myofibrils. Other myofiber nuclei (MFN) can be seen at the periphery of the myofibers. Note that they exhibit a similar chromatin pattern as the three nuclei previously described. Also present in this micrograph is a capillary (C) coursing along the center of the micrograph. In this plane of section, it is difficult to clearly distinguish between the endothelial cell nuclei and nuclei of fibroblasts in the endomysium. Perhaps the most significant feature of a longitudinal section of a muscle fiber is the striations that they exhibit. The inset shows at higher magnification the banding pattern of the myofiber. The dark staining lines represent the A-band. The light staining area is the I-band which is bisected by the dark staining Z-line.
micrograph. They correspond to the dot-like structures seen in the inset of the cross-sectioned myofibers above. Note that adjadcent myofibrils are aligned with one another with respect to their banding pattern and also that they exhibit different widths. Each myofibril is essentially a cylindrical structure much like a dowel, thus when sectioned in a longitudinal plane, the width of each myofibril will vary depending on what portion of the cylindrical structure has been cut. The sarcoplasmic reticulum, a membrane system that is present between myofibrils and the nature of the bands in a myofibril are shown to advantage in the next plate.
• KEY C, capillaries E, endomysium ECN, endothelial cell nuclei FN, fibroblast nuclei MF, muscle fibers MFN, muscle fiber nuclei M, myofibers My, myofibrils MyN, myofiber nuclei N, nucleus
The work effected by skeletal muscle to allow body movement is through tendons to which the muscle fibers are attached. The site of attach-ment between a muscle fiber and the collagen of the tendon is referred to as the myotendinal junction. The muscle fibers at the junctional site end in numerous fingerlike cytoplasmic projections. At the ends of each projection and between these projections, the collagen fibrils of the tendon attach to the cell at its basal lamina (see electron micrograph in adjacent plate). In the light microscope, these fingerlike projections appear to merge into the tendon. The detailed relationship is seen at the electron microscope level. The last sarcomeres in the muscle fiber end where the fingerlike projections begin. At this point, the ending sarcomere lacks its Z-line and the actin filaments from the A-band con-tinue into the cytoplasmic fingers ending at the sarcolemma.
Myotendinal junction, monkey, H&E, ×365. collagenous bundles of the tendon. Several of the muscle fibers (MF’) are seen at the point where they terminate and are attached to the tendon This light micrograph reveals a tendon (T) and adjacent to it fibers. The area in the rectangle is seen in higher magnification in the mi-several muscle fibers (MF). The tendon contains dispersed crograph below.
tendinocytes whose nuclei (N) are compressed between the
Myotendinal junction, monkey, H&E, ×1,560. end of the muscle fiber are clearly seen. Between the fingerlike structures are the collagen fibers of the tendon. The nuclei of the tendinocytes (T) are seen The muscle fiber (MF) in this micrograph is seen at the point in the tendon where it continues from the muscle fiber.