Biology Reference
In-Depth Information
4.2.2 Changes in tissue thyroid hormone concentration
Changes in tissue thyroid hormone concentrations in whole fi sh body
during metamorphosis of the Japanese fl ounder (
Paralichthys olivaceus
) were
studied by using radioimmunoassay (Miwa et al., 1988; Tagawa et al., 1990).
In the eggs of the fl ounder, a low level of thyroxine (T4 ) and a fairly high
level of triiodothyronine (T3) are detected just after fertilization, and the
concentrations of these hormones gradually lower towards hatching. After
hatching, tissue T3 concentration sharply decreases and both hormone levels
become very low (Tagawa et al., 1990). Because the thyroid follicles develop
after hatching, these hormones in eggs are apparently of maternal origin.
Tissue T4 concentration of the larval fl ounder is kept rather low during
premetamorphosis, but it increases gradually during prometamorphosis.
Then, it rises sharply at the beginning of climax, and the high level is
maintained throughout climax but it decreases to about one half of the
maximum level at postclimax (Fig. 3), thus, creating a clear surge of tissue T4
concentration in metamorphic climax (Miwa et al., 1988; Tagawa et al., 1990).
This change in tissue T4 concentration during metamorphosis, especially
the surge in metamorphic climax, well coincides with previously mentioned
histological activation of the thyroid gland. Larval tissue T3 concentration
is much lower (about one-tenth of that of T4) and shows similar change to
that in T4 during metamorphosis (Fig. 3).
Figure 3.
Changes in whole body tissue T
4
and T
3
concentrations during metamorphosis of
the Japanese fl ounder. Each point represents the average of the duplicate determination of
pooled samples. [From Tagawa et al., 1990, with permission].
