FIGURE 11.16 • Structure of cardiac muscle fiber. a. This scanning electron micrograph shows the heart muscle tissue preparation obtained from the monkey right ventricle. The sample was ultrasonicated within the sodium hydroxide that resulted in the digestion of collagen fibers and separation of cardiac myocytes at the intercalated discs. Note the branching pattern of myocytes and clearly visible transverse and lateral components of intercalated disc. 32,000. b. Three-dimensional drawing of an intercalated disc, which represents a highly specialized attachment site between adjacent cardiac muscle cells. The intercalated disc is composed of the transverse component (blue area) that crosses the fibers at a right angle to the myofibrils (analogous to the risers of a stairway) and a lateral component (pink area) that occupies a series of surfaces perpendicular to the transverse component and parallel to the myofibrils (analogous to the steps of a stairway). The fascia adherens is the major constituent of the transverse component. It holds the cardiac muscle cells at their ends and serves as the attachment site for thin filaments. The maculae adherentes reinforce the fascia adherens and are also found in the lateral components. The gap junctions are found only in the lateral component of the intercalated disc. c. This electron micrograph reveals portions of two cardiac muscle cells joined by an intercalated disc. The line of junction between the two cells takes an irregular, steplike course, making a number of nearly right-angle turns. In its course, different parts of the intercalated disc are evident. These include the transverse components (fascia adherens and maculae adherentes) and lateral components (gap junctions and maculae adherentes). The macula adherens (MA) is enlarged in inset 1 (62,000). The fascia adherens (FA) is more extensive than the macula adherens, being disposed in a larger area of irregular outline. The fascia adherens is enlarged in inset 3 (62,000). The fascia adherens of the intercalated disc corresponds to the zonula adherens of other tissues. The gap junction (GJ) is enlarged in inset 2 (62,000). Other features typical of cardiac muscle are also present: mitochondria (Mi), sarcoplasmic reticulum (SR), and components of the sarcomere, including Z lines (Z), M line (M), and myofilaments. This particular specimen is in a highly contracted state, and consequently, the I band is practically obscured. 30,000. (Part A reprinted with permission from Zhang L, Ina K, Kitamura H, Campbell GR, Shimada T. The intercalated discs of monkey myocardial cells and Purkinje fibers as revealed by scanning electron microscopy. Arch Histol Cytol 1996;59:453–465.) preparations. It holds the cardiac muscle cells at their ends to form the functional cardiac muscle fiber (see Fig. 5.20, page 131). It always appears as a transverse boundary between the cardiac muscle cells. The TEM reveals an intercellular space between the adjacent cells that is filled with electron-dense material resembling the material found in the zonula adherens of epithelia. The fascia adherens serves as the site at which the thin filaments in the terminal sarcomere anchor onto the plasma membrane. In this way, the fascia adherens is functionally similar to the epithelial zonula adherens, where actin filaments of the terminal web are also anchored.
 Maculae adherentes (desmosomes) bind the individual muscle cells to one another. Maculae adherentes help prevent the cells from pulling apart under the strain of regular repetitive contractions. They reinforce the fascia adherens and are found in both the transverse and lateral components of the intercalated discs.