomously folding actin binding fragment from fly moesin. S econd, we use a biomechanical approach to examine the distribution of stiff ness/tension during dorsal closure by following the response of the various tissues to cutting by an ultraviolet laser. We tested our previous model ( Young, P.E., A.M. Richman, A.S. Ketchum, and D.P. Kiehart. 1993. Genes Dev. 7:29-41) that the leading edge of the lateral epidermis is a contractile p urse-string that provides force for dorsal closure. We show that this struc ture is under tension and behaves as a supracellular purse-string, however, we provide evidence that it alone cannot account for the forces responsibl e for dorsal closure. In addition, we show that there is isotropic stiffnes s/tension in the amnioserosa and anisotropic stiffness/tension in the later al epidermis. Tension in the amnioserosa may contribute force for dorsal cl osure, but tension in the lateral epidermis opposes it. Third, we examine t he role of various tissues in dorsal closure by repeated ablation of cells in the amnioserosa and the leading edge of the lateral epidermis. Our data provide strong evidence that both tissues appear to contribute to normal do rsal closure in living embryos, but surprisingly, neither is absolutely req uired for dorsal closure. Finally, we establish that the Drosophila epiderm is rapidly and reproducibly heals from both mechanical and ultraviolet lase r wounds, even those delivered repeatedly. During healing, actin is rapidly recruited to the margins of the wound and a newly formed, supracellular pu rse-string contracts during wound healing. This result establishes the Dros ophila embryo as an excellent system for the investigation of wound healing . Moreover, our observations demonstrate that wound healing in this insect epidermal system parallel wound healing in vertebrate tissues in situ and v ertebrate cells in culture.