Fig. 1

The early development of skeletal muscle in vertebrates (based on rodent models). At the early embryonic stage, mature somites differentiate into the ventral sclerotome (SCL) and the dorsal dermomyotome (DM). Myogenic progenitors derived from the DM give rise to most of the skeletal musculature except craniofacial muscles which are formed from unsegmented mesoderm. Cells from the DM delaminate downward to form a layer of myotome, which is further divided as epaxial or hypaxial myotome depending on their location. Cells from the epaxial myotome give rise to the deep muscle on the dorsal surface, while cells from the hypaxial myotome develop into muscle in the ventral body wall and limbs. Primary myofibers are formed within the myotome and function as scaffolds for myogenesis at fetal and postnatal stages. In addition, Pax7⁺ fetal myoblasts and satellite cells are also formed at this stage which remain undifferentiated until the fetal stage. Later, fetal myoblasts expand rapidly and fuse with primary myofiber to construct fetal myofibers while satellite cells resident within the interstitial spaces of myofibers at the late stage of gestation. Most myofibers are formed at the prenatal stage and there is only growth in the size but not number of myofibers contributed by satellite cells after birth. Note: The arrows indicate the developmental relationships between myogenic progenitors and skeletal muscle at different locations. NTB: Neural tubes; NOTO: notochord