Transition terminal bronchiolus - respiratory bronchiolus (human)

Stain: Azan. A longitudinal section shows the transition from the lumen of a terminal bronchiolus (1) into that of a respiratory bronchiolus (2). The epithelium of the terminal bronchiolus is regularly columnar and ciliated. The respiratory bronchiolus has an irregular lining and is covered by low columnar-cuboidal cells (↑); an alveolar pouch (3a) and few areas within the same lumen are lined by the common thin alveolar epithelium (small arrow heads) as found in the surrounding alveoli (3b). At (4) darker stained spots of irregular smooth muscle. (5) indicates a pulmonary artery and (*) smaller arterial branches. (6) showed carbon particles in macrophages.