Directionality Of Developing Skeletal Muscles Is Set By Mechanical Forces

Formation of oriented myofibrils is a key event in the development of a functional musculoskeletal system. However, the mechanisms that control orientation of myocytes, their fusion and the resulting directionality of adult muscles remain enigmatic. Here, we utilized in vivo and in vitro live imaging, CAS9/CRISPR-mediated mutagenesis in fish, genetic experiments in mice and single cell transcriptomics to demonstrate that individual myocyte polarization and subsequent orientation depend on cell stretch imposed by skeletal expansion. Our data revealed that upon migration, individual facial myocytes form unpolarized clusters corresponding to future muscle groups. These clusters undergo oriented stretch and alignment during embryonic growth. Experimental in vivo perturbations of cartilage shape, size and distribution caused disruptions in directionality and number of myofibrils. Controlled in vitro 2D and 3D experiments applying continuous tension via artificial attachment points demonstrated a sufficiency for mechanical forces to instruct coherent polarization of myocyte populations. Consistently, perturbations of cartilage extension revealed a role of the developing skeleton in the directional outgrowth of non-muscle soft tissues during limb and facial morphogenesis. We performed bulk RNA seq to confirm single cell transcriptomic analysis of cells collected daily from soft and hard culture conditions.