Abstract
The morphogenesis of tissues, like the deformation of an object, results from the interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognized, whereas the importance of tissue material properties, in particular stiffness, has received much less attention. Using Caenorhabditis elegans as a model, we examine how both aspects contribute to embryonic elongation. Measuring the opening shape of the epidermal actin cortex after laser nano-ablation, we assess the spatiotemporal changes of actomyosin-dependent force and stiffness along the antero-posterior and dorso-ventral axis. Experimental data and analytical modelling show that myosin-II-dependent force anisotropy within the lateral epidermis, and stiffness anisotropy within the fiber-reinforced dorso-ventral epidermis are critical in driving embryonic elongation. Together, our results establish a quantitative link between cortical tension, material properties and morphogenesis of an entire embryo.
Highlights
Morphogenesis and organ formation rely on force distribution and tissue material properties, which are often heterogeneous and evolve over time
We developed a novel analysis method to derive mechanical stress, based on the equilibrium shape of a thin cut in an infinite elastic isotropic plane, subjected to biaxial loading(Theocaris, 1986)
Our results strongly suggest that the stress anisotropy correlates with morphological changes. We found that both seam and DV epidermal cells contribute to the changes in embryo diameter, irrespective of their level of active myosin II
Summary
Morphogenesis and organ formation rely on force distribution and tissue material properties, which are often heterogeneous and evolve over time. The embryo evolves from a lima-bean to a typical cylindrical shape with a four-fold increase in length, without cell migration, cell division, or a notable change in embryonic volume (Sulston, 1983, Priess and Hirsh, 1986) (figure 1a) This process requires the epidermal actomyosin cytoskeleton, which acts mostly in the lateral epidermis ( called seam cells), while the dorso-ventral (DV) epidermal cells may remain passive (Appendix 1)(Wissmann, 1997, Wissmann, 1999, Shelton, 1999, Piekny, 2003, Diogon, 2007, Gally, 2009, Chan, 2015, Vuong-Brender, 2016). Our data and modelling highlight that the distribution of forces in the seam cells and the stiffness in the DV epidermis must be polarized along the circumferential axis (or DV axis) to drive elongation This supplementary gives the proofs of the various relations used in the main paper to extract the residual stresses in the epithelial cells. Aiming to estimate residual stresses from the shape aperture, we identify the correspondence between the linear elastic coefficients of anisotropic elasticity and the parameters of a fiber model, at low strains
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