Human Reproduction Vol.17, No.8 pp. 2160–2164, 2002
Somatic and embryonic cell nucleus transfer into intact
and enucleated immature mouse oocytes
J.Fulka Jr1,2,6, F.Martinez3, O.Tepla4, M.Mrazek4 and J.Tesarik5
1Institute
of Animal Production, 2Center for Cell Therapy and Tissue Repair, Prague, Czech Republic, 3University of Granada,
Department of Biochemistry and Molecular Biology, Campus Fuente Nueva, Granada, Spain, 4ISCARE IVF, Prague, Czech Republic
and 5MAR & Gen, Molecular Assisted Reproduction and Genetics, Granada, Spain
6To
whom correspondence should be addressed at: Institute of Animal Production, POB 1, 104 01 Prague 10, Czech Republic.
E-mail: fulka@vuzv.cz
BACKGROUND: The aim of our study was to evaluate the possibility of embryonic or somatic cell haploidization
after fusion with intact or enucleated immature oocytes which were subsequently cultured in vitro. Embryonic or
somatic cell nuclei do not undergo premature chromosome condensation when fused to intact or enucleated
immature oocytes whose maturation is prevented by dibutyryl cyclic AMP (dbcAMP). The presence of dbcAMP
permits, however, the completion of DNA replication in somatic cell nuclei. METHODS AND RESULTS: The
chromosomes condensed when the reconstructed cells were released from the dbcAMP block. When somatic or
embryonic nuclei were introduced into intact immature meiotically competent oocytes and subsequently cultured
their chromosomes assembled on a common spindle with meiotic chromosomes and proceeded through the meioticlike division, judged according to the presence of the first polar body extruded. When embryonic cell nuclei were
introduced into cytoplasts obtained from immature meiotically competent oocytes, polar bodies were extruded in
about 75% of reconstructed cells but the metaphase plates were abnormal in almost all cases. When somatic cell
nuclei were inserted into the above cytoplasts, polar bodies were extruded only very exceptionally and in these cells
chromosomes were arranged in abortive metaphase plates. CONCLUSIONS: Our results suggest that somatic cell
nuclei are unable to proceed through the reduction division (haploidization) when introduced into an immature
oocyte meiotic cytoplasm.
Key words: haploidization/mouse/nuclear transfer/nucleus/oocyte
Introduction
It is proposed that the haploidization of patient somatic cell
diploid chromosome complement, within enucleated donor
oocytes, may result in the production of cells with half the
number of chromosomes which could then be used as gametes
with their own genetic identity (male, female) for the treatment
of certain forms of infertility (Tsai et al., 2000; Tesarik et al.,
2001; Trounson, 2001). This assumption is based on previous
observations studying the nucleocytoplasmic interactions in
fused oocytes and also the behaviour of embryonic or somatic
cell nuclei introduced into a meiotic cytoplasm. It was shown
that nuclei always respond to the given cytoplasmic chromosome condensation signal; however, for the correct cell cycle
transition the corresponding nucleus cell cycle stage is very
important (Fulka et al., 1993, 1995a). When meiotic cell
nuclei, which are still diploid, are introduced into immature
or maturing oocytes, the cytoplasm induces the reduction
division of their chromosome complement. The cells with a
haploid chromosome number are able to support the complete
embryonic development. It was demonstrated that somatic cell
nuclei introduced into immature mouse oocytes underwent the
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meiotic-like division, albeit without the expulsion of the first
polar body (Kubelka and Moor, 1997). Thus in some cells two
groups with a half number of chromosomes were observed.
Basically two experimental schemes may result in the production of haploidized somatic cells. First, the somatic cell G2
stage nuclei will be introduced into an immature oocyte
cytoplasm and subsequently matured in vitro up to the second
metaphase. Following the oocyte activation a haploid pronucleus could theoretically only be formed when the fully replicated originally paternally and maternally derived homologous
chromosomes from the diploid nucleus separated faithfully
with unseparated sister chromatids to opposite spindle poles
in the absence of a physical attachment by chiasmata. The
second scheme proposed the introduction of G1 stage nuclei
into a mature oocyte (MII) cytoplasm. After oocyte activation
the second polar body will be extruded and the oocyte
cytoplasm may contain a haploid chromosome set (Kubiak
and Johnson, 2001), provided the unreplicated and unattached
two homologous chromatids that were originally derived from
the parents segregate from each other. It must be assumed,
however, that meiotic and mitotic divisions are completely
© European Society of Human Reproduction and Embryology
Nucleus transfer
different. During the early meiotic stage (zygotene), homologous chromosomes pair through the synaptonemal complex and
undergo recombination (pachytene). The crossing-over, which
can be seen as chiasmata, then holds the paired homologues
together (Simchen and Hugerat, 1993). In meiosis, during the
first division, homologues move to opposite spindle poles
(reductional segregation) and, at the second division, sister
chromatids move to opposite poles as during mitosis (Kleckner,
1996). Also the attachment of meiotic or mitotic chromosomes
to the spindle differs. In the first meiotic division sister
chromatid kinetochores lie side by side and thus they attach
to microtubules from the same pole. In the second meiotic
division kinetochores lie back to back and they attach to
microtubules from opposite poles, similarly to mitotic chromosomes (Paliulis and Nicklas, 2000). Thus the physical connection and the chromosome position on the spindle seem to be
crucial for the successful progress through the final stages of
oocyte meiotic maturation. In our experiments, we have studied
the first suggested scheme in the mouse model. Our results show
that the haploidization through this approach was unsuccessful.
Materials and methods
Mouse oocytes were released into M2 medium containing dibutyryl
cyclic AMP (dbcAMP) (150 µg/ml) from large antral follicles of ICR
females stimulated previously with 5 IU of pregnant mare serum
gonadotrophin (PMSG). Their cumulus cells were removed by pipetting and only those oocytes containing distinct germinal vesicles
(GV) were used. Zonae pellucidae were removed by pronase treatment
(0.5%). For enucleation, oocytes were incubated in M2 supplemented
with dbcAMP, cytochalasin D (5 µg/ml) and nocodazole (3 µg/ml)
for 30 min. Thereafter they were elongated in a very narrow pipette
and the oocyte part containing GV was cut off with a glass needle
(Karnikova et al., 1998). Nuclei (karyoplasts) for transfer were
obtained by microsurgical removal from two-cell stage embryos
collected from oviduct on the next day after the vaginal plug was
detected (embryonic nuclei). Embryonic nuclei (karyoplasts) or whole
somatic cells [dissociated cells (trypsin-EDTA) from cumulus cell
cultures] were agglutinated with either intact or enucleated oocytes
in phosphate buffered saline (PBS) supplemented with phytohaemmaglutinin (200 µg/ml), washed in M2 and transferred into polyethyleneglycol solution (PEG, relative molecular weight 1000) for 50 s
(Fulka et al., 1995b). Then they were washed several times in M2 and
cultured in medium M199 containing bovine serum albumin (BSA)
(4 mg/ml), Na-pyruvate (0.2 mmol/l) and gentamicin (50 µg/ml) in
an atmosphere 5% CO2 in air and 37°C for up to 14–16 h. The
efficiency of fusion was evaluated 30 min post-induction and those
cells which did not fuse were discarded. At the end of culture the
fusion products were evaluated under the inverted microscope, then
fixed in acetalcohol and stained with aceto-orcein. In order to assess
if somatic cell replicate DNA when fused to immature oocytes
bromodeoxyuridine was added to the culture medium (Ouhibi et al.,
1994). Oocytes were then fixed in methanol, labelled with anti
BrDU antibody (Dako, Glostrup, Denmark) and examined under the
fluorescence microscope. Unless otherwise stated all chemicals were
purchased from Sigma.
Results
In total 127 (efficiency 127/165; 77%) somatic cells were
fused to intact GV staged oocytes, 132 somatic cells to 195
Table I. Maturation of intact and enucleated mouse oocytes fused to
somatic or embryonic cells
Type of fusion
Total no.
cells fused
No. of oocytes
with polar bodies
Sc⫻intact GV oocyte
Sc⫻enucleated GV oocyte
Bn⫻intact GV oocyte
Bn⫻enucleated GV oocyte
127/165 (77%)
132/195 (68%)
27/37 (72%)
30/42 (71%)
40/127 (31%)
2/132 (1%)
20/27 (74%)
23/30 (77%)
GV ⫽ germinal vesicle; Sc ⫽ somatic cell; Bn ⫽ blastomere nucleus.
enucleated oocytes (68%), 27 embryonic karyoplasts to 37
intact GV staged oocytes (72%) and 30 embryonic karyoplasts
to 42 enucleated oocytes (71%; Table I). When somatic cells
are fused to intact or enucleated oocytes and the fusion products
are kept in medium with dbcAMP the introduced nuclei
remained intact (Figure 1). This situation persists as long as
dbcAMP is present in the medium but nuclei slightly increase
their diameter (Figure 2).
We have assumed that for successful haploidization the
nuclei introduced into oocytes must be at G2 phase. First we
labelled somatic cell cultures with BrDU to assess the percentage of cells in S-phase. After evaluation we found 51% of
cells with positive labelling (51/100). Thus we assumed that
only a minimum of cell nuclei will be in G2 phase when
randomly selected. For this reason another strategy was chosen.
The randomly selected somatic cells were fused to intact or
enucleated GV staged oocytes and cultured for 24 h in the
medium with dbcAMP and BrDU and thereafter processed for
fluorescence microscopy. When evaluated, 48% of fused cells
showed positive labelling (27/56). However, when BrDU was
added after 24 h of culture only two cells from 52 showed
positive signal. These results indicate that the immature oocyte
cytoplasm does not prevent the replication of DNA and within
a 24 h lasting culture somatic cell nuclei can be synchronized
in G2 phase. This has been further confirmed according to the
metaphase chromosome morphology as already described (Rao
et al., 1977).
When released from dbcAMP block the meiotic cell cycle
progression is under the control of the oocyte cytoplasm. This
means that the nuclear envelope breakdown and chromosome
condensation is typically detected within 1 h of culture in
dbcAMP free medium in both intact and enucleated fused
cells. The first meiotic spindle could be clearly detected after
6 h of culture. Typically, only one spindle was detected in
cells produced by fusion of a somatic cell to an intact
oocyte. This spindle contained both the meiotic and mitotic
chromosomes resulting from the unification of both groups.
When evaluated after 14–16 h of culture in dbcAMP free
medium 31% of reconstructed oocytes (somatic cell⫻intact
oocyte) extruded the first polar body (40/127; Table I); however,
the evaluation of the second metaphase plates was extremely
difficult as they contained both the meiotic and mitotic chromosomes. We may however assume that they were mostly
abnormal. This assumption came from the evaluation of those
oocytes which did not extrude the first polar bodies. While the
organization of meiotic chromosomes was almost exclusively
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J.Fulka et al.
Figure 1. Somatic cell nucleus introduced into an enucleated germinal vesicle (GV) stage mouse oocyte. Fixed 1 h post-induction of fusion.
Phase contrast, ⫻600.
Figure 2. Somatic cell nucleus introduced into an enucleated GV stage mouse oocyte. Fixed 20 h post-induction of fusion. Note the nucleus
enlargement and a well visible nucleolus. Phase contrast, ⫻600.
Figure 3. Mouse oocyte metaphase I spindle with an equatorial arrangement of meiotic chromosomes while the mitotic chromosomes are
located on spindle poles. Phase contrast, ⫻600.
Figure 4. Mouse anaphase–telophase I oocyte with somatic cell chromosomes located outside the spindle (arrow). Phase contrast, ⫻600.
Figure 5. Enucleated mouse oocyte fused to somatic cell and matured for 16 h. Note that in this case the first polar body was extruded but
the metaphase II plate is evidently abnormal (arrow). Phase contrast, ⫻600.
Figure 6. Enucleated mouse oocytes fused to somatic cells and thereafter matured in vitro did not extrude the first polar body and are
arrested in metaphase I-like stage with chromosomes dispersed chaotically on the spindle. Phase contrast, ⫻600.
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Nucleus transfer
normal, the somatic chromosomes were allocated, in most
cases, outside the spindle (Figures 3, 4). The incompatibility
of a meiotic spindle and mitotic chromosomes was even more
evident when somatic cells were fused to enucleated oocytes.
Here only two oocytes (2/132) extruded the first polar bodies
(Table I). When examined after staining, the chromosomes
remaining in the cytoplasm were rather abnormal and formed
a cluster of chromatin (Figure 5). The oocytes without polar
bodies were also stained and evaluated. Figure 6 shows the
most typical configuration of chromatin where chromosomes
are randomly allocated on the spindle.
Next the behaviour of early embryo chromosomes in a
meiotic cytoplasm were evaluated. Nuclei were isolated from
two-cell stage embryos on the next day after the detection of
a vaginal plug because it is known that they are G2-phase
staged. This experiment was designed to exclude the possibility
that our somatic cell cultures have an adverse effect on cultured
cells. Fused cells were cultured with dbcAMP for 1 h and
thereafter in an inhibitor free medium for 14–16 h. In general,
the frequency of oocytes with polar bodies was evidently
higher compared with somatic cell fusion—in both groups
⬎70% of reconstructed cells exhibited the polar body—intact
oocyte⫻embryonic karyoplast (20/27); enucleated oocyte⫻
embryonic karyoplast (23/30; Table I). These polar bodies
were only slightly smaller than the oocyte cytoplasm. When
these cells were evaluated after staining, again the configuration
of mitotic chromosomes showed gross abnormalities which
were typically seen as a cluster or patches of chromatin. In
conclusion these results show rather the inability of the mitotic
cell nucleus (chromosomes) to undergo haploidization in a
meiotic cytoplasm. This resulted typically in an abnormal
allocation of the chromatin on the meiotic spindle and thus
the inability to secure the proper separation of mitotic chromosomes.
Discussion
The possibility of somatic cell nucleus haploidization in the
meiotic cytoplasm has been suggested in several articles. Our
results, however, showed that under the experimental scheme
and conditions used, the expected haploidization was not
possible. The abnormal organization of chromosomes on the
first meiotic spindle was the main problem when somatic cells
were transferred into immature cytoplasts. The chromosomes
only exceptionally formed the regular metaphase plate, instead
they were typically arranged along the whole spindle. This
abnormal organization prevented the anaphase to telophase I
transition. Only in two cases were the second metaphases
detected. The reason for the improper chromosome organization is not known. It is, however, interesting that the same
phenomenon was already observed when early metaphase I
oocytes were fused to anaphase-telophase I oocytes. Here, too,
the anaphase–telophase chromosomes were dispersed on their
original spindle. Similar abortive organization or dispersion
was documented in some cases (Tarkowski and Balakier, 1980;
Grabarek and Zernicka-Goetz, 2000) when maturing mouse
oocytes were fused to G2 phase blastomeres or to follicular
cells, and some pictures from the article published by Kubelka
and Moor also resemble the situation commonly observed in
our fused cells (Kubelka and Moor, 1997).
The abnormal chromosome organization on the first meiotic
spindle is not easy to explain; however, some recent results
indicate that the absence of chromosome synapsis plays a
crucial role. In mouse oocytes from animals homozygous for
a targeted disruption of the DNA mismatch repair gene Mlh1,
the absence of MLH1 protein dramatically reduces the meiotic
recombination. The chromosomes in maturing oocytes are
present as univalents and are unable to establish the correct
spindle attachment (Woods et al., 1999). Also the absence of
Spo 11p results in the defects of chromosome synapsis and a
random segregation at meiosis I (Lichten, 2001). The mouse
meiotic mutation mei1 disrupts chromosome synapsis but some
oocytes progress to metaphase I; their chromosomes are,
however, unpaired and not properly organized on the spindle
(Libby et al., 2002). These results indicate that the meiotic
recombination ensures the correct attachment and segregation
of chromosomes during meiosis and is essential for its progression, but certainly some other factors may play an important
role in the chromosome spindle arrangement and subsequent
segregation (Bernard et al., 2001; Kaplan et al., 2001). On the
other hand, when grasshopper spermatocytes in metaphase I
were fused to spermatocytes in metaphase II and a single
chromosome was moved from one spindle to the other,
chromosomes placed on the spindle of a different meiotic
division behaved as they do on their native spindle. Thus
metaphase II chromosomes attached to the metaphase I spindle
and in anaphase I individual chromatids were separated
(Paliulis and Nicklas, 2000). This phenomenon has been
observed also in fused metaphase I to metaphase II mouse
oocytes (Fulka et al., 1995a). However, in both these cases
the chromosomes in fused cells still belong to a category of
‘meiotic chromosomes’. It may be possible that chromosomes
in mitotic cells are further modified and thus incompetent to
undergo the proper congression and attachment to the spindle.
The frequency of polar bodies extruded was higher when
G2 blastomere nuclei were introduced into immature cytoplasts,
but the resulting metaphase plates were again abnormal. This
higher frequency may be influenced by the absence of cell
cycle checkpoint controls (Fulka et al., 2000).
It is not surprising that, after fusion of either somatic or
embryonic cells to intact oocytes, polar bodies were frequently
observed. It has been shown recently (Fulka et al., 1997;
Rieder et al., 1997) that the cell cycle progression in oocytes
with two chromosome groups (spindles) is under the control
of the more advanced (or normal) spindle. In the mouse both
groups of chromosomes form a single common spindle, on
the other hand both spindles are separated in fused pig oocytes
(Fulka, 1983) and also in bovine oocytes. Thus, Salamone
et al. postulated the successful haploidization of somatic
cells fused to GV stage bovine oocytes (Salamone et al.,
2001, 2002).
It is interesting to note that the behaviour of meiotic cells
injected or fused to immature or maturing oocytes is completely
different. Normal metaphase plates are frequently formed and
the number of chromosomes is reduced (Ogura et al., 1998;
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J.Fulka et al.
Sasagawa et al., 1998). When somatic cells were fused to
post-metaphase I oocytes, it was shown that the compatibility
between this type of cytoplasm and a somatic cell is much
better and newly formed metaphase plates seem to be normally
organized (unpublished results). This is supported by earlier
studies when G2-phase blastomere nuclei were introduced into
chemically enucleated oocytes (post-telophase I). Here the
metaphase plates were normal and chromosomes segregated
properly into their sister chromatids (equatorial division). Other
studies claimed the successful haploidization of somatic cells
after the injection of their nuclei into mature oocytes which
were subsequently activated (Lacham-Kaplan and Daniels,
2001; Tesarik et al., 2001). It must be stressed that a meiotic
division is not simply a condensation or movement of chromosomes. The first meiotic division requires the pairing and
separation of homologous chromosomes; during the second
meiotic division the equal distribution of corresponding
chromatids must be secured. Our observations suggest that the
haploidization of somatic cell by their transition through the
‘whole’ meiotic cell cycle was unsuccessful due to intrinsic
characteristics of somatic chromosomes.
Acknowledgements
J.F. Jr thanks John C.Schimenti from The Jackson Laboratory for his
generous help. J.F. Jr’s lab is supported by GACR 524/02/0032, MzeMO2-99–01 and MSMT LN 00A 065.
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Submitted on November 23, 2001; resubmitted on March 15, 2002; accepted
on April 10, 2002