Cross-striations are evident in H&E–stained preparations of longitudinal sections of muscle fibers. They may also be seen in unstained preparations of living muscle fibers examined with a phase contrast or polarizing microscope, in which they appear as alternating light and dark bands. These bands are termed the A band and the I band (see Fig. 11.4).
In polarizing microscopy, the dark bands are birefringent (i.e., they alter the polarized light in two planes). Therefore, the dark bands, being doubly refractive, are anisotropic and are given the name A band. The light bands are monorefringent (i.e., they do not alter the plane of polarized light). Therefore, they are isotropic and are given the name I band.
Both the A and I bands are bisected by narrow regions of contrasting density (see Fig. 11.4). The light I band is bisected by a dense line, the Z line, also called the Z disc [Ger. Zwischenscheibe, between discs]. The dark A band is bisected by a less dense, or light, region called the H band [Ger. Hell, light]. Furthermore, bisecting the light H band is a narrow dense line called the M line [Ger. Mitte, middle]. The M line is best demonstrated in electron micrographs (Fig. 11.5), although in ideal H&E preparations it can be detected in the light microscope.
As noted above, the cross-banding pattern of striated muscle is caused by the arrangement of the two kinds of myofilaments. To understand the mechanism of contraction, this banding pattern must be considered in functional terms.
muscle fascicle (composed of muscle fibers) myofibril (composed of myofilaments) muscle fiber (composed of myofibrils) sarcomere Z lineZ line
FIGURE 11.4 • Organization of a skeletal muscle. A skeletal muscle consists of bundles of muscle fibers called fascicles. In turn, each fascicle consists of a bundle of elongate muscle fibers (cells). The muscle fiber represents a collection of longitudinal units, the myofibrils, which in turn are composed of myofilaments of two types: thick (myosin) filaments and thin (actin) filaments. The myofilaments are organized in a specific manner that imparts a cross-striated appearance to the myofibril and to the fiber. The functional unit of the myofibril is the sarcomere; it extends in both directions from one Z line to the next Z line. The A band marks the extent of the myosin filaments. Actin filaments extend from the Z line into the region of the A band, where they interdigitate with the myosin filaments as shown. The cross sections through different regions of the sarcomere are also shown (from left to right): Through thin filaments of the I band; through thick filaments of the H band; through the center of the A band, where adjacent thick filaments are linked to form the M line; and through the A band, where thin and thick filaments overlap. Note that each thick filament is within the center of a hexagonal array of thin filaments.
FIGURE 11.5 • Electron micrograph of skeletal muscle fiber. This low-magnification electron micrograph shows the general organization of skeletal muscle fibers. Small portions of three muscle fibers in longitudinal profile are included in this micrograph. The muscle fiber on the right reveals a nucleus at its periphery. Two fibers—one in the middle and another on the left—exhibit regular profiles of myofibrils separated by a thin layer of surrounding sarcoplasm (Sr). Each repeating part of the myofibril between adjacent Z lines is a sarcomere (S). The cross-banded pattern visible on this micrograph reflects the arrangement, in register, of the individual myofibrils (M); a similar pattern found in the myofibril reflects the arrangement of myofilaments. The detailed features of a sarcomere are shown at higher magnification in Figure 11.7a. The presence of the connective tissue in the extracellular space between the fibers constitutes the endomysium of the muscle. 6,500.