Length regulation of epithelial cell junctions

Morphogenesis of epithelial tissues involve precise spatiotemporal coordination of cell shape changes. In vivo, many morphogenetic events are driven by pulsatile contractions of intercellular junctions, producing irreversible deformations at tissue scales. The biophysical functions of these oscillatory contractions and the significance of their temporal structure remain unknown. Here, we combine theory and experiments to demonstrate that pulsatory contractions serve as a mechanical ratchet to guide directed morphogenesis. We propose a new theory for epithelial tissue mechanics that highlights two essential properties of intercellular junctions. First, epithelial junctions must overcome a critical strain threshold to trigger irreversible tension remodelling and junction length changes. Second, continuous relaxation of junctional strain promotes gradual loss of mechanical memory from the system, which makes the system refractory to continuous input, enabling frequency-dependent modulation of cell shape changes via junctional ratcheting. Taken together, the combination of mechanosensitive tension remodelling and strain relaxation provides a robust mechanism for directed tissue morphogenesis and the maintenance of tissue homeostasis in response to small amplitude contractions.