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Dorsal closure is a key process during Drosophila morphogenesis that models cell sheet movements in chordates, including neural tube closure, palate formation, and wound healing. Closure occurs midway through embryogenesis and entails circumferential elongation of lateral epidermal cell sheets that close a dorsal hole filled with amnioserosa cells. Signaling pathways regulate the function of cellular structures and processes, including Actomyosin and microtubule cytoskeletons, cell-cell/cell-matrix adhesion complexes, and endocytosis/vesicle trafficking. These orchestrate complex shape changes and movements that entail interactions between five distinct cell types. Genetic and laser perturbation studies establish that closure is robust, resilient, and the consequence of redundancy that contributes to four distinct biophysical processes: contraction of the amnioserosa, contraction of supracellular Actomyosin cables, elongation (stretching?) of the lateral epidermis, and zipping together of two converging cell sheets. What triggers closure and what the emergent properties are that give rise to its extraordinary resilience and fidelity remain key, extant questions.
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Supplemental Video 1: Drosophila embryogenesis, starting at about 3 hrs after fertilization during ventral furrow formation (reproduced with permission from Tomer et al 2012). Nuclei are labeled through the expression of GFP-labeled histone. A white dot (upper left corner, 9:55:00 – 9:57:00) marks the approximate end of germ band retraction; a green dot (11:30:00 – 11:32:00) marks the approximate time of the onset of dorsal closure; and a red dot (14:45:00 – 14:47:00) marks the approximate end of closure. Supplemental Video 2: The single cell layered amnioserosa is shown adhering to the surface of the yolk syncytium (reproduced with permission from Reed et al 2004). The end of the video sequence has an arrowhead pointing to the amnioserosa about midway through the apposition process and a Y indicating yolk. The embryo is expressing GFP-cadherin, GFP-actin and the GFP yolk membrane tag G289 PTT (see Figure 2). Supplemental Video 3: This video starts towards the end of germ band retraction (a previously unpublished video from Ruth Montague, Verietta Williams, and Dan Kiehart). F-actin, labeled with a protein that fuses GFP to the actin binding domain of Drosophila Moesin (GFP-Moe-ABD, called sGMCA in Kiehart et al 2000) accumulates at the initially scalloped, leading edge of the DME cells. As F-Actin accumulates, the scalloped leading edge transforms into a smooth curve (see Figure 4). Supplemental Video 4: The PAS cells remain associated with the DME cells (not shown) as closure progresses (reproduced with permission from Rodriguez-Diaz et al. 2008). F-Actin is labeled primarily in the amnioserosa, with a protein that fuses GFP to the Actin binding domain of Drosophila Moesin (GFP-Moe-ABD). Bright spheres shown accumulating toward the end of the video are phagocytosed fragments of amnioserosa cells that underwent apoptosis and were engulfed by macrophage-like hemocytes in the fly embryo (see Figure 5 a-e).