The myofibril is the structural and functional subunit of a muscle fiber. They are best seen at higher magnification in the light microscope in a cross section of the cell where they appear as dot-like structures. The overall effect is a stippled appearance of the cytoplasm. Each myofibril is composed of a bundle of myofilaments that are of two types. One type is the myosin II thick filament. The other is actin and its associated proteins that make up the thin filaments. It is the arrangement of the thick and thin filaments that produce density differences that in turn cre-ate the cross-striations of the myofibril when viewed in longitudinal section (see diagram). As evident in the diagram, the site of overlap of thin and thick filaments produces the dark A-band. The light appearing I-band contains the thin filaments. Careful examination of the A-band in the light microscope reveals a light staining area in the middle of the A-band. This is referred to as the H-band. It is an area that is occupied by thin filaments and is devoid of thick filaments. At the middle of each I-band is the thin dense Z-line to which the thin filaments are attached. The distance between Z-lines is referred to as a sarcomere. When a muscle contracts, the sarcomere and I-band shorten. The filaments, however, maintain a constant length, thus the contraction is produced by an increase in the overlap between the two filament types.
Skeletal muscle, human, H&E, ×512; inset ×985.
This micrograph reveals a cross section of a muscle fascicle. The individual muscle fibers (MF) exhibit a polygonal shape, but vary only slightly in width. Of the many nuclei that can be observed in this plane of section, only some belong to the muscle fibers. The muscle fiber nuclei (MFN) appear to be embedded within the extreme periphery of the fiber. In contrast, fibroblast nuclei (FN) that belong to the endomysium lie clearly outside of the muscle fiber, are typically smaller and exhibit greater density than the nuclei of
Skeletal muscle, human, H&E, ×512; inset ×985.
This micrograph, a longitudinal section of a gluteraldehyde fixed, plastic embedded specimen reveals four myofibers (M). Although they appear to be markedly different in width, the difference is due mainly to the plane of section through each of the fibers. Because the nuclei of the myofibers are located at the periphery of the cell, their location is variable when observed in a longitudinal section. For example, three nuclei (N) are seen in what appears to be a central location of a fiber. This is due to the section grazing the periphery of this fiber. The clear space at either end of two of these nuclei represents the cytoplasmic portion of the cell that contains organelles and is devoid of
Skeletal muscle, human, electron micrograph, ×5,000.
The low power electron micrograph shown here should be compared to the inset of the longitudinally sectioned myofibers above. It reveals portions of three myofibers (M), two of which exhibit a nucleus (N). Between cells, various amounts of collagenous fibers are present representing the endomy sium (E). The micrograph illustrates the banding pattern of the myofibrils to advantage. In contrast to the longitudinally sectioned muscle in the inset above, individual myofibrils (My) can be identified in this electron the muscle fibers. Also present between the muscle fibers are capillaries (C). The endothelial cell nuclei (ECN) are also relatively dense. Other nuclei that may be present, but are very difficult to identify belong to satellite cells. The inset which shows the boxed area reveals several nuclei, two of which belong to the muscle fibers (MF). The small, very dense nucleus (FN) probably belongs to a fibroblast of the endomysium. Also clearly evident here is a cross-sectioned capillary (C). The more striking feature at this magnification is the appearance of the muscle cells’ myofibrils which appear as the punctate or dot-like structures.