Premyofibril model of myofibrillogenesis. Assembly begins at the edges of muscle cells with premyofibrils composed of minisarcomeres (bounded by alpha-actinin containing z-bodies) with muscle actin and actin-binding proteins and the nonmuscle isoform of myosin II. Muscle myosin and titin are added and z-bodies align in register to form nascent myofibrils with beaded Z-bands. As mature myofibrils form, the beaded Z-bands gradually become linear, nonmuscle myosin is lost, and myosin-binding proteins (C-protein and myomesin) are incorporated into aligned A-bands [5–8]. Diagram modified from Stout et al. [8].
