Biology Reference
In-Depth Information
thyroid hormone in
E. coqui
as in indirect developers. This critical period
was called “cryptic metamorphosis” (
Callery & Elinson, 2000a
).
Embryos are staged according to
Townsend and Stewart (1985)
.
Townsend and Stewart divided the development into 15 stages from unc-
leaved egg to the froglet, which hatches from its jelly capsule (
Fig. 9.1
). Each
stage takes about 1.5 days at 23
C. When thyroid hormone synthesis is
inhibited with methimazole, the embryos arrest at TS12. They have a
frog-like appearance of head, eyes, legs, and general body morphology, but
development is incomplete of skin, body musculature, jaw cartilage, and di-
gestive tract (
Callery & Elinson, 2000a; Singamsetty & Elinson, 2010
). The
embryos do not hatch and remain in the aqueous environment, contained
within their fertilization envelope and jelly capsules. When removed from
their jelly capsules, these methimazole-inhibited embryos stay in water and
do not climb onto land.
Two further features that require thyroid hormone late in development
are tail and nutritional endoderm. The modified tail is used for respiration,
rather than swimming, and thyroid hormone is required for its regression,
around the time of hatching at TS15 (
Callery & Elinson, 2000a
). Nutritional
endoderm is a specialized, yolk-rich, cellularized tissue which provides nu-
trition in late embryogenesis and for the first few days after hatching
(
Buchholz et al., 2007
). Utilization of yolk and subsequent disappearance
of the nutritional endoderm requires thyroid hormone (
Singamsetty &
Elinson, 2010
). All of the effects of methimazole can be reversed by adding
thyroid hormone (
Callery & Elinson, 2000a; Singamsetty & Elinson, 2010
).
Nath, Fisher, & Elinson. (2013)
recently showed that the cardiac myosin
gene,
myh6
, is expressed in legs and other muscle-containing tissues prior
to thyroid hormone synthesis not only in
E. coqui
embryos but also in
Xenopus tropicalis
tadpoles. The microRNA,
miR-208
, is contained in an
myh6
intron, and its co-expression in mouse mediates thyroid hormone de-
pendent heart muscle growth (
van Rooij et al., 2007
). This raises the pos-
sibility that
myh6
and its intronic
miR-208
is involved in responses of
different muscles to thyroid hormone in frog embryos and tadpoles.
Themechanisms of thyroid hormone production anduse in
E. coqui
parallel
those found in metamorphosis of tadpoles. The thyroid gland differentiates
around TS10 (
Jennings & Hanken, 1998
), and activation of the thyroid gland
depends on hypothalamic stimulation of the pituitary by corticotrophin-
releasing factor (CRF), as in tadpoles (
Kulkarni, Singamsetty, & Buchholz,
2010
). The expressionof one of the thyroid hormone receptors,
thrb
,isinduced
by thyroid hormone (
Callery, 1999; Singamsetty & Elinson, 2010
)asin
Xenopus laevis
,and
thrb
is upregulated at TS10 (
Callery & Elinson, 2000a
),
