No cell is an island, especially when it's embedded in an epithelial layer of a fruit fly embryo. As Martin et al. reveal, forces transmitted among surrounding cells restrict how the cell can change shape during development.
The bending, twisting, and folding of embryonic development often entails changes in cell shape. A column-shaped cell can compress its upper section, becoming a wedge or cone—a process called apical constriction. When neighboring cells perform this maneuver in concert, epithelial tissue folds or buckles. Apical constriction helps relocate future mesoderm cells during gastrulation in Drosophila, for example. However, apical constriction occurs predominantly in the ventral–lateral (side-to-side) direction, not the anterior–posterior (front-to-back) direction. It's possible that the cell contracts in just one dimension or that the surrounding tissue exerts a force that limits the cell to constricting ventro-laterally.
Martin et al. found evidence for the second alternative when they observed Drosophila mesoderm tissue that normally folds to form a structure called the ventral furrow. Extending throughout the tissue is a mesh of actin and myosin that travels from one cell to another via adherens junctions. The researchers weakened intercellular connections by reducing the number of these junctions. Cells pulled apart, and the tissue often tore along the anterior–posterior axis. After such tears, which appear to release epithelial tension, cells began to constrict in both directions, not just side-to-side. The results suggest that tension in the tissue, which spreads through the actin-myosin network, normally prevents apical constriction from occurring in both directions.
