section_path: Development
section_name: Development
paragraph_text: Development
  
  All muscles are derived from paraxial mesoderm. During embryonic development in the process of somitogenesis the paraxial mesoderm is divided along the embryo 's length to form somites, corresponding to the segmentation of the body most obviously seen in the vertebral column. Each somite has three divisions, sclerotome (which forms vertebrae), dermatome (which forms skin), and myotome (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form epaxial and hypaxial muscles, respectively. The only epaxial muscles in humans are the erector spinae and small vertebral muscles, and are innervated by the dorsal rami of the spinal nerves. All other muscles, including those of the limbs are hypaxial, and innervated by the ventral rami of the spinal nerves.
  
  During development, myoblasts (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation of connective tissue frameworks, usually formed from the somatic lateral plate mesoderm. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongated multinucleated skeletal muscle cells.
  
  Between the tenth and the eighteenth weeks of gestation, all muscle cells have fast myosin heavy chains; two myotube types become distinguished in the developing fetus – both expressing fast chains but one expressing fast and slow chains. Between 10 and 40 per cent of the fibers express the slow myosin chain.
  
  Fiber types are established during embryonic development and are remodelled later in the adult by neural and hormonal influences. The population of satellite cells present underneath the basal lamina is necessary for the postnatal development of muscle cells.