Fig. 2.

The assembly of myofibrils. a Formation of premyofibrils. Proteins assemble into structures known as premyofibrils, which characteristically contain α-actinin along the periphery of the cell, and nonmuscle myosin (NMHC) scattered between the actin. b Formation of nascent myofibrils. Muscle-specific myosin II isoforms and stabilizing proteins become incorporated into the myofibrils, with the myosin heavy-chain (MHC) proteins replacing NMHC. Titin is also expressed in the myofibril at this stage. c Formation of mature myofibrils. Nascent myofibrils fuse to form mature myofibrils, forming a highly organized sarcomeric structure