Effects of hormones on functional differentiation of mouse respiratory epithelium

The effects of corticosteroids and thyroxine on morphogenesis and differentiation of fetal respiratory epithelium were tested in serum-free culture. Strips from the margins of 16-day fetal mouse lungs, containing future respiratory branches, were cultured in medium containing growth factors plus different concentrations of dexamethasone and/or thyroxine. After 5 days, some of the cultures were processed for electron microscopy and the rest were used to quantitate unsaturated and saturated phosphatidylcholine. In the absence of thyroxine and dexamethasone, the lung tissue developed the equivalent of one more day in utero during the 5 days of culture, perhaps because of residual hormone in the tissue. Thyroxine and dexamethasone in concentrations compatible with normal physiological conditions permitted normal development from the pseudoglandular stage to the saccular stage, differentiation of respiratory cell types, and normal surfactant accumulation. Thyroxine alone gave essentially the same lack of morphological maturation and accumulation of disaturated phosphatidylcholine as culture in the absence of all hormone, but columnar epithelial cells held increased numbers of lamellar bodies. Dexamethasone alone at 10(-6) M gave essentially normal development. Dexamethasone at 10(-9) M was sufficient to permit almost normal development when combined with 10(-7) M thyroxine. We conclude that dexamethasone is required for normal maturation of the fetal lung and that thyroxine acts synergistically to lower the dexamethasone requirement. The presence of apparently authentic lamellar bodies under some conditions of low disaturated phosphatidylcholine accumulation make this morphological criterion suspect as a test for differentiation of type II cells.