This figure shows cross-sectioned cardiac muscle fibers. Many have rounded or smooth-contoured polygonal profiles. Some fibers, however, are generally more irregular and elongate in profile. These probably reflect a profile of both a fiber and a branch of the fiber. The more lightly stained region in the center of many fibers represents the myofibril-free region of the cell already re- may not be able to depend on these structures for identifying cardiac muscle. Intercalated discs are opposing cell-to-cell contacts. Thus, cardiac muscle fibers differ in a very fundamental respect from fibers of skeletal muscle. The cardiac muscle fiber consists of an end-to-end alignment of individual cells; in contrast, the skeletal muscle fiber is a single multinucleated protoplasmic unit. In examining a longitudinal section of cardiac muscle, it is useful to scan specific fibers along their long axes. By doing so, one can find places where the fibers obviously branch. Two such branchings are indicated by the arrows in this figure.
above or below the “in-focus” plane of section. Often, the staining of muscle cell nuclei in a specific specimen is very characteristic, especially when seen in face view as here. Notice, in the nucleus between the asterisks, the well-stained nucleolus and the delicate pattern of the remainder of the nucleus. Once such features have been characterized for a particular specimen, it becomes easy to identify nuclei with similar staining characteristics throughout the specimen. For example, survey the field in figure on left for nuclei with similar features. Having done this, it is substantially easier to identify nuclei of connective tissue cells (CT), which display different staining properties and are not positioned in the same relationship to the muscle cells.
ferred to above and indicated by the asterisks in top right figure. Delicate connective tissue surrounds the individual muscle fibers. This contains capillaries and sometimes larger vessels, such as the venule (V) in the center of the bundle of muscle fibers. Larger amounts of connective tissue (CT) surround bundles of fibers, and this tissue contains larger blood vessels, such as the arteriole (A) marked in the figure.
Cardiac muscle, heart, human, H&E ×400.
At higher magnification, it is possible to see the cut ends of the myofibrils. These appear as the numerous red areas that give the cut face of the muscle cell a stippled appearance. The nuclei (N) occupy a central position surrounded by myofibrils.
Remember, in contrast, that nuclei of skeletal muscle fibers are located at the periphery of the cell. Note, also, that as mentioned, the nucleus-free central area of the cell, devoid of myofibrils, shows areas of perinuclear cytoplasm similar to that marked with asterisks in figure directly above.
KEY A, arteriole C, capillaries CT, connective tissue ID, intercalated discs N, nuclei of cardiac muscle cells V, venule arrows, sites where fibers branch asterisks, perinuclear cytoplasmic areas
PLATE 25 Cardiac Muscle, Purkinje Fibers