Elongation of the body axis during embryonic development is driven by physical forces that influence changes in cell shape. Signals must be coordinated between multiple tissues to enable proper morphogenesis. In Caenorhabditis elegans embryos, elongation involves waves of muscle contractions that trigger cytoskeleton reorganisation in the epidermis. How these signals are established and integrated across different tissues remains unclear. Here, Flora Llense, Michel Labouesse and colleagues use calcium imaging to visualise the pattern and intensity of muscle contractions in C. elegans embryos. This reveals that two key cells in each muscle quadrant of the embryo coordinate the waves of contractions. Ablation of these leader cells hinders muscle contraction and subsequently impairs embryo elongation. The authors then use an RNAi screen to identify proteins involved in triggering elongation in response to the muscle contractions. They find two innexin proteins (components of invertebrate cell-cell junctions) and two channel proteins that regulate this coordinated muscle activity and facilitate normal elongation. Unexpectedly, one of these key innexin proteins is expressed in the intestine rather than the muscle, uncovering a role for the intestine in coordinating morphogenesis. This work provides new insights into the regulation of embryonic muscle contractions before synapses have formed and uncovers pathways that facilitate signal transduction between mechanically coupled tissues during axial elongation.
