Biology Reference
In-Depth Information
1. HORMONAL CONTROL OF METAMORPHOSIS
Metamorphosis is a stage of the amphibian life cycle that is character-
ized by dramatic morphological transformation accompanied by a transition
in ecological niche and behavioral mode. Hormones orchestrate the diverse
morphological and physiological changes that occur during metamorphosis,
and also function as mediators of environmental effects on development. A
striking characteristic of amphibian metamorphosis is that a single signaling
molecule produced by the thyroid gland (thyroid hormone, TH) can or-
chestrate the entire suite of molecular, biochemical, and morphological
changes. TH is required for amphibian metamorphosis (
Brown & Cai,
2007
); the hormone initiates gene expression programs in diverse tissues that
lead to cell proliferation, death, differentiation, or migration (
Brown & Cai,
2007
). Hormones produced by the anterior pituitary gland and the interrenal
glands (amphibian homologs of the mammalian adrenal cortex; corticoste-
roids, CSs) influence the rate of metamorphosis by controlling TH produc-
tion and action on target tissues. Neurohormones produced in the
hypothalamus control hormone biosynthesis and secretion by the pituitary
gland, and the hypothalamus mediates the interaction between the external
and internal environments, and the production of hormones that control
metamorphosis.
1.1. Thyroid hormone
The thyroid gland develops early in the amphibian embryo and matures
functionally at the time of hatching when it separates into two distinct lobes
and is essentially completely developed by the onset of metamorphosis
(
Dodd & Dodd, 1976; Kaye, 1959, 1961; Nieuwkoop & Faber, 1956;
Regard, Taurog, & Nakashima, 1978; Saxen, Saxen, Toivonen, &
Salimaki, 1957a, 1957b
). Thyroid activity increases markedly during
prometamorphosis, peaks at metamorphic climax, and declines thereafter
to reach an “adult” level of activity (
Dodd & Dodd, 1976; Kaye,
1959, 1960; Kikuyama, Kawamura, Tanaka, & Yamamoto, 1993; Regard
et al., 1978
). The major product of the amphibian thyroid gland is
3,5,3
0
5
0
-tetraiodothyronine (thyroxine; T
4
) with minor amounts of
3,5,3
0
-triiodothyronine (T
3
) produced (
Buscaglia, Leloup, & De Luze,
1985; Rosenkilde, 1978
). Coincident with measures of thyroid activity,
plasma concentration and whole-body content of T
3
and T
4
increase
throughout prometamorphosis and peak at metamorphic climax (
Denver,
