Histology of the Heart
The major part of the heart is constituted of cardiac muscle. This kind of muscle is found only in the heart and in the tunica media of the terminal parts of the venae cavae as they enter the heart. The layer of the heart consisting of cardiac muscle is called the myocardium. Its inner surface of the myocardium is lined with endocardium, and the outer surface with epicardium.
Cardiac muscle is similar to skeletal muscle in many ways. Both types appear striated as a result of the arrangement of the actin and myosin filaments in the sarcomeres of the muscle fibres. The sarcolemma (plasma membrane) of both types has invaginations called T-tubules that spread depolarization throughout the cell. The signal is transmitted to the terminal cisternae of the sarcoplasmic reticulum, which are adjacent to the T-tubules. The release of calcium from the cisternae allows contraction to take place. The mechanism of muscle contraction (sliding filament) is the same in the cardiac and skeletal muscle.
There are differences in cardiac and skeletal muscle observable at the light microscope and ultrastructural level. Cardiac muscles fibres are of smaller diameter (about 15 micrometers) than most skeletal muscle fibres (10-100 micrometers). Cardiac muscle fibres are formed by individual muscle cells with one or two centrally placed nuclei, while skeletal muscle fibres are multinucleated protoplasmic units in which the nuclei are peripherally located. Cardiac muscle fibres branch and anastomose, skeletal muscle fibres do not. Cardiac muscle fibres are arranged in a linear array, each fibre is about 85-100 micrometers long. The junction between two cardiac muscle cells, called an intercalated disk, is another distintguishing feature. The intercalated disk is made up of three types of cell junctions: fascia adherentes, desmosomes and gap junctions.
At the ultrastructural level, the arrangement of T-tubules is more regularly organized in skeletal muscle, and they are found at the A-I junction, in contrast to the Z-line in cardiac muscle. T-tubules are usually associated with two terminal cisternae (triad) in skeletal muscle, versus one (diad) in cardiac muscle. The cisternae of skeletal muscle are much larger than those of cardiac muscle. Cardiac muscle is more vascularized and has more abundant mitochondria than does skeletal muscle (40% of volume vs. 2%), it also contains glycogen granules between the myofibrils. Physiologically, cardiac muscle is intrinsically rhythmic (contracts without outside stimulation) although it is regulated through nervous and hormonal mechanisms. The rate of cardiac muscle contraction is set by the sinoatrial node, whose intinsic rhythm is the most rapid.
Figure 1 shows cardiac muscle in longitudinal section. (It is taken from slide 94.) The striations can be seen along the length of the muscle fibres. [The striations are easier to see when looking through the microscope, they are not as obvious on these scanned computer images.] The nuclei of the cardiac muscle cells lie in the middle of the cells. In a good face-view section, the nucleolus is well-stained and the rest of the nucleus has a delicate pattern. The myofibrils separate to bypass the nucleus, and there is often a perinuclear region in which no striations are seen. This region contains cytoplasmic organelles not directly involved in contraction.
Each muscle fibre is surrounded by an endomysium of delicate connective tissue with a rich capillary network. Although the reticular fibres of the endomysium are not usually seen, you will see the nuclei of fibroblasts between the muscle fibres and also many capillaries running alongside them. Fibroblast nuclei tend to be more flattened and darker staining than those of cardiac muscle cells and are of course peripherally located. [In your sections, you wont be able to identify each nucleus. Find a good cardiac muscle nucleus with the features described above, then look for ones that look similar.]
Intercalated disks appear as slightly darker lines perpendicular to the length of the cardiac muscle fibres. Depending upon the preparation and the staining, intercalated disks can be obvious or barely identifiable.
Figure 2 shows another longitudinal section of cardiac muscle (from slide 23). In this section, several cardiac fibres are seen branching.
Figure 3 shows cardiac muscle fibres in cross section. The cut ends of the myofibrils appear stippled. When cross sections of myofibrils appear irregular, it probably indicates an area of branching. The nuclei of cardiac fibres are found near the middle of the cross section, sometimes a paler perinuclear region can be seen. Connective tissue runs between bundles of muscle cells, these bundles may become more widely separated during tissue preparation. Fibroblast nuclei will be found within the connective tissue or at the periphery of a muscle fibre (since each muscle fibre is also individually wrapped in endomysium). Many capillaries can be seen among the cardiac muscle fibres. The small empty circles among the muscle fibres are all capillaries (except for those larger than one RBC, they represent pre- or post-capillary vessels). The thickened areas of the capillary walls are endothelial cell nuclei. A large blood vessel, containing RBCs, is wrapped around a muscle bundle to the right of the figure, and a much smaller vessel (but too large to be a capillary), is a bit to the left of the large vessel. The endothelial cell nucleus in the smaller vessel is prominent.
The heart has a "skeleton" , which is the site for the origin and insertion of cardiac muscle. It consists of a fibrous ring surrounding each of the four orifices (aortic, pulmonary, tircuspid and mitral). The heart valves are attached to the cardiac skeleton.
Impulses originating in the sinoatrial node (SA node or pacemaker) pass along the cardiac muscle fibres of the atria and along internodal tracts of modified muscle fibres to the atrioventricular node (AV node) near the tricuspid valve. The AV node provides the only bridge between atrial and ventricular muscle. From the AV node, impulses pass across the fibrous skeleton of the heart to the ventricles via the AV bundle of His. The bundle of His divides into a right and left branch (the latter with 2 fascicles) which travel along the ventricular septum to the apex of the heart and then reverse their direction.
The branches of the bundle of His give off fibres, called Purkinje fibres, which are modified cardiac muscle cells with a diameter about twice that of regular fibres (30 vs. 15 micrometers). Purkinje fibres contain fewer myofibrils than regular cardiac muscle fibres and have large concentrations of glycogen. Their nuclei tend to be surrounded by a large perinuclear space with the myofibrils well toward the periphery of the muscle fibre. Purkinje fibres are much faster conducting than regular cardiac muscle fibres, with which they make contact via gap junctions. The impulse initiated in the SA node cause the atria to contract first and expel blood into the ventricles. The impulse is also carried along the internodal fibres to the AV node, bundle of His and its branches and then to the Purkinje fibres. Contraction of the ventricles begins at the apex and continues in a wavelike fashion toward the base, forcing blood into the aorta and pulmonary trunk.
Figure 4 shows Purkinje fibres in longitudinal section. Because of the lower density of myofibrils, Purkinje fibres appear paler than regular cardiac muscle fibres (a few of which can be seen at the bottom of the figure). A prominent perinuclear region is seen around several nuclei, and intercalated disks are evident. As in other cardiac muscle, capillaries are abundant. A capillary can be seen branching from a larger vessel near the middle, top third of the figure.
The Purkinje fibres shown in Figure 4 were scanned from slide 8. In this slide, collagen stains blue (Masson trichromic). This blue might be helpful in guiding you to the Purkinje fibres when you look at the slides under your microscope, as they are found in a layer of CT called the subendocardial layer (see below). (Little CT was visible in Figure 4, it would have been toward the top of the figure beyond the field of view).
Figure 5 (also from slide 8) shows Purkinje fibres in cross or oblique section embedded in the CT of the subendocardial layer. The wide diameter of the fibres and the large perinuclear region devoid of myofibrils can be seen clearly.
Figure 6 is also scanned from slide 8, at exactly the same magnification as Figure 5. Figure 6 shows regular cardiac muscle fibres in cross section. It can be seen that the fibres are of smaller diameter, stain more darkly because of a higher density of myofibrils, and have a smaller perinuclear region than the Purkinje fibres of Figure 5.
The endocardium lies on the luminal side of the myocardium. Its inner surface is covered with endothelial cells the squamous epithelium lining the inside of the heart and blood vessels. Beneath the endothelium is a layer of fairly loose, well-vascularized connective tissue, this becomes a bit denser closer to the myocardium. The thickness of the endocardium varies inversely with the thickness of the myocardium. In other words, it is thicker in the atria than in the ventricles, as the muscular walls are more substantial in the ventricles. The layer of CT closest to the myocardium is slightly looser and is called the subendocardial layer. It contains veins and nerves, as well as the Purkinje fibres when present.
Figures 7 and 8 show the endocardial layer of the atrium and ventricle, respectively. At this magnification, the nuclei of the endothelial cells are barely distinguishable. The difference in the thickness of the endocardium between the atrium and ventricle is evident, both figures were scanned at the same magnification. Part if the myocardium is visible in both figures. No veins or nerves (or Purkinje fibres) are seen in the fields of view.
The epicardium is the delicate, inner visceral layer of the pericardium. We do not see the outer, fibroelastic parietal layer of the pericardium on our slides. The outer part of the epicardium is lined with mesothelium: the epithelium lining the walls and contents of the closed cavities of the body, such as the thoracic, pericardial and abdominal cavities. Large blood vessels and nerves are found in the epicardium, and adipose tissue can be abundant.
Figure 9 shows a low power view of the epicardium of the ventricle. Part of the myocardium is also visible. In the field of view shown here, there is a large amount of adipose tissue within the connective tissue of the epicardium. A nerve bundle and several blood vessels can also be seen. The mesothelial lining (at the top) is not really distinguishable.
A higher power view of the epicardium is shown in Figure 10. The nerve is the same as the one in Figure 9 and can be used for orientation. Blood vessels are more easily identified (brighter red due to RBCs). The nuclei of the mesothelial cells can be distinguished (albeit with difficulty) at this magnification.
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