Muscle

Skeletal Muscle

Skeletal muscle, cross and longitudinally sectioned
Study the striations of skeletal muscle. Locate a site where muscle fibers are clearly longitudinally sectioned. Compare your observations here with electron micrographs of skeletal muscle in your text and atlas, particularly the structure of a longitudinally sectioned sarcomere. Note the location and content of A bands and I bands. Note the location of the nuclei. In the cross-sectioned skeletal muscle of the specimen, locate a group of cells (a fascicle) and look for nuclei near the edge (sarcolemmal region) of the muscle fibers (cells). Identify endomysium, perimysium , and epimysium.

Skeletal muscle, iron hematoxylin stain
This specimen has been stained to enhance the striations. Observe A bands and I bands in this specimen.

Tongue
Reinforce your ability to recognize the special histological features of skeletal muscle. Note striations, nuclei, and fiber arrangement. Note the characteristic three planes of skeletal muscle in the tongue.

Spermatic cord
Again examine skeletal muscle of this specimen and observe the striations, nuclei, and fiber arrangement.

Skeletal muscle, rat, frozen section
Observe the special preservation of skeletal muscle in this specimen. The freezing technique used in preparation of this specimen tends to preserve the cells more representative of the natural living state. Therefore with the better preservation in these specimens you can better observe the endomysium and perimysium because of the absence of artifact space between muscle cells.

Skeletal muscle, human, frozen section
Examine the skeletal muscle of this specimen and its histological features, including the endomysium and location of nuclei.

Smooth Muscle

Esophagus
Find the smooth muscle in the outer perimeter of this section. It is represented here as two layers: an outer longitudinal layer demonstrates the smooth muscle cells of that layer in cross-section orientation (since the esophagus is in cross-section) and an inner circular layer demonstrating the appearance of smooth muscle cells in longitudinal orientation.

Pampiniform plexus
Smooth muscle surrounding the lumen of the ductus deferens appears in three discrete layers. Observe that the smooth muscle cells of the inner layer and outer layer are longitudinally-oriented along the long axis of the lumen, thus causing the cells to appear in cross-section. The cells of the middle layer encircle the lumen, thus causing the muscle cells to appear longitudinally-oriented. Smooth muscle is also present in the walls of muscular arteries, in longitudinal orientation, since the cells encircle the lumen of the vessel. Smooth muscle is also present in the walls of several veins present. In some large veins, smooth muscle cells are in the inner circular layer, and there are also smooth muscle cells in an outer longitudinal layer (observed in cross-section).

Uterus
The muscular wall of the uterus displays an abundance of smooth muscle cells in various planes of orientation. Note that the elongate nuclei of smooth muscle cells are located centrally in the cells.

Skeletal vs. Smooth Muscle on the same section

Prostate and prostatic urethra
In this section smooth muscle and skeletal muscle are both present. Compare and contrast the two types of muscle. The smooth muscle is present as an integral part of the prostate surrounding the islands of epithelium and the skeletal muscle is present off to one side of the tissue section (apparently a small portion of one of the skeletal muscles in the lower abdominal wall). Most of the skeletal muscle cells (commonly referred to as skeletal muscle fibers) seen are in cross section. Note the position of the nuclei in the cross sectioned fibers as well as the few longitudinally sectioned fibers present in the specimen. Note the delicate endomysium.

Cardiac Muscle

Heart, muscular interventricular septum.
Note the acidophilic cardiac muscle fibers that change direction frequently. The striations are difficult to observe in this specimen. Observe fiber bundles which are cut in longitudinal, oblique and transverse section. Observe the centrally located nuclei of cardiac muscle. Locate a large area of longitudinally sectioned fibers and note:
1. The walls of individual fibers are parallel for short distances only.
2. The variation in width is produced by frequent branching and anastomosing of adjacent fibers.
3. The branches arising from individual fibers are variable in diameter.

Because of its fibrous appearance, cardiac muscle bears a superficial resemblance to skeletal muscle and dense connective tissue (regular and irregular). In longitudinal section, the regular branching and anastomosing pattern is the most distinctive and characteristic feature of cardiac muscle. Higher magnification will be required to confirm the presence or absence of myofibrils and cross striations.

Observe an appropriate branching area and examine the fibers carefully and note:
1. Careful examination of the fibers and branches reveals the presence of myofibrils and cross striations.
2. The presence of intercalated disks.
3. The nuclei frequently appear to be located in a mid-fiber position. Note the sarcoplasmic cone which is the pale-staining area flanking the nucleus that is devoid of myofibrils.

The presence of myofibrils and cross striations in branching fibers indicates the contractile nature of this tissue and confirms the diagnosis of cardiac muscle. Purkinje fibers are modified cardiac muscle cells specialized for impulse conduction. Because they are larger and have lower density of myofibrils, intercalated disks and myofibrils are easily observed.