Lineage segregation during gastrulation has long been thought to be driven by differential cell adhesion and cortical tension among cells, which would together lead to a differential tissue surface tension (TST) and the spatial segregation of specific cell types. However, this long-standing hypothesis is mainly based on in vitro work, and it is as yet unclear whether it holds true in vivo. Now, on p. 1798 Carl-Philipp Heisenberg and colleagues assess the role of differential TST in lineage segregation and find that, contrary to in vitro work, differential TST is insufficient to explain progenitor cell segregation and germ layer formation within the in vivo gastrulating zebrafish embryo. In the study, the authors describe their unique version of video force microscopy called 3D CellFit, which allows them to analyse surface tensions in 3D within a living organism. Using this method, the authors show that ectoderm and mesoderm tissues do not, in fact, exhibit differential TST in the gastrula. They further present evidence that the apparent discrepancy between the in vitro and in vivo results is due to a difference in osmolarity between the culture medium and the interstitial fluid that surround the cells. Finally, by inhibiting the function of the small GTPase Rac, a key regulator of protrusion-driven cell migration, the authors show that directed cell migration, rather than differential TST, provides the major mechanism that determines the segregation of the germ layer progenitors.
- © 2017. Published by The Company of Biologists Ltd