Although thyroid hormone (T3) influences epithelial cell differentiation during late fetal lung development, its effects on early lung morphogenesis are unknown. We hypothesized that T3 would alter embryonic lung airway branching and temporal-spatial differentiation of the lung epithelium and mesenchyme. Gestational day 11.5 embryonic mouse lungs were cultured for 72 h in BGJb serum-free medium without or with added T 3 (0.2, 2.0, 10.0, or 100 nM). Evaluation of terminal bud counts showed a dose-and time-dependent decrease in branching morphogenesis. Cell proliferation was also significantly decreased with higher doses of T 3. Morphometric analysis of lung histology showed that T 3 caused a dose-dependent decrease in mesenchyme and increase in cuboidal epithelia and airway space. Immunocytochemistry showed that with T 3 treatment, Nkx2.1 and surfactant protein SP-C proteins became progressively localized to cuboidal epithelial cells and mesenchymal expression of Hoxb5 was reduced, a pattern resembling late fetal lung development. We conclude that exogenous T 3 treatment during early lung development accelerated epithelial and mesenchymal cell differentiation at the expense of premature reduction in new branch formation and lung growth. triiodothyronine; lung development; Nkx2.1; Hoxb5; surfactant protein-C DURING FETAL DEVELOPMENT, lung morphology and differentiation of the airway lining epithelium change with advancing gestation. In the mouse, lung morphogenesis starts at gestational day 9.5 (Gd 9.5) with the evagination of the lung primordium from the ventral foregut to form the prospective trachea and two lung buds followed by the dichotomous branching of each lung bud through the process of branching morphogenesis. Differentiation of this lung primordium into the bronchial and pulmonary acinar systems begins during this period. The restriction of cell types and in some cases loss of cell-specific markers herald changes in the stage of bronchiolar versus distal epithelial cell fate and differentiation (48,58). These processes of airway branching and cytodifferentiation require a close interaction between endodermal epithelium and adjacent mesenchyme and are affected by specific factors and hormonal systems active in the developing embryo and fetus (1,22,24,35,38). In particular, in rodent lung organ cultures and human lung explant cultures, triiodothyronine (T 3 ), the active form of thyroid hormone, influences late fetal lung development by accelerating epithelial cell differentiation leading to surfactant phospholipid synthesis and by regulating lung structural development and alveolar septation (9,11,26,38). Little is known about the cellular expression pattern of T 3 receptors during fetal lung development, but they are present in rodent and human lung tissue during the pseudoglandular period and increase in quantity and binding affinity as lung development progresses (6,13,18,20,23). Although there are relatively low levels of T 3 and its receptors in early fetal life, maternal hypo-or hyperthyroidism le...
