Cell adhesion and cortex tension are known to regulate cell–cell contact formation. Heisenberg, Paluch and colleagues now analyse the contributions of cell adhesion and cortex tension in contact formation and sorting of zebrafish progenitor cells (Sciencehttp://doi.org/jcp; 2012).

Building on previously published models of cell–cell adhesion and sorting, the authors developed a theoretical description of the shape of two progenitor cells adhering to each other. They then used dual micropipette aspiration assays to separate adhering progenitor cells from zebrafish embryos ex vivo and determined that cortex tension controls interfacial tension at the cell–cell contact, and thereby regulates cell–cell contact expansion. In contrast, cell adhesion was not involved in determining cell–cell contact size. Instead, the authors demonstrated that following mechanical separation of adhering cells, the linkage of cadherin to the actin cytoskeleton was crucial in limiting the mechanical resistance of adhesive bonds to pulling forces. The cytoskeletal anchoring of cadherins was further shown to be important for correct progenitor cell sorting within cell aggregates in vitro, and also during the segregation of progenitor cells in gastrulating zebrafish embryos in vivo. Thus, by combining theoretical, biophysical and live imaging experiments, the authors showed that cell adhesion is necessary to mechanically couple the cortices of adhering cells to support cell sorting.