Cardiac Histology Histology > Cardiovascular and Respiratory > Cardiovascular and Respiratory

NOTES

HEART WALL Comprises three layers

• Endocardium, Myocardium, Epicardium

Endocardium:

• Innermost layer of endothelial cells.

• Its function is to line the chambers and valves of the heart, creating a smooth surface to reduce friction with the blood.

• Site of endocarditis

Myocardium:

• Muscle cells contract to pump blood from the heart; are arranged in bundles wrapped in connective tissue.

• Myoendocrine cells that produce atrial natriuretic peptide (aka, factor or hormone) in response to high blood pressure.

• Nodal cardiocytes control the rhythmic contraction of the myocardium (these cells are concentrated in the sinoatrial and atrioventricular nodes).

Details of cardiac muscle cells:

• Striated.

• Have 1-2 centrally located nuclei.

• Cells branch to connect with multiple adjacent cells.

This arrangement increases intercellular connections, which are marked by intercalated discs, for efficient signal conduction.

• Intercalated discs comprise interdigitating processes that hold adjacent cells together via complex junctions to rapidly spread contraction signals throughout the myocardium.

• Purkinje fibers

Are lighter in the center and lie nearer the endocardium than do muscle cells Purkinje fibers are part of conduction

1 / 3 system of the heart. Epicardium:

• Contributes to the serous layer of the pericardium, the protective sac that envelops the heart.

• Comprises simple squamous epithelium.

• Subepicardial connective tissue comprises adipose and other connective tissues, passage for coronary blood vessels and nerves.

FULL-LENGTH TEXT

• Here we will learn the histology of the three layers of the heart wall.

• To begin, outline the heart and great vessels;

• Label the endocardium, which is the innermost layer of endothelial cells; its function is line the chambers and valves of the heart, creating a smooth surface to reduce friction with the blood.

For anatomical context, show that the great vessels are continuous with the heart; elsewhere, we'll learn that the tunics of the blood vessels are analogous with those of the heart.

• Next, indicate the thick myocardium, which comprises:

- Cardiac muscle cells, aka, fibers, that contract to pump blood from the heart,

- Myoendocrine cells that produce atrial natriuretic peptide (aka, factor or hormone) in response to high blood pressure,

- And, nodal cardiocytes that control the rhythmic contraction of the myocardium (these cells are concentrated in the sinoatrial and atrioventricular nodes).

• Lastly, draw the outer layer, the epicardium, which comprises simple squamous epithelium.

- Though omitted from our diagram for simplicity, recognize that the epicardium also contributes to the serous layer of the pericardium, the protective sac that envelops the heart.

Now, let's look at histological samples to learn some microscopic details.

• First, indicate that cardiac muscle cells are characterized by striations and 1-2 centrally located nuclei

• The cells branch to connect with multiple adjacent cells;

2 / 3 • This arrangement increases intercellular connections, which are marked by intercalated discs, for efficient signal conduction.

• The intercalated discs comprise interdigitating processes that hold adjacent cells together via complex junctions to rapidly spread contraction signals throughout the myocardium.

Next, let's look at the epicardium and myocardium:

• First, label the epicardium, which is identified by its simple squamous epithelium.

• Then, indicate the sub-epicardial connective tissue, which comprises adipose and other connective tissues to provide passage for coronary blood vessels and nerves.

• Deep to this, indicate the myocardium, and label the muscle fibers; in this sample, they are cut in cross section, and we see they are arranged in bundles and wrapped in connective tissue.

- The myoendocrine cells and nodal cardiocytes of the myocardium are better seen with electron microscopy.

• In our final histological sample, identify the endocardium, and indicate that it lines the chambers of the heart.

• Deep to the endothelium, label the myocardium, and indicate the muscle fibers.

• Compare these to the Purkinje fibers, which are lighter in the center and lie nearer the endocardium.

Images: Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)