Which feature is last to disappear as the epithelium changes along the respiratory tract?

• A. Mucous gland
• B. Cilia
• C. Goblet cells
• D. Cartilage

Answer: (B)

As you move down the airways, the lining undergoes a stepwise change from a tall, ciliated, mucus-producing surface to a simpler, non-ciliated one. Submucosal mucus glands are abundant in the trachea and larger bronchi but dwindle in the smaller airways. Goblet cells, which secrete mucus, are plentiful in the trachea and bronchi but disappear as you reach the bronchioles. Cartilage rings support the trachea and bronchi, but disappear as you progress into bronchioles. Cilia, the tiny hair-like projections that move mucus out of the airway, persist through the larger conducting airways and are only absent once you arrive at the non-ciliated, simple cuboidal epithelium of the terminal bronchioles. Because they endure longest before disappearing, cilia are the feature that lasts the latest along the respiratory tract.