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of local TH availability during organogenesis (
Arjona, de Vrieze, Visser,
Flik, & Klaren, 2011
).
It has to be mentioned that despite the molecular characterization of
transporters during
X. tropicalis
metamorphosis (
Connors, Korte, Anderson,
& Degitz, 2010
), no information is available yet on the potential role of
TH transporters in the developing
Xenopus
embryo or tadpole of either spe-
cies. Nevertheless, an efficient TH transporter, 4F2hc-IU12, was character-
ized by overexpression in
X. laevis
oocytes (
Ritchie, Peter, Shi, & Taylor,
1999
). The transcript encoding the IU12 subunit was previously demon-
strated to be an early T
3
responding gene during intestinal development
(
Liang, Sedgwick, & Shi, 1997
). The expression of IU12 is also upregulated
in the head and limb during metamorphosis (
Liang et al., 1997
).
3. BOTH T
3
AND T
4
ARE PRESENT DURING EMBRYONIC
STAGES
Taken together, the data reviewed above show that TRs, deiodinases,
and transporters are present and functional in all vertebrate embryos exam-
ined to date. In some species, including humans, changed phenotypes can be
linked to impaired TH signaling during early development, raising the cen-
tral question of ligand availability during this period. A related question is
how TH delivered to the embryonic tissues that require TH signaling
and degraded in those that do not? There were several attempts to visualize
TH availability and action using reporter systems (
Flamant & Samarut, 1998;
Quignodon et al., 2004; Wallis et al., 2010
). For example, Flamant and
Samarut used a cocultivation assay in which different parts of the early chick
embryo (from day 1 of incubation) were cocultured with a T
3
sensitive re-
porter cell line. Region-specific and developmental stage-specific produc-
tion of T
3
was found. T
3
was produced successively by the blastoderm
(
area opaca
), Hensen's node (the equivalent of Spemann's organiser in
Xenopus
), and then by the caudal part of the embryo. However, the meth-
odological bias of this approach is that TH can be detected only in tissue
where the TH signaling activates transcription and where cofactors required
for the specific construct used are present. The most straightforward method
to date to test for TH presence in tissue is RIA. The main drawback of this
method is that the precise spatial distribution of TH within tissues is lost.
Another means of addressing the question is the use of autoradiography
using Iodine isotope in order to visualize TH distribution. This technique
is the only one providing any clue on spatial distribution of TH within
