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 and chondroitin sulfate acids, heparin). Acidic forms form flexible bonds with metabolites and participate in their transport.
- Glycoproteins, which include mucins (the basis of mucous membrane and glandular secretions) and mucoids (structural components of various tissues).
Methods of morphological identification
Special stains of histological sections are used for the precise detection of carbohydrates:
- PAS reaction (Periodic acid-Schiff reaction). The initial compounds are oxidized by periodic acid or potassium periodate to aldehydes, which upon contact with Schiff's reagent yield a persistent red color.
- Glycogen differentiation. The PAS reaction is used in combination with enzymatic control (pretreatment with amylase) or independent staining with 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 change in the original color of the dye).
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 hypothalamo-hypophyseal system, pancreas, thyroid gland, and adrenal glands.
Glucose metabolism disorders lead to intracellular glycogen accumulations:
- In diabetes mellitus, against the background of hyperglycemia and glucosuria, glycogen accumulates in the epithelium of the renal loop of Henle, hepatocytes, and pancreatic $\beta$-cells.
- In glycogenoses (hereditary storage enzymopathies), due to a deficiency of cleavage enzymes, glycogen is deposited in the liver, kidneys, muscles, gastrointestinal tract, and erythrocytes.
| Glycogenosis type | Disease name | Glycogen structure |
|---|---|---|
| Type I | Von Gierke disease | Unchanged |
| Type II | Pompe disease | Unchanged |
| Type III | Cori disease | Sharply altered |
| Type IV | Andersen disease | Sharply altered |
| Type V | McArdle disease | Unchanged |
| Type VI | Hers disease | Unchanged |
Disorders of glycoprotein metabolism (mucous and colloid dystrophy)
Mucous dystrophy is characterized by the accumulation of mucins and mucoids in cells or the stroma. The process is accompanied by hypersecretion of mucus, changes in its viscosity, death and desquamation of the epithelium, obstruction of glands with the development of cysts, and secondary inflammation. The main cause is catarrhal inflammation of mucous membranes, as well as systemic hereditary diseases (cystic fibrosis). A clinical example is the blockage of bronchi by mucus with the formation of atelectasis and pneumonia.
In colloid dystrophy, pseudomucins—mucus-like substances that condense and acquire a colloid-like appearance—accumulate in glandular tissue (typical for colloid goiter).
Briefly: answers to topical questions
Which specific enzymes deficiency causes type I, II, and III glycogenoses?
Glycogenoses of types I, II, and III develop due to a hereditary deficiency of specific enzymes involved in glycogen metabolism.
- Type I glycogenosis (von Gierke disease) is caused by a deficiency of glucose-6-phosphatase (in subtype Ia) or glucose-6-phosphate translocase (in subtype Ib).
- Type II glycogenosis (Pompe disease) occurs due to a defect in alpha-1,4-glucosidase (acid maltase), which leads to the accumulation of excess glycogen in lysosomes.
- Type III glycogenosis (Cori disease) is caused by a deficiency of amylo-1,6-glucosidase.
PAS and Best's carmine stain red—glycogen has no match! (The PAS reaction and Best's carmine stain red when detecting glycogen).
Class and exam questions
How does labile glycogen differ from stable glycogen?
Labile glycogen is stored in the liver and skeletal muscles and is easily used up to meet energy demands. Stable glycogen is part of the heart's conduction system, nervous tissue, and cartilage, maintaining a constant level.
In which parts of the kidneys does glycogen deposit in diabetes mellitus?
Due to severe glucosuria, glycogen deposits are detected intracellularly in the epithelium of the loop of Henle (the thin segment and the distal tubules).
What is the metachromasia phenomenon?
This is the ability of chromotropic substances (glycosaminoglycans and mucins) to alter the original color of a dye (e.g., toluidine blue or methylene blue) during histochemical examination.
What else to study on this topic
- Neuroendocrine regulation of glycogen metabolism (role of insulin, adrenaline, glucocorticoids)
- Differential diagnosis of types I–VI hereditary glycogenoses
- Pathomorphology of catarrhal inflammation of mucous membranes
- Morphogenesis of colloid goiter in colloid dystrophy
- Complications of cystic fibrosis and bronchial mucus obstruction
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