(A) and (B) The tissue surface tensiometer (TST) and calculation of surface tension. An aggregate (A), placed between two parallel plates, is deformed by moving the plates closer together at time zero (B) (i). The upper (UCP) and lower compression plate (LCP) are not to scale and are magnified for better illustration (panel A). The UCP is connected to an electrobalance (B) through a nickel-chromium wire (NCW). The balance ensures that the UCP’s position is kept constant, i.e., the deformation initiated on the aggregate by moving the LCP up (at *t*=0) is kept constant over time, and the change in force is recorded (B) (ii). For a detailed description of the working principle, see Materials and Methods. From the geometry of the aggregate (B) (i) and the force at equilibrium, i.e., the force plateau in panel (B) (ii), the surface tension can be calculated by application of the Young–Laplace equation (B) (iii). *R*_{1} and *R*_{2} are the two primary radii of curvature, at the aggregate’s equator and in a plane through its axis of symmetry, respectively. *R*_{3} is the radius of the contact circle with either compression plate. *H* is the distance between upper and lower compression plates (from ). (B) (iv) shows a zebrafish aggregate before compression (top), under compression (middle), and 1 sec after release from a 1.5 h-compression (bottom). The aggregate remains flattened right after release and only rounds up again slowly after force removal. (C) Surface tension is independent of aggregate size. (C) shows that surface tension values are independent of the size of the aggregate (volume) for the mesendodermal tissue. Data points are in green, and the mean surface tension is drawn as a dashed blue line. *r* denotes the correlation coefficient, whose low negative value shows that there exists a negligible negative correlation between aggregate volume and surface tension. This confirms that the calculated aggregate surface tension is a material property. (D) Surface tension values of the different zebrafish tissues. Error bars represent standard error of the mean σ values. Sample numbers for MZoep, Lefty, E-cadMO, and Cyclops represent the number of compressions performed for each data set and are 35, 38, 39, and 35, respectively. Statistical analysis was by ANOVA and Newman-Keul’s multiple comparisons test. Statistical difference (*p*<0.001) in σ-values was found between MZoep and MZoep+E-cadMO, MZoep and Cyclops, Lefty and MZoep+E-cadMO, and Lefty and Cyclops, respectively.

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