Mechanics of morphogenesis during cell sheet movements

We have been investigating the mechanics of dorsal closure, a stage of \textit{Drosophila} embryogenesis. Over 2-3 hours a ``hole'' in the dorsal surface changes its 2-D geometry from an ellipse to an eye shape, which eventually closes edge to edge. This hole initially is filled with a monolayer of amnioserosa cells, a transient tissue under tension. Beyond the dorsal hole are two flanks of epithelial tissue, also under tension, which are zipped together at each ``corner of the eye.'' The net result of dorsal closure is to form a continuous epithelium on the outer surface of the embryo. High-resolution, \textit{in vivo} images of amnioserosa cells will be presented. Experimental time series of apical shape changes have been assessed with the methods of signal analysis to quantify a band of reversible oscillations and a set of ingression processes. A generalized-force model was formulated to account for changes in cross-sectional areas. High-resolution, 3-D images of dorsal closure also will be presented. The amnioserosa was observed to bulge outwards, where the asymmetric dome was analyzed with Laplace's formula to quantify the turgor pressure. The 3-D zipping process includes substantial remodeling of tissue interfaces and significant intracellular remodeling.