Making Germ Cells from Human Embryonic Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 63

The following sections will summarize the current literature, outlining mouse and
human ES cell investigations and their findings, in a chronological perspective lead-
ing up to the current status of this field and an examination of what the future holds.

3.4.1 Similarities Between ES Cells and Germ Cells

There are numerous similarities between embryonic stem cells and primordial germ
cells, including morphological, genetic, and developmental features. Primordial
germ cells have large nuclei with dispersed chromatin, prominent nucleoli, and very
little cytoplasm or a high nucleus-to-cytoplasm ratio (Motta et al. 1997 ). Interestingly,
human ES cells share these same ultrastructural cellular characteristics (Thomson
et al. 1998 ; Sathananthan et al. 2002 ).
Both PGCs and ES cells also have great self-renewal capabilities and potential
for pluripotency. Although PGCs are not pluripotent in vivo , remarkably when they
are removed from the gonad and cultured in vitro they generate pluripotent embry-
onic germ cell (EGC) lines with nearly identical potential and morphology to
hESCs (see Chap. 1 of this volume) (Donovan and De Miguel 2003 ; Shamblott
et al. 1998 ). Exciting recent work has successfully used germline stem cell lines
derived from the neonatal ovary to produce offspring in mice (Zou et al. 2009 ).
Furthermore, recent studies in mice and humans have shown that adult spermatogo-
nial stem cells (SSCs) can be cultured in vitro and that a subpopulation has the
ability to be “reprogrammed” to a state of multipotency or pluripotency (see Chap. 2
and Part II of this volume). Although it was once thought that only fetal PGCs
have the potential for pluripotency and giving rise to germ cell lines, these adult
mouse multipotent germline stem cells (mGSCs) obtained from the testis can spon-
taneously differentiate into derivatives of all three primary germ layers and the
germ line and contribute to chimeras (Brinster 2002 ; Guan et al. 2006, 2009 ;
Kanatsu-Shinohara et al. 2004 ; Seandel et al. 2007 ). Notably, human adult germline
stem cells derived from testicular biopsies have been propagated and differentiated
into the three germ layers (Conrad et al. 2008 ; Kossack et al. 2009 ; Payne and
Braun 2008 ). Some current studies in our laboratory are focused on expanding
spermatogonia from small testicular biopsies, optimizing the conditions required to
obtain hESC-like cells, and comparing the differentiation potential of these hESC-
like cells with other pluripotent cell lines (Clavijo and Reijo Pera, unpublished).
Of great interest is the observation that both PGCs and hESCs express a unique
combination of genes, referred to as stem/germ cell genes, that are exclusive to cells
of the preimplantation embryo, inner cell mass, ooyctes, and/or human embryonic
carcinoma cells (hECCs) (Clark et al. 2004b ; Bortvin et al. 2003 ). Many proteins
expressed in early germ cells are also expressed in mouse and human ES cells
including, the transcription factor OCT4 (Niwa et al. 2000 ), PUM2 (Moore et al.
2003 ), and NANOS1 (Clark et al. 2004a ; Jaruzelska et al. 2003 ). Human and mouse
PGCs and ES cells also display strong cell surface staining for TNAP (Gaskell et al.
2004 ). Further, three additional human genes, NANOG (the human homolog of