Transition of secondary bronchus to bronchiole

A secondary bronchus is identified by a pseudostratified columnar epithelium with cilia and goblet cells. The lamina propria possesses a smooth muscle layer and mixed glands (not present here). Cartilage plates maintain patency.

A bronchiole is lined with a lower pseudostratified columnar epithelium with cilia and goblet cells. A smooth muscle continues in the lamina propria, but neither cartilage nor mixed glands persist.

Smooth muscle is present in both passageways, but cartilage plates are present only in the secondary bronchus and not in the bronchiole.

Smooth muscle is present in both passageways, but cartilage plates are present only in the secondary bronchus and not in the bronchiole.

Alveoli are visible in the field and therefore these passageways are intrapulmonary. However, the alveolar lumens are not continuous with those of the larger passageways and thus neither the secondary bronchus nor bronchiole are capable of gas exchange.

Macrophages with phagocytosed carbon particles lie in the connective tissue adjacent to both passageways. These cells are often referred to as dust cells.