Mucoid swelling: reversible phase
This is the initial and fully reversible stage of connective tissue disorganization. It is based on the accumulation of hydrophilic substances (primarily hyaluronic acid) and plasma proteins, especially globulins.
The cascade of reactions is triggered by microcirculatory damage. Due to the resulting tissue hypoxia, the enzyme hyaluronidase is activated, leading to the weakening of chemical bonds between glycosaminoglycans (GAGs) and proteins. Simultaneously, fibroblasts begin to actively synthesize new GAGs.
Histological picture:
- The ground substance swells, pushing the stromal cells apart.
- Collagen fibers split and swell, but retain their bundle structure.
- Macroscopically, organs are unchanged at this stage.
The main diagnostic sign is the metachromatic reaction. When stained with cationic dyes (e.g., toluidine blue), free acidic GAGs shift the medium pH, causing the tissue to acquire a lilac or purple color instead of blue.
Historical background: The term "mucoid swelling" was introduced by A.I. Strukov. Foreign pathologists practically do not use this term, preferring the concepts of "chromotropic edema" (proposed by V.T. Talalayev) or "Talalayev mucoid".
Fibrinoid swelling: point of no return
If the pathogenic factor continues to act, the process progresses to a profound and irreversible stage. Total disintegration of collagen, fibronectin, and depolymerization of GAGs occur. Vascular permeability increases sharply.
A fibrinoid forms in the lesion focus—a complex mass consisting of proteins from decaying fibers, nucleoproteins from destroyed cells, and plasma proteins. Fibrin is an obligatory component.
Microscopic changes:
- Collagen fibers merge into homogeneous masses.
- The tissue becomes sharply eosinophilic (plasma proteins block GAG residues, shifting the pH toward the alkaline side).
- Metachromasia is completely absent because glycosaminoglycans are destroyed.
- The PAS reaction (PAS) is sharply positive due to the abundance of glycoproteins.
The process is often accompanied by a cellular reaction (macrophages, plasmacytes) and can be systemic (in rheumatic diseases) or local (spasm and damage to arterioles in hypertension).
Vascular and tissue hyalinosis
Hyalinosis is the finale of profound destructive processes. It is always preceded by plasmarrhagia, fibrinoid swelling, inflammation, or necrosis.
The tissue undergoes irreversible compaction, vessel walls thicken and become homogeneous, and their lumen narrows. This inevitably leads to hypoxia, parenchymal atrophy, and organ sclerosis.
Classification of vascular hyaline:
- Simple hyaline. Formed from slightly altered plasma components. Found in benign hypertension, atherosclerosis, and sometimes in healthy individuals.
- Lipohyaline. Contains lipids and $\beta$-lipoproteins. Specific for patients with diabetes mellitus.
- Complex hyaline. Consists of immune complexes, fibrin, and cellular detritus. Characteristic of diseases with an immunopathological component (e.g., rheumatism).
Clinically, hyalinosis and the preceding fibrinoid necrosis lead to severe consequences: the formation of heart valve defects, loss of joint mobility, and the development of acute renal failure (when glomerular damage occurs against the background of malignant hypertension).
Briefly: answers to topical questions
In which diseases does complex vascular hyaline form?
Complex vascular hyaline forms in diseases with immunopathological disorders, particularly in rheumatic diseases. This type of hyaline is formed as a result of immunopathological processes accompanied by the destruction of fibrous structures and increased tissue-vascular permeability. Its composition includes immune complexes, antibodies, fibrin, and degenerating structures of the vessel wall.
What do organs look like macroscopically in pronounced hyalinosis?
Macroscopically, in hyalinosis, dense, homogeneous, translucent masses resembling hyaline cartilage are identified in tissues. The appearance of organs depends on the localization of the process:
- Glazed spleen (lien glacatus) — the organ capsule is sharply thickened, dense, whitish, translucent, and acquires a characteristic shiny, lacquered appearance.
- Hyalinosis of heart valves — the cusps are compacted, thickened, with adhesions forming between them, creating a "fishmouth" type of defect.
How is hyalinosis classified depending on localization?
Depending on the localization of hyaline masses, the process is classified into two main types.
- Hyalinosis of proper connective tissue — damage to the organ stroma, fibrous capsules, and scars.
- Vascular hyalinosis — damage predominantly to small arteries and arterioles.
Also, depending on the prevalence of the process, systemic (generalized) hyalinosis affecting many organs and local hyalinosis affecting a single organ or its capsule are distinguished.
What histological stains are used to detect fibrinoid swelling in addition to the PAS reaction?
In addition to the PAS reaction, hematoxylin and eosin staining, fibrin staining, and toluidine blue staining are used to detect fibrinoid swelling. With hematoxylin and eosin staining, bundles of collagen fibers appear as homogeneous eosinophilic masses. Staining for fibrin always yields a positive result, as fibrin is an obligatory component of fibrinoid. With toluidine blue staining, the metachromatic reaction is absent, which is associated with the almost complete destruction of glycosaminoglycans.
To memorize the stains: Mucoid — Metachromasia (tissue retains acidic GAGs and becomes lilac). Fibrinoid — Fixed (structures are destroyed, metachromasia disappears, tissue is brightly eosinophilic).
Class and exam questions
What is the fundamental difference between mucoid and fibrinoid swelling?
Mucoid swelling is a reversible process in which collagen bundles retain their structure and the tissue exhibits pronounced metachromasia. Fibrinoid swelling is irreversible; the fibers are completely destroyed, metachromasia disappears, and fibrin invariably appears in the tissue.
Why does metachromasia disappear in fibrinoid swelling?
This is due to the almost complete destruction and depolymerization of glycosaminoglycans. Their acidic radicals are blocked by plasma proteins, causing the tissue to become eosinophilic.
What type of vascular hyaline is detected in diabetes mellitus?
Lipohyaline, which contains lipids and $\beta$-lipoproteins, is most characteristic of diabetes mellitus.
What else to study on this topic
- Detailed mechanisms of tissue hypoxia development in microcirculatory disorders.
- Features of the cellular inflammatory response at the stages of fibrinoid swelling and necrosis.
- Pathogenesis of malignant arterial hypertension and its effect on the glomerular apparatus of the kidneys.
- Principles of histochemical detection of glycosaminoglycans using alcian blue and colloidal iron.
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