Place in order the three layers of the heart wall, listing the deepest layer first.

Epicardium

The epicardium is the outermost layer of the heart. It is actually the visceral layer of the serous pericardium, which adheres to the myocardium of the heart. Histologically, it is made of mesothelial cells, the same as the parietal pericardium. 

Below the mesothelial cells is a layer of adipose and connective tissue that binds the epicardium to the myocardium and cushions the heart. Nerves and blood vessels that supply the heart are found in the epicardium. At the roots of great vessels, the epicardium reflects back and continues as the parietal pericardium, forming an enclosed pericardial sac. The sac is filled with serous pericardial fluid that prevents friction during heart contractions.

Myocardium

The myocardium is functionally the main constituent of the heart and the thickest layer of all three heart layers. It is a muscle layer that enables heart contractions. Histologically, the myocardium is comprised of cardiomyocytes. Cardiomyocytes have a single nucleus in the center of the cell, which helps to distinguish them from skeletal muscle cells that have multiple nuclei dispersed in the periphery of the cell.

Cardiomyocytes are very rich in glycogen deposits and mitochondria. This has a great functional significance since the myocardium is constantly contracting and needs a great amount of energy at all times. Cardiomyocytes also contain yellow lipofuscin granules. They don’t have any particular functional significance, but they are interesting since they are age markers for the cell. The older the cell, the more lipofuscin it has. 

Cardiomyocytes communicate through special intercellular bridges called intercalated discs. The discs comprise of three components: adherens junction (fascia adherens), desmosomes (maculae adherentes) and gap junctions (communicating junctions). These three components enable mechanical unity of cardiomyocytes and a direct pathway of spreading action potentials. Because of this, the myocardium is observed as a form of syncytium, rather than a group of somewhat independent cells.

Although the structure of myocardium is the same in the atria and ventricles, it is thicker in the ventricles. This is due to the greater hydrostatic pressure that the ventricles must overcome when pumping the blood into the systemic vessels. 

Endocardium

The endocardium is the innermost layer of the heart. It lines the inner surfaces of the heart chambers, including the heart valves. The endocardium has two layers. The inner layer lines the heart chambers and is made of endothelial cells. Superiorly, is the second layer: a subendocardial connective tissue which is continuous with the connective tissue of the myocardium. Branches of the heart’s conduction system are immersed into the subendocardial layer.

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Clinical relations: Endocarditis

Endocarditis is an inflammation of the endocardium. Usually, it has an infectious etiology. It mostly affects already damaged heart valves and in most cases, is caused by the bacteria Streptococcus viridans. In the case of a healthy heart, the pathogenic bacteria is Streptococcus aureus. No matter what bacteria is the cause, pathogenesis is the same. Once the bacteria carried by the bloodstream reaches the heart valves, they invade the endocardium. Then they cause the destruction of heart tissue, which leads to the formation of friable vegetations composed of necrotic debris (destructed heart cells), thrombi, and microorganisms. Parts of the vegetation can be loosened by the bloodstream and disseminated throughout the body, forming secondary deposits of infection. If the causing bacteria is very virulent, the endocarditis has a fulminant course and often a fatal outcome. On the other hand, when the bacteria are not so virulent they can persist for some time in the heart causing valves deformities. 

It is interesting that Streptococcus viridans is normally found in the oral cavity, where the easiest way for it to reach the heart and form vegetations on damaged valves is during a stomathology procedure. For this reason, it is necessary to inform your doctor if you have a valve problem, so they could prescribe antibiotics for precautions. 

Usual symptoms of endocarditis are flu-like signs, heart murmur, night sweating, shortness of breath, and chest pain during breathing. Infectious endocarditis is treated by intravenous antibiotics for 2 to 6 weeks. If the disease has significantly damaged the heart valves, it is indicated to perform a valve replacement or repair surgery.
 

Sources

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References:

• Ross, M. H., Pawlina, W. (2011). Histology (6th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
• Mescher, A. L. (2013). Junqueira’s Basic Histology (13th ed.). New York, NY: McGraw-Hill Education.
• Kumar, V., Abbas, A. K., Aster, J. C. (2013). Robbins Basic Pathology (9th ed.). Philadelphia, PA: Elsevier Saunders.