Biology Reference
In-Depth Information
stage 7 of metamorphosis. Following the completion of metamorphosis, IRD
activity falls to moderate levels in parasitic-phase juveniles and upstream-
migrant adults (Eales et al
.
, 2000). The sharp decline in T
4
ORD and a surge
in T
3
IRD activity may contribute to the decline in serum TH levels that
is observed at the start of metamorphosis and, thus, may act to further
minimize the level of active hormone available throughout the metamorphic
process. The cDNA for
P. marinus
D2 has recently been cloned and real-time
PCR expression data for the intestine, liver and kidney are consistent with
T
4
ORD activity data (R.G. Manzon, unpublished data).
Collectively, all data pertaining to the lamprey thyroid axis, including
serum TH concentrations during spontaneous metamorphosis, various
ablation and replacement experiments, exogenous TH treatments, and
both THDP and TH deiodinase data, are consistent with the hypothesis
that a suppression of the thyroid system is important for the normal
progression of metamorphosis in lampreys. Although these data are very
convincing and strongly support the need for a decline in serum TH
concentrations during lamprey metamorphosis, they do not represent the
complete picture. A close examination of serum TH levels demonstrates
that despite the decline in serum TH concentrations during spontaneous
metamorphosis, these relatively low levels might be comparable to peak
levels during anuran metamorphosis. In larval lamprey serum, T
4
and T
3
concentrations peak at values in the range of 1550-3153 µg/dl and 7800
ng/dl, respectively (Lintlop and Youson, 1983a; Youson et al
.
, 1994). By
stage 3 of metamorphosis, TH concentrations drop to 965 µg/dl and 860
ng/dl for T
4
and T
3
, respectively, and are at their lowest levels of 98 µg/
dl of T
4
and 61 ng/dl of T
3
at stage 6 of metamorphosis. Relative to larval
lamprey, the TH concentrations measured during metamorphosis are very
low, however, they are comparable to the peak T
3
concentrations of 75-150
ng/dl as measured during metamorphic climax in
R. catesbeiana
and
X.
laevis
(White and Nicoll, 1981; Tata
et al
.
, 1993).
One view is that the decline in serum TH levels during spontaneous
lamprey metamorphosis is a result of a decrease in TH synthesis and
secretion in conjunction with an increase in cellular uptake and nuclear
binding at sites where TH act to regulate morphogenesis. Thus, despite
the decrease in serum concentrations, the function of TH in lamprey
metamorphosis might be fundamentally similar at the level of the gene
to the situation in teleost and anuran metamorphoses. There are data that
suggest that the nuclear binding of T
3
by hepatocytes increases during
P.
marinus
metamorphosis (Lintlop and Youson, 1983a). Using
in vitro
binding
assays, hepatocyte nuclei where shown to bind T
3
with high affi nity (K
d
2.69 X10
-10
M) and their binding capacity increased slightly from 1.89 pg
T
3
/µg DNA in larvae to peak values of 2.62 pg T
3
/µg DNA at metamorphic
stage 5 and 6 and then decreased in parasitic adults and upstream migrants
