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Layers of the heart

Recommended video: Myocardium [01:42]
Definition, function and anatomy of the myocardium.

This article will discuss the layers of the heart (the epicardium, the myocardium and the endocardium) and any clinical relations pertaining to them.

In the same way that vehicles have their fuel pumps, our body has the heart. The heart is a muscular organ found in the middle mediastinum that pumps blood throughout the body. It is housed in the pericardial sac, which protects it and assists with its mechanics.

Recalling from the heart anatomy, it has two atria and two ventricles that make up elements and important steps for the heart cycle. But let’s take a look at the heart histology; it is essential for understanding the function of the heart. Histologically, the heart is made of three layers of tissue: epicardium, myocardium, and endocardium.

Key facts about the layers of the heart
Epicardium Visceral layer of serous pericardium
Comprised of mesothelial cells and fat and connective tissues
Myocardium Muscle layer
Comprised of cardiomyocytes
Endocardium Lines inner surface of heart chambers and valves
Comprised of a layer of endothelial cells, and a layer of subendocardial connective tissue
Clinical relation Endocarditis
  1. Epicardium
  2. Myocardium
  3. Endocardium
  4. Clinical relations: Endocarditis
  5. Sources
+ Show all


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.

Once you've finished learning about the layers of the heart, start studying the main structures of the heart with our handy diagrams, worksheets and quizzes


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. 


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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