Transition of respiratory bronchiolus into alveolar duct in the lung (human)

Stain: Azan. The alveolar duct (1↔) exhibits an alveolated wall but at a few places (3) bronchiolar characteristics are evident such as cuboidal epithelium covering small areas of smooth muscle and connective tissue. They differ from the characteristic alveolar tips (2↓) of neighboring thin-walled alveoli (A). The tips (2) contain elastin masked by collagen (blue-stained) and are covered by thin alveolar cells type I.