Phenotypic markers of human small airway epithelial cells (SAECs) cultured at an air-liquid interface. Day 0 represents the first day of air-liquid interface which is generally 48 hours after seeding on the membrane, and all representative images were taken at day 7. A: Immunofluorescence imaging of MUC5AC (green) demonstrating the presence of mucous granules. B: Immunofluorescence imaging of β-tubulin IV (green) demonstrates the presence of ciliated cells. C: Immunofluorescence imaging of zonula occludens-1 (ZO-1) (green), a key protein present in the intercellular tight junctions. Cell nuclei were counterstained blue by DAPI. D: Transepithelial electrical resistance (TER) was measured every other day from 2 days of ALI. TER data represent the mean response from 2 different donors (12 monolayers from each donor). Scale bar: 10 μm.