The heart and associated structures are broadly broken into 6 functionally and structurally distinct compartments: the pericardium/epicardium, endocardium, myocardium, valves, fibrous skeleton, and conduction system.
The pericardium has fibrous and serous components. The fibrous component forms the outermost layer of the pericardial sac and is composed of dense collagenous tissue. The serous pericardium lines the pericardial cavity, and is covered by mesothelial cells which secrete the pericardial fluid. Quiescent mesothelial cells are flattened under routine light microscopy. Following injury, these cells can become cuboidal or even columnar.
The parietal (outer-most) layer forms the pericardial sac which is readily apparent grossly. This is continuous with the visceral layer (synonymous with the epicardium) which is a thin layer covering the surface of the heart. Beneath the visceral pericardium, there is often a thin layer of adipose containing vessels and lymphatics. While the parietal pericardium is capable of adapting to changes in size of the heart, it has limited ability to expand rapidly (such as following acute pericardial effusion). Chronic distension of the pericardial sac is better tolerated as collagen fibers in the pericardium presumably remodel.
The endocardium is the inner most lining of the heart and is the cardiac equivalent of the tunica intima. It is lined by endothelial cells and supported by a small amount of fibrous stroma. The endocardium is continuous with the atrioventricular valves and also covers chordae tendinae, papillary muscles, and chordae tendinae. The endocardium may contain a small amount of adipose, lymphatic vessels, and nerves. Purkinje fibers, important in electrical transmission, are also contained within the endocardium.
Like skeletal muscle, cardiac muscle is striated with many of the same ultrastructural features, including Z-lines, A bands, and I bands (described in Chapter 4 Muscle). Key differences from skeletal muscle are:
- Branching of cardiomyocytes
- Prominent connective tissue
- Cardiomyocytes have centrally located nuclei with a perinuclear halo
- Transversely orientated intercalated discs
A unique feature of atrial cardiomyocytes is the presence of cytoplasmic granules containing atrial natriuretic factor (ANF). Secretion of ANF counteracts aldosterone, and induces natriuresis, vasodilation, and suppression of the renin-angiotensin-aldosterone system.
The intercalated discs are especially important for cardiac muscle function, and are junctional complexes between terminal Z lines. The intercalated disc contains desmosomes, gap junctions, and fasciae adherentes. Cardiomyocytes are particularly rich in mitochondria and have large quantities of myoglobin. Ventricular cardiomyocytes are arranged into outer, middle and inner layers that form a convoluted helix around the heart. These layers are difficult to appreciate in routine sections. Cardiac myocytes also form a thin sleeve of tissue that extend from the left atrium into the walls of pulmonary veins. These cells are thought to be an important focus of ectopic activity in certain disease states (such as in atrial fibrillation).
Valves are covered by endothelium which has a similar function to vascular endothelium elsewhere in the body. Different layers of the tricuspid and mitral valves include the atrialis (on the atrial side), spongiosa, fibrosa, and ventricularis (on the ventricular side). Beyond the proximal ½ of the valve, the valve is avascular and these different layers are not readily appreciable. The valve stroma is composed of a mixture of collagen, elastic fibers, and proteoglycans. Valve interstitial cells (VICs) are the primary stromal cell and are responsible for homeostasis of the extracellular matrix. VICs are also activated in valvulopathies, and play an important role in progression of diseases such as myxomatous valvular degeneration (‘endocardiosis’).
Heart rate is determined by a collection of sinoatrial nodal cells (‘pacemaker cells’) located near the confluence of the cranial vena cava and right atrium, in the subepicardial myocardium. While these cells are essentially modified cardiomyocytes, they are slightly smaller and interspersed by dense fibrous connective tissue. Sinoatrial node cells are often found in close association to the sinus node artery. They are extensively modified by autonomic ganglia which are located in the adjacent epicardium. The atrioventricular node is microscopically similar to the sinoatrial node, and can be found slightly cranial (anterior) to the coronary sinus, and immediately dorsal to the tricuspid annulus. Electrical impulses continue to be conducted through Purkinje fibers which branch at the basal aspect of the interventricular septum, and send ventricular ramifications in the subendocardium. Purkinje cells are considerably larger than regular cardiomyocytes and stain lightly eosinophilic. Electrical conduction is facilitated by desmosomes and gap junctions. Intercalated discs are not present between Purkinje cells.
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