Like the skeleton, the body's soft tissues are pulled and moulded into shape by forces akin to those that determine the shapes of liquid droplets, researchers announce in Physical Review Letters 1 . Mark Zajac and colleagues from the University of Notre Dame in Indiana considered the process of 'convergent extension'. This sequence of cell rearrangements occurs during embryo growth for many different animals, including fruit flies, sea urchins and frogs. It takes place in tissues made up of cells sitting side by side like bubbles in foam.
Convergent extension has three stages.
Elongated cells then creep over one another to become intercalated, like playing cards pushed into a stack. This shortens the tissue in the direction of cell elongation, but lengthens it in the perpendicular direction, along the axis of the 'stack'. So the tissue extends at right angles to the direction of cell elongation. But what triggers the initial deformation of the cells, or what controls the way they reshuffle is still a mystery. 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.