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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 ac-
cumulating 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.
1. INTRODUCTION
In this review, we revisit two principal concepts that have framed current
thinking on thyroid hormone (TH) signaling during embryonic development
over the past two decades: first, TH levels are too low prior to the induction of
the thyroid gland to activate target gene transcription and, second, that as recep-
tors, TRs are expressed during early development and their principal role is to
act as aporeceptors to repress target gene transcription (
Chassande, 2003
). These
ideas arise in some part from the clear distinction of premetamorphic states from
the marked TH-dependent prometamorphic and metamorphic phases (see
Chapter 10
). Although metamorphosis in anuran amphibians, and to some
extent in flat fish, has received themost attention, most vertebrate species display
a peak of TH during a key phase around birth or hatching (see
Chapter 14
). As
discussed byHolzer andLaudet, in amniotes with nonclassical metamorphosis, a
TH peak is also associated with drastic morphological and functional changes
(
Paris, Brunet, Markov, Schubert, & Laudet, 2008
).
In mammals, embryonic development can be defined as the process that
covers development from the zygote until birth including organogenesis and
early morphogenesis. Similarly, in birds and reptiles, hatching is regarded as
the end of embryonic development. In the context of aquatic vertebrate devel-
opment, the limits are less well defined. Hatching can be used to define the end
of embryonic development, but inmany species, embryogenesis is not finalized
at this stage, and the embryo is still yolk-feeding. Consequently, in the Euro-
pean Union, the legal definition for aquatic species is to consider that the em-
bryonic period terminates within dependent feeding (2010/63/UE).
In this chapter, we review current knowledge on TH signaling during
early development and focus on the following ideas:
During embryonic development in
Xenopus
and other species, not only
are TH receptors (TRs) and all the deiodinases expressed but also TH
