Biology Reference
In-Depth Information
agonist/antagonist NH-3 (
Lim, Nguyen, Yang, Scanlan, & Furlow, 2002
),
also observing effects on eye formation.
In chicken embryos, TRs are present during early embryogenesis, par-
ticularly in neuroectoderm, and TH-activated reporter gene expression was
found in several embryonic tissues, including Hensen's node and the prim-
itive streak (
Flamant & Samarut, 1998
). TRs are expressed from early devel-
opment, with TR
a
exhibiting a quite ubiquitous pattern but variable
expression levels, whereas TR
b
expression is restricted to some organs such
as the brain, kidney, eye, yolk sac, and liver (
Forrest, Sjoberg, & Vennstrom,
1990
). The same group also demonstrated that both receptors are expressed
(but with distinct patterns) in the cerebellum at E9, and questioned the pres-
ence of liganded receptor at this stage (
Forrest, Hallbook, Persson, &
Vennstrom, 1991
).
In teleost fishes, TRs are also detected in eggs and are expressed during
early development (see
Power et al., 2001
, reviewing T
3
/thyroxine (T
4
)
contents of many teleosts species). The role of unliganded receptor was dem-
onstrated soon after the cloning of the TRs in zebrafish (
Essner, Breuer,
Essner, Fahrenkrug, & Hackett, 1997
), but the question of the importance
of liganded TRs during early development remains unsolved. Nevertheless,
it was demonstrated, as it was in
Xenopus
, that all the actors of TH metab-
olism and signaling are present and functional during zebrafish embryonic
development (
Walpita, Van der Geyten, Rurangwa, &Darras, 2007
). How-
ever, here again, there is no demonstration of a developmental role for an
“endogenous-liganded” receptor.
In lamprey, two TR isoforms are present but they are resulted from a
duplication that is specific to agnathes (
Laudet, 2011
). In contrast to tetra-
pods and teleost fishes, where a peak of TH is mandatory for metamorphosis
to occur, lamprey metamorphosis is dependent on a decrease in TH
(
Manzon & Youson, 1997; Youson, Manzon, Peck, & Holmes, 1997
).
TRs and TH are present in most lamprey embryonic tissues such as the liver,
kidney, and gill, with particularly high levels in intestine (reviewed in
Youson & Manzon, 2012
). The lamprey could therefore provide a valuable
model for studying the role of a liganded TR during embryonic develop-
ment. However, the molecular pathways activated by TH in lamprey
embryo are still to be analyzed.
The increasing number of sequenced genomes shows that TRs are pre-
sent in most bilateria (
Laudet, 2011
). TH can have a biological action in
Aplysia and sea urchin, and TR homologs have been described in both
groups, but their role during embryogenesis has not been investigated
