Figure 1.

Schematic overview of the different cell types within the respiratory epithelium. Trachea to bronchi: the epithelial surfaces of the trachea and proximal airways are lined by a pseudostratified columnar epithelium consisting of basal, brush, ciliated, goblet, and club cells, along with the less frequent pulmonary neuroendocrine cells (PNECs) and ionocytes. Bronchioles to alveoli: small airways are lined by a simple columnar or cuboidal epithelium that consists of ciliated, club, and a few goblet cells. Alveoli: the alveoli are lined by either squamous alveolar type 1 (AT1) or cuboidal alveolar type 2 (AT2) cell.