Biology Reference
In-Depth Information
the plaice,
Pleuronectes platessa
(Brooks and Johnston, 1993) and turbot,
Scophthalmus maximus
(Focant et al., 2000). However, in these fi shes, these
muscle changes do not occur in metamorphosis but in a different growth
stage after the metamorphosis from juveniles to adults (Focant et al.,
2000).
Administration of T4 to premetamorphic larvae of the Japanese fl ounder
precociously induced the morphological change in the muscle from larval
to adult type: muscle fi bers became thicker with abundant myofi brils,
while TU administration inhibited the change (Yamano et al., 1991). T4
administration also precociously induced transition of the muscle protein
components such as isoforms of myosin DTNB light chains and troponin-T
(Yamano et al., 1994b). Thus, it is evident that thyroid hormone plays the
primary role in controlling morphological and biochemical changes in the
muscle during metamorphosis of the Japanese fl ounder.
As mentioned above, the transition of DTNB light chains during
metamorphosis was markedly infl uenced by thyroid state. However, TU
administration to juveniles of the fl ounder neither restored the larval type
DTNB light chain nor depressed the expression of adult-type (Yamano et
al., 1994b). In higher vertebrates, thyroid hormone induces fetal to adult
transition of cardiac myosin heavy chains (MHCs) (Izumo et al., 1986). In
contrast to the case of DTNB light chains in the Japanese fl ounder, however,
the response of MHCs to thyroid hormone is reversible depending on
the thyroidal states (Izumo et al., 1986). The mechanism of the thyroidal
control on gene expression for the cardiac MHCs is well documented: the
complex of thyroid hormone and its receptor directly binds to the thyroid-
response element of the gene to regulate the expression of the gene either
positively or negatively (Izumo and Mahdavi, 1988). In contrast, in fl ounder
metamorphosis, neither the molecular mechanism of thyroid-hormone
action on DTNB light chains nor the reason why the responses of DTNB
light chains to thyroid hormone are irreversible is not known.
4.2.5.4 Chloride cell development
Sequence of chloride cell development and its control by thyroid hormone
during metamorphosis is briefl y presented here. A thorough review has
been written on the osmoregulatory system of fl atfi shes during early
development and metamorphosis (Schreiber, 2001).
It is well known that chloride cells (mitochondria-rich cells (MRCs)
in the gills are responsible for the secretion of excess Na+ and Cl- in the
body fl uid in adult marine fi shes (Zadunaisky, 1984). On the other hand,
cutaneous chloride cells located on the epithelium covering yolk and body
have been suggested as the ion-secreting site during early life stages in
teleosts (Shelbourne, 1957; Roberts et al., 1973; Hwang, 1989; Hwang and
