A putative structural transition underlies reliable formation of skeletal muscle

During organ development cells undergo significant morphological and positional changes. Yet by the end of organogenesis, internal organ structure is typically reliably defined. It remains an open question how the internal structure of an organ emerges, particularly in the presence of dynamic boundary constraints. Here, we utilize live imaging and morphological measures of the developing zebrafish to unravel how early muscle organization materializes. We observed that myocytes undergo an ordered chiral twist according to their position within the myotome. Cell movement undergoes a rapid speed reduction once the cells span the myotome segment. Biophysical perturbation reveals cells are mechanically plastic and can adapt to changes in boundaries and environment. Combining these results, we propose that the early myotome may be undergoing a structural transition from a fluid-like to a solid-like state, which resembles a frozen system. Cell shape plasticity combined with structural changes in the material behavior could be a general mechanism for organ morphogenesis in dense tissues.