Biology Reference
In-Depth Information
2011
). This difference of profile indicates that, even if both are implicated,
TR
do not play the same role during metamorphosis. The peak of
TRb
is a key event in triggering the whole process and could be used as a
signature of metamorphosis in Anurans.
Treatment with exogenous TH induces a precocious metamorphosis.
On the contrary, tadpole treated by goitrogen becomes much bigger than
the control and fails to undergo metamorphosis, indicating that the tadpoles
still grow but are not able to achieve their postembryonic development (see
Buchholz et al., 2006; Laudet, 2011
; and references therein). THs are pro-
duced only by the thyroid gland and are distributed by the blood stream in
the peripheral organs of the animal. As mentioned above, locally, the
amount of THs and particularly the amount of T3 are controlled by
deiodinase activity that has been shown as important to regulate the remo-
deling of tissues in amphibian metamorphosis such as the tail resorption or
limb growth (
Huang, Cai, Remo, & Brown, 2001
). The action of
deiodinases explains at least in part the difference in the timing of metamor-
phosis between various tadpole organs, such as the limbs that appear early
and the tail that shrinks at the final stages.
During amphibian metamorphosis, most organs undergo complex and
coordinated changes and this event is an excellent model to study the effects
of THs at different levels. The intestine undergoes spectacular changes in-
cluding both apoptosis and cell proliferation (for review, see
Ishizuya-Oka,
Hasebe, & Shi, 2010
). In
X. laevis
, the adult intestine exhibits villosities and is
90% shorter than the long and simple larval intestine which consists of a
monolayer of larval epithelial cells, with thin layers of surrounding connec-
tive tissue and muscles. When T3 levels surge the vast majority, if not all, of
the larval epithelial cells undergo apoptosis, and concurrently, adult epithe-
lial stem/progenitor cells appear
de novo
and rapidly proliferate (
Ishizuya-
Oka & Ueda, 1996; Shi, Hasebe, Fu, Fujimoto, & Ishizuya-Oka, 2011
).
Thus, the intestine undergoes opposite changes at the morphological and
cellular level, triggered by T3. The brain also exhibits dramatic changes,
with a massive apoptosis of certain central nervous system neurons
(
Coen, Le Blay, Rowe, & Demeneix, 2007
) as well as motor and sensory
neurons of the resorbing tail. Other organs, such as gills that disappear or
skin that acquire features necessary for a terrestrial life are drastically
remodeled during metamorphosis (for review, see
Ishizuya-Oka et al.,
2010
). We will not review in detail the action of TH at the cellular and gene
regulation levels during
Xenopus
metamorphosis as several excellent reviews
are available on this topic (see Chapters 2, 4 and 9 in this issue).
a
and
TR
b
