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probably does not occur until 20 weeks or later and in the fetal plasma, TH
levels remain low until about 4 months prior to birth. The TH levels reach a
peak around birth and remain high several months after birth. This post-
embryonic/perinatal period in mammals such as human and mouse mimics
anuran metamorphosis in many aspects (
Atkinson, 1994; Shi, 1999; Tata,
1993
). Among them are the transition from aquatic (living in the amniotic
fluid) to terrestrial habitat with air-breathing after birth as the lung matures,
the switching of the hemoglobin from the fetal to adult types, the increased
albumin expression and the induction of urea cycle enzymes in the liver, the
maturation of the intestine to the adult form, skin keratinization, and the
developmental progression and restructuring of the central and peripheral
nervous systems, etc. Thus, it may not be surprising that TH deficiency dur-
ing neonatal period leads to irreversible, profound neurological deficit, and
mental retardation as well as other defects in human (
Hetzel, 1989; Hsu &
Brent, 1998; Porterfield &Hendrich, 1993
). While the importance of TH in
mammalian development has long been established, it is difficult to manip-
ulate the uterus-enclosed mammalian embryos for studying the mechanisms
of TH action. This contrasts sharply with the total control of anuran meta-
morphosis by TH and easiness to manipulate this process in tadpoles or
organ/primary cell cultures for functional and mechanistic analyses.
Natural metamorphosis in anurans takes place at precise developmental
stages, which are defined based on animal morphology, including the pres-
ence or absence of the limb or tail or their morphology, etc. (
Dodd &Dodd,
1976; Shi, 1999
). In
Xenopus
, the onset of metamorphosis is around
stage 55 when endogenous TH becomes detectable (
Fig. 10.1
)(
Dodd &
Dodd, 1976; Nieuwkoop & Faber, 1956
). Thus, TH synthesis is the primary
determinant for the timing of the onset of metamorphosis. TH is synthesized
in the thyroid gland as either T3 or T4. In
Xenopus laevis
, the gland first
appears in the embryo as a median thickening of the pharyngeal epithelium
at the time of tadpole hatching (stages 35/36) (
Dodd & Dodd, 1976
). This
rudiment then develops into a functional larval thyroid gland around stage
53. Subsequently, the plasma TH concentrations increase, reaching peak
levels around stage 60, the climax of metamorphosis. Currently, the mech-
anisms underlying the developmental regulation of the plasma TH levels are
unknown. However, as in mammals, many factors appear to be involved. As
have been reviewed extensively, these factors include those that influence
the growth and maturation of the thyroid gland and hormonal clues that reg-
ulate TH synthesis and release from the gland (
Denver, 2013; Dodd &
Dodd, 1976; Gilbert et al., 1996; Shi, 1999
). Regardless of the regulation
