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PGCs convert into EG cells, since the latter set of culture conditions were not
successful for conversion of SSCs. These conclusions raised the intriguing question
of whether SSCs should generally be considered capable of “dedifferentiation” and
therefore inherently multipotent or alternatively whether a small primitive subpopu-
lation of germ cells in the postnatal testis could be responsible for the production
of ES-like cells in culture. To answer this, genetic marking was used to demonstrate
that single SSC clones could give rise both to continually self-renewing SSC
cultures and to ES-like cells (Kanatsu-Shinohara et al.
2008a
). Of note, the partial
androgenetic pattern of imprinting in SSCs was invoked as an indication that epi-
genetic instability in culture could contribute to the change in fate of SSCs, upon
conversion into ES-like cells (Kanatsu-Shinohara et al.
2004
).
Several studies subsequently confirmed the general concept that pluripotent stem
cells could be derived from the testis and specifically found that even the adult testis
harbors cells with such capability. Guan et al. (
2006
) found that, after a very brief
period of culture, the Stra8
+
population of adult testicular cells was able to generate
ES-like cells (Guan et al.
2006
). However, the published nomenclature provided did
not unequivocally distinguish the identity of the parental population from the ES-like
derivatives. Interestingly, the authors found that even the parental SSCs could
directly contribute to chimerism in blastocyst injection assays, although no SSC
transplantation data were shown to establish the purity and functionality of the starting
population of germ cells, which were precultured for 1 week prior to initiation of
ES-like induction culture conditions. The authors suggested that the presence of
somatic factors such as GDNF could serve to inhibit plasticity both
in vitro
and
in vivo
. This hypothesis is in contrast with the results obtained by another group
(Huang et al.
2009
). The latter reported the derivation of alkaline phosphatase-positive
pluripotent cells from unselected neonatal testicular cell suspensions in short-term
culture (1 week), but they proposed that the testicular somatic cells present in the
culture, specifically the Leydig cells, are responsible for the production of Igf1
which, through Akt signaling, maintains pluripotency of SSCs. In this study, there is
no distinction between SSC and pluripotent cells, but rather they were proposed to
be the same cell type able to contribute both to spermatogenesis after transplantation
in busulfan-treated testes and to chimera formation upon blastocyst injection.
Notably, these cells did not form teratomas when transplanted into the testis, whereas
they did form teratomas when transplanted subcutaneously in NOD-SCID mice.
Another group reported the development of pluripotent stem cells during short-term
culture of neonatal and adult testicular cells from
Oct4/GFP
-reporter mice (Izadyar
et al.
2008
). This report also did not distinguish between the different populations of
germline-derived cells in question (i.e., SSCs and ES-like cells) in the culture system,
a similar semantic and experimental issue as in the Guan et al. (
2006
) study (Izadyar
et al.
2008
). In fact, both populations appeared to coexist (based on heterogeneous
morphology of colonies in the images provided). Perhaps not surprisingly, the
authors found a remarkably similar expression profile and imprinting pattern
between germ cells before and after culture. This could be due to dilution of ES-like
cells by a majority of SSC-like cells in the culture (which could also explain the
absence of teratoma formation), although this issue was not specifically addressed.
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