Breaking the dogma on C. elegans gonad morphogenesis by a combination of biophysical modelling, genetic manipulations, and live fluorescence microscopy

Gonad development in C. elegans is an established model for organogenesis. While many of the genes involved have been discovered, the cellular and mechanobiological aspects underlying gonad elongation and turning remain poorly understood. Traditionally, it has been assumed that the somatic distal tip cell (DTC) actively migrates as a leader cell and that germ cells follow in its path. However, a mechanism for DTC propulsion has not been identified. Here, we used live-imaging, laser ablations, DTC-specific genetic manipulations, and a qualitative physical model, to show that the gonad does not elongates by a pulling force from the leader cell, but rather due to a pushing force generated by the proliferating germ cells, which are confined by a basement membrane behind the DTC. Local release of matrix-degrading metalloproteases by the DTC determines the direction of gonad elongation. How the DTC turns and the gonad folds into its typical U-shape has been a long-standing question in the field. We show that the DTC concentrates integrin-mediated cell-matrix adhesions on the inside of the turn and that this temporally controlled asymmetric adhesion, coupled with pushing forces from the back, generates a bending moment, as predicted by our model. Genetic perturbations that interfere with adhesion polarity lead to turning defects, including no turn and reversal of turn direction. Our findings challenge the dogma regarding DTC migration and offer a novel framework to understand normal tissue morphogenesis as well as cancer metastasis, in cases where cells are confined by a basement membrane and perform directed invasion.