Formation of skeletal muscle is called myogenesis. Precursor cells called myoblasts originate in the somitic mesoderm. Limb and abdominal muscles develop from myoblasts migrating out of somites, whereas back muscles develop from nonmigrating myoblasts. Multinucleated skeletal muscle cells are formed from fusion of mononu-cleated myoblasts into myotubes. Subsequent synthesis of contractile myofibrils and organization into sarcomeres within myotubes result in maturation into myofibers. Myogenesis occurs in a primary wave during embryonic development followed by a secondary wave during early fetal developments. Primary and secondary fibers are predisposed to form slow and fast contraction fibers, respectively. Innervation occurs concurrently with maturation of muscle fibers and subsequently plays an important role in survival and determination of myofiber type. Groups of myofibers separate into individual muscles surrounded by connective tissue as development continues. Myofiber number becomes fixed near birth, although additional myonuclei are added as the fibers enlarge. Nuclei are added to existing fibers by fusion of additional myoblasts called satellite cells. Myofibers grow in diameter by adding new circumferential contractile filaments and grow in length by adding new sarcomeres to the end of existing filaments. Postnatal development of contractile and metabolic properties involves sequential replacement of many fiber-type specific proteins within existing myofibers.
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