Biology Reference
In-Depth Information
Unilateral Gonadectomy (ULG) entails surgical removal of one gonad
and leaving the other intact; the ULG provides an opportunity to know
whether (i) the fi sh has retained the fl exibility of differentiation/maturation
of gametes during the course of gametogenesis and (ii) the remaining gonad
is able to compensate and if so partially or completely the hormone and/or
gametogenesis in response to the negative feedback. With the presence of
iteroparous and semelparous fi shes, the Unilateral Ovariectomy (ULO) may
also provide an opportunity to know whether the ULO induces divisions
of oogonia with concomitant generation of stem cells. In iteroparous
species such as
Heteropneustes fossilis
and
Oncorhynchus mykiss,
eggs are
produced in more than one clutch or breeding season, and at a given
period, their ovaries consist of follicles at different stages of oogenesis. But
in semelparous species like
O. kisutch,
which reproduce only once in a life
time, synchronized oogenesis occurs and most of the available follicles are
recruited into a single clutch. Following ULO and with the availability of
ingredients for egg production, the compensatory ovary may opt for one or
more of the following pathways: 1. Produce larger eggs from the existing
pool of vitellogenic follicles, 2. Maximize the fecundity by reducing the
number of atretic follicles and 3. Increase the number of dividing oogonia
with concomitant production of stem cells.
In all fi shes, the ULO is typically accompanied by a compensatory
course. Firstly, it is accompanied by elevated FSH levels (e.g., Tyler et al.,
1997). In iteroparous fi shes, the compensatory ovarian hypertrophy is
accompanined by the options 2 and 3, i.e., by recruiting additional follicles
into the vitellogenesis and reducing the number of atretric oocytes and/
or increasing the number of dividing oogonia, implying the incidental
generation of additional stem cells (e.g.,
H. fossilis
, Goswami and Sundararaj,
1968;
O. mykiss,
Tyler et al., 1994). Contrastingly, the compensatory ovarian
hypertrophy of the semelparous
O. kisutch
has limited options of 1 and 2, i.e.,
producing larger oocytes from the existing pool of follicles and maximizing
the fecundity by reducing the number of atretic oocytes (Luckenback et
al., 2008). Understandably, the semelparous fi sh, which are to die after
spawning, do not generate additional dividing oocytes and more stem
cells.
On the other hand, Unilateral Castration (ULC) of the Atlantic salmon
Salmo salar
and stickleback
Gasterosteus aculeatus
suppresses androgen levels,
for instance, the plasma level of 11-KT to 56-63% (Hellqvist et al., 2002).
The rapid and complete compensation of androgen levels, especially that
of testosterone (see Table 1) observed in the ULC mammals is not found
in the breeding males of salmon and stickleback. This observation clearly
suggests that the ULC is not regulated by negative feedback.
With availability of more information, a review and critical analysis of
regeneration of the ovary and testis in gonadectomized fi shes has become
