In vivo, bronchial epithelial cells differentiate along an abnormal squamous pathway under conditions of retinoid deficiency. The squamous phenotype is characterized by the induction of specific markers such as keratin 13, and the enzymes transglutaminase type I and cholesterol sulfotransferase.

These markers have been used to demonstrate that primary bronchial epithelial cells subcultured onto plastic dishes will also differentiate along the squamous pathway if grown in the absence of retinoids. Furthermore, it has been shown that physiologic amounts of retinoic acid inhibits the expression of these squamous markers in their in vitro cultures, just as in the in vivo system. This is not characterization performed in house.

For most studies, Lonza recommends culturing cells with Retinoic Acid, however, if a researcher is looking to specifically study squamous cell growth or differentiation, or if a researcher believes that Retinoic Acid may otherwise interfere with their assay, Lonza also offers cells bronchial/tracheal epithelial cells without Retinoic Acid (catalog no. CC-2541).