Mechanisms of chronic lung disease development
In pathomorphology, there are three main mechanisms leading to severe chronic processes in the respiratory system:
- Bronchogenic mechanism. Its basis is the impairment of pulmonary drainage function and bronchial patency. It is through this pathway that obstructive diseases develop: chronic bronchitis, bronchiectasis, and chronic obstructive pulmonary emphysema.
- Pneumoniogenic mechanism. Closely associated with previous bronchopneumonia, lobar pneumonia, and their complications (e.g., acute abscess or carnification). The outcome is the formation of a chronic abscess or chronic pneumonia. A distinctive feature is a pronounced restrictive component.
- Pneumonitogenic mechanism. The pathological process is localized in the interstitium of the respiratory compartments of the lungs. It is characterized by the development of chronic inflammation and fibrosis, which is typical for interstitial lung diseases.
Regardless of the mechanism, the finale of these processes is universal: pneumosclerosis (pneumocirrhosis), secondary pulmonary hypertension, right ventricular hypertrophy (forming "cor pulmonale"), and cardiopulmonary failure develop. In addition, a background condition for the development of lung cancer is created.
Etiology and molecular pathogenesis
The development of chronic bronchitis is triggered by exogenous and endogenous factors (ethnic characteristics, genetic predisposition). The most important factor is smoking (active and passive).
The pathogenesis of tobacco smoke exposure includes:
- Suppression of bronchial epithelial cilia activity, which leads to impaired mucociliary clearance.
- An inhibitory effect on alveolar macrophages, which disrupts the mechanisms of local pulmonary defense.
- Direct damage to the surface epithelium followed by the development of squamous cell metaplasia.
- A secondary compensatory reaction — hyperplasia of mucus-producing cells.
Other factors include atmospheric pollutants (sulfur dioxide and nitrogen oxide emissions in urbanized areas) and occupational hazards (contact with organic and mineral dusts, toxic gases).
At the molecular level, these triggers cause the stimulation of nerve endings by acetylcholine, the expression of mucociliary clearance and mucin genes, mucous hyperproduction, and the release of transforming growth factor beta (TGF-beta).
Pathomorphological changes
The morphological picture consists of two parallel lines: epithelial changes and stromal inflammation. The most pronounced changes are recorded at the level of small bronchi.
Macroscopically: Bronchial walls appear thickened, surrounded by connective tissue layers, and deformed. With prolonged progression, bronchiectasis forms — saccular or cylindrical expansions of the bronchial lumens.
Microscopically: The process proceeds as chronic mucous or purulent catarrhal inflammation.
- Changes in epithelium and glands: Begin with damage and desquamation (shedding). This is followed by hyperplasia of bronchial glands and goblet cells. Metaplasia develops: mucous (increase in the number of goblet cells) and squamous (replacement of ciliated epithelium with stratified squamous). The cascade is completed by dysplasia — a violation of cell structure and differentiation, which is a precancerous process.
- Changes in the stroma: Characterized by pronounced inflammatory infiltration. Infiltration cells release inflammatory and tissue-damaging mediators: proteases (destroying tissues), reactive oxygen species, tumor necrosis factor (TNF), and interleukin-8 (IL-8). An antiprotease imbalance occurs. Granulation tissue proliferates in the wall (sometimes with the formation of inflammatory mucosal polyps), and the muscular layer undergoes sclerosis and atrophy. Driven by cytokines and growth factors, fibrosis progresses.
Morphometry and complications
To objectively assess mucous gland hyperplasia (one of the main signs of the disease), the Reid index is used. This is the ratio of the thickness of the submucosal glands to the thickness of the entire bronchial wall.
- Normally, the index is 0.44 ± 0.09.
- In pathology, it increases to 0.52 ± 0.08.
Note: at the level of small bronchi, it is the assessment of goblet cell hyperplasia that has diagnostic significance.
Complications: The course of the disease can be complicated by secondary infection with the development of bronchopneumonia. Impaired bronchial patency leads to the formation of atelectasis foci and obstructive pulmonary emphysema. Ultimately, pneumofibrosis develops.
Briefly: answers to topical questions
What clinical and morphological forms of chronic bronchitis are distinguished depending on the nature of the exudate?
In chronic bronchitis, microscopic changes are associated with the development of two variants of catarrhal inflammation:
- chronic mucous catarrhal inflammation;
- chronic purulent catarrhal inflammation.
The "DMPH" rule will help remember the epithelial change cascade: Damage -> Metaplasia -> Dysplasia -> Hyperplasia.
Class and exam questions
Which part of the respiratory tract suffers the most in chronic bronchitis?
The most pronounced pathomorphological changes are localized at the level of small bronchi.
What is the Reid index and what is it used for?
This is a morphometric indicator — the ratio of the thickness of the submucosal glands to the thickness of the entire bronchial wall. It allows for an objective assessment of the degree of glandular hyperplasia.
Why does chronic bronchitis increase the risk of lung cancer?
Due to constant epithelial damage, squamous cell metaplasia develops, which eventually transitions into dysplasia — a precancerous process. Therefore, the disease is considered a background condition for oncology.
What else to study on this topic
- Bronchiectasis: types and morphology of bronchiectasis
- Chronic obstructive pulmonary emphysema
- Pneumoniogenic and pneumonitogenic mechanisms of pulmonary pathology
- Morphology of cor pulmonale and pulmonary hypertension
- Interstitial lung diseases and the development of fibrosis
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