Biology Reference
In-Depth Information
Figure 17.
Dark-fi eld photomicrographs showing changes in TRĪ±gene expression in the
developing mucosal epithelium of the stomach of a larva at prematamorphosis (A), a larva at
prometamorphosis (B), a larva at metamorphic climax (C), and a post climax specimen (D) of the
Japanese fl ounder. Arrowheads indicate melanophores. Asterisks indicate stomach epithelium
from which the gastric glands would differentiate. These sections were treated simultaneously
with the same hybridization solution. e, direction of esophagus; g, gastric gland; l, stomach
lumen. Bar=0.5 mm. [From Yamano and Miwa, 1998, with permission].
Kobayashi, 1958; Kaltenbach, 1958). These hormones accelerate both T4-and
T3-induced shrinkage of the cultured tail fi n discs (Kikuyama et al., 1983;
Kaltenbach, 1985). A similar permissive effect of cortisol, the major corticoid
in teleosts, on thyroid hormone action during metamorphosis is seen in the
Japanese fl ounder (de Jesus et al., 1990). Addition of cortisol alone to the
culture medium did not affect shortening of the fi n rays
in vitro
. However,
when cortisol was added together with T4 or T3, it enhanced the rate of
shortening compared with that of fi n rays treated with T4 or T3 alone. On
the other hand, the permissive effect of cortisol on thyroid hormone was
not seen
in vivo
; when the prometamorphic larvae were reared in seawater
containing both cortisol and T4, cortisol did not enhance the metamorphosis-
promoting effect of thyroid hormone (de Jesus et al., 1990). This may be
because endogenous cortisol level is suffi cient so that addition of exogenous
cortisol is not effective. In fact, a considerable amount of cortisol is detected
in whole body extracts during fl ounder metamorphosis. Tissue cortisol
concentration shows a change parallel to that in tissue T4 concentration;
the concentration remains low during premetamorphosis, increases during
prometamorphic stage, reaching a peak level at metamorphic climax and
decreases to about half of the maximal level at postclimax (de Jesus et
