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In-Depth Information
CRF and not TRH that controls TSH level during metamorphosis, and as
CRF is controlling the corticoid axis, this explains the coupling between
stress and metamorphosis (see
Laudet, 2011
and references therein).
3. TH CONTROLS METAMORPHOSIS IN AMPHIBIANS
3.1. Metamorphosis in anurans
Anurans (frogs and toads) provided the historical models in which the link
between metamorphosis and TH was established. The first hints were given
when tadpole fed with horse thyroid extract underwent a precocious meta-
morphosis compared to others fed with nonthyroid tissue (liver, muscle,
etc.) (
Gudernatsch, 1912
). Since this experiment, a lot of publications inves-
tigating metamorphosis used the toad
Xenopus tropicalis
or
X. laevis
as model
organism, and TH levels are today well known along the various stages in
these models. Several authors have shown that a peak of T4 and T3 occurs
at the climax of metamorphosis, its most anatomically spectacular phase
when the animal exhibits both regressing larval and developing adult features
(
Leloup & Buscaglia, 1977
). In a similar way, there is a peak of
TR
b
mRNA,
but not
TR
mRNA, at the climax of metamorphosis (
Fig. 14.2
). Interest-
ingly, the comparison of
TR
a
expression levels between different toad spe-
cies have shown that a higher expression correlates with an earlier and faster
metamorphosis suggesting that the level of this receptor is important for the
control of the onset of metamorphosis (
Hollar, Choi, Grimm, & Buchholz,
a
T3
T4
100
mRNA TR
a
75
50
mRNA TR
b
25
35/36
45
51
55
58
61
63
66
Developmental stages
1
Climax
Figure 14.2 THs and TRs mRNA level during Xenopus laevis development. Adapted from
Leloup and Buscaglia (1977)
. Stage illustrations are adapted from
Nieuwkoop and Faber
(1994)
.
