Clinical-Functional Characteristics and Morphogenesis
The pathology exhibits specific clinical features. The disease often manifests following antibiotic intake and presents with a subacute onset. The clinical course is typically fluctuating, yet generally regarded as relatively favorable. Pulmonary function testing reveals prominent obstructive changes linked to small airway involvement.
Pathogenesis follows a strict sequence of three stages:
- Initial pathological activation of alveolar macrophages directly within the bronchioles.
- Pronounced exudation, characterized by fluid and cellular migration into the lumens of bronchioles and alveoli.
- Final organization of the accumulated exudate, involving its gradual replacement by connective tissue.
Pathology and Staging of the Process
Bronchiolitis obliterans organizing pneumonia (also known as cryptogenic organizing pneumonia) differs fundamentally from other forms of idiopathic fibrosing alveolitis. The main distinction lies in the precise localization: the process strictly affects the terminal bronchioles and alveolar ducts.
Characteristic morphological features include:
- A combination of carnification foci in the distal respiratory tract (bronchioles, alveolar ducts, and alveoli).
- Presence of chronic, moderately severe interstitial inflammation.
- Preservation of the overall lung tissue architecture despite inflammation.
- Uniformity of microscopic lesions.
The pathological process is divided into two stages:
- Early stage. Characterized predominantly by intra-alveolar edema and marked exudation within alveolar and bronchiolar lumens. The tissue is edematous, showing lymphohistiocytic infiltration mixed with neutrophils. Young granulation tissue begins to form around the peribronchiolar zone (near small bronchi and terminal bronchioles), accompanied by prominent basal cell hyperplasia.
- Late stage. Characterized by peribronchial sclerosis. Fully developed carnification foci form, wherein alveoli are irreversibly replaced by connective tissue. Bronchioles become deformed; their inner surfaces are lined by hyperplastic basal cells exhibiting atypical features. At the molecular level, there is upregulated expression of growth factors—TGFβ and FGFb—by both epithelial and interstitial cells.
Dysregenerative Epithelial Changes
A critical component of BOOP pathology is the remodeling of the epithelial lining. Dysregenerative changes manifest as three types of hyperplasia: goblet cell, basal cell, and atypical basal cell.
- Goblet cell hyperplasia. Normal epithelial cells are replaced by goblet cells, which are small with nuclei compressed toward the basement membrane. Their primary pathological function is the hyperproduction of Alcian-positive mucus. Interspersed among goblet cells are columnar cells with dark cytoplasm and small mucous granules in their apical regions.
- Basal cell hyperplasia. Manifests as an increased number of basal cell layers (ranging from 2 to 8 layers) located directly beneath the columnar epithelium. The cells feature relatively large, centrally located, dark-staining nuclei with a fine chromatin network and smooth nuclear membranes. The cytoplasm forms a thin basophilic rim. This type of hyperplasia shows a clear maturation gradient from the basement membrane toward the surface: as cells move upward, nuclei become lighter and the cytoplasmic rim widens.
- Atypical basal cell hyperplasia. This form exhibits clear signs of cellular atypia, including marked cellular and nuclear pleomorphism. Both cell and nuclear sizes are pathologically enlarged, and nucleoli become distinctly visible under microscopy.