Skeletal Muscle, Dystrophy and Myopathy

Abstract

Skeletal muscle cells, commonly called muscle fibers, are multinucleated syncytia formed during development by fusion of mononucleated precursor cells, the myoblasts. They measure 10–100 μm in diameter and can have lengths from a few millimeters up to almost a meter. The hundreds of nuclei are located in the cells’ periphery close to the plasma membrane, whereas most of the cytoplasm is occupied by the longitudinally arrayed myofibrils composed of the myofilaments: thick filaments assembled from myosin II molecules containing rod-shaped segments and globular heads projecting out of the filaments, and thin filaments composed of actin. The contractile myofilaments myosin and actin are organized by specialized proteins of the “sarcomeric cytoskeleton,” which include alpha-actinin and myomesin, as well as the giant proteins nebulin, obscurin, and titin. They fulfill structural and mechanical functions and have roles in signaling. The electron micrograph shows part of the cytoplasm of a striated muscle fiber with parallel arrays of myofibrils. Mitochondria are lined in the small cytoplasmic cords between the myofibrils, where glycogen is accumulated. Profiles of the sarcoplasmic reticulum forming triads (arrows; cf. also Fig. 170) are visible.