The tracheobronchial tree undergoes metaplasia—normally columnar epithelium with mucociliary clearance converting to squamous epithelium—as a protective adaptive response to chronic irritants such as cigarette smoke; this metaplastic pathway is distinct from the esophagus, whose native squamous epithelium can progress directly to squamous cell carcinoma from smoking without an intervening metaplastic change (in contrast to Barrett's esophagus, a metaplastic change driven by acid reflux). This region is also anatomically the site of characteristic proximal infections (e.g., parainfluenza-associated croup and Klebsiella-associated bronchopneumonia) and gives rise to centrally located bronchogenic carcinomas (squamous cell, small cell, and adenocarcinoma), while chronic bronchitis is formally defined by three consecutive months of productive cough over two consecutive years. This is a topic within pulmonary pathology relating epithelial adaptation, infection localization, and carcinogenesis along the respiratory tree.
The tracheobronchial tree undergoes metaplasia—normally columnar epithelium with mucociliary clearance converting to squamous epithelium—as a protective adaptive response to chronic irritants such as…