Myotubes and muscle fibres are bundled by connective tissue into fascicles. Several fascicles become grouped together to form muscles. Until week 16, most muscle cells are still myotubes, to which further myoblasts become attached. At week 18, most of the muscle fibres look similar to those of the adult. Most myotubes disappear by the end of month 7.
During the late fetal period, the final population of myoblasts appears. These become satellite cells in the adult muscle. The cells are located outside the sarcolemma but inside the basal lamina of the muscle fibre and are usually inactive. However, following intensive exercise or in the case of muscular injuries, they start to divide and to differentiate; they serve as a reservoir for muscle regeneration.