Tissue morphogenesis is driven by cell contractility transmitted to other cells or the extracellular matrix (ECM). The ECM can also sculpt developing tissues, raising the question of how the physical properties of cells and the ECM are integrated during development. Magali Suzanne and colleagues now present the eversion of the Drosophila leg disc as a powerful system for interrogating morphogenetic mechanics. The leg disc comprises two epithelia – one that will give rise to the leg itself, and an overlying peripodial epithelium (PE) sitting on a basement membrane ECM. Time-lapse analysis of cultured leg discs reveals that, during the process of eversion, the PE undergoes a stereotyped sequence of elongation, opening and withdrawal from the leg epithelium. A dramatic reorganisation of myosin and an increase in tension precedes PE opening, and the ECM is concomitantly extensively remodelled, progressively deteriorating as tension in the PE increases. Dual live imaging of the PE and the underlying matrix indicates that the two tissues become uncoupled during leg elongation, an observation confirmed by electron microscopy. This physical uncoupling reflects a functional uncoupling: preventing matrix degradation does not prevent PE opening. The morphogenesis of the PE is thus defined by increased tissue tension and cell-ECM uncoupling.
