Pathology · patient case question
Chronic smoke exposure causes metaplasia of the respiratory epithelium. What does it change INTO?
Patient case: Long-standing chronic obstructive pulmonary disease
Patient: Male, 67 years old
Chief Complaint: “I get short of breath when you lean the chair back.”
Background: COPD from a 45 pack-year smoking history. Uses tiotropium and a salmeterol/fluticasone inhaler. Chronic productive cough.
Vital Signs: BP 138/84 mmHg · HR 88 bpm · RR 22 · SpO₂ 93% on room air · barrel chest, pursed-lip breathing
Findings: Xerostomia with cervical caries. Palatal erythema consistent with candidiasis. Cannot tolerate a supine position.
Show answer and explanation
Correct answer: B. Stratified squamous epithelium
- A. Simple squamous epithelium is the single flat layer of alveoli, endothelium and mesothelium — built for exchange, not protection, and the opposite of what a chronically injured airway needs.
- B. CORRECT. METAPLASIA is the reversible replacement of one differentiated cell type by another better able to withstand a chronic stress. In the airway, the normal PSEUDOSTRATIFIED CILIATED COLUMNAR epithelium with goblet cells is replaced by STRATIFIED SQUAMOUS epithelium, which is tougher but has NO CILIA and NO mucus clearance. That loss of the mucociliary escalator is why secretions accumulate and why these patients get recurrent infections. Persistent injury on a metaplastic field is the substrate on which DYSPLASIA and ultimately carcinoma develop.
- C. Simple cuboidal epithelium lines ducts and renal tubules and provides no protective advantage against inhaled irritants.
- D. Transitional (urothelial) epithelium is confined to the urinary tract, where it accommodates distension.
Note: Two other metaplasias worth carrying: BARRETT oesophagus, where squamous becomes columnar intestinal-type under acid reflux — the mirror image of this one — and squamous metaplasia of salivary ducts after obstruction or irradiation, which can be mistaken for carcinoma histologically.