Figure 1.

Major cell types of the lung epithelium. In the large airways (2⁰ to 2⁵ branches), the major cells types are ciliated, undifferentiated columnar, secretory, and basal cells. In the small airways (2⁶ to 2²³ branches), the cell types are similar, with relatively more ciliated cells, and the secretory cells shift to the Clara cell type. After 2²³ branches, the airway epithelium merges with the alveolar epithelium, with type I and type II cells. Not shown are a variety of less common cell types, such as cartilage cells and mucus glands in the large airways, and neuroendocrine cells.