Invagination of the epithelial cell sheet of the prospective mesoderm in Drosophila gastrulation is a well-studied, relatively simple morphogenetic event that results from dynamic cell shape changes and cell movements. However, these cell behaviors have not been followed at a sufficiently short time resolution. We examined mesoderm invagination in living wild-type embryos by real-time imaging of fluorescently labeled cell-cell adherens junctions, which are located at the apical zones of cell-cell contact. Low-light fluorescence video microscopy directly visualized the onset and progression of invagination. In an initial period of approximately 2 minutes, cells around the ventral midline reduced their apical surface areas slowly in a rather synchronous manner. Next, the central and more lateral cells stochastically accelerated or initiated their apical constriction, giving rise to random arrangements of cells with small and relatively large apices. Thus, we found that mesoderm invagination began with slow synchronous and subsequent fast stochastic phases of cell apex constriction. Furthermore, we showed that the mesoderm invagination of folded gastrulation mutant embryos lacked the normal two constriction phases, and instead began with asynchronous, feeble cell shape changes. Our observations suggested that Folded gastrulation-mediated signaling enabled synchronous activation of the contractile cortex, causing competition among the individual mesodermal cells for apical constriction. Movies available on-line: http://www.biologists.com/JCS/movies/jcs2073.html

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