Hematoxylin-eosin staining and the principle of distinction
The overwhelming majority of histological slides are stained with the hematoxylin-eosin combination. Each of these stains has a strict chemical affinity, and understanding this logic is the foundation of reading the slide.
- Hematoxylin stains acidic structures blue and blue-violet. Since the main cellular acids are DNA and RNA, primarily the nuclei will be stained blue, as well as areas of the cytoplasm rich in ribosomes.
- Eosin stains basic structures in shades of red and pink. These are predominantly cytoplasmic proteins, including myofibrils, collagen fibers, and erythrocytes.
A key skill for a histologist is distinction, i.e., the ability to differentiate subtle shades: red and carmine-red, basophilic blue and violet, eosinophilic pink and intense red. Without developed distinction, it is impossible to correctly identify tissues and cellular elements.
Brief information on cardiac embryology
Understanding cardiac embryogenesis facilitates the study of its histology and clinical defects. The embryo develops from three germ layers, among which mesenchyme, a derivative of the mesoderm, plays a key role for the cardiovascular system.
During lateral folding of the embryo, the mesenchymal cords close together, forming a cavity and the primordium of the primitive heart tube. This S-shaped structure is divided into two sections:
- posterior section — the future atrium;
- anterior section — the future ventricle.
Knowledge of this scheme subsequently allows us to understand the mechanisms underlying congenital heart defects studied in anatomy, pathology, and clinical departments.
Three tunics of the heart: endocardium, myocardium, epicardium
The heart wall consists of three consecutive layers:
- Endocardium — the inner tunic, contacting the blood;
- Myocardium — the middle muscular tunic, providing contractile function;
- Epicardium — the outer tunic, the visceral layer of the pericardium.
The structural logic of the endocardium is determined by the need to prevent contact activation of coagulation. If erythrocytes were to come into contact with a rough surface, intracardiac thrombi would develop, followed by pulmonary embolism and other vessel blockages. Therefore, the inner lining of the heart is constructed on the principle of a vascular wall.
Layered structure of the endocardium and valves
The endocardium includes four layers (from inside out):
- Endothelium — simple squamous epithelium, analogous to vascular endothelium;
- Subendothelial layer — loose fibrous connective tissue;
- Musculo-elastic layer — contains smooth myocytes and elastic fibers;
- Outer connective tissue layer — loose connective tissue bordering the myocardium.
Important feature: the endocardium practically lacks its own blood vessels and is nourished diffusely from the heart chambers.
Heart valves are folds of the endocardium and have a simpler structure: they are covered with endothelium on both sides, and the core of the valve consists of dense fibrous connective tissue, providing mechanical strength.
Myocardium and epicardium: structure and clinical correlations
Myocardium is built of cardiac striated muscle tissue. Unlike skeletal muscle, cardiomyocyte nuclei are centrally located, and the cells themselves are interconnected via intercalated discs. A distinction is made between contractile and conducting cardiomyocytes, the latter forming the conduction system of the heart.
The layering of the myocardium varies:
- in the atria — two layers (outer transverse and inner longitudinal);
- in the ventricles — three layers (outer oblique, middle circular, inner longitudinal).
With age, the pigment lipofuscin accumulates in cardiomyocytes as a sign of physiological aging; in parallel, partial replacement of muscle tissue by adipose tissue occurs.
Epicardium is externally covered by mesothelium — simple squamous epithelium of serous membranes. Beneath the mesothelium lies connective tissue with a large number of vessels and nerves. Blood supply to the heart is delivered precisely via the epicardium: coronary arteries branch from the epicardium deep into the myocardium and further to the endocardium.
Clinical consequence of this architecture: the endocardium is most vulnerable to hypoxia. Therefore, myocardial ischemia and infarction more frequently begin in the subendocardial regions, and it is precisely there that microfocal ("non-transmural") infarctions form.
Key points
- Hematoxylin stains acids (nuclei, RNA) blue, while eosin stains protein structures red-pink.
- The heart develops from mesenchyme: the primitive heart tube is divided into posterior (atrium) and anterior (ventricle) segments.
- The heart wall consists of the endocardium, myocardium, and epicardium; the endocardium structurally replicates the vascular wall to prevent thrombosis.
- The endocardium has four layers: endothelium, subendothelial, musculo-elastic, and outer connective tissue layers.
- Heart valves contain dense fibrous connective tissue covered by endothelium.
- Atrial myocardium is two-layered, ventricular myocardium is three-layered; cardiomyocytes are connected by intercalated discs.
- The epicardium is covered with mesothelium and rich in blood vessels, which explains the predominantly subendocardial localization of ischemia.
- Age-related changes in the myocardium include the accumulation of lipofuscin and fatty replacement.