General Architecture of the Heart Wall and Valves
Embryological development results in a four-chambered heart consisting of 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 correlates with functional load. The myocardium of the left ventricle performs the highest workload and measures 10–15 mm in thickness. The right ventricular wall is thinner at 5–8 mm, while the atria, experiencing minimal load, measure only 2–3 mm.
Valvular structures are located within the chambers. Atrioventricular and semilunar valves (located at the junctions of ventricles and great vessels) are anchored to fibrous rings, which restrict the orifices and serve as a skeletal framework.
- The right atrioventricular valve is the tricuspid valve (valvula tricuspidalis), anchored by chordae tendineae originating from three papillary muscles in the right ventricle.
- The left atrioventricular valve is the bicuspid (mitral) valve (valvula mitralis), corresponding to two more robust papillary muscles.
Papillary muscles, developing from the myocardium, use tendinous cords (chordae tendineae) to hold the cusps in place, preventing their inversion into the atria during ventricular contraction.
Histology of the Endocardium
The endocardium continuously lines all heart chambers from the inside. Embryologically and histologically, this membrane is analogous to a blood vessel (artery) wall. Four consecutive layers are identified in the endocardium (from the chamber lumen outward):
- Endothelium — the innermost layer resting on a basement membrane and facing the chamber lumen.
- Subendothelial layer — formed by loose connective tissue.
- Myoelastic layer — contains smooth myocytes and elastic fibers, providing elasticity and tone to the membrane.
- Outer connective tissue layer — the deepest stratum, bordering directly on the myocardium.
A unique feature of the endocardium is its trophic (nutritional) mechanism. Blood vessels are exclusively present in the deep outer connective tissue layer. The remaining layers lack intrinsic vessels and receive nutrients via diffusion from the blood within the heart chambers and from vessels of the outer layer.
Structure of Heart Valves
The core of a valve cusp consists of dense fibrous connective tissue. Both sides of the cusp are covered by endothelium. A key feature of the valves is the tissue asymmetry of their surfaces:
- The atrial side is characterized by a predominance of elastic fibers (visualized well withorcein staining), providing elasticity.
- The ventricular side is formed predominantly of collagen fibers. Tendinous cords from the papillary muscles insert precisely into this side, ensuring high mechanical tensile strength.
The substance of the atrioventricular valve cusps lacks blood vessels. Tissue nutrition is achieved through the diffusion of substances directly from the blood flowing through the chambers (similar to the trophic supply of the superficial endocardial layers).
Epicardium: The Outer Layer
The epicardium is a serous membrane 0.3–0.4 mm thick, representing the visceral layer of the pericardium. It is firmly fused to the myocardium. Structurally, the epicardium comprises (from superficial to deep):
- Mesothelium — a simple squamous epithelium of mesodermal origin.
- Thin connective tissue plate — containing alternating layers of collagen and elastic fibers, as well as blood vessels.
Subepicardial adipose tissue may lie deep to the connective tissue plate. If this adipose layer is absent, the epicardium directly abuts the myocardium.
At the base of the great vessels (aorta, pulmonary trunk, venae cavae), the epicardium reflects to become the parietal layer of the serous pericardial sac. Between the epicardium and the parietal pericardium lies the slit-like pericardial cavity, which represents a remnant of the embryonic coelomic cavity.