Figure: Three-dimensional rendering of myofibrils in growing Drosophila flight muscles enabled the
automatic tracking of sarcomeres: each sarcomere is bordered by two Z-discs (red dots on
the grey sticks) with myosin filament stacks (blue spheres) located in between the Z-discs
(yellow spheres).
Muscle cells can be several centimetres long, connecting two neighbouring skeletal elements. The force-producing sarcomeres on the other hand have a stereotypic length of only 2-3 µm. Thus, thousands of sarcomeres are arrayed in myofibril chains, which span across the long cells in adult animals. Since all animals start off small, their embryonic muscle cells are short and contain only few sarcomeres. The same is true for the juvenile mammalian heart. Muscles and in particular the heart are under continuous forces and never stop beating. How are new sarcomeres inserted into contracting myofibrils without breaking them? This question was studied by Clement Rodier and colleagues at IBDM in Marseille using the developing Drosophila muscles as model. In collaboration with Benjamin Friedrich’s group at TU Dresden, Clement was able to automatically measure the length of the individual sarcomeres in growing flight muscles (see image). This revealed a surprising class of “too-long” sarcomeres, containing not 1 but 2 myosin filament stacks in its centre, suggesting an ongoing division of the myosin filament stack. Live imaging of muscle growth in vivo confirmed that one sarcomere indeed divides into two daughters by tension-induced sliding of the myosin filament stack into two daughter stacks. This establishes of a new Z-disc in the middle, linking the 2 daughter sarcomeres. Thus, muscle length growth induces the division of one mother sarcomere into two daughters, while maintaining muscle contractility. The results are now featured on the cover of Science Advances.
Rodier C, Estabrook ID, Chan EH, Rice G, Loreau V, Raunser S, Görlich D, Friedrich BM, Schnorrer F. Muscle growth by sarcomere divisions. Sci Adv. 2025 Jul 11;11(28):eadw9445.
