A pivotal event during gastrulation is mesoderm migration. In Drosophila embryos, presumptive mesoderm cells invaginate, undergo an epithelial-to-mesenchymal transition (EMT), and then spread over the ectoderm to form a monolayer. On , Murray and Saint investigate what types of cell rearrangements occur during this example of mesoderm spreading. They express photoactivatable GFP fused to α-Tubulin in all the cells of fly embryos, photoactivate sections of mesoderm or small numbers of mesodermal cells as gastrulation begins, and then follow the migration of these fluorescent cells over non-fluorescent ectodermal cells. The researchers find that those cells in contact with the ectoderm immediately after the EMT migrate dorsolaterally as a group, but are sometimes overtaken by cells not in contact with the ectoderm. Murray and Saint conclude that mesodermal cells use a combination of strategies to form a monolayer: directional dorsolateral migration (presumably towards chemoattractants expressed in the dorsal ectoderm), strong adhesion between mesodermal and ectodermal cells, and some intercalation during the final stages.
