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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 junction 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.
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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.
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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
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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
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provides an anterior-posterior pulling force that contributes to germ-band extension through passive cell shape changes. Although anterior-posterior patterning mutants fail to fully extend their germ-bands, during the fast phase the elongation length is normal despite defects in polarized cell
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embryo at the start of germ-band elongation and 30 minutes later. The germ-band (grey) is posterior to the cephalic furrow (curved-line) and folds dorsally (red arrow) upon cell intercalation. Each rectangle represents a field of cells before (0 min) and after (30 min) convergent extension.
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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
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mutants are defective in germ-band extension, which supports the idea that polarized protein localization is critical for cell rearrangements. One mechanism by which myosin II might promote polarized cell remodeling is through contractile activity that creates tension orienting junctional
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The source that establishes planar polarity during germ-band extension remains elusive. Polarized intercalation is largely unaffected in mutant embryos that lack dorsal-ventral cell types. Yet, mutations that disrupt segmental patterning along the anterior-posterior axis, such as
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In addition, there is evidence that mechanical tension is necessary and sufficient for the cortical localization of myosin II. Thus, not only can myosin II generate tension but it may also be up-regulated by tensile forces, creating a
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intercalation. Time-lapse analysis revealed that an increase in cell shape stretching in the anterior-posterior axis was compensating for aberrant cell intercalation, independent of anterior-posterior patterning. Furthermore, during
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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
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localizes to the anterior-posterior boundaries of cells, destabilizing adherens junctions, whereas the
Bazooka/Par-3 complex localizes to dorsal-ventral boundaries, stabilizing adherens junctions. Moreover,
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Butler, L.C.; Blanchard, G.B.; Kabla, A.J.; Lawrence, N.J.; Welchman, D.P.; Mahadevan, L.; Adams, R.J.; Sanson, B. (2009). "Cell shape changes indicate a role for extrinsic tensile forces in
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of eve or runt is sufficient to locally reorient the polarity of nearby cells. This evidence argues that planar polarity is established by cell-cell interactions, and not by a
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development, it has been suggested that intercalary cell behavior relaxes the stress imposed on the germ-band, allowing stretched cells to restore to isometric shapes.
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The dorsal-ventral intercalation of cells during germ-band extension ultimately arises from the asymmetric localization of proteins within individual cells.
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involve oriented junctional remodeling, which indicates that the intercalating cells are intrinsically polarized within the plane of the epithelium.
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disassembly. However, the precise mechanism in which asymmetrically localized protein complexes encourage directed intercalation remains disputed.
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Bertet, C.; Sulak, L.; Lecuit, T. (2004). "Myosin-dependent junction remodelling controls planar cell intercalation and axis elongation".
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Asymmetric localization of proteins to reciprocal cell borders in the apical plane of a polarized epithelial.
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loop that allows cells to dynamically respond to fluctuations in their mechanical environment.
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Fernandez-Gonzalez, R.; de Matos Simoes, S.; Röper J.-C.; Eaton, S.; Zallen, J.A. (2009).
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Colored nuclei arbitrarily mark rows of cells in order to visualize tissue morphogenesis.
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425:"Multicellular rosette formation links planar cell polarity to tissue morphogenesis"
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Blankenship, J.T.; Backovic, S.T.; Sanny, J.S.; Weitz, O.; Zallen, J.A. (2006).
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germ-band extension and its regulation by pair-rule segmentation genes"
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axis while subsequently narrowing along the dorsal-ventral axis.
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In order for cells to intercalate between one another the
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349:"Mechanisms of elongation in embryogenesis"
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60:Germ-band extension begins shortly after
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16:Morphogenic process during embryogenesis
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105:tissue must be dynamically remodeled.
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467:Zallen, J.A.; Wieschaus, E. (2004).
514:"Animal development: Crowd control"
101:that maintain the integrity of the
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490:10.1016/s1534-5807(04)00060-7
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217:"Cell intercalation during
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39:process widely studied in
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176:Researchers suggest that
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107:Time-lapse microscopy
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162:ectopic expression
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