Biology Reference
In-Depth Information
postembryonic development is dependent upon TH and resembles organ metamor-
phosis in amphibians.
1. INTRODUCTION
Thyroid hormone (TH) plays critical roles in vertebrate development
and adult organ function (
Atkinson, 1994; Franklyn & Gammage, 1996;
Freake & Oppenheimer, 1995; Hetzel, 1989; Lazar, 1993; Shi, 1999; Silva,
1995; Tata, 1993; Yen, 2001
). The most dramatic developmental process
controlled by TH is the metamorphosis in anurans, when an aquatic,
herbivorous tadpole is transformed into a terrestrial, carnivorous frog
(
Dodd & Dodd, 1976; Gilbert, Tata, & Atkinson, 1996; Shi, 1999
). During
anuran metamorphosis, different tissues undergo distinct changes (
Dodd &
Dodd, 1976; Gilbert et al., 1996; Shi, 1999
). Some, like the tail and gills,
are completely resorbed. Others, such as the limbs, are formed
de novo
. Most
of the organs, however, undergo partial, often drastic remodeling involving
both larval cell death and adult tissue development. For example, in the larval
intestine, the major tissue characteristic of the organ, the larval epithelium,
degenerates during metamorphosis by undergoing apoptosis (
Dodd &
Dodd, 1976; Shi, Hasebe, Fu, Fujimoto, & Ishizuya-Oka, 2011; Shi &
Ishizuya-Oka, 1996
). Concurrently, the adult epithelial stem cells are formed
de novo
through dedifferentiation of some larval epithelial cells through yet
unknown mechanisms. These cells then proliferate and differentiate to
form the adult intestinal epithelium with a self-renewing system resem-
bling that in the mammalian adult intestine (
Ishizuya-Oka & Shi, 2011;
Sun & Shi, 2012
).
Extensive studies as early as a century ago have firmly established that
TH is the causative agent of amphibian metamorphosis (
Allen, 1918;
Gilbert et al., 1996; Gudernatch, 1912; Shi, 1999
). There are two natural
THs, T3 (3, 5, 3
0
-triiodothyronine) and T4 (3, 5, 3
0
,5
0
-tetraiodothyronine,
or thyroxine). T4 is biologically less active and often considered as a pre-
hormone, whose deiodination gives to the more active hormone T3.
The plasma levels of both T3 and T4 are low or undetectable in embryos
or premetamorphic tadpoles and rise to peak at the climax of metamorphosis
(
Fig. 10.1
)(
Leloup & Buscaglia, 1977
).
Although other vertebrates do not undergo such a complete
TH-dependent transformation, TH is known to also play similarly critical
