Is thyroid hormone signaling relevant for vertebrate embryogenesis?

Classically, thyroid hormones (THs) have been primarily associated with postembryonic development (Tata, 1968), notably metamorphosis in anuran amphibians and flat fish. This period is parallel to the perinatal period in man and many marked developmental transitions in other species. As amply described in other chapters, metamorphosis is characterized by a peak of thyroxine (T(4)) and triiodothyronine (T(3)) that is synchronous with the metamorphic climax. In contrast, the developmental period that characterizes embryonic development prior to the significant production of TH by the endogenous thyroid gland has received little attention. Furthermore, the prevailing concepts of TH physiology during this period have been framed by two observations in amphibians and mammals: first, TRs are expressed, while circulating TH levels are much lower than those during metamorphosis and, second, extrapolating from the knowledge largely obtained from in vitro models, in the absence of TH, the aporeceptor represses target gene transcription during premetamorphic development. We propose to revisit both concepts in the light of accumulating data, first, on TH availability both in eggs and in embryos and, second, on the increasing knowledge of the complexity of TR and TH control of transcription.