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stimulation by thyroid hormone, as the limbs do not elongate fully in its
absence (
Callery & Elinson, 2000a
). Nonetheless, the limbs develop carti-
lage and muscle without thyroid hormone synthesis by the thyroid gland
(
Callery & Elinson, 2000a
). Attempts to accelerate limb development in
E. coqui
with thyroid hormone, either
in vivo
or
in vitro
, were not successful
(
Elinson, 1994
), but CRF
in vivo
accelerated development, including
hindlimb length (
Kulkarni et al., 2010
).
E. coqui
limb buds, when
transplanted to embryos of the frog
Rana pipiens
, continue development
including cartilage elements, while the host's limb buds remain small
andundifferentiated(
Elinson, 1994
). These results indicate that most of
limb development in
E. coqui
does not require embryo-derived thyroid
hormone.
6. A POSSIBLE ROLE FOR MATERNAL THYROID
HORMONE AND THYROID HORMONE RECEPTOR
Development of eyes and limbs in species with tadpoles, but not in
direct developers, requires thyroid hormone synthesis. There are two pos-
sibilities for the derived, direct developer condition. The first is that the eyes
and limbs have lost the requirement for regulation by thyroid hormone.
Alternatively, thyroid hormone and its receptor are supplied maternally in
the egg cytoplasm.
One change of early eye and limb development in
E. coqui
that is prob-
ably independent of thyroid hormone is their initial large sizes. Both eyes
and limbs are proportionately larger when they first appear in direct devel-
opers compared to tadpoles. These large initial sizes are likely due to large
embryonic fields, specified by inductive interactions involving growth fac-
tors and other ligands. Beyond that the activity of maternal stores of thyroid
hormone and receptors needs to be explored.
The possibility that thyroid hormone is used in the early development is
strongly suggested by the presence of thyroid hormone in the eggs of shark
(
McComb, Gelsleichter, Manire, Brinn, & Brown, 2005
), teleosts (
Power
et al., 2001
), amphibians (
Morvan-Dubois et al., 2006; Weber, Farrar,
Tom, & Grau, 1994
), and birds (
Flamant & Samarut, 1998; Ho, Reed, &
Burggren, 2011; McNabb & Wilson, 1997; Prati, Calvo, Morreale, &
Morreale de Escobar, 1992
). In addition, mRNA or protein for thyroid
hormone receptors is present in the eggs of zebrafish (
Essner, Breuer,
Esner, Fahrenkrug, & Hackett, 1997; Liu, Lo, & Chan, 2000
), chicken
