Myogenesis involves three populations of precursor cells, embryonic and fetal myoblasts and postnatal satellite cells, that appear sequentially during development (Fig. 1). Embryonic myoblasts undergo extensive proliferation at the presumptive location of muscles, and then fuse into primary myofibers. Fetal myoblasts form secondary fibers and add nuclei to growing primary myofibers. Satellite cells lie beneath the basal lamina of myofibers, contribute DNA to growing myofibers, and serve as a precursor pool for muscle repair following injury.

Myoblast differentiation is accompanied by cell-cycle withdrawal followed by fusion to form myotubes with central nuclei. Contractile protein accumulation, displacement of nuclei to the periphery, and innervation result in maturation of myofibers. Embryonic myoblasts differentiate to form primary muscle fibers in early gestation, before individual muscles can be discerned. Fetal myoblasts use the surface of primary myofibers as a scaffold to align and form secondary fibers. In mammals, all primary fibers are initially slow fibers with some becoming fast fibers in fast twitch muscles. Most secondary fibers are initially fast fibers. Since there are 5 to 20 times more secondary than primary fibers, this gives rise to a common histological pattern of a small number of slow fibers surrounded by a larger number of fast fibers. The majority of myofiber formation is completed by the third trimester of development in most mammalian species. In birds, individual embryonic myoblasts are committed before fusion to forming slow, fast, or mixed primary fibers. Avian secondary fibers may also be slow, fast, or mixed. Satellite cells, which represent approximately 30% of muscle nuclei in neonates and approximately 4% in adults, do not express fiber-type characteristics until fusion with a myofiber. Postnatal muscle has a limited capacity to generate new fibers from satellite cells after injury, although a few new fibers may form one to two months after birth.

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