Two tissue movements - convergence and extension - are essential for axial morphogenesis in vertebrate and invertebrate embryos. But what generates the tensile forces that drive the intercalation of cells that underlies these two movements? On p. 2435, Skoglund and colleagues report that in Xenopus laevis embryos, convergence and extension at gastrulation require a myosin IIB-dependent cortical actin network. Using morpholino knockdown, they show that myosin IIB (a cytoskeletal myosin that crosslinks actin filaments and acts as a molecular motor) is needed during gastrulation to maintain a stereotypical cortical actin cytoskeleton. This network is polarized relative to the embryonic axis, the researchers report, and cyclically lengthens and shortens during gastrulation. Depletion of myosin IIB also results in the loss of the polarized protrusive activity usually seen in intercalating cells, the loss of cell-cell and cell-matrix adhesion, and failure of blastopore closure. Together, these findings reveal how a molecular-scale motor protein can generate the tensile forces that drive tissue-scale embryonic morphogenesis.
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