Biology Reference
In-Depth Information
It had long been thought that maternal TH could not reach the fetus as
the placenta expresses high levels of the inactivating deiodinase, D3. How-
ever,
Vulsma, Gons, and de Vijlder (1989)
demonstrated that T
4
did reach
the fetus by observing infants with a total organification defect. Total T
4
concentrations in maternal serum and amniotic fluid at 6-12 weeks of de-
velopment are, respectively, 11.3
m
g and 1.6 ng/dL, (
Burrow et al., 1994
),
but the differences in total hormone have to take account the high amounts
of distributor proteins in the maternal serum. The fetal thyroid gland and
pituitary-thyroid axis become functional by 10 weeks after conception
(
Thorpe-Beeston, Nicolaides, & McGregor, 1992
).
Surprisingly, in a study of homogenized human brain tissues at 10 and
12 weeks of gestation, T
3
was detected, but not T
4
(
Bernal & Pekonen,
1984
). This corroborates the study of
Obregon, Calvo, Del Rey, and de
Escobar (2007)
on the presence of T
3
and T
4
in rat brain tissue before thyroid
gland formation. In addition, a number of clinical studies show that low T
4
maternal concentrations during the first trimester of pregnancy are associated
with impaired neurological and visuospatial development of fetus (
Haddow
et al., 1999; Pop et al., 1999
) and that the consequences of low maternal TH
levels in the first trimester are worser than lower maternal TH levels in the
second and third trimesters. These results in humans reflect data from studies
on rodents showing that underactive maternal thyroid function in early ges-
tation (before the fetal thyroid is formed) is associated with impaired fetal
brain growth (
Obregon et al., 2007
).
3.2. T
3
and T
4
are present in the egg and early embryo of
vertebrates
The first reports of TH presence in vertebrate eggs date from over 50 years
ago. T
3
and T
4
were reported in the vitellus of different amphibian species as
early as the 1950s (
Volpert, Michel, & Roche, 1958
) results that were con-
firmed later (
Gancedo, Alonso-Gomez, de Pedro, Delgado, & Alonso-
Bedate, 1997
). THs of maternal origin are present in the egg yolk of birds
(
McNabb, 2006; Power et al., 2001; Prati, Calvo, Morreale, & Morreale
de Escobar, 1992
), reptiles (
Shepherdley, Richardson, Evans, Kuhn, &
Darras, 2002
), and fish eggs where TH is present in variable amount
(
Power et al., 2001; Szisch, Papandroulakis, Fanouraki, & Pavlidis, 2005,
Walpita, Crawford, Janssens, Van der Geyten, & Darras, 2009
). In the latter
study, the authors demonstrated that TH content decreased during embry-
onic development. If the presence of T
4
and T
3
in teleost fish embryo is well
documented, the role(s) of TH signaling during embryogenesis remains to
