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Germ-band extension is a morphological process widely studied in ''Drosophila melanogaster'' in which the germ-band, which develops into the segmented trunk of the embryo, approximately doubles in length along the anterior-posterior axis while subsequently narrowing along the dorsal-ventral axis.〔Irvine, K., and Wieschaus, E. (1994). Cell intercalation during Drosophila germ-band extension and its regulation by pair-rule segmentation genes. Development 120, 827–841.〕 Germ-band extension begins shortly after gastrulation and is divided into two phases. The fast phase, in which most of the extension occurs, takes about 25 minutes. The remaining extension continues during the slow phase and is completed in the following 70 minutes.〔da Silva, S. M. & Vincent, J. P. (2007). Oriented cell divisions in the extending germband of Drosophila. Development 134, 3049–3054.〕 During this process the ventral germ-band extends around the posterior end of the embryo, effectively folding over onto the dorsal side of the egg. Multiple individual cells intercalating mediolateral to the anterior-posterior axis drive the resulting global elongation of the embryo. In addition, cell shape changes, and oriented cell divisions in the posterior germ-band are in part required for full elongation.〔Butler, L.C., et al. (2009). Cell shape changes indicate a role for extrinsic tensile forces in Drosophila germ-band extension. Nature Cell Biology 11, 859-864.〕 However, elongation of the body axis seems to be primarily linked to changes in cell neighbor relations. This article describes axis elongation in ''Drosophila''. However, the basis of germ-band elongation is applicable to many organisms including other invertebrates and vertebrates alike.〔Keller, R. (2006). Mechanisms of elongation in embryogenesis. Development 133, 2291–2302.〕 ==Cellular basis== In order for cells to intercalate between one another the adherens junctions that maintain the integrity of the epithelial tissue must be dynamically remodeled. Time-lapse microscopy has captured this process of cell neighbor exchange, which is schematically represented to the right. In the type 1 configuration, two cells contact each other along the anterior-posterior axis, whereas two dorsal-ventral cells do not directly contact. Next, the cell boundary between the two anterior-posterior neighbors selectively shrinks, resulting in an obligatory intermediate type 2 junction, where the four cells share a vertex. Upon resolution of the type 2 junction, a new type 3 junctions forms perpendicular to the original type 1 configuration. During this process the two dorsal-ventral cells have become neighbors.〔Bertet, C., Sulak, L. & Lecuit, T. (2004). Myosin-dependent junction remodelling controls planar cell intercalation and axis elongation. Nature 429, 667–671.〕 When multiple clusters of cells intercalate in the dorsal-ventral axis, through junctional neighbor exchange, the outcome is an extension of germ-band in the anterior-posterior axis. In addition to the simple neighbor exchange, higher-ordered rosette formations have been observed in which five or more cells meet at a vertex. Multicellular rosettes form and resolve in a directional fashion that promotes germ-band elongation.〔Blankenship, J. T., Backovic, S. T., Sanny, J. S., Weitz, O. and Zallen, J. A. (2006). Multicellular rosette formation links planar cell polarity to tissue morphogenesis. Dev. Cell 11, 459-470.〕 Neighbor exchange and multicellular rosette formation involve oriented junctional remodeling, which indicates that the intercalating cells are intrinsically polarized within the plane of the epithelium. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Germ-band extension」の詳細全文を読む スポンサード リンク
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