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Slides of skeletal muscle come in a variety of forms. You can have a slide of pure skeletal muscle, or slides of various organs, such as tongue or larynx, that contain skeletal muscle. Obtain a slide that contains skeletal muscle cut in longitudinal section and prepare to examine it microscopically. Before placing your slide on the microscope stage, remember to read the label, examine the slide with your eye and note any visible macroscopic features that might help your examination.
Fig. 1. Skeletal Muscle (H & E).
Bring the specimen into focus with the 4X objective and locate the skeletal muscle fibers in long section. Center the fibers in your field of view, then immediately turn to the 10X objective for further examination.
Fig. 2. Skeletal Muscle (H & E).
Examine fibers cut in long section with the low-power objective. Individual skeletal muscke cells are called muscle fibers. In long section, one can appreciate the linear nature of skeltal muscle fibers. Note that individual skeletal muscle fibers are linear, cylindrical cells with a diamater of approximately 10 - 100 microns. In a skeletal muscle organ, each fiber runs parallel to the long axis of the organ, and each cell runs the entire length of the organ, from origin to insertion, without branching or making intracellular contacts with neighboring cells.
The external lamina, which is a basal lamina that surrounds each cell, is unresolvable. Each muscle fiber is wrapped by a layer of loose connective tissue called endomycium. Endomycium is a loose connective tissue sheath that consists of a network of fine reticular fibers embedded in ground substance. Due to the lack of significant amounts of elastic or collagen fibers, the endomycium is unstained in routine H & E preparations. The endomycium not only lends physical support to each muscle fiber, but also supports the small blood vessels and nerve arborizations that supply the tissue.
PHOTOGRAPHS. Take several photomicrographs of skeletal muscle in long section at this magnification. Always take more photographs than you will need so you have a group of pictures from which you can select which you want to include in your e-atlas.
Fig. 3a. Skeletal Muscle (H & E).
Fig. 3b. Skeletal Muscle (H & E).
Rotate the high-power objective into position to study the shape and position of the nuclei. Skeletal muscle cells are multinucleated cells. Their multinucleation develops from the fusion of embryonic muscle precursor cells, called myoblasts, during their development; each mature muscle cell is, thus, a true syncytium.
Note that the nuclei of skeletal muscle cells are flattened and are located along the periphery of the cell. Some of the nuclei you observe might actually belong to satellite cells. Satellite cells are stem cells that proliferate to form new myoblasts after minor skeletal muscle injury. They are small cells situated between skeletal muscle fibers and the basal laminae. Depending on the quality of your specimen and your microscope, the striations that are characteristic of skeletal muscle cells may be visible under high-power. You may need to close down your aperture diaphragm to increase the contrast and see them. If so, note their appearance.
PHOTOGRAPHS. Take several photomicrographs of skeletal muscle in long section at high magnification. Always take more photographs than you will need so you have a group of pictures from which you can select which you want to include in your e-atlas.
Now change to a slide of skeletal muscle that is stained to show the striations. Beginning wit the scanning objective, find myofibers in long section. Systematically work your way through the low mag and high mag objectives to find fibers that dispaly nice striations. Then turn to your 100X oil-immersion objective to study the details of the striations. Note that the striations that characterize skeletal muscle cells, and all striated muscle cells for that matter, are alternating light and dark bands that run perpendicular to the long axis of the cell. The dark bands are known as A bands. (A stands for anisotropic, an optical property characterized by the bands' capacity to alter of the path of polarized light.) The light bands are known as the I bands. (I stands for isotropic.) Each A band is a stack of thick filaments (thick filaments contain the contractile protein myosin) and includes whatever length of thin filaments interdigitate with the thick (thin filaments contain the contractile protein actin, among other proteins). Each I band consists of thin filaments between A bands.
The A and I bands each have a thin bands running down their centers. These smaller bands can be very difficult to resolve with a standard bright-field microscope. Try to resolve a pale H zone running down the center of each A band. The H zone is the central portion of an A band that does not overlap with thin filaments. Also try to resolve a dark Z-line, or Z disk, running down the center of each I band. The Z line consists of the actin binding protein alpha-actinin, a protein that anchors thin filaments from the two apposing sides of the Z line. If you can resolve Z lines, then you can delineate individual sarcomeres, the units of contraction of striated muscle, that extend from one Z line to the next. The striations reflect the abundance and organization of the contractile proteins within the cytoplasm of the skeletal muscle cells.
PHOTOGRAPHS. Take several photomicrographs of skeletal muscle in long section at the oil-immersion magnification. Make sure the photographs clearly show the A bands, the I bands and the Z lines. It is a bonus if your pictures show the H zone. Always take more photographs than you will need so you have a group of pictures from which you can select which you want to include in your e-atlas.
Fig. 4. Skeletal Muscle (H & E).
Now that you have examined long section of skeletal muscle at all magnifications, reverse your study and see what details can be resolved at the lower magnifications.
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Copyright © 2011 by Stephen Gallik, Ph. D. | Author: Stephen Gallik, Ph. D.
