Thyroid hormone affects distal airway formation during the late pseudoglandular period of mouse lung development.

We recently showed that T3 treatment of cultured gestational day 11.5 early pseudoglandular period mouse lungs, accelerated terminal airway development at the expense of decreased branching morphogenesis. As the ability of T3 to influence epithelial cell differentiation increases with advancing development, we hypothesized that in the late pseudoglandular period, T3 would cause further premature changes in the morphology of the distal airways leading to abnormal saccular development. Gestational day 13.5 embryonic mouse lungs were cultured for 3 and 7 days without or with added T3. Increasing T3 dose and time in culture resulted in progressive development of thin walled, abnormal saccules, an increase in cuboidal and flattened epithelia and airway space with a concomitant decrease in mesenchymal cell volume. Consistent with increased cuboidal and flattened epithelial cell volume identified by morphometry, immunostaining suggested increased cell proliferation detected by localization of proliferating cell nuclear antigen (PCNA) in epithelial cells of T3 treated lungs. T3 decreased mesenchymal expression of Hoxb-5 protein and caused progressive localization of Nkx2.1 and SP-C proteins to distal cuboidal epithelia of early abnormal saccules, evidence that T3 prematurely and abnormally advanced mesenchymal and epithelial cell differentiation. Western blot showed a T3-dependent decrease in Hoxb-5 and a trend towards decreased Nkx2.1 and SP-C, after 3 and 7 days of culture, respectively. We conclude that exogenous T3 treatment during the late pseudoglandular period prematurely and abnormally accelerates terminal saccular development. This may lead to abnormal mesenchymal and epithelial cell fate.