The neuromuscular junction
At an age of 9 weeks (begin of the fetal period) the first neuromuscular junctions appear on the newly created myotubes. With the fusing of several myotubes a syncytium is constructed in which a certain striation (i.e., ordered contractile elements) can be recognized. Soon the myotubes receive the first impulse from the motoneurons. This causes the muscle fibers to begin contracting.
Originally the myotubes are innervated by different nerves (polyneurally) on different portions of their surfaces (multiply). Only after birth the superfluous connections are undone and the innervation becomes mononeural.
Now the muscle fibers have attained their adult structure in that they are united through connective tissue sheaths to fascicles and the fascicles to muscles. These sheaths stem from fibroblasts that surround these muscle fibers. They form the internal and external perimysium as well as the epimysium. The endomysium stems from the muscle fibers themselves.
Adult musculature and its structures
With the differentiation to muscle fibers through the fusing of several myotubes the muscle cells lose their ability to divide. However, satellite cells remain as stem cells at the surface of the muscle fibers.