Figure 2. Composition of the airway epithelium varies with airway diameter.

The conducting airways include the air passages from the nasal cavity to the terminal bronchioles. The same major cell types compose the airway epithelium lining in all these parts, but the relative proportion of each cell type varies with the airway diameter, which is consistent with differences in functional requirements. This is also evident when comparing the cellular composition of the human conducting airways with mouse airways. Mucous goblet cells are rare in the mouse airway epithelium beyond the upper airway; similarly, club cells are rare in human airways larger than the terminal bronchioles. The pie charts indicate approximate proportions of the major cell types at different locations of the respiratory system (compiled from references^9,154).