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Science 6 December 2002:
Vol. 298. no. 5600, pp. 1950 - 1954
DOI: 10.1126/science.1079478
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Shaping the Vertebrate Body Plan by Polarized Embryonic Cell Movements
Ray Keller
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Supplementary Material
To view these movies, download a QuickTime viewer.
- Movie S1
A movie of gastrulation of the frog, Xenopus laevis, shows the role of the powerful convergence and extension movements of the dorsal mesodermal and neural tissues in shaping the body plan. The central, light area of relatively large cells is the vegetal endoderm, which will be enclosed. Around the vegetal endoderm lies the mesodermal marginal zone, which will roll inside, or involute, beginning with the dark area at the top, which is the newly formed blastopore. As the marginal zone rolls inside, it will narrow or converge around the vegetal endoderm, and extend backwards, across the vegetal endoderm. These combined convergence and extension events close the blastopore. You will not be able to see the mesodermal marginal zone converge and extend, as it does so on the inside. However, the future posterior neural tissue will undergo similar movements on the outside. As the dorsal marginal zone involutes, the neural tissue will come into view at the top of the embryo, and converge toward the midline and extend very dramatically, across the vegetal endoderm. Finally, neural folds will form and move to the midline where they fuse to form the neural tube. The broad area at the top of the movie is future forebrain and head, and the narrow, rapidly extending part at the bottom of the frame is the future hindbrain/spinal cord. The events shown in the movie take about 8 hours at laboratory temperatures. [Movie courtesy of Dave Shook]
- Movie S2
A movie of a cultured explant of the dorsal part of the early gastrula shows the convergence and extension movements are mechanically independent of the rest of the embryo and are driven by internal forces. In this explant, the dorsal mesodermal tissues (future notochord and somitic mesoderm) occupy the bottom third of the explant and the posterior neural tissues (hindbrain/spinal cord) lie in the middle third; the top third is the future forebrain. Note that both the mesodermal tissues and the posterior neural tissues converge and extend; however, in this explant, they do so serially, end-to-end, rather than in parallel, with the mesoderm beneath the neural tissue. The mesodermal component bends to one side because of asymmetry late in the convergence and extension process. These explants converge and extend on nonadhesive substrates and do not require external substrates for traction. [Movie by Ray Keller]
- Movie S3
This movie shows the process of convergence and extension by mediolateral intercalation of mesodermal cells. The explant will converge in the horizontal aspect of the frame and extend in the vertical axis. The posterior part of the explant is out of sight off the top of the frame and is not free to move; therefore, the explant extends anteriorly off the bottom of the frame. The cells are epi-illuminated at a low angle to shadow the cells. Note that the cells are elongated and aligned mediolaterally and that they slowly shuffle between one another (intercalate) along the mediolateral axis to form a longer, narrower array. [Movie courtesy of John Shih]
- Movie S4
This movie shows that the presumptive notochordal (green) and somitic (red) mesodermal cells of the zebrafish embryo intercalate mediolaterally to form a narrower, longer array during convergence and extension. This movie was generated by a powerful new method of time-lapse confocal imaging in which cells can be tracked in three dimensions in space, and through time, in the clear zebrafish embryo. It is made like an ordinary time-lapse movie, but at each time interval, a “z-series” stack of multiple images through a volume is gathered. The three-dimensional movements of many cells can then be traced as they move through the embryo. Here, the cells are represented by colored spheres. [Movie courtesy of Richard Adams, Charles Kimmel, and Natalia Glickman; see the article by these authors in Development, in press]
- Movie S5
A time-lapse, laser-scanning fluorescence confocal movie shows somitic mesodermal cells polarized in the mediolateral axis. Several of the cells in the explant are labeled with a membrane-targeted GFP (green fluorescent protein) and imaged at two planes; the red image is a sectional view at the substrate, which in this case is fibronectin, and the green image is a sectional view at about 5  m above the substrate. Note that the protrusive activity of the cells is highly polarized, with aggressive filo-lamelliform protrusive activity concentrated at narrowed medial and lateral ends of the cells. These protrusions are thought to exert the traction on adjacent cells, and perhaps on the endogenous fibronectin substrate as well, to pull the cells between one another. In contrast, the elongated anterior and posterior aspects of the cells show less of this activity and more small, local, filiform extensions. These are thought to hold the cells together to form a stiff array. They are dynamic, a characteristic better seen in movie S6. [Movie courtesy of Lance Davidson]
- Movie S6
A time-lapse confocal movie of fluorescently labeled intercalating mesodermal cells shows the numerous, very dynamic, small filiform protrusions and contacts on their elongated anterior and posterior surfaces. The mediolateral axis runs from the upper left to lower right. These dynamic contacts are thought to allow cell intercalation while binding the cells together in an array that is stiff and can support the compression load essential for exerting a pushing force. [Movie courtesy of Lance Davidson]
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