Although there is some indication that in embryos inactive cells at the edge of the elongating group determine the direction of 'stacking', convergent extension can also occur in cells grown on a uniform surface and so appears to be an intrinsic property of cells once they start to elongate. Zajac's group wondered whether, rather than being under close genetic control, the process might be directed by cruder, mechanical forces acting between cells. Cells tend to stick together because of 'adhesion molecules' at their surfaces that act like a kind of Velcro. Cells acquire a kind of surface tension that pulls them together rather as two liquid droplets are pulled into one by forces acting at their surface. The coalescence of droplets by surface tension lowers their total energy and so stabilizes the liquid.
Surfaces cost energy, and a coalesced drop has less surface area than two separate ones.
Cells don't coalesce, but they do pull and deform each other in an effort to lower their total surface energy. Zajac's group assumed simply that elongated cells have different densities of adhesion molecules on their long and short sides, and so different surface tensions. This, they show, implies that a long side pressing up against a short side will have a different energetic cost from two short sides in contact, even though the contact area is the same for both. Consequently, elongated cells may tend to line up rather than remain in a disorderly jumble. When the researchers took into account the influence of the boundary surrounding the active cells, they found that, once aligned, stretched-out cells intercalate so as to expose more short sides than long ones at the boundary.