Biology Reference
In-Depth Information
not as dramatic as metamorphosis in anurans, multiple tissues undergo hor-
monally regulated changes in cellular composition, biochemistry, and mor-
phology during salamander metamorphosis (
Dodd & Dodd, 1976
). These
changes include degeneration of external gills and tailfins; remodeling of
skeletal elements and integument; shortening of the intestine; modification
of pigmentation; muscle degeneration and differentiation; and changes in
nitrogen excretion, urogenital function, and oxygen transport (
Duellman &
Trueb, 1986
). Also, new structures arise
de novo
, including eyelids and skin
glands. All of these changes adapt aquatic larvae for survival and reproduction
as adults in more terrestrial environments.
Although salamanders have a more protracted and gradual metamor-
phosis than anurans, TH is the primary metamorphic hormone in both am-
phibian groups. Serum TH in the form of tetraiodothyroine (T4) is low
during embryonic and early larval development, but as larvae approach
metamorphosis, T4 levels increase (
Norman, Carr, & Norris, 1987
). This
pattern of hormone secretion is not unlike that observed for other examples
of hormonally regulated, postembryonic development (e.g., insect and fish
metamorphosis;
Gilbert, Tata, & Atkinson, 1996; Laudet, 2011; Power
et al., 2001
). The rise in T4 during amphibian larval development is asso-
ciated with maturation and stimulation of brain regions that regulate thy-
roid activity, in response to extrinsic and intrinsic cues. These brain regions
include the hypothalamus and pituitary gland. Increasing levels of T4 dur-
ing larval development stimulate corticotropin-releasing hormone (CRH)
release from modified nerve endings within the median eminence. CRH
travels through capillaries to the anterior pituitary where it stimulates
thyrotropes to release thyrotropin (TSH) into the blood stream. TSH in
turn stimulates the thyroid gland to release T4. The TSH-releasing factor
in amphibians is CRH, which also stimulates the release of corticotropin
(ACTH) and subsequently, glucocorticoids (
Denver & Licht, 1989
). The
potent, synergistic action of corticoids and TH in promoting anuran and
salamander metamorphosis clearly implicates the hypothalamic-pituitary-
thyroid (HPT) and HP-interrenal (HPI) axes in the physiological
regulation of metamorphosis (
Buscaglia, Leloup, & De Luze, 1985; Dodd &
Dodd, 1976; Galton, 1991, 1992; Gancedo et al., 1992; Hayes, Chan, &
Licht, 1993; Kuhn, De Groef, Van der Geyten, & Darras, 2005
). Exactly
how T4 levels initially increase at appropriate times during amphibian
development to trigger the onset of metamorphosis, and sustain T4 at
high levels during metamorphosis,
is not fully understood (
Manzon &
Denver, 2004
).
