Endocardium and epicardium

General architecture of the heart wall and valves

During development, a four-chambered heart is formed, consisting of the right and left atria and ventricles. The heart wall has a classic three-layered structure: the inner layer is the endocardium, the middle (muscular) layer is the myocardium, and the outer (serous) layer is the epicardium.

The thickness of the muscular wall directly depends on the functional load. The myocardium of the left ventricle performs the most work and has a thickness of 10–15 mm. The wall of the right ventricle is thinner at 5–8 mm, while the atria, experiencing minimal load, are only 2–3 mm thick.

The valvular apparatus is located inside the cavities. Atrioventricular and semilunar valves (located at the junction of the ventricles and large vessels) are anchored to the fibrous ring, which bounds the orifices and serves as a supporting skeleton.

• The right atrioventricular valve is the tricuspid valve (valvula tricuspidalis), to which chordae tendineae attach from the three papillary muscles of the right ventricle.
• The left atrioventricular valve is the bicuspid (mitral) valve (valvula mitralis). It corresponds to two more powerful papillary muscles.

Papillary muscles, developing from the myocardium, hold the cusps via tendinous cords (chordae tendineae) to prevent their inversion into the atria during contraction.

Histology of the endocardium

The endocardium continuously lines all heart cavities from the inside. By embryonic origin and tissue structure, this membrane is analogous to the wall of a blood vessel (artery). The endocardium consistently distinguishes four layers (from the chamber lumen inward):

1. Endothelium — the innermost layer, resting on the basal membrane and facing the chamber lumen.
2. Subendothelial layer — formed by loose connective tissue.
3. Musculo-elastic layer — contains smooth myocytes and elastic fibers. It provides elasticity and tone to the membrane.
4. Outer connective tissue layer — the deepest stratum, bordering directly on the myocardium.

A feature of the endocardium is the mechanism of its nutrition (trophism). Blood vessels are present exclusively in the deep outer connective tissue layer. The remaining layers lack their own vessels and receive nutrition by diffusion — from the blood in the heart chambers and from the vessels of the outer layer.

Structure of the heart valves

The base of a valve cusp consists of dense fibrous connective tissue. Both sides of the cusp are covered with endothelium. A key feature of the valves is the tissue asymmetry of their surfaces:

• Atrial surface is characterized by a predominance of elastic fibers (well visualized when stained with orcein). This gives the valve elasticity.
• Ventricular surface is formed predominantly by collagen fibers. It is into this side that the chordae tendineae from the papillary muscles insert, ensuring high mechanical tensile strength.

There are no blood vessels within the cusps of the atrioventricular valves. Tissue nutrition is provided by diffusion of substances directly from the blood flowing through the chambers (similar to the trophism of the superficial layers of the endocardium).

Epicardium: the outer layer

The epicardium is a serous membrane 0.3–0.4 mm thick, which represents the visceral layer of the pericardium. It is firmly fused with the myocardium. Structurally, the epicardium includes (from outside to inside):

1. Mesothelium — a simple squamous epithelium of mesodermal origin.
2. Thin connective tissue plate — contains alternating layers of collagen and elastic fibers, as well as blood vessels.

Subepicardial adipose tissue may be located beneath the connective tissue plate. If the adipose layer is absent, the epicardium lies directly against the myocardium.

At the base of large vessels (aorta, pulmonary trunk, venae cavae), the epicardium transitions into the parietal layer — the serous layer of the pericardial sac. Between the epicardium and pericardium is the slit-like pericardial cavity, which is a remnant of the embryonic coelomic cavity.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of embryonic development of papillary muscles from the myocardium.
• Histophysiology of the musculo-elastic layer of the endocardium.
• Structural features of the semilunar valves of the aorta and pulmonary trunk.
• Differences between the parietal and visceral layers of the pericardium.

Other topics in the section "Cardiovascular system: veins, lymphatic vessels, heart"

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