Biomedical Engineering Reference
In-Depth Information
this time makes the creation of pluripotent stem cells impossible (Matsui et al.
1992
; Resnick et al.
1992
; Stevens
1966
). More specifically, this period in PGC
development coincides with the cessation of mitosis in both the male and female
germline, the erasure of imprints that occurs in the germline, and the onset of sexual
differentiation in the somatic lineages. Unfortunately, to date the role that any of
these mechanisms play in EG derivation remains unclear.
The derivation of EG cells from mice was soon followed by the derivation of the
same cell type from humans and other species (Shamblott et al.
1998, 2001
;
Turnpenny et al.
2003
). Like their murine counterparts, EG cell lines derived from
human PGCs were also found to be pluripotent based on their ability to form tera-
tomas containing cells from all three primary germ layers when injected into
immune-deficient mice. Interestingly, human EG cells could be derived from
embryos that had passed the point of sexual differentiation based on the observation
of sex cords in male embryos (Shamblott et al.
1998
; Turnpenny et al.
2003
).
Whether this implies a difference in mechanisms of PGC development between
mice and humans remains unclear. Notably though, human EG cell lines have
proven to be much more difficult to propagate than their murine counterparts and
this has, unfortunately, limited their usefulness in regenerative medicine (Turnpenny
et al.
2006
).
1.4
Probing the Mechanisms of Pluripotency
The ability to make pluripotent stem cells from a source other than the ICM
provided a new system with which to probe the mechanisms controlling develop-
mental potency. Importantly, ES cells are derived from a population of cells that are
already themselves pluripotent. Therefore, creation of ES cells may require altera-
tions in cell cycle control or a clock that times cell division but not necessarily
alterations in developmental potency. But both EC and EG cells are derived from a
population of cells, PGCs, that are specialized. Functional analyses of PGC potency
have been carried out in which genetically marked PGCs carrying specific coat
color alleles or green fluorescent reporters were introduced into the blastocoel cav-
ity of the pre-implantation embryo. This experiment has been carried out by several
investigators and the donor PGCs did not give rise to coat color or germline chime-
ras (Donovan et al. unpublished observations; Stewart, personal communication;
Papaioannou, personal communication; Durcova-Hills et al.
2006
). Thus, the con-
clusion from these studies is that PGCs can be considered a specialized cell type
that is highly restricted in developmental potency.
Interestingly, when PGCs are converted into EG cells not all PGCs give rise to
EG cells. We estimated that between 8 and 23% of the cultured PGCs gave rise to EG
cells (Resnick et al.
1992
), suggesting heterogeneity in the PGC population. This
conclusion is also supported by analysis of the morphology, cell surface antigen stain-
ing, and expression of the germ cell marker Ddx4 in PGCs isolated from the embryo
(Durcova-Hills et al.
2006
). Studies of PGCs have also revealed heterogeneity for a6
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