Biology Reference
In-Depth Information
be defined (
Power et al., 2001; Raine & Leatherland, 2000
). TRs are present
in teleost fish embryos and it has been demonstrated that increasing rainbow
trout oocyte TH content can affect later embryo TR gene expression, which
suggests that both maternal THs and
in ovo
TH manipulation can affect sub-
sequent fish development (
Power et al., 2001; Raine & Leatherland, 2000
).
In
X. laevis
embryo, early work on THs only investigated TH signaling
pathways from NF50 (premetamorphosis) onward, plasma TH concentra-
tions at stages earlier than NF52 being assumed to be too low to measure
(
Leloup & Buscaglia, 1977; Regard, Taurog, & Nakashima, 1978
). The fact
that activating deiodinases are functional at early larval stages (NF35-37),
prior to thyroid organogenesis and free feeding raised the question of the
ligand/substrate availability during
Xenopus
development. We have mea-
sured the quantities of T
3
and T
4
both in eggs and embryos at progressive
stages. Eggs from different females show quite variable TH content (
Fini,
Le M´vel, et al., 2012; Morvan Dubois et al., 2006
). However, the most
important finding is that all NF35-37 embryos from different females exhibit
measurable, physiological relevant amounts in the femtomole range, with
both hormones being found in eggs and embryos. These amounts are com-
patible with the affinity levels of deiodinases.
Krain and Denver (2004)
have
reported levels of T
4
and T
3
in NF50
Xenopus
tadpoles. These authors found
very low levels of T
4
, in the order of 0.2 fmol/tadpole and 1.0 fmol T
3
/tad-
pole at NF50. Measuring total TH levels in whole tadpoles does not take
into account tissue-specific changes. Indeed, the fact that overexpressing
mutant TRs, unable to recruit corepressors, produces an eye-specific phe-
notype (
Havis et al., 2006
), underscores tissue specificity of TR and TH
signaling in early development. There were no significant variations in
T
4
levels between eggs and embryos. In contrast, a significant difference
(
P
<
0.05) was observed between the mean of T
3
measured in eggs and
that found in NF35-37 embryos. T
3
is increased (threefold) at stage
35-37 compared to eggs (
Fini, Le M´vel, et al., 2012; Morvan Dubois
et al., 2006
), suggesting an endogenous conversion of T
4
into T
3
by the
embryonic-activating deiodinases. As this pathway is energy consuming,
it should therefore be of interest to examine if this aspect of thyroid signaling
during embryonic development is conserved through evolution.
In oviparous species, TH is maternally supplied. For instance, it is well
documented that TH accumulates in the avian yolk (
Roche, Michel,
Volpert, & Sanz, 1957
), and T
3
response signals were found in early trans-
genic chick embryos (
Flamant & Samarut, 1998
). We also reviewed data on
T
4
and T
3
measurements in reptiles, fishes, and amphibians. The most likely
