Classification of Tissue Carbohydrates
In histochemical practice, three key groups of carbohydrate compounds are distinguished:
- Polysaccharides, the main representative of which in animal tissues is glycogen.
- Glycosaminoglycans (mucopolysaccharides): neutral (tightly bound to protein molecules) and acidic (hyaluronic acid, chondroitin sulfate, heparin). Acidic forms establish flexible bonds with metabolites and participate in their transport.
- Glycoproteins, which include mucins (the basis of secretions in mucous membranes and glands) and mucoids (structural components of various tissues).
Methods of Morphological Identification
Special stains on histological sections are used for precise carbohydrate detection:
- PAS reaction (Periodic acid–Schiff reaction). Initial compounds are oxidized by periodic acid or potassium periodate to aldehydes, which yield a persistent red color upon contact with Schiff's reagent.
- Glycogen differentiation. The PAS reaction is combined with enzymatic control (preliminary amylase treatment) or performed using Best's carmine, which stains glycogen red.
- Detection of mucins and glycosaminoglycans. Toluidine blue or methylene blue is used. The presence of chromotropic substances is judged by the phenomenon of metachromasia (a shift in the original dye color).
Glycogen Metabolism Pathology and Glycogenoses
Normally, labile glycogen (liver, skeletal muscle) is mobilized upon metabolic demand, whereas stable glycogen (nerve cells, cardiac conduction system, endothelium, aorta, cartilage) remains a constant component. Regulation is carried out by the hypothalamic-pituitary system, pancreas, thyroid, and adrenal glands.
Disturbances in glucose metabolism lead to intracellular accumulation of glycogen:
- In diabetes mellitus, against the background of hyperglycemia and glucosuria, glycogen accumulates in the epithelium of the loop of Henle in the kidneys, hepatocytes, and pancreatic $\beta$-cells.
- In glycogenoses (inherited storage enzymopathies), due to a deficiency of cleavage enzymes, glycogen deposits in the liver, kidneys, muscles, gastrointestinal tract, and erythrocytes.
| Type of glycogenosis | Disease name | Glycogen structure |
|---|---|---|
| Type I | von Gierke disease | Unaltered |
| Type II | Pompe disease | Unaltered |
| Type III | Cori disease | Markedly altered |
| Type IV | Andersen disease | Markedly altered |
| Type V | McArdle disease | Unaltered |
| Type VI | Hers disease | Unaltered |
Glycoprotein Metabolism Disorders (Mucous and Colloid Dystrophy)
Mucous dystrophy is characterized by the accumulation of mucins and mucoids within cells or the stroma. The process is accompanied by mucus hypersecretion, altered viscosity, epithelial death and desquamation, gland obstruction with cyst formation, and secondary inflammation. The primary cause is catarrhal inflammation of mucous membranes, as well as systemic hereditary diseases (cystic fibrosis). A clinical example is bronchial obstruction by mucus leading to atelectasis and pneumonia.
In colloid dystrophy, glandular tissue accumulates pseudomucins—mucus-like substances that thicken and acquire a colloid-like appearance (typical for colloid goiter).