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Myocardium

Myocardium

For medical students2 min readUpdated 2026-10-10

Myocardium is the middle and thickest layer of the heart wall, composed of striated cardiac muscle tissue. It forms a three-dimensional reticular structure that ensures rhythmic and continuous contractions.

ArchitectonicsThe atria have 2 muscle layers, while the ventricles have 3.
Difference from skeletal muscleCardiomyocyte nuclei are always centrally located within the cell.
Energy metabolismFatty acids are the primary substrate for ventricular function.
AdaptationIn response to increased load, cells increase in volume rather than divide (hypertrophy).

Layer Architectonics and Stroma

Muscle tissue attaches to the fibrous rings—the cardiac skeleton surrounding the atrioventricular orifices. The layer orientation varies by chamber:

Interspersed between muscle elements is a stroma of loose connective tissue containing blood capillaries, lymphatic vessels, and larger arteries and veins that ensure an intensive blood supply.

Histology of Cardiomyocytes

The myocardium is primarily composed of contractile cardiomyocytes organized into "functional fibers." Unlike skeletal muscle tissue, these are not true multinucleated syncytia, but chains of individual cells connected by lateral anastomoses.

Key diagnostic features on histological slides:

  1. Cross-striation: results from the strictly ordered arrangement of myofibrils.
  2. Central nuclei: positioned in the center of the cell (1–2 nuclei each), as myofibrils are less densely packed around the nucleus.
  3. Intercalated discs: distinct dark transverse lines at cell boundaries (especially visible with iron hematoxylin staining).

Intercalated Discs and Cell Junctions

Intercalated discs provide mechanical strength and synchronous myocardial contraction. Three types of intercellular junctions are found in these zones:

Specifics of Ventricular and Atrial Cells

Ventricular cardiomyocytes are cylindrical in shape. The contractile apparatus (myofibrils) occupies about 40% of their volume. To support aerobic metabolism, the cells contain an abundance of mitochondria (35–38%) forming a "mitochondrial reticulum." Coupling of excitation and contraction is mediated by T-tubules and the L-system ($Ca^{2+}$ stores).

Atrial cardiomyocytes are classified into working and myoendocrine cells. Secretory cells (predominantly in the right atrium) have a branching or stellate shape with fewer myofibrils and mitochondria, but well-developed rough endoplasmic reticulum and Golgi complexes. They perform an endocrine function: synthesizing an anticoagulant glycoprotein and atrial natriuretic peptide (ANP), which lowers blood pressure in response to excessive blood volume.

Regeneration and Hypertrophy

Despite the presence of rare stem cells, physiological regeneration of the myocardium via cell division virtually ceases after adolescence.

The primary adaptation mechanism to increased workload (hypertension, exercise, post-infarction state) is hypertrophy. The cell count remains unchanged (absence of hyperplasia), but cell volume increases due to nuclear polyploidization, expanded sarcoplasm, and synthesis of new mitochondria and myofibrils.

Mnemonic

To remember the layers of the ventricular myocardium, use the rule "L-C-L" (Longitudinal — Circular — Longitudinal): the inner and outer layers are longitudinal, while the thick middle layer is circular.

Frequently asked questions

What types of cardiomyocytes form the cardiac conduction system?

The cardiac conduction system is formed by atypical cardiomyocytes. The following types are well-characterized:

  • P cells (pacemaker cells) — small, polygonal cells lacking T-tubules. They form the core of the sinoatrial node and are present in the atrioventricular node. They act as true autorhythmic cells, generating the heart rate (60–70 impulses per minute).
  • Transitional cells — cylindrical cells with short T-tubules. They form the basis of the atrioventricular node and are found at the periphery of the sinoatrial node. They occupy an intermediate position between typical and atypical cardiomyocytes, possessing both automaticity and contractility.

The conduction system also includes the bundle of His and its branches—Purkinje fibers.

From which embryonic germ layer does the myocardium develop?

The myocardium develops from the myoepicardial mantle, which is a thickening of the visceral layer of the lateral plate mesoderm (splanchnopleure). Development begins before embryo folding, when two myoepicardial plates appear in the splanchnopleure, surrounding the mesenchymal endothelial heart tubes. During lateral folding of the embryo, these primordia approach each other and fuse into a single tube, forming the middle layer of the primary heart tube—the myocardium.

Is a cardiac muscle fiber a true syncytium?

No, it is a functional fiber. It consists of individual cells (cardiomyocytes) connected by intercalated discs, forming a unified network.

What is the purpose of intercalated discs in the myocardium?

They mechanically bind cardiomyocytes together (via desmosomes and interdigitations) and ensure rapid impulse propagation from cell to cell (via gap junctions).

What endocrine function does the myocardium perform?

Atrial secretory cells release atrial natriuretic peptide (ANP), which stimulates renal excretion of sodium and water, resulting in decreased blood pressure.

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