Layer architectonics and stroma
The muscle tissue attaches to the fibrous rings—a kind of heart "skeleton" surrounding the atrioventricular orifices. The layers have different orientations depending on the chamber:
- In the atria, two layers are distinguished: internal (longitudinal) and external (circular).
- In the ventricles, the wall is thicker and consists of three layers: thin internal and external (both longitudinal) and the most powerful middle layer, whose fibers have a circular orientation.
Between the muscle elements lies a stroma of loose fibrous connective tissue. Blood capillaries, lymphatic vessels, and larger arteries and veins pass here, ensuring intensive nourishment.
Cardiomyocyte histology
The basis of the myocardium is formed by contractile cardiomyocytes combined into "functional fibers." Unlike skeletal muscle tissue, these are not true multinucleated symplasts, but chains of individual cells connected by lateral anastomoses.
Key diagnostic features on micropreparations:
- Cross-striation: arises due to the strictly ordered arrangement of myofibrils.
- Central nuclei: located in the center of the cell (1–2 per cell), as myofibrils are packed less densely around the nucleus.
- Intercalated discs: distinct dark transverse stripes at cell boundaries (especially well visible with iron hematoxylin staining).
Intercalated discs and junctions
Intercalated discs provide mechanical strength and synchrony of myocardial work. Three types of intercellular junctions are distinguished in these zones:
- Interdigitations — finger-like membrane invaginations that increase the contact area.
- Desmosomes — responsible for mechanical fixation; myofibrils attach to the plasmalemma in this zone.
- Nexuses (gap junctions) — unite the cytoplasm of neighboring cells, ensuring instantaneous conduction of electrical excitation.
Specifics of ventricular and atrial cells
Ventricular cardiomyocytes have a cylindrical shape. The contractile apparatus (myofibrils) occupies about 40% of their volume. To provide energy for aerobic metabolism, cells contain a huge number of mitochondria (35–38%), forming a "mitochondrial reticulum." A system of T-tubules and the L-system ($Ca^{2+}$ depot) is used to couple impulse and contraction.
Atrial cardiomyocytes can be working or myosecretory. Secretory cells (primarily in the right atrium) have a branching shape; myofibrils and mitochondria are less developed in them, but the granular ER and Golgi apparatus are well expressed. They perform an endocrine function: they synthesize an anticoagulant glycoprotein and atrial natriuretic factor (ANF), which lowers blood pressure in case of blood volume overload.
Regeneration and hypertrophy
Despite the presence of isolated stem cells, after adolescence, physiological regeneration of the myocardium via cell division practically ceases.
The main mechanism of adaptation to increased load (hypertension, sports, post-infarction state) is hypertrophy. In this case, the number of cells does not change (hyperplasia is absent), but their volume increases: nuclear polyploidization occurs, the volume of sarcoplasm grows, and new mitochondria and myofibrils are synthesized.
Briefly: answers to topical questions
What types of cardiomyocytes form the conduction system of the heart?
The conduction system of the heart is formed by atypical cardiomyocytes. The following types are described in detail in sources:
- P-cells (pacemaker cells) — small polygonal cells without T-tubules. They form the basis of the sinus node and are present in the atrioventricular node. They are a true self-oscillating system and generate rhythm (60–70 impulses per minute).
- Transition cells — cylindrical cells with short T-tubules. They form the basis of the atrioventricular node and are found at the periphery of the sinus node. They occupy an intermediate position between typical and atypical cardiomyocytes, possessing automaticity and contractility.
Also, the conduction system 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 plate, which is a thickening of the visceral leaf of the splanchnotome. Development begins even before the embryo folds, when two myoepicardial plates appear in the splanchnotome leaf, surrounding the endothelial tubes from mesenchyme. During lateral folding of the embryo, these primordia approach each other. After the closure of the trunk folds, they merge into a single tube, forming the middle layer of the primary heart model—the myocardium.
The "LCP" rule (Longitudinal — Circular — Longitudinal) helps to remember the layers of the ventricular myocardium: the inner and outer layers are longitudinal, and the powerful middle layer is circular.
Class and exam questions
Is the cardiac muscle fiber a true symplast?
No, this is a functional fiber. It consists of individual cells (cardiomyocytes) connected to each other by intercalated discs, forming a single network.
Why are intercalated discs needed in the myocardium?
They mechanically fasten cardiomyocytes (via desmosomes and interdigitations) and ensure rapid impulse conduction from cell to cell (via nexuses).
What endocrine function does the myocardium perform?
Atrial secretory cells secrete atrial natriuretic factor (ANF). It stimulates the excretion of sodium and water by the kidneys, leading to a decrease in blood pressure.
What else to study on this topic
- Mechanism of excitation-contraction coupling (interaction of T-tubules and L-system)
- Energy metabolism of the myocardium: the role of fatty acids and ketone bodies
- Ultrastructure and biochemistry of myosecretory cardiomyocytes
- Intracellular remodeling during working myocardial hypertrophy
- Prospects for using autologous stem cells for post-infarction repair
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