Biomedical Engineering Reference
In-Depth Information
Stevens made the startling observation that even transplantation of gonadal tissue
out of the embryonic microenvironment and into the adult testis was sufficient to
induce teratomas in mice of the proper genetic background (Stevens
1964
). In the
1990s, a series of experimental conditions were established to efficiently obtain
pluripotent clones, known as embryonic germ (EG) cells, by simply transferring
murine primordial germ cells (PGCs) during a precise developmental window from
the gonadal niche to an
in vitro
milieu defined by specific growth factors and feeder
cells, as reviewed elsewhere in this volume (Matsui et al.
1992
). These observa-
tions, in conjunction with the fact that the solitary task of the germline is to transmit
the genetic and epigenetic information required for embryogenesis, all pointed to
the possibility that postnatal germ cells could be predisposed to pluripotency. Data
supporting this hypothesis has now been published by multiple groups of investigators,
following a landmark study from T. Shinohara's laboratory in 2004 (Kanatsu-
Shinohara et al.
2004
).
In this chapter, we first introduce the mammalian spermatogonial stem cell
(SSC), the cell type from which pluripotent stem cells are believed to arise, and
discuss the technology that has facilitated investigation of this phenomenon. The
unique properties of SSCs are highlighted in comparison to somatic cells and
embryonic stem (ES) cells. We then address the factors that may predispose SSCs
to pluripotency and review the studies in which murine and human germ cells have
been observed to become pluripotent spontaneously
in vitro
, a phenomenon that is
not observed with somatic cells in culture. Finally, we discuss the implications of
the most recent findings related to male germline stem cells and we compare the
properties of the germline-derived pluripotent cells with those of pluripotent cells
generated from somatic cells through the delivery of exogenous pluripotency
factors [induced pluripotent stem (iPS) cells].
2.2
The Putative Precursors: Spermatogonial
Stem Cells (SSCs)
The SSC, responsible for maintaining near life-long spermatogenesis in mammals,
is contained within the population of undifferentiated spermatogonia, along the
basement membrane of the seminiferous tubule, but represents only about 0.03% of
germ cells in mice (Tegelenbosch and de Rooij
1993
). While morphologic criteria
were previously used to define these stem cells, the advent of technology to trans-
plant and later to expand them in culture has allowed a series of investigations into
the molecular features that define SSCs, as reviewed elsewhere in this volume
(Brinster and Zimmermann
1994
; Kanatsu-Shinohara et al.
2003
). The notion that
postnatal testicular cells are predisposed to pluripotency remained untestable prior
to the advent of technology to accurately identify and propagate SSCs. In 2003, the
Shinohara group described a set of culture conditions that allowed long-term culture
of SSCs, by employment of mouse embryonic fibroblast (MEF) feeder cells, in
conjunction with a rich culture medium supplemented with several recombinant
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