This portion of a muscle fiber represents longitudinal sections of myofibrils #1, #2, #3 with numerous mitochondria #1, #2, #3 between them; there are also cisterns of the sarcoplasmic reticulum. The myofibrils also include numerous glycogen granules. Any myofibril distinctly consists of sarcomeres that are structural units between the neigh boring Z lines #1, #2. Dark bands (anisotropic or A bands) are significantly more abundant than pale bands #1, #2 (isotropic or I bands); this means that the myofibrils are contracted.

This is a fragment of two muscle fibers #1, #2 consisting of closely packed myofibrils #1, #2, #3. The myofibrils form strictly parallel rows. Dark (anisotropic or A) and pale (isotropic or I) bands of the myofibrils match the same structures of neighboring myofibrils precisely, so that the entire fiber is distinctly striated.

From the outside, muscle fibers are covered by a basement membrane (basal lamina) formed by an amorphous substance and fine collagen (or reticular) fibrils. Thus, a connection is provided between neighboring muscle fibers as well as tendons. Myosatellite cells are found in myosyncytial plasma membrane pits #1, #2 under the basement membrane. Their elongated nuclei are rich in electron-dense heterochromatin #1, #2 and are surrounded by thin rims of cytoplasm with few organelles. The cells represent cambial store for skeletal muscles. They are activated in case of muscle fiber damage. The sarcoplasm in a muscle fiber has numerous myofibrils.