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interpretation would be that T
4
in the avian and
Xenopus
embryos is exclu-
sively of maternal origin. However,
Tindall et al. (2007)
have shown that
thyroid peroxidase (TPO) is also expressed during embryonic
Xenopus
de-
velopment, raising the possibility that some synthesis occurs, but for this to
be the case thyroglobulin would also most probably have to be present.
However, up to now no functional study has been carried out to show
whether this enzyme is active during
Xenopus
development. Nevertheless,
expression patterns of TPO during
Xenopus
embryogenesis are similar to
those observed for deiodinases.
Tindall et al. (2007)
show that TPO is
expressed well before thyroid gland anlage formation. They show that
blocking TPO with methimazole during embryogenesis affects both mor-
phogenesis and organogenesis.
4. WHAT ROLES DOES TH SIGNALING HAVE DURING
EARLY DEVELOPMENT?
As described above, many components of TH signaling pathways,
including transporters, receptors, ligands, and deiodinases, have been shown
to be present in the early stages of embryonic development in chicken,
mouse, and human. Given their presence in these species and the presence
of at least the ligands in the eggs of other vertebrates, it is feasible that TH
signaling is also likely to be functional during early development of many
vertebrates. Despite the demonstrations of the three components men-
tioned, other elements of the pathway are not characterized yet, such as
transporter and distributor protein expression during early development.
Furthermore, the actual functions and mechanisms of TH signaling are how-
ever very difficult to investigate, especially because of the difficulty of con-
trolling TH availability with precision. Valuable insight came from the
analysis of neurological disorders due to impaired TH availability during
early pregnancy and from the careful analysis of endocrine disruptors effect
on embryonic development. Using the
Xenopus
model, we recently showed
that
Xenopus
are competent to respond to T
3
prior to NF40, and that TH
may have a role in neuronal differentiation.
4.1. Reduced maternal TH levels affect neuromotor
development in the child
The first demonstrations that treating young patients with thyroid extract
could improve the mental retardation characteristic dated from the end of
the nineteenth century (
Railton, 1894; Smith, 1894
). However, even at that
