Pseudostratified ciliated columnar becomes simple squamous as the airway narrows
Expansion
| Level | Epithelium | Cartilage | Smooth muscle | Glands |
|---|---|---|---|---|
| Trachea and bronchi | Pseudostratified ciliated columnar with goblet cells | Yes | Present | Yes |
| Bronchioles | Simple ciliated columnar to cuboidal, club cells replacing goblet cells | No | Relatively most | No |
| Terminal bronchiole | Simple cuboidal | No | Yes | No |
| Respiratory bronchiole | Cuboidal with alveoli in the wall | No | Sparse | No |
| Alveolus | Simple squamous (type I pneumocyte) | No | No | No |
The progressive changes are systematic: cartilage disappears, goblet cells are replaced by club cells, cilia are lost, the epithelium thins, and the proportion of smooth muscle rises relative to wall thickness.
Two clinical consequences: the bronchioles have the highest proportion of smooth muscle and no cartilage, which is why they are the site of bronchoconstriction and of collapse in emphysema; and the loss of cilia distally means particles reaching the alveoli are cleared by macrophages rather than the mucociliary escalator.
Club (Clara) cells secrete surfactant protein, detoxify inhaled agents via cytochrome P450, and act as progenitor cells for bronchiolar repair.