Biology Reference
In-Depth Information
The fi rst indication of TH involvement in lamprey metamorphosis
was the paradoxical induction of a partial metamorphosis in larval year
classes 1, 2, and 3 of L. planeri following immersion in the anti-thyroid agent
(goitrogen) potassium perchlorate (KClO 4 ) (Hoheisel and Sterba, 1963) (Fig.
1). The effects of goitrogens on endostyle morphology and iodine metabolism
suggested that KClO 4 inhibited thyroidal activity (Jones, 1947; Klenner
and Schipper, 1954; Clements-merlini, 1962a,1962b), but the relationship
to metamorphosis remained unclear. This idea that thyroidal inhibition
might be involved in lamprey metamorphosis was later supported by the
observations that serum T 4 (Wright and Youson, 1977) and T 3 (Lintlop and
Youson, 1983b) concentrations declined coincident with the fi rst external signs
of metamorphosis in P. marinus . Subsequent studies on L. reissneri claimed
a complete metamorphosis following treatment with either KClO 4 , sodium
perchlorate (NaClO 4 ), proplythiouracil (PTU) or thiourea (TU) (Suzuki,
1986,1987,1989). Conversely, precocious metamorphosis was not observed in
G. australis following 70 days of PTU treatment despite signifi cant decreases
in both serum T 4 and T 3 concentrations (Leatherland et al . , 1990). These results
led Leatherland et al . (1990) to further suggest that, since the decline in TH
occurs after the fi rst external signs of metamorphosis are visible, the decline
in TH could not be the stimulus that initiates metamorphosis.
These early studies raised an important question regarding the role of
TH in lamprey metamorphosis: Is the decline in serum TH levels required
for the initiation of metamorphosis or is it simply a consequence thereof?
These two possibilities need not be mutually exclusive, for instance the
decline, in fact, might be a consequence of metamorphosis with the resultant
lower TH a requirement for metamorphosis. The transformation of the larval
endostyle to a follicular thyroid during metamorphosis may be responsible
for the observed decline in serum TH levels (Youson, 2007). Unlike all
other vertebrates, but more like that seen in some protochordates, TH
synthesis in larval lampreys occurs in the subpharyngeal endostyle rather
the typical vertebrate thyroid follicles (or tubules). The transformation
of the endostyle commences at stage 1-2 of metamorphosis with typical
follicles fi rst appearing at stage 4 of metamorphosis (Wright and Youson,
1976,1980a). Although the transforming endostyle contains thyroglobulin
and binds iodide throughout metamorphosis, there appears to be a reduced
capacity to bind iodide at this time which might equate to a decrease in
TH synthetic capacity (Wright and Youson, 1976; Wright et al . , 1980). It is
possible that the elevated TH levels observed in pre-metamorphic larvae
(see above) are required to ensure suffi cient reserves for metamorphosis
when synthetic capacity is reduced during transformation of the thyroid
tissue. To help resolve the puzzle over the importance of the decline in serum
TH levels for metamorphosis a series of thyroid ablation and replacement
experiments were conducted on larval lamprey.
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