Biology Reference
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a 2-day interval accelerated metamorphosis as evidenced by shortening of
elongated fi n rays (Fig. 7) and right eye migration. In conclusion, it is clear
that TSH stimulates thyroid organ to secrete T4 from prometamorphosis
to climax and this pituitary-thyroid axis plays the major role in controlling
metamorphosis of the Japanese fl ounder.
To examine the presence of negative feedback regulation in pituitary-
thyroid axis, Miwa and Inui (1987a) treated premetamorphic larvae of the
fl ounder with T4 and TU for 14 days. The thyroid follicles of larvae treated
with TU for 2 weeks were apparently hyperactive as evidenced by tall follicle
cells and well developed vacuole- like structure in the cytoplasm of the cells,
but T4 was not detected immunocytochemically in the thyroid (Fig. 8-2c),
indicating the inhibition of T4 synthesis by TU. On the other hand, TSH
cells in the pituitary of the TU-treated larvae markedly proliferated and
were stained more intensely with immunocytochemistry using anti-TSH
serum compared to those of control animals (Fig. 8-1a, 1c). In contrast, T4
treatment induced typical inactivation of thyroid follicles (Fig. 8-2b) and
almost completely abolished immunocytochemical reaction of pituitary
to anti-TSH serum (Fig. 8-1b). These facts clearly indicate that a negative
feedback regulation is functioning in the pituitary-thyroid axis during
metamorphosis of the Japanese fl ounder.
4.2.5 Role of thyroid hormone in development of organs during
metamorphosis
Amphibian metamorphosis is accompanied by extensive morphological
and physiological remodeling of various organs (Gilbert and Frieden,
1981), and these structural and functional developments are thought
to result from tissue-specifi c reprogramming of gene expression that is
stimulated directly by thyroid hormones (Atkinson et al., 1994). Studies
on fl atfi sh metamorphosis, especially on the Japanese fl ounder in the last 2
decades disclosed that various internal organs of fl ounder larvae undergo
dramatic morphological, biochemical and functional development and
changes during metamorphosis, and that these development and changes
are controlled by thyroid hormones.
4.2.5.1 Shift of erythrocyte population
In higher vertebrates such as some mammals (Bloom and Bartelmez,
1940; Craig and Russel, 1964) and birds (Fraser, 1963), there is a shift of
erythrocyte populations from primitive erythrocytes, which are specifi c
to embryos, to defi nitive erythrocytes during embryonic development. In
anuran amphibians, such a shift of erythrocytes is known to occur during
metamorphosis from tadpoles to froglets (Broyles, 1981). A similar change
in erythrocyte populations during early development has also been reported
