scientific
report
scientificreport
Oct4 is required for primordial germ cell survival
James Kehler 1, Elena Tolkunova 2, Birgit Koschorz 2, Maurizio Pesce 3, Luca Gentile 4, Michele Boiani1w,
Hilda Lomelı´5,6, Andras Nagy 6, K. John McLaughlin7, Hans R. Schöler 1w+ & Alexey Tomilin1,2
1Germline
Development Group, Center for Animal Transgenesis and Germ Cell Research, New Bolton Center, University of
Pennsylvania, Pennsylvania, USA, 2Department of Developmental Biology, Max Planck Institute of Immunobiology, Freiburg,
Germany, 3Laboratorio di Biologia Vascolare e Terapia Genica, Centro Cardiologico, Milan, Italy, 4Department of Cell and
Developmental Biology, Max Planck Institute for Molecular Biomedicine, Münster, Germany, 5Departamento de Genética y Fisiologia
Molecular, Instituto de Biotecnologı́a, Universidad Nacional Autónoma de Mexico, Cuernavaca, Mexico, 6Samuel Lunenfeld Research
Institute, Mount Sinai Hospital, Toronto, Ontario, Canada, and 7Developmental Epigenetics Group, Center for Animal Transgenesis
and Germ Cell Research, New Bolton Center, University of Pennsylvania, Kennett Square, Pennsylvania, USA
Previous studies have shown that Oct4 has an essential role in
maintaining pluripotency of cells of the inner cell mass (ICM) and
embryonic stem cells. However, Oct4 null homozygous embryos
die around the time of implantation, thus precluding further
analysis of gene function during development. We have used the
conditional Cre/loxP gene targeting strategy to assess Oct4
function in primordial germ cells (PGCs). Loss of Oct4 function
leads to apoptosis of PGCs rather than to differentiation into a
trophectodermal lineage, as has been described for Oct4deficient ICM cells. These new results suggest a previously
unknown function of Oct4 in maintaining viability of mammalian
germline.
Keywords: Oct4 function; primordial germ cells; conditional gene
targeting; Cre/loxP; apoptosis
EMBO reports (2004) 5, 1078–1083. doi:10.1038/sj.embor.7400279
1Germline
Development, Center for Animal Transgenesis and Germ Cell Research,
New Bolton Center, University of Pennsylvania, 382 W. Street Road, Kennett Square,
Pennsylvania 19348, USA
2
Department of Developmental Biology, Max Planck Institute of Immunobiology,
Stübeweg 51, 79108 Freiburg, Germany
3Laboratorio di Biologia Vascolare e Terapia Genica, Centro Cardiologico,
Fondazione I Monzino, Parea 4, I-20138, Milan, Italy
4Department of Cell and Developmental Biology, Max Planck Institute for Molecular
Biomedicine, Mendelstrasse 7, Münster 48149, Germany
5Departamento de Genética y Fisiologia Molecular, Instituto de Biotecnologı́a,
Universidad Nacional Autónoma de Mexico, A.P. 510-3, Cuernavaca, Morelos 62271,
Mexico
6
Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 381–600 University
Avenue, Toronto, Ontario, Canada M5G 1X5
7
Developmental Epigenetics Group, Center for Animal Transgenesis and Germ Cell
Research, New Bolton Center, University of Pennsylvania, W. Street Road,
Kennett Square, Pennsylvania 19348, USA
w
Present address: Department of Cell and Developmental Biology, Max Planck
Institute for Molecular Biomedicine, Mendelstrasse 7, Münster 48149, Germany
+
Corresponding author. Tel: þ 49 251 980 2866; Fax: þ 49 251 980 2894;
E-mail: schoeler@mpi-muenster.mpg.de
Received 24 August 2004; revised 20 September 2004; accepted 21 September 2004;
published online 15 October 2004
1 0 7 8 EMBO reports
VOL 5 | NO 11 | 2004
INTRODUCTION
The POU domain transcription factor Oct4 shows a remarkable
expression pattern in mouse ontogeny. Maternal Oct4 RNA and
protein are present in fertilized oocytes until the two-cell stage,
and zygotic Oct4 gene expression starts at the four- to eight-cell
stage (Rosner et al, 1990; Schöler et al, 1990; Yeom et al, 1996).
During early cleavage, uniform amounts of Oct4 RNA are found
in all blastomeres, but the levels decrease in the outer cells of the
morula as they polarize and form the trophectoderm. In the
3.5 days postcoitum (dpc) blastocyst, Oct4 RNA and protein levels
are low in this epithelial cell layer and become undetectable 1 day
later (Schöler et al, 1990; Palmieri et al, 1994). In contrast, Oct4
expression is maintained in the inner cell mass (ICM) of the
blastocyst. Differential expression of Oct4 is observed again during
embryogenesis, when the ICM differentiates at 4.5 dpc into epiblast
(primitive ectoderm, embryonic ectoderm) and hypoblast (primitive
endoderm, embryonic endoderm). Oct4 expression is maintained
in the epiblast but, as hypoblast cells differentiate into visceral and
parietal endoderm, Oct4 protein levels transiently increase and
then decrease to undetectable levels. During gastrulation, Oct4
expression is progressively repressed in the epiblast and by 7.5 dpc
is confined exclusively to newly established primordial germ cells
(PGCs; Schöler et al, 1990; Yeom et al, 1996). PGCs continue to
express Oct4 as they proliferate and migrate to the forming genital
ridges. In female PGCs, Oct4 is repressed by the onset of meiotic
prophase I (13–14 dpc) and is then re-expressed after birth,
coincident with the growth phase of oocytes. In male embryos,
Oct4 expression persists in germ cells throughout fetal development. After birth, it is maintained in proliferating gonocytes,
pro-spermatogonia and later in undifferentiated spermatogonia
(Pesce et al, 1998a; Tadokoro et al, 2002). In addition, embryonic
stem (ES) cells, embryonal carcinoma (EC) cells and embryonic
germ (EG) cells, pluripotent cell lines derived from the ICM,
epiblast and PGCs, respectively, also express Oct4 as long as they
remain undifferentiated (for reviews, see Pesce et al, 1998b; Surani,
2001; Donovan & de Miguel, 2003).
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
Oct4 and primordial germ cell survival
J. Kehler et al
This highly restricted, cyclic expression pattern in the germ
line, referred to as the ‘totipotent cycle’, has been taken as an
indication that Oct4 may have a role in maintaining the
pluripotency and germline potential of pluripotent embryonic
cells. Indeed, gene targeting showed that Oct4-deficient embryos
survived through the morula stage, but could not form an ICM and
also failed to give rise to ES colonies in vitro (Nichols et al, 1998).
These results suggested that differentiation of the ICM into
trophoblast cells was due to the absence of Oct4 function, rather
than to a reduction in viability or decreased proliferation. Further
functional tests involved ES cells whose self-renewal and
pluripotent state relied on the inducible expression of Oct4 (Niwa
et al, 2000). In these experiments, alteration in Oct4 expression
above or below a twofold threshold level was sufficient to trigger
differentiation into hypoblast and trophoblast cells, respectively.
These results were taken as an indication that Oct4 may also act
in a dosage-dependent manner in the early mouse embryo, during
the formation of the first three germ layers. Although a defined
level of Oct4 has been demonstrated to be crucial to maintain
pluripotency in ICM and ES cells, the molecular read-out required
for this important function is still unknown.
Peri-implantation lethality of Oct4-deficient embryos precluded determination of its function in germ cells. In the present
study, we circumvented this limitation by germ-cell-specific
deletion of Oct4 and show that PGCs undergo apoptosis without
Oct4. Therefore, the loss of Oct4 function at different developmental stages and in different cell-type contexts (ICM/ES versus
PGC) exerts different physiological effects.
RESULTS AND DISCUSSION
Oct4 locus targeting
To circumvent the peri-implantation lethality of Oct4 null
homozygous embryos (Nichols et al, 1998) and to assess Oct4
function in developing germ cells, we applied the conditional
Cre/loxP gene targeting approach (supplementary Fig 1 online).
Conditionally targeted Oct4flox mice were eventually generated
and used for subsequent mating with TNAPCre mice. The latter
express Cre recombinase in PGCs due to an insertion of the Cre
coding sequence into the Tissue Non-specific Alkaline Phosphatase locus. Previously, we evaluated the specificity of this Cre
model and found that before 10.5 dpc Cre activity is detected
exclusively in PGCs (Lomeli et al, 2000). After 10.5 dpc, some
embryos showed Cre expression additionally in somatic tissue.
However, given that after 7.5 dpc the Oct4 gene is restricted to
germ cells, we presumed that deletion of Oct4 in somatic cells
would be irrelevant and that the phenotype anticipated in PGCs
should be cell-autonomous.
Oct4 phenotype in postnatal gonads
To ablate Oct4 function in PGCs through flox-D biallelic
conversion, we set up a multistep mating scheme, as outlined in
supplementary Fig 2 online. Postnatal D/D animals of both sexes
did not show any gross behavioural, anatomical or physiological
anomalies. However, they were partially or completely infertile,
correlating with various degrees of germ cell deficiency in their
gonads. In comparison to ovaries of pre-oestral (3-week old)
control D/ þ females, the ovaries of D/D females contained about
half the number of growing follicles. Even more striking was the
difference in the number of primordial follicles, with 25–100 times
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
scientific report
fewer primordial follicles in D/D ovaries (compare Fig 1A and B).
Our interpretation is that compensatory recruitment of primordial
follicles into the pool of maturing follicles soon depletes the
reservoir. Consequently, 6-week-old D/D ovaries contained hardly
any primordial follicles and very few growing follicles, whereas
the D/ þ females of the same age maintained a large number of
ovarian follicles (compare Fig 1E and F). Follicles were mostly
absent in D/D ovaries older than 6 weeks (compare Fig 1G and H).
Thus, by the time D/D females had reached breeding age
(B6 weeks), their gonads were essentially free of germ cells,
which explains their sterility.
In contrast to the full sterility of all matured D/D females, the
fertility of adult D/D males was always impaired but varied in
severity, even among littermates. Accordingly, histological analyses showed complete or partial depletion of spermatogenic cells
in D/D testes, manifested as germ-cell-free seminiferous tubules
with only somatic cells present, alongside unaffected tubules with
all stages of spermatogenesis present (Fig 1I,M). The percentage of
germ-cell-free tubules varied in different D/D males from 30% to
100% (n ¼ 7), correlating with partially or completely impaired
fertility of these animals. In contrast to the D/D females, the
phenotypic penetrance in D/D males did not correlate with age
(data not shown). Another visible feature of D/D testes was a
reduction in the diameter of seminiferous tubules lacking germ
cells and hyperplasia of the surrounding Leydig cells (Fig 1I,M),
similar to other germ-cell-depleted phenotypes. The presence of
some surviving spermatogonia in D/D testes throughout the
postnatal period and a few Oct4-positive oocytes in young D/D
ovaries (Fig 1C,K) is consistent with the idea that these cells are
descendants of PGCs that escaped Cre-mediated recombination.
Oct4 phenotype in PGCs
Cre recombinase expression under TNAP locus control and
consequently Oct4flox allelic excision can occur in germ cells
presumably between 7.25 and 15.5 dpc, as suggested by previous
studies (Ginsburg et al, 1990; Anderson et al, 2000; Lomeli et al,
2000). Therefore, gamete deficiency in postnatal gonads can be
attributed to loss of PGCs within this period of fetal development.
To examine this possibility, we stained PGCs for alkaline
phosphatase (AP; Ginsburg et al, 1990), at different stages of
embryonic development. Examination of D/D embryos showed
a PGC population comparable in size to that of D/ þ embryos at
the premigratory (8.5 dpc) and early migratory (9.5 dpc) stages
(Fig 2C–F). In contrast, the number of PGCs in D/D embryos was
markedly lower than in D/ þ littermates in the late migratory
(10.5 dpc) and postmigratory (12.5 dpc) phases of PGC development (Fig 2A,B,G–J). Thus our data support the notion that the
primary gamete deficiencies observed in adult mice were caused
by the loss of PGCs between 9.5 and 10.5 dpc of embryonic
development.
The presence of oocytes and sperm in the ovaries and testes
of some adult D/D animals is consistent with the persistence of
some postmigratory PGCs in the genital ridges of D/D embryos
(Fig 2G,I). These PGCs may result from either the dispensability
of Oct4 function or its function being undisturbed in these cells.
We consider the first possibility to be unlikely for two reasons.
First, we observed variability in the number of germ cells within
genital ridges of the same stage, which reflects variability in Oct4
dependence. Second, TNAPCre has limited efficiency like most
EMBO reports VOL 5 | NO 11 | 2004 1 0 7 9
scientific report
Oct4 and primordial germ cell survival
J. Kehler et al
Fig 1 | Oct4 phenotype in adult gonads. Immunostaining with Oct-4 antibody (C,D,K,L) and haematoxylin and eosin (A,B,E–J,M) of ovaries of indicated
ages (A–H) and 10-week-old testes (I–M), isolated from D/D (A,C,E,G,I,K,M) and the control group D/ þ (B,D,F,H,J,L) mice, is shown. The arrows and
arrowheads point to primordial (arrows) and growing (arrowheads) follicles in the ovarian sections. In the testicular sections, representative germ-celldeficient seminiferous tubules are labelled as D/D and those harbouring normal spermatogenic cells of all stages as D/ þ ; the arrowheads point to
Oct4-positive spermatogonia type B. Shown are mildly (70%; I,K) and severely (100% of germ-cell-free tubules; M) affected testes. Scale bars in (A) for
(A,B,E–M) and in (C) for (C,D) represent 100 mm.
Cre-expressing models. Previously, we found that on average 60%
of 13.5 dpc PGCs in TNAPCre Z/AP crosses underwent Cremediated recombination of the floxed reporter allele (Lomeli et al,
2000). In the Oct4D/ þ ;TNAPCre/ þ Oct4flox/flox crosses, we find
a similar average rate of recombination, inferred from the marked
increase in apoptosis seen in 10.5 dpc D/D embryos that never
reached 100% efficiency (see quantitative data in supplementary
Figs 3 and 4 online). This latter possibility is further supported by
two observations: (1) the presence of Oct4-positive germ cells
in young D/D ovaries (Fig 1C) and in mildly affected D/D testes
(Fig 1K) and (2) the fact that these D/D males transmit the intact
flox allele in their sperm. We found this allele at an expected
frequency in the progeny by natural backcrossing of mildly
1 0 8 0 EMBO reports
VOL 5 | NO 11 | 2004
affected adult D/D males and by intracytoplasmic sperm injection
(ICSI) of the few spermatozoa recovered from the epididymides of
severely affected infertile males into oocytes (data not shown).
Therefore, in a subpopulation of PGCs, probably those that
survived through embryonic stages (Fig 2G,I) and subsequently
gave rise to gametes in adult D/D animals (Fig 1C,K), Cre
recombinase failed to catalyse the excision of the flox allele.
Taken together, the above data suggest that development of PGCs
does not proceed without Oct4.
Fate of Oct4-deficient PGCs
We considered at least four explanations that might account for
the loss of PGCs following disruption of Oct4 function. In the first,
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
Oct4 and primordial germ cell survival
J. Kehler et al
scientific report
Fig 2 | Oct4 phenotype in PGCs. Alkaline phosphatase (AP) staining of D/D (A,C,E,G,I) and D/ þ embryos (B,D,F,H,J) is shown. The 8.5 dpc (C,D) and
9.5 dpc (E,F) embryos in both groups show no difference in the number of PGCs (arrows) that are located at the base of allantois within the hindgut
pocket (8.5 dpc) and, subsequently, in the hindgut endoderm (9.5 dpc). As opposed to the further increase in the number of PGCs in 10.5 dpc D/ þ
embryos (B,H), the D/D littermates show a marked decrease in the number of PGCs (A,G), which is further apparent in 12.5 dpc genital ridges
(I versus J). Photographs were taken from the ventral (C,D,G,H) or lateral (A,B,E,F) sides of embryos. Scale bars in (C) for (C,D) and in (E) for (E,F)
represent 100 mm, and scale bar in (I) for (I,J) represents 200 mm.
somatic differentiation would be analogous to the trophoblast
differentiation seen in Oct4-deficient ICM and ES cells (Nichols
et al, 1998; Niwa et al, 2000). However, other than into prooogonia or gonocytes, differentiation fates have not been reported
for mammalian PGCs. Accordingly, we were not able to detect the
expression of early (Cdx-2) or mature (TROMA-1) trophoblast
markers in D/D embryos between 9.5 and 10.5 dpc within the
dorsal mesentery, which is the site of PGC migration (data not
shown). Our results further argue against a second scenario, where
Oct4 loss of function could interfere with PGC migration by
disrupting their ability to receive a chemotactic signal and/or
migrate towards the developing genital ridges. Whereas ectopic
PGCs are known to undergo apoptosis in wild-type and mutant
mouse models (Stallock et al, 2003), we did not observe any
difference in the distribution of PGCs between genotypes, and all
of the apoptotic PGCs that we analysed were isolated from the
dorsal mesentery. Third, we hypothesized that Oct4 loss in PGCs
might trigger premature meiosis, due to the observation that Oct4
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
is downregulated in female PGCs on their entry into meiosis and
coincides with a massive wave of cell death in the fetal ovary at
13.5 dpc (Pesce et al, 1998a). However, as we did not observe
clear nuclear signs of meiosis in D/D PGCs before 13.5 dpc (data
not shown), we excluded the third possibility as well.
Because programmed cell death, apoptosis, is the only fate
ascribed to PGCs that leave the germ lineage in response to
genetic mutations or missing cues from the somatic environment
(for reviews, see McLaren, 2000; Zhao & Garbers, 2002; Tres et al,
2004), we assessed whether it may also account for PGC loss
following Oct4 withdrawal. To this end, histological analyses of
sections of the aorta–gonad–mesonephros (AGM) region of
10.5 dpc D/D embryos were performed. Immunostaining for the
Oct4 protein again confirmed its loss by Cre-mediated recombination of the flox locus (compare Fig 3A and B). Importantly, near
the genital ridge, we observed compact clusters of cells with dark,
fragmented nuclei, which is a hallmark of apoptosis (data not
shown). These clusters showed co-staining with the stage-specific
EMBO reports VOL 5 | NO 11 | 2004 1 0 8 1
scientific report
Oct4 and primordial germ cell survival
J. Kehler et al
and early migratory PGCs but becomes essential by 10 dpc, when
PGC proliferation is highest (Tam & Snow, 1981; Gomperts et al,
1994). An inducible Cre model might help to discriminate
between the two possibilities by precise control of the timing of
recombination in the Oct4flox locus.
The germ cell phenotype in D/D males did not seem to progress
with age. On the contrary, the germ cell deficiency in D/D females
was exacerbated by age. The population of resting oocytes,
already impaired at the prepubertal age, was further depleted
during the first ovulatory cycles and/or by subsequent ovarian
atresia. This difference might be due to the fact that spermatogenesis relies on self-renewing stem cells, whereas the oocyte pool is
mainly established during fetal development.
Speculations
Fig 3 | Apoptosis of Oct4-deficient PGCs. Labelling with Oct-4 antibody
(A,B), and double TUNEL (green)/SSEA-1 antibody (red) staining (C,D)
of the D/D (A,C) and control D/ þ (B,D) 10.5 dpc embryo sections are
shown. Significant reduction in Oct4 protein staining can be seen in the
D/D sections. Shown in (C,D) is the AGM region of the same embryos as
in Fig 2G,H. Note a strong TUNEL signal in the nuclei of the same
clustered cells showing residual SSEA-1 staining on the cell membrane
(C and inset). The SSEA-1 signal was notably weaker than that from
viable PGCs of the þ /D embryos (D) yet sufficiently strong to be
distinguished from the background. Scale bars in (A) for (A,B) and in
(C) for (C,D) represent 50 mm. The inset is a twofold magnification of the
PGC/apoptotic area in (C). Arrowhead, PGC; da, dorsal aorta; dm, dorsal
mesentery; gr, genital ridge.
embryonic antigen 1 (SSEA-1), a marker of migratory PGCs, and
with TUNEL staining (Fig 3C and inset), implying that they
represent bona fide PGCs undergoing apoptosis. We additionally
confirmed the result as well as quantified PGC loss in D/D
embryos, using fluorescence-activated cell sorting analyses
(supplementary Figs 3 and 4 online).
In sum, our results show that the ablation of Oct4 expression in
PGCs impairs their maintenance starting by 10 dpc. In fact, lack of
Oct4 resulted in a massive (as high as 70%; supplementary Figs 3
and 4 online) wave of premature apoptosis in PGC before their
colonization of the developing gonadal ridges, contributing to
postnatal gonads depleted of pro-spermatogonia and oocytes.
Although Oct4 and TNAP are coexpressed in PGCs during their
initial formation in the proximal epiblast at 7.25 dpc, the
phenotype is not evident until 10 dpc. A possible explanation for
this finding is that Cre recombinase requires 1 or 2 days after the
onset of its expression to reach a critical threshold level to catalyse
recombination in vivo. In agreement with this, Cre excision
activity in crosses between the TNAPCre and the Cre reporter Z/AP
mice was first detected at 9 dpc (Lomeli et al, 2000). Therefore, the
Oct4flox allele may remain intact until 9 dpc. The subsequent
degradation of Oct4 mRNA and protein may further delay the
manifestation of the Oct4 phenotype until about 10 dpc, when the
majority of D/D PGCs showed features of apoptosis. An alternative
explanation is that Oct4 function is dispensable in premigratory
1 0 8 2 EMBO reports
VOL 5 | NO 11 | 2004
Apoptosis is the only fate ascribed to PGCs that leave the germ
lineage in response to genetic mutations or missing cues from the
somatic environment (for reviews, see McLaren, 2000; Zhao &
Garbers, 2002; Tres et al, 2004). In line with these observations,
ablation of Oct4 in migratory PGCs promotes their apoptosis. This
is a remarkably distinct function in germ cells from its previously
demonstrated role as a gatekeeper of pluripotency in ICM cells
and their cultured counterparts, ES cells (Nichols et al, 1998;
Niwa et al, 2000). However, our data do not preclude the
potential superimposition of several pathways. For example, loss
of Oct4 might initially trigger PGCs to differentiate into somatic
cell types other than trophoblast (see above), and then undergo
apoptosis due to a lack of appropriate survival signals. These two
phenomena may also serve as different solutions to the same
problem. The Oct4 gene is downregulated whenever lineages split
off from the germ line, namely into the trophoblast and hypoblast,
and in each of the three somatic lineages (Pesce et al, 1998b).
As PGCs are not known to exit the germ line spontaneously by
differentiation, the forced loss of Oct4 function in our experimental model may establish a conflict that can be only resolved
by apoptosis. In more specific terms, however, the different
physiological read-outs of Oct4 deficiency in ICM cells and PGCs
should reflect distinct sets of target genes and/or partner proteins
recruited by Oct4 at the two different stages of germline
development.
The Fgf/Frfr, LIF/gp130/Jak/Stat3 and Steel/c-kit/AKT/mTOR/
Bax transduction pathways have been shown to operate in
migratory PGCs to promote their growth and survival (for reviews,
see McLaren, 2000; De Miguel et al, 2002; Zhao & Garbers, 2002;
Stallock et al, 2003). Whether and how Oct4 interacts with the
elements of these pathways in PGCs remains to be determined.
This is not a trivial task, however, considering scant PGC number,
their short lifespan in vitro and the lack of a suitable long-term
culture model. The EG cells, despite their PGC origin, are more
closely related to the ES cells in both growth factor dependence
and developmental capacity (for reviews, see Surani, 2001;
Donovan & De Miguel, 2003). Accordingly, the EG cells we
established from migratory 8.5 dpc Oct-4flox/flox PGCs differentiated into trophoblast rather than having underwent apoptosis,
after Oct4 function had been abolished by an inducible Cre (data
not shown). The recent demonstration of retroviral transduction of
PGCs might provide a feasible approach for designing future
experiments to investigate potential pathways downstream of
Oct4 (De Miguel et al, 2002).
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
Oct4 and primordial germ cell survival
J. Kehler et al
Conclusion
Given these two essential roles of Oct4 in maintaining pluripotency in early embryonic cells and viability in PGCs, unravelling
other possible functions of Oct4 has become a highly relevant
pursuit. Specifically, comparing differential gene expression in
Oct4 null and wild-type PGCs, as well as with ES cells, could
identify potential Oct4 target genes involved in PGC survival. This
knowledge should help us to understand how Oct4 exerts its
pleiotropic effects during mouse development, as well as to draw
general principles that define the commonalities and differences
between ES cells and germ cells.
METHODS
Targeting vector design, ES cell electroporation, genotyping
procedures and mouse experimentation are described in the
legend to supplementary Fig 1 online. Histological procedures are
described in supplementary information online.
Supplementary information is available at EMBO reports online
(http://www.emboreports.org).
ACKNOWLEDGEMENTS
We thank P.-O. Angrand and F. Stewart for the pHC-Cre and pNPKCreAR3 plasmids, S. Schlatt for advice on adult germ cells, D. Groff,
A. Leu, V. Koehlin, M. Volkov and R. Turek for excellent technical
assistance, A. Malapetsas for editing the manuscript and lab members for
useful discussions. This work was supported by National Institutes of
Health (NIH) grants RO1HD42011-01 to H.R.S. and RO1-HD-4406601A1 to K.J.M., the Marion Dilley and David George Jones Funds and the
Commonwealth and General Assembly of Pennsylvania to H.R.S. and
K.J.M., and by Max-Planck Gesellschaft to A.T. J.K. was supported in part
by the NIH Medical Scientist Training Program (T32-GM07170).
REFERENCES
Anderson R, Copeland TK, Schöler H, Heasman J, Wylie C (2000) The onset
of germ cell migration in the mouse embryo. Mech Dev 91: 61–68
De Miguel MP, Cheng L, Holland EC, Federspiel MJ, Donovan PJ (2002)
Dissection of the c-Kit signaling pathway in mouse primordial germ
cells by retroviral mediated gene transfer. Proc Natl Acad Sci USA 99:
10458–10463
Donovan PJ, de Miguel MP (2003) Turning germ cells into stem cells. Curr
Opin Genet Dev 13: 463–471
Ginsburg M, Snow MH, McLaren A (1990) Primordial germ cells in the mouse
embryo during gastrulation. Development 110: 521–528
&2004 EUROPEAN MOLECULAR BIOLOGY ORGANIZATION
scientific report
Gomperts M, Garcia-Castro M, Wylie C, Heasman J (1994) Interactions
between primordial germ cells play a role in their migration in mouse
embryos. Development 120: 135–141
Lomeli H, Ramos-Mejia V, Gertsenstein M, Lobe CG, Nagy A (2000) Targeted
insertion of Cre recombinase into the TNAP gene: excision in primordial
germ cells. Genesis 26: 116–117
McLaren A (2000) Germ and somatic cell lineages in the developing gonad.
Mol Cell Endocrinol 163: 3–9
Nichols J, Zevnik B, Anastassiadis K, Niwa H, Klewe-Nebenius D, Chambers I,
Schöler H, Smith A (1998) Formation of pluripotent stem cells in the
mammalian embryo depends on the POU transcription factor Oct4. Cell
95: 379–391
Niwa H, Miyazaki J, Smith AG (2000) Quantitative expression of Oct-3/4
defines differentiation, dedifferentiation or self-renewal of ES cells. Nat
Genet 24: 372–376
Palmieri SL, Peter W, Hess H, Schöler HR (1994) Oct-4 transcription factor is
differentially expressed in the mouse embryo during establishment of the
first two extraembryonic cell lineages involved in implantation. Dev Biol
166: 259–267
Pesce M, Wang X, Wolgemuth DJ, Schöler HR (1998a) Differential expression
of the Oct-4 transcription factor during mouse germ cell differentiation.
Mech Dev 71: 89–98
Pesce M, Gross MK, Schöler HR (1998b) In line with our ancestors: Oct-4 and
the mammalian germ. BioEssays 20: 722–732
Rosner MH, Vigano MA, Ozato K, Timmons PM, Poirier F, Rigby PW,
Staudt LM (1990) A POU-domain transcription factor in early stem cells
and germ cells of the mammalian embryo. Nature 345: 686–692
Schöler HR, Dressler GR, Balling R, Rohdewohld H, Gruss P (1990) Oct-4: a
germline-specific transcription factor mapping to the mouse t-complex.
EMBO J 9: 2185
Stallock J, Molyneaux K, Schaible K, Knudson CM, Wylie C (2003) The proapoptotic gene Bax is required for the death of ectopic primordial germ
cells during their migration in the mouse embryo. Development 130:
6589–6597
Surani MA (2001) Reprogramming of genome function through epigenetic
inheritance. Nature 414: 122–128
Tadokoro Y, Yomogida K, Ohta H, Tohda A, Nishimune Y (2002)
Homeostatic regulation of germinal stem cell proliferation by the
GDNF/FSH pathway. Mech Dev 113: 29–39
Tam PP, Snow MH (1981) Proliferation and migration of primordial germ cells
during compensatory growth in mouse embryos. J Embryol Exp Morphol
64: 133–147
Tres LL, Rosselot C, Kierszenbaum AL (2004) Primordial germ cells: what does
it take to be alive? Mol Reprod Dev 68: 1–4
Yeom YI, Fuhrmann G, Ovitt CE, Brehm A, Ohbo K, Gross M, Hübner K,
Schöler HR (1996) Germline regulatory element of Oct-4 specific for the
totipotent cycle of embryonal cells. Development 122: 881–894
Zhao GQ, Garbers DL (2002) Male germ cell specification and differentiation.
Dev Cell 2: 537–547
EMBO reports VOL 5 | NO 11 | 2004 1 0 8 3