(A) Flow patterns observed in a liquid contained in a circular domain in response to a locally applied force (the red arrow). Although the force is applied locally, the fluid moves everywhere as result of viscous interactions. In biological tissues these long range interactions could be mediated by adhesive interactions between cells or interactions of the cells with the matrix. (B) Schematic cross section through an early embryo. Epiblast and hypoblast cells are in contact with a complex basal lamina through specific lamina integrin interactions (green double arrows). In principle basally extended lamellipodia can get traction from this matrix, through coupling to integrins to the internal actin cytoskeleton, as long as the matrix is stationary with respect to the cells. If the cells cannot get traction from the matrix, they can only get traction from each other either through protrusions that make cell-cell contacts or through directional relaxation and contraction of the actin cables associated with the adhesive junctional complexes (red double arrows). (C) Scheme outlining how retraction of specific junctions (red) in one direction (blue arrows) followed by relaxation of these junctions is a direction perpendicular to the original direction of contraction (blue arrows) results in a tissue deformation and reorientation. This process involves rosette formation as an intermediate stage.

(A) Overview image of a 3 h incubated (EG XII) embryo showing the hypoblast cells stained in green (as detected with a HNK1 antibody, which recognizes a cell surface carbohydrate epitope strongly expressed on hypoblast cells). The anterior posterior axis of the embryo is oriented vertically with posterior pole of the embryo located in the bottom of the image. (B) High magnification image of the epiblast showing the cell boundaries of the epiblast cells in green [phalloidin staining of filamentous actin, mainly associated with the apical adherens junctions, see Fig. 2C] and the nuclei of these cells in red (Propidium-iodide staining of DNA). Note the presence of four mitotic cells indicated by white arrows. Cells in mitosis round off, retract, in between other cells in the epiblast and in doing so pull the apical sides of adjacent cells into typical rosette structures, where more than three neighboring cells meet. (C) Section through the embryo shown in (B), showing individual epiblast cells transfected with GFP. Note the tall shape of the epiblast cells connecting overlaying a discontinuous layer of hypoblast cells. (D) Cell flow patterns observed by tracing individual GFP transfected cells in the epiblast over the course of 10 h. Movement direction is from yellow to green. The green part of the track is the movement recorded over the last hour of the experiment.