Cell shape change analysis during <i>Drosophila</i> axis extension.

<p>(A) Scanning electron microscopy micrographs (from Flybase, [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002292#pbio.1002292.ref036" target="_blank">36</a>]) showing lateral views of gastrulating <i>Drosophila</i> embryos at stage six, seven, and eight. Anterior is to the left. The part of the germband undergoing convergent extension is labelled in purple and the main direction of extension indicated with a yellow arrow. The following landmarks or morphogenetic events are indicated: PC, pole cells; CF, cephalic furrow; ATF, anterior transverse furrow; PTF, posterior transverse furrow; PMI, posterior midgut invagination (also called posterior endoderm invagination). The box represents the ventral surface that we optically sectioned by confocal microscopy. (A’) Scanning electron microscopy micrograph showing a ventral view of a gastrulating embryo at stage eight, with the approximate position of the anterior and posterior field of views that we analyzed. Both views are bissected by the ventral furrow (VF) through which the mesoderm invaginates. (B) Schematics representing the small neighborhood of cells considered by the tracking algorithms. Cell shape changes (strain rates) are calculated by comparing two timepoints before and after a given time. A strain rate is the ratio of the change in length to the original length, divided by the time interval, with units of proportion per minute (pp/min). The cell shape change is represented here by two orthogonal vectors, showing elongation in one direction (blue, positive) and shrinkage in the perpendicular direction (red, negative). (C, C’) Example frames of movies of wild-type embryos at t = 10 min after GBE onset labeled with <i>ubi-DE-cad-GFP</i>, showing an anterior (C, wtLB009) and a posterior view (C’ wtCL051010). (C) For anterior views, the cephalic furrow (arrow) is used as an anterior landmark, and the scale shows distance from this landmark. The purple shading shows the region removed from the analysis, where cells stretch behind the cephalic furrow. (C’) For posterior views, the posterior-most edge of the embryo seen in the confocal stack is used as posterior landmark (arrow), with distance from it indicated in the scale. (D, D’) Outcome of tracking for anterior (D) and posterior views (D’), showing the polygons describing the cell outlines and the cell centroids from which are drawn the tracks giving the cell positions for the previous 2.5 min (5 timepoints). The tracks shown are track retained for the analysis, after removing tracks from mesodermal cells, mesectoderm cells, and for anterior views, from cells deformed by the cephalic furrow (corresponding to purple region in C). (E, E’) Spatiotemporal heat maps summarizing AP cell length change contributing to GBE, over the first 30 min of GBE (<i>y</i>-axis) and as a function of cell position in the AP axis (<i>x</i>-axis), for anterior (E) and posterior views (E’), averaged for five and four embryos, respectively. (F, F’) Graphs summarizing AP cell length change as a function of time for the first 30 min of GBE, for anterior (F) and posterior views (F’), averaged for five and four embryos, respectively. The ribbon’s width shows the standard error (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002292#sec009" target="_blank">Materials and Methods</a>). Data associated with this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002292#pbio.1002292.s001" target="_blank">S1 Data</a>.