In a study of respiratory mucosa, which feature is last to disappear as the airway becomes distal?

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

Answer: (D)

In the conducting airways, multiple protective features are lost in a stepwise fashion as you move distally. The wall and lining shift from a rugged, mucus-clearing setup to a thinner, less protected lining as oxygen exchange becomes the focus.

Cartilage provides structural support in the trachea and larger bronchi, but as you go toward smaller airways, the cartilage rings are lost, and the airways become more collapsible. Submucosal mucous glands, which secrete mucus to trap particles, also disappear with distal progression. Goblet cells—the mucus-producing cells in the epithelium—are abundant in the trachea and larger bronchi but decline as the epithelium becomes simpler toward smaller airways and are largely absent in the distal bronchioles. Through all of these changes, the ciliated epithelial cells that sweep mucus toward the pharynx persist the longest, maintaining mucociliary clearance up to the distal conducting airways. Only near the very end of the conducting pathway do ciliated cells disappear, giving way to nonciliated, secretory Clara cells in the smaller airways and, beyond that, to respiratory units without cilia.

Thus, the feature that remains longest as the airway becomes distal is the cilia, making it the last to disappear.