• at 4 weeks of age, mice lack the apical cilia characteristic of ciliated cells in large bronchioles and small bronchioles

• during the first 4 weeks of life, mice show a progressive and extensive loss of normal conducting airway epithelial cells

• however, no pulmonary inflammation is observed and mice survive normally for >6 months

• at 4 weeks of age, a single layer of low cuboidal-to-squamous epithelium lacking the ''domed'' morphology characteristic of Clara cells, the apical cilia characteristic of ciliated cells, and goblet shape of mucus producing cells, is present in large bronchioles and small bronchioles

• at 4 weeks of age, the proliferation index of bronchiolar cells is reduced by 42% relative to that in wild-type lungs

• cell width increases 13%, cell height decreases 33%, and cell density decreases 12%, consistent with a shift from columnar to cuboidal-squamous morphology

• by 6 weeks of age, few ciliated cells remain in the bronchioles

• no Clara cells are identified at birth and thereafter

• loss of Clara cells occurs faster than loss of ciliated cells

• no allergen-induced goblet cell differentiation or mucus production is observed in the bronchiolar epithelium

• at 4 weeks of age, mice lack goblet cells

• goblets cells fail to form following allergen exposure

• at 4 weeks of age, mice lack the apical cilia characteristic of ciliated cells in large bronchioles and small bronchioles