page_title: Germ-band extension
section_title: Cellular basis
hierarchical_section_title: Germ-band extension / Cellular basis
caption_attribution_description: English: T1 Junction exchange. Neighbor exchange.
context_page_description: 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.
  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. 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. However, elongation of the body axis seems to be primarily linked to changes in cell neighbor relations.
context_section_description: 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. 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. 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.
caption: Schematic of neighbor exchange or an elementary T1 process involving four cells.  T1-magenta cells are in direct contact.  T2-all cells share a common vertex. T3-resolution results in green cells sharing a common boundary.
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