Apical extracellular matrix regulates fold morphogenesis in the Drosophila wing disc

Inside every animal, from the smallest insect to the largest mammal, complex organs are built from sheets of cells that bend, twist, and fold into intricate shapes. This process of tissue folding is not merely a matter of cells moving around; it is a mechanical feat where layers of living material must curve and hold their form to create functional structures like lungs, brains, or wings. For these folds to work, they must be stable enough to maintain their shape during growth, yet flexible enough to change when the time comes to transform into a final adult form. Understanding how nature achieves this balance—how a soft, living sheet can be held in a precise curve and then released to flatten out again—is a central question in developmental biology. It reveals the physical rules that govern how life builds itself, moving from simple layers into the complex, three-dimensional organs that keep us alive.

Researchers have turned their attention to the developing wing of a fruit fly to uncover the mechanics behind this phenomenon. The fruit fly's wing begins as a flat, hollow sac of cells called a wing disc. As the larva grows, this sac does not remain flat; instead, it develops deep, narrow grooves that fold the tissue inward. These folds are essential, but they must eventually disappear. When the larva is ready to become a pupa, the tissue must unfold and remodel itself into a thin, bilayered sheet that will eventually harden into the adult wing. The mystery lay in understanding what holds these folds in place and what triggers them to let go. To solve this, scientists created a detailed three-dimensional map of the inner surface of the wing disc, tracking its shape from the larval stages when the folds are growing, all the way to the early pupal stage when the tissue begins to unfold.

By measuring the depth and width of these folds on the curved surface of the tissue, the researchers discovered a hidden structural element holding the folds together. On the inner, or apical, side of the cells, they found a network of fibrous material known as the apical extracellular matrix. This matrix acts like a physical bridge, connecting the two opposing walls of each fold. It is this connection that stabilizes the curve, preventing the tissue from collapsing or flattening prematurely. To test how this material functions, the team built a computer model of the folding tissue. In this simulation, they represented the tissue as a collection of cells and added a sticky layer to mimic the extracellular matrix. The model predicted that for the tissue to unfold, this connecting layer must be removed first.

The team then tested this prediction in living flies. When they genetically altered the flies so that this fibrous matrix could not form, the folds became unstable and took on abnormal shapes. Conversely, when they prevented the removal of this matrix at the time the fly should have been unfolding, the tissue remained stuck in its folded state and failed to remodel. These experiments confirmed that the presence of the matrix keeps the folds stable during growth, while its removal is the necessary trigger that allows the tissue to flatten out. The consequences of getting this mechanical process wrong extend beyond the larval stage; flies with these disruptions in the matrix developed adult wings with visible deformities, proving that the precise timing of this material's assembly and removal is critical for the final shape of the organ.

This work establishes that the extracellular matrix is not just a passive filler between cells, but an active mechanical regulator that dictates the shape and stability of developing tissues. It shows that the transition from a folded larval structure to a flat adult organ relies on a specific mechanical switch: the construction of a stabilizing bridge to hold the shape, followed by its deliberate dismantling to allow for change. By identifying this mechanism, the study provides a clear explanation of how epithelial tissues can be mechanically stabilized during development, offering a concrete example of how physical connections between cells drive the complex architecture of living organisms.