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Comparison of expression patterns of mRNAs for THRα and THRβ
revealed rather ubiquitous expression of THRα and developmental stage-
and tissue-specifi c expression of THRβ in chicken (Forrest et al., 1990, 1991)
and rat (Bradley et al., 1989, 1992, 1994; Mellstrom et al., 1991). In amphibian
metamorphosis, a rapid increase in THRβ mRNA has been found during
spontaneous metamorphosis and also in response to exogenous thyroid
hormone (Yaoita and Brown, 1990; Kawahara et al., 1991). Yamano and Miwa
(1998) studied developmental changes in the expression levels and regional
expression patterns of THRα and THRβ genes during metamorphosis of
the Japanese fl ounder. Although the expression levels of fl ounder THR
gene transcripts are very low in fertilized eggs, substantial amount of
THR mRNAs are present in premetamorphic larvae, except for THRαB,
which is kept low throughout larval development. THRα-gene transcripts
increase in metamorphic climax and decrease rapidly in postclimax, while
Figure 15. Developmental changes in the expression of TR genes during larval growth and
metamorphosis of the Japanese fl ounder. One hundred units correspond to 36.5 amol of
TR cDNA reverse transcribed from 1µg of mRNA. Changes in T4 and T3 concentrations at
corresponding metamorphic stages (Tagawa et al., 1990) are shown in the uppergraph. [From
Yamano and Miwa, 1998, with permission].
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