Muscle: Skeletal Muscle

Skeletal muscle is composed of large, cylindrical, multinucleated cells. The most striking feature of skeletal muscle fibers is the presence of striations that are conspicuous in longitudinally sectioned fibers. The striations are due to the presence of myofibrils, which are cylindrical bundles of thick and thin myofilaments, organized into units of contraction called sarcomeres. The orderly arrangement of these repeating units within the myofibrils gives rise to the characteristic pattern of transverse banding.

#15 Skeletal muscle (Phosphotungstic Acid – Hematoxylin, PTAH)

    Open with WebViewer #15 Skeletal muscle (Phosphotungstic Acid – Hematoxylin, PTAH)

Some slides also have a section stained with H & E. First, examine with the scanning objective the PTAH-stained preparation and note that the muscle fibers are grouped into bundles (or fascicles). The spaces between the bundles contain the perimysium, in which, under slightly higher magnification, brownish-orange connective tissue fibers, adipose cells, blood vessels and nerves can be identified. Next, under moderately high magnification, examine the individual skeletal muscle fibers, which have been cut lengthwise and appear as solitary, long and relatively thick, strongly stained bands separated by a delicate loose connective tissue (endomysium). Then, lower the condenser lens and/or close the iris diaphragm to see the myofibrillar cross- banding (alternating dark or A bands and light or I bands). Lastly, under immersion oil, notice additional markings, such as a thin Z band bisecting each I band and, in occasional fibers, a light H band within each A band. Note that in many regions the muscle fibers have contracted beyond their physiological limit due to fixation. This produces artifactual “contraction bands” in which the organization within the myofibril is distorted such that the individual bands overlap. In this case you simply see alternating light and dark bands.

Note: the shape and position of the pale-staining muscle nuclei, which should not be confused with the flattened, more elongate fibrocyte nuclei within the neighboring endo- or perimysium.

#3 Muscle and tendon junction, Human or Rabbit

    Open with WebViewer #3 Muscle and tendon junction, Human or Rabbit

Examine this preparation with the scanning objective and identify bundles of skeletal muscle fibers and sheets of tendinous tissue (dense regular connective tissue). Under higher magnification, skeletal muscle fibers may be recognized by their cross-striations, which can be made more evident by closing the iris diaphragm or by lowering the condenser lens. In this preparation most tendinous tissue has a homogenous, almost glassy, appearance (this is a diagnostic feature). The cells of the tendon (fibroblasts or fibrocytes) occur in rows, squeezed between the thick collagenous fibers; only their flattened, rod-like basophilic nuclei show well. The zone of insertion of the skeletal muscle into the tendon is obvious, but higher resolution (with the electron microscope) is necessary to see the detailed structure of the junction. What has the electron microscope revealed about the relation of skeletal muscle to tendon?

#32 Upper Esophagus, Human or Rabbit

    Open with WebViewer #32 Upper Esophagus, Human or Rabbit

This is a portion of a transverse section through the wall of the esophagus at or near the junction of its upper and middle segments. For orientation purposes, identify with the scanning objective the following successive layers in the esophageal wall: (1) an inner lining of non- keratinized stratified squamous epithelium; (2) an adjacent loose connective tissue layer; (3) a layer formed by smooth muscle fibers (which have been cut in cross-section); (4) a subjacent connective tissue layer in which lie large blood vessels; (5) two thick layers of skeletal muscle: an inner layer of fibers in cut in longitudinal or oblique section, and an outer layer rich in adipose cells and blood vessels. Under higher magnification, examine the two skeletal muscle layers and notice that, owing to the level of the esophagus at which the section was taken, the musculature is not entirely skeletal and does contain some bundles of smooth muscle fibers. If your section passes through a lower region of the esophagus, the proportions of smooth and skeletal muscle will change. In the skeletal muscle, note the following: strong acidophilia and peripheral nuclei. In longitudinal sections identify cross-striations (which can be intensified by lowering the condenser lens and/or by almost closing the iris diaphragm). In cross-section note: the muscle fibers appear as more or less rounded polygons separated by the endomysium; the nuclei are clearly visible at their periphery; and cross-sections of myofibrils are visible as distinct dots that are often grouped in clusters, an artifact of fixation.

Note: This is a good slide for comparing the morphological characteristics of smooth and skeletal muscles in both cross and longitudinal section, since these two types of muscle lie side by side within the same area.

#116 Tongue, Human H&E

    Open with WebViewer #116 Tongue, Human H&E

Observe numerous bundles (fascicles) of skeletal muscle fibers cut in different planes (this is a unique feature of the tongue). Identify the fatty connective tissue (perimysium), which surrounds each muscle bundle. From the perimysium partitions of loose connective tissue (endomysium) can be seen to penetrate into the bundle separating the individual muscle fibers. Under higher magnification, examine some skeletal muscle fibers in longitudinal section; striations may show poorly (if at all) but will become more conspicuous with the condenser lens lowered and/or the iris diaphragm almost closed. Nuclei are clearly visible at the periphery of muscle fibers cut in cross-section; in fibers sectioned lengthwise, they may appear to occupy any position with respect to fiber breadth, but more are seen at the sides and all actually occupy marginal regions of the fibers.

Define a sarcomere. Be sure you know what the electron microscope has revealed about its fine structure. Know the structural changes that occur in a sarcomere during contraction and the theory that has evolved from electron microscopic studies to explain muscle contraction.


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