Biology Reference
In-Depth Information
4.2 Hormonal Control of Metamorphosis
4.2.1 Dynamics of thyroid gland activity and pituitary thyroid-
stimulating hormone (TSH) cells during metamorphosis
One of the most spectacular actions of thyroid hormone is the stimulation of
metamorphosis in amphibian larvae (Dodd and Dodd, 1976). It had also long
been known that the thyroid gland is activated during the metamorphosis
of eel and plaice, (Sklower, 1930). However, before the series of studies
on the hormonal control of the metamorphosis of the Japanese fl ounder,
Paralichthys olivaceus
(Inui and Miwa, 1985), the role of the thyroid hormone
in fi sh development had been ambiguous (Eales, 1979).
Miwa and Inui (1987a) found conspicuous histological activation of the
thyroid organ and pituitary TSH cells toward the metamorphic climax in
Japanese fl ounder larvae. In the larvae, immunoreactivity to anti-thyroid-
hormone serum is already present in the subpharyngeal region 36 h after
hatching and a few thyroid follicles have become evident in 4-day old
fi sh (Fig. 2-3a). Thyroid follicles increase both in size and in number from
premetamaorphosis to metamorphic climax, and the activity of the follicles
judged by the follicular epithelial cell height becomes highest at climax (Fig.
2-3d). Thereafter, the number of the follicles decreases, and at post climax,
the epithelial cells become fl attened (Fig. 2-3e), indicating typical inactive
state of the gland. Similar morphological activation in the thyroid follicles
and strong immunoreactivity to T4 and T3 at metamorphic climax have
been reported for developing Senegalese sole (
Solea senegalensis
), though
the study did not refer to inactivation after metamorphosis (Ortiz Delgado
et al., 2006).
On the other hand, immunoreactive (Ir) TSH cells are fi rst found in
the pituitary of 4 day old larvae of the Japanese fl ounder (Fig. 2-2a). The
area occupied by the Ir TSH cells increases from dorsal part to ventral part
of proximal pars distalis of the pituitary, and staining intensity of the cells
also increases toward prometamorphosis. The area of the cells becomes
largest, and the cells are stained very intensely in prometamorphosis (Fig.
2-2c), indicating that the amount of TSH stored in the cells reaches a peak
at this time. The IrTSH cells is much decreased in number and in staining
intensity in metamorphic climax (Fig. 2-2d), suggesting a rapid release of
TSH from the cells, and show the least activity in post climax (Fig. 2-2e).
These morphological changes in TSH cells suggest a strong stimulatory
action of pituitary TSH on the thyroid gland in metamorphic climax.
