(C22-26) Slide 139: Monkey heart - H & E and iron hematoxylin.

There are two longitudinal sections of papillary muscle (which will be discussed in the Cardiovascular section) on this slide. One is stained with H & E (C22), the other with iron hematoxylin alone (C23). (Glass slide #23 contains a cross section of cardiac muscle.)

As you can see in this preparation, it is not possible to make out the exact limits of the individual myocytes. There are several features, however, which make it possible to identify this striated muscle as cardiac. While most myocytes have a single nucleus, there are a significant number that are binucleate. The nuclei are large and euchromatic, and usually have a single, large nucleolus. The nuclei are centrally placed in the myocyte (C24). At either end of the nucleus, there is a conical shaped region without sarcomeres that contain granules. These are the very large cardiac mitochondria (C24&25).

The diagnostic structural feature of cardiac muscle in longitudinal section is the intercalated disk. These disks represent the junctions between adjacent myocytes (the dark line at the center of C25). Often, these disks have the appearance of a flight of stairs (C26) in the section. The dense material is applied to the inner surface of the sarcolemma and it is a very large fascia adherens. If you look carefully, you should be able to see that the intercalated disk has the appearance of a wide Z band, occupying that position in the banding sequence (C25). As in skeletal muscle, you should be able to see A, H, I, and Z bands in this preparation (review these on C25).

Cardiac muscle is highly vascular, so there are lots of endothelial nuclei (top center, C25). There are also a few fibroblast nuclei. These two nuclear types cannot be easily distinguished; however, both are much smaller and more heterochromatic than the myocyte nuclei.

The only manifestation of the endomysium clearly visible in this preparation are the capillaries. Although it cannot be seen in this preparation, it should be recognized that the myocytes are organized into very long bundles, or fascicles, which are separated from adjacent fascicles by connective tissue which is functionally equivalent to the perimysium of skeletal muscle.