Androgen-induced proliferation in the developing larynx of Xenopus laevis is regulated by thyroid hormone

Exposure to exogenous androgen regulates cell number in the developing larynx of Xenopus laevis and hormone-regulated laryngeal development requires secretion of thyroid hormone (TH). We sought to determine whether exposure to TH is both sufficient and necessary for androgen-evoked cell proliferation (androgen competency) in developing larynx. Androgen competency was not observed in the premetamorphic larynx (tadpole stage 53, before TH secretion) but was present just prior to metamorphic climax (stage 58, during TH secretion). However, when TH is administered precociously (between stages 48 and 50), androgen competency can be observed at stage 53. The stage 52 larynx expresses high levels of the mRNA for TH receptor alpha. The duration of TH exposure required at tadpole stage 48 is greater than 2 days; studies in juveniles indicate that TH exposure need not be maintained in order for androgen competency to persist. The effects of exposure to TH on androgen competency are long lasting and perhaps permanent. While organotypic cultures obtained from tadpoles during premetamorphosis (stage 52) can proliferate in vitro and proliferation is augmented by TH exposure as it is in vivo, precocious exposure to TH does not induce androgen competency. In contrast, androgen does evoke cell proliferation in cultures obtained from metamorphosing (stage 58) tadpoles; proliferation is confined to the cartilage component. Thus, unlike larynges in vivo, muscle will not proliferate in response to androgen, indicating the necessity for an additional factor not present in vitro. Androgen receptor mRNA expression, believed required for androgen competency, was assessed in vivo and in vitro. The tadpole larynx strongly expresses AR mRNA, expression does not require exposure to TH nor is expression diminished in culture.