Fig. 1.

Full differentiation of primary tracheobronchial epithelial (TBE) cells in air-liquid interface (ALI) culture conditions requires all-trans retinoic acid (ATRA). Human bronchial epithelial HBE1 and primary TBE cells were grown in ALI culture conditions with or without ATRA for 7 days followed by fixation in 4% paraformaldehyde and embedded in paraffin. Cross sections of membrane-adherent cells were then stained with hematoxylin and eosin for morphological analysis. HBE1 cells grown without ATRA (A) displayed a stratified squamous epithelial phenotype compared with HBE1 cells treated with ATRA (B), which displayed a simple squamous or thin stratified squamous epithelial phenotype. C: primary TBE cells grown without ATRA displayed a keratinized stratified squamous epithelial phenotype, whereas those treated with ATRA (D) displayed a pseudostratified columnar ciliated epithelial phenotype similar to respiratory epithelium in vivo. Black bar represents 10 μm.