For Heisenberg, “The big question is what factors are important in cell sorting and tissue organization during development.” Differential adhesion among cells has been the most prominent hypothesis. Differences in cell cortex tension, which is produced by actomyosin contraction, have never been carefully measured before.
Using an atomic force microscopy probe, the authors measured the resistance to deformation in cells from each layer of the embryo. They found that tension was highest in the outermost layer, the ectoderm, followed by the endoderm and finally the mesoderm. By inhibiting myosin, the authors reduced ectoderm tension to match that of the other cell types. Ectoderm tension was also reduced when cells were treated with a growth factor that diverted them to a nonectodermal fate.
Cells sorted according to their tension rather than adhesion levels. When mixed in vitro, ectoderm cells partitioned to the inside, surrounded by meso- and endoderm cells. This inside-out arrangement is probably due to the absence of yolk cells and other in vivo factors that interact preferentially with each layer. Treatment with actomyosin inhibitors, which did not change adhesion, prevented ectoderm cells from burrowing inwards, indicating that cells rely on tension differences for sorting.
“In textbooks, you will read that differential intercellular adhesion of cells is sufficient to explain their sorting behavior,” says Heisenberg. “But it's not just the adhesion that's doing it. It's a combination of several factors, including adhesion and tension.”