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immunoreactivity to anti-pepsinogen serum. In contrast, immunoreactivity
of pepsinogen was maintained at a very low level in TU treated fi sh (Miwa
et al., 1992). The results clearly show that development of functional
stomach in the Japanese fl ounder during metamorphosis is stimulated by
thyroid hormone. Precocious induction of gastric glands and pepsinogen
by T4 and inhibition of the gastric development by TU in metamorphosing
larvae have also been shown in the summer fl ounder,
Paralichthys dentatus
(Huang et al., 1998; Soffi entino and Specker, 2001). Soffi entino and Specker
(2001), who examined the effect of exogenous T4 on the proliferation
rate of stomach fundic mucosa, suggest that early increase of tissue T4
concentration stimulates proliferation of fundic mucosa and later peak T4
level promotes differentiation to gastric glands in metamorphosis of the
summer fl ounder.
For amphibians, it is well known that the tadpole alimentary tract
is reorganized in association with the transition from herbivorous to
carnivorous feeding during metamorphosis (Dauca and Hourdry, 1985).
Administration of T4 also induces precocious differentiation of the stomach
in vivo
(Lipson and Kaltenbach, 1965) and
in vitro
(Pouyet et al., 1983).
Thus, thyroid hormone stimulates differentiation of gastric gland also in
amphibians. In contrast, adrenocorticoid rather than thyroid hormone seems
to be directly responsible to the gastric development of mammals. Injection
of ACTH or cortisol to neonatal rats causes precocious development of
pepsinogen in rat gastric mucosa (Furihata et al., 1972). Although thyroid
hormone accelerates functional development of gastric glands in neonetal
rats, the effect is mediated by corticosterone (Theng and Johnson, 1986;
Theng et al., 1987a, b). On the other hand, it must be noted that rat gastric
glands have already functionally differentiated to a certain degree at birth
(Helander, 1969; Defi ze, 1989; Yahav, 1989). Hence, it is still possible that rat
fetuses have a different hormonal regulatory system from their neonates.
4.2.5.3 Muscle development
Flatfi sh larvae are pelagic and rather slow swimmer, while juveniles,
although usually sedentary on the bottom, swim quickly at feeding. The
change in mobility during metamorphosis is accompanied by drastic
changes in the skeletal muscle in the Japanese fl ounder (Yamano et al., 1991,
1994b). The muscle of premetamorphic larvae of the Japanese fl ounder
consists of thin muscle fi bers with a small number of thin myofi brils (Fig.
14-a, b). During metamorphic climax, the larval type muscle transforms into
the adult type, which is characterized by thick muscle fi bers with abundant
myofi brils (Fig. 14-e, f) (Yamano et al., 1991). Corresponding with these
morphological changes, protein components of the muscle also change.
A myosin molecule of the larval type muscle is composed of two heavy
