Figure 1.

Summary of steps involved in myocyte differentiation and myofibrillogenesis, with regards to actin dynamics. (a) Proliferating myoblasts are derived from determined myotome cells and possess an unspecialized actin cytoskeleton (grey lines). (b) As differentiation begins, these cells aggregate, characterized by the formation of localized stress fibers in a cortical actin wall (insert). Contractile function of these fibers is provided by nonmuscle myosin (NMM, arrow). (c) Myoblasts align themselves concurrent to substrate attachment and the elaboration of protocostameres (arrow). The stress-fiber-like cortical actin and NMM will form the premyofibril templates for subsequent myofibril assembly. (d) Fusion occurs, resulting in the formation of multinucleated myotubes. Myofibrils begin to form at the cell periphery, centered on costamere attachment points (arrow), constructed from premyofibril templates (insert). Premyofibrils consist of alternating bands of membrane-associated α-actinin (circles) and NMM (arrow). (e) As the myocyte matures, additional myofibrils will fill all available space, interconnected with one another and with organelles by desmin intermediate filaments. New myoblasts will continue to fuse to the terminal ends of the myotube to create a growing myofiber. Artwork by A. Pete.