25, 26 Using a model of epithelial damage induced by intratracheal detergent or SO 2 inhalation, Borthwick and colleagues 25 identified a population of 5-bromo-2 0 -deoxyuridine label-retaining cells that were localized to the gland ducts of the upper trachea and were able to reconstitute a surface-like tracheal epithelium.