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not activate the receptor. We demonstrated that TRIAC, a T3 derivative,
plays a T3-like role in this species: binding the amphioxus TR and inducing
metamorphosis (
Paris et al., 2008, 2010
). Taken together, these data support
the idea that the TH pathway is involved in metamorphosis in the
cephalochordate amphioxus. Given both the phylogenetic position of
amphioxus and the strong evidence for TH-controlled metamorphosis in uro-
chordates (
Patricolo, Cammarata, &D'Agati, 2001
), TH-regulated metamor-
phosis appears thus as an ancient process. This, together with the observations
of variations on the way metamorphosis occurs in development of some spe-
cies as the neotenic
Necturus
or the direct developer
E. coqui
(
Laudet, 2011
),
leads to the suggestion that TH-regulated metamorphosis is a shared ancestral
character common to all chordates (
Laudet, 2011; Paris et al., 2008; Paris &
Laudet, 2008
).
According to this model, vertebrate species that do not exhibit a clear
metamorphosis during postembryonic development would either be direct
developers, with intra-egg metamorphosis and a juvenile hatching from the
egg, or alternatively, undergo a morphologically cryptic posthatching meta-
morphosis such as observed in
Necturus
. These ideas challenge the classical
view of vertebrate postembryonic development, a rather neglected develop-
mental period, especially in species that apparently do not undergo metamor-
phosis. In the remaining part of this chapter, we will thus concentrate on
amniotes (
Fig. 14.3
), their postembryonic development, and the role of TH.
5. MAMMALS
If indeed there is a role of TH in the control of postembryonic develop-
ment in mammals, similar to the one observed in biphasic species such as
Xenopus
or flatfishes, we should observe two phenomena: (i) a peak of THdur-
ing this period and (ii) a role of TH in the remodeling of organs and tissues dur-
ing this peak.Wewill thus analyze the situation inmammals, taken intoaccount
that the actual data available are extremely biased toward mouse and human.
5.1. TH levels in eutherian mammals
In mice (
Mus musculus
), TH levels are relatively low at birth and a peak of
circulating TH is observed at 15 days postpartum. T4 levels increase about
10-fold and T3 levels increase even more (
Hadj-Sahraoui, Seugnet,
Ghorbel, & Demeneix, 2000
). In rat, the situation is very similar to the
mouse (
Babu et al., 2011
). In human (
Homo sapiens
), embryonic blood
TH levels increase during the gestation in contrast with maternal levels that
remain stable (
Thorpe-Beeston, Nicolaides, Felton, Butler, & McGregor,
