Biology Reference
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in the fetal ovary has yet to enter meiosis, but has retained expression of
pluripotency factors and remained proliferative (
Stoop et al., 2005
).
Indeed, meiosis onset in human fetal ovary is considered to be highly
asynchronous (
Skrzypczak, Pisarski, Biczysko, & Kedzia, 1981
), compared
to mouse in which female germ cells enter meiosis in a synchronized
manner at E13.5. It is possible that some cells in this PGC-like
subpopulation may escape meiosis and apoptosis (
Fulton, Martins da
Silva, Bayne, & Anderson, 2005
) and are a possible source of the recently
reported mitotically active germ cells purified from adult human ovaries
(
White et al., 2012
).
Despite these collective findings across a range of different species, the
exact function of pluripotency genes in the context of PGCs remains
unclear. Nanog expression in the preimplantation epiblast correlates with
X-chromosome reactivation (
Silva et al., 2009
) and a direct role for
pluripotency genes in regulating X-chromosome inactivation status has
been reported (
Donohoe, Silva, Pinter, Xu, & Lee, 2009; Navarro et al.,
2008, 2010
). It is therefore possible that reexpression of pluripotency genes
is involved in the early reprogramming events, such as X-chromosome
reactivation, in PGCs. They may also act, in parallel with Blimp1,
to prevent somatic differentiation of early PGCs. Further mechanistic studies
in the mouse combined with careful observations in other mammalian
species will be particularly informative to begin to uncover their precise role.
2.4. Other early PGC factors
Nanos proteins are evolutionarily conserved RNA-binding proteins
that regulate germline development (
Tsuda et al., 2003
). In
Drosophila
, nanos
is involved in appropriate migration of germ cells to the gonad (
Kobayashi,
Yamada, Asaoka, & Kitamura, 1996
). In mice, Nanos2 is required for male
gametogenesis (
Suzuki & Saga, 2008
), whereas Nanos3 plays an earlier role
in PGCs. Although first expressed as early as E7.5, the requirement Nanos3
becomes evident during migration at around E9.5 when PGC numbers are
decreased in
Nanos3
null embryos and by E12.5 few germ cells can be found
in mutants (
Suzuki, Tsuda, Kiso, & Saga, 2008
). This loss of germ cells is due
to apoptosis and appears to be partially dependent on Bax. Double knockout
Nanos3
mice are fertile but they have vastly reduced germ-
cell numbers, suggesting multiple pathways regulate PGC survival (
Suzuki
et al., 2008
).
/
;
Bax
/
