where it ends. Note the banding pattern of the muscle fiber. At this magnification, the fingerlike projections (arrows) at the • MYOTE N DI NAL J U NCTION KEY MF, muscle fibers MF’, terminating muscle fibers N, nuclei S, sarcomere T, tendon Tc, tendinocytes Myotendinal junction, monkey, electron micrograph, ×24,000. This electron micrograph shows the end of part of a muscle. Note that the last sarcomere (S) lacks a Z-line. The actin filaments appear to extend from the A-band and continue along the length of the finger projections and seemingly attach to the sarcolemma. Between the finger projec-tions are the collagen fibrils (arrows) that make up the tendon. (Micrograph courtesy of Dr. Douglas Kelly.)
Cardiac muscle consists of fibers that possess the same arrangement of contractile filaments and thus the same cross-banding patterns that are present in striated skeletal and visceral muscle. Although cardiac muscle is, therefore, also striated, it differs in many significant respects from skeletal and striated visceral muscle. Cardiac muscle consists of individual cells that are joined by complex junctions to form a functional unit (fiber). The histologically obvious differences between cardiac and the other striated muscle fibers are the presence in cardiac muscle of intercalated discs (the light microscopic representation of the complex junctions), the location of cardiac muscle cell nuclei in the center of the fiber, and the branching of the cardiac muscle fibers. All of these characteristics are evident in a well-prepared longitudinal section of the muscle.
Cardiac muscle, heart, human, H&E ×160.
This figure shows a longitudinal section of cardiac muscle. The muscle fibers are disposed horizontally in the illustration and show cross-striations. In addition to the regular cross-striations (those of greater frequency), however, there is another group of very pronounced cross-bands, namely, the interca346 lated discs (ID). Intercalated discs most often appear as a straight band, but sometimes they are arranged in a stepwise manner (see also figure on right). These discs are not always displayed in routine H&E sections; therefore, one
Cardiac muscle, heart, human, H&E ×400.
Like skeletal muscle, the cardiac muscle is composed of linear contractile units, the myofibrils. These are evident in this figure as the longitudinally disposed linear structures that extend through the length of the cell. The myofibrils separate to bypass the nuclei, and in doing so, they delineate a perinuclear region of cytoplasm that is free of myofibrils and their cross-striations. These perinuclear cytoplasmic areas (asterisks) contain the cytoplasmic organelles that are not directly involved in the contractile process. Many cardiac muscle cells are binucleate; both nuclei typically occupy the myofibrilfree region of cytoplasm, as shown in the cell marked by the asterisks. The third nucleus in this region appears to belong to the connective tissue either
Cardiac muscle, heart, human, H&E ×160.