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ooplasma and a dense nucleus with a large nucleolus (Aroua et al., 2005).
Stage III corresponds to the previous “intermediate” or “yellow/silver”
stage with 0.4%≤GSI<1.2%; oocytes are larger and a few lipidic vesicles are
observed in the ooplasma, which indicates the initiation of the incorporation
of lipidic stores in the oocytes, also referred as “endogenous vitellogenesis”
(Aroua et al., 2005). Stages IV and V correspond to the previous “silver”
stage with a GSI≥1.2%; oocytes are further enlarged with a large nucleus
and small nucleoli at a peripheral position and numerous lipidic vesicles
in the ooplasma, with is the oil-droplet stage of early vitellogenesis (Aroua
et al., 2005). In the most advanced stage of silvering, vitellogenin can be
observed in the ooplasm, as well as in the plasma, which corresponds to
the start of exogenous vitellogenesis.
Considering the start of gonadal maturation, silvering should be
considered as an initiation of puberty. As the development of gonads and
sexual maturity are blocked at this stage and until the occurrence of oceanic
reproductive migration, our group defi ned eel silvering as a prepuberty.
These data concerning gonadal maturation show that eel silvering is quite
different from salmon smoltifi cation, which occurs before a growth phase
and is not associated with changes related to reproduction.
7.3 Endocrine/internal Factors Involved in the Control of
Secondary Metamorphosis
As indicated previously, silvering consists of various morphological,
physiological and behavioural changes. Among the modifi ed organs,
some are related to sensory organs, others to hydrostatic pressure or
seawater adaptation, similarly to changes observed during smoltifi cation,
which traditionally led to eel silvering being defi ned as a metamorphosis.
However, unlike smoltifi cation, silvering also includes some changes related
to an onset of sexual maturation such as gonad development, which led
to the hypothesis that silvering corresponds to a pubertal event. Puberty,
the major post-embryonic developmental event in the life cycle of all
vertebrates, encompasses various morpho-physiological and behavioural
changes, which unlike metamorphic changes are induced by sexual
steroids (for review, see Romeo, 2003). While metamorphosis is mostly
triggered in vertebrates by the activation of the thyrotropic axis, puberty
is triggered by the activation of the gonadotropic axis. In order to assess
which neuroendocrine axis may be involved in the induction of silvering,
we analyzed the profi les of pituitary and peripheral hormones during the
transition from yellow to silver eels.
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