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detectable at 3-4 months postnatally but these normally disappear thereafter
(Rajpert-De et al.
2004
).
A similar picture has emerged for
Oct4
expression in cultured SSCs. Multiple
studies confirmed expression by RT-PCR in neonatal SSCs (Kanatsu-Shinohara
et al.
2004, 2005b
). However, a quantitative analysis of neonatal SSCs later demon-
strated sharply lower Oct4 expression by either mRNA or protein, compared to ES
cells (Imamura et al.
2006
). Not surprisingly, the same pattern was seen in adult
SSCs in long-term culture (Seandel et al.
2007
). Notably, this heterogeneous and
relatively low magnitude of Oct4 expression has recently been found to be function-
ally important in self-renewal and survival of cultured SSCs (Dann et al.
2008
).
A second major pluripotency gene studied in the male germ line is
Sox2
(SRY [sex
determining region Y] - box 2, which, like
Oct4
is expressed in the germline)
(Avilion et al.
2003
). In mice, germline expression of
Sox2
is lost by E15.5 (Western
et al.
2005
). In humans,
SOX2
mRNA was detected in adult testis in two studies (Gure
et al.
2000
; Schmitz et al.
2007
). However, using more rigorous methods SOX2 was
later shown to be absent even in human PGCs and also absent in adult testis by both
message and protein (Perrett et al.
2008
; de Jong et al.
2008
). In cultured neonatal
murine SSCs, no Sox2 protein was detectable despite significant transcript levels
(Imamura et al.
2006
; Shi et al.
2006
). In contrast, Seandel et al. (
2007
) found that
adult SSCs in culture did not even express
Sox2
message (Seandel et al.
2007
).
Therefore, in both mice and human functional SOX2 protein is not likely to be pres-
ent beyond an early developmental window, and this decline in expression maybe
paralleled in cultured SSC lines derived from mice of increasing age.
The third canonical pluripotency-associated transcription factor is Nanog, another
homeodomain-containing protein strongly expressed in ES cells (Chambers et al.
2003
). Beyond the mouse blastocyst stage,
Nanog
expression is present in the male
germ lineage, and the protein is detectable through E16.5, at which time it is largely
down-regulated coincident with mitotic arrest (Chambers et al.
2003
; Yamaguchi
et al.
2005
). No Nanog protein was detected in the adult mouse testis (Hoei-Hansen
et al.
2005
). Similarly, in the human testis, NANOG protein is present through 19
weeks of gestation, but the rare positive cells that remain at 3-4 months postnatally
are completely absent by childhood and also in adults (Hoei-Hansen et al.
2005
).
Perhaps not surprisingly cultured murine SSCs from neonatal or adult stages do not
express Nanog (Kanatsu-Shinohara et al.
2004
; Seandel et al.
2007
).
As mentioned above, OCT4, SOX2, and NANOG are thought to form a core
regulatory network in ES cells (Boyer et al.
2005
). Moreover, both OCT4 and
SOX2 are key transcription factors in the cocktail of genes used to generate iPS
cells starting from somatic cells, with OCT4 being the most critical of the two
(Takahashi and Yamanaka
2006
; Kim et al.
2009c
). However, based on the studies
described above, these proteins, with the exception of OCT4, are not present either
in postnatal germ cells or in cultured SSCs (Fig.
2.1
). Therefore, it is unlikely that
a resident subpopulation of ES-like cells in the postnatal testis could give rise to
pluripotent stem cells
in vitro
. Furthermore, based on their absence in the precursor
population, it seems that neither SOX2 nor NANOG-driven signals are likely to be
the most proximal mediators in the signaling pathway leading to the conversion of
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