1 Monash IVF, Clayton, Victoria, Australia, 3168 and 2 Monash Institute of Reproduction and Development, Monash University, Clayton, Victoria, Australia, 3168
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Abstract |
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Key words: blastocyst transfer/fertilization/IVF/parthenogenetic activation/single pronucleus
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Introduction |
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The experimentally induced parthenogenetic nucleus is incapable of supporting normal early development. Likewise, embryonic development of human parthenogenetic oocytes would be expected to become impaired after activation of the embryonic genome, at ~2 days post-insemination, thus leading to early embryo arrest. Standard IVF culture to day 2 or 3 does not provide an opportunity to observe embryo development once the maternal message has ceased. Oocytes that appeared initially as 1PN but are actually fertilized, cannot be distinguished from parthenogenetic embryos at this stage. By extending in-vitro culture to day 6, further embryonic development and formation of a morphologically normal blastocyst may possibly be used as an indicator of fertilization and normal developmental capacity.
This case report involves a full-term pregnancy and birth resulting from transfer of a blastocyst observed to have only one PN at 18 h post-insemination, but which developed to form a morphologically normal blastocyst after 6 days of in-vitro culture.
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Case report |
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A total of 19 oocytes was retrieved, of which seven (37%) were fertilized as shown by the presence of two PN. One oocyte was observed to have only one PN with fragmented polar bodies. Culture of the seven fertilized oocytes produced embryos ranging from two to eight cells on day 3; however, all these embryos were either arrested or totally fragmented by day 6 post-insemination and therefore not suitable for transfer. Culture of the single 1PN oocyte produced an eight-cell embryo on day 3 and a morphologically normal early nascent blastocyst on day 6. Since no other embryos were suitable for transfer to the patient and this embryo exhibited apparently normal developmental capacity in vitro, this embryo was transferred to the patient. On day 16 after oocyte collection, pregnancy was confirmed with a positive blood pregnancy test and 6 weeks later ultrasound showed a viable singleton intrauterine gestational sac and embryo dimensions consistent with transfer dates. The pregnancy proceeded uneventfully and an ultrasound scan at 23 weeks revealed a normal baby with no congenital abnormalities. An uneventful delivery at 38 weeks resulted in birth of a 3860 g normal healthy boy.
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Discussion |
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Pronuclear fusion is exhibited in species where fertilization occurs after completion of meiosis. In humans, however, this event has never been observed and PN have only been identified as separate bodies prior to nuclear membrane breakdown. It has been suggested (Levron et al., 1995) that a normal variant of fertilization may involve formation of a common pronuclear membrane during syngamy, leading to a diploid zygote appearing with only one PN. The single 1PN oocyte that resulted in the livebirth in this case report was assessed only once for fertilization 18 h after insemination and no second PN was observed. Therefore no conclusion can be drawn as to either pronuclear asynchrony or fusion being the mechanism leading to normal development in this case.
Analysis of the chromosome constitution of 1PN oocytes after standard IVF and intracytoplasmic sperm injection (ICSI) further supports the presence of fertilization in some 1PN oocytes. Y-specific hybridization signals have been revealed in 35.6% of embryos (Staessen et al, 1997), therefore indicating that in 7075% of such cases a spermatozoon had penetrated the oocyte and that only 2530% of the 1PN oocytes were actually parthenogenetic. Since 48.7% of IVF and 27.9% of ICSI-derived 1PN oocytes were found to be diploid, with equal ratios of XX and XY embryos, formation of a single PN in a diploid embryo was interpreted as indicative of normal fertilization followed by asynchronous formation of PN.
In-vitro culture of oocytes observed as 1PN reveals that although the majority arrest in early cleavage stage, a small proportion are capable of further development and are observed to form morphologically normal blastocysts indistinguishable from other 2PN-derived blastocysts. It has been observed (Plachot et al.,, 1992) that 77% of 1PN oocytes proceeding to the two to four-cell stage by day 2, whereas 14.3% can reach early blastocyst by day 5 in culture. Those 1PN-derived embryos capable of blastulation with adequately developed inner cell mass may in fact be derived from fertilized oocytes with a diploid complement. Differences in cleavage performance of diploid and haploid 1PN embryos have been observed (Levron et al., 1995
) with a higher incidence of haploidy noted in zygote stage and the rarity of haploid 1PN embryos after 3 days of culture. This provides evidence of selection against haploid eggs during culture.
Diploid 1PN-derived embryos may indicate fertilization, or result from non-extrusion of the second polar body or spontaneous diploidization. Observations of these diploid parthenogenomes are rarely reported; however, the existence has been reported (Staessen et al., 1997) of a possible diploid, androgenetic one PN-derived eight-cell embryo with two Y and two 18-specific signals. It is possible that diploid androgenetic oocytes do occur but this is most likely to be very rare, since the fertilizing spermatozoa would need to be diploid and the female chromosomes lost (no female PN formed). It would seem more likely that these oocytes would contain two (diploid sperm and haploid oocyte chromatin) or even three (two haploid sperm and haploid oocyte chromatin) PN. Consequently, it is probably less likely that diploid androgenomes occur when only one PN is observed, than when two or more PN are observed. The developmental competence of diploid androgenomes to the blastocyst stage is unknown but could be expected to be irregular because of possible multiple sperm centrioles (Sathananthan et al., 1991
).
Culture of normal fertilized oocytes to the blastocyst stage has been shown to allow a more objective selection of viable embryos for transfer (Jones et al., 1998). This extended culture beyond 23 days in vitro allows elimination of embryos incapable of development once maternal message ceases and activation of the embryonic genome is initiated (Braude et al., 1988
). Extension of in-vitro culture to day 6 provides the opportunity to observe further embryonic development of these embryos and to identify those capable of blastocyst formation and therefore most likely to be normally fertilized.
In this report, transfer of a blastocyst derived from a 1PN oocyte resulted in a full-term pregnancy and birth of a normal, healthy boy at 38 weeks. It appears that the initial fertilization check observation of one PN did not reflect the developmental capacity of the embryo. This is not the first report of birth following transfer of 1PN embryos. The occurrence has been reported (Staessen et al., 1997) of 38 single embryo transfers where the only embryo available for transfer originated from a 1PN oocyte, resulting in three pregnancies with two livebirths and one preclinical abortion. Pregnancy and livebirth following transfer of 1PN embryos following round spermatid injection at ICSI have also been reported (Barak et al., 1998
). However, all these embryos were transferred after culture only to early cleavage stage, allowing no further assessment. By extending the culture period to allow for blastocyst formation, an opportunity is provided to select further those 1PN-derived embryos with the greatest developmental potential. Further culture to establish blastulation may provide a better opportunity to determine fertilization. These blastocysts may then provide an additional source of embryos for the patient, particularly if no other embryos are available.
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Notes |
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References |
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Submitted on December 4, 1998; accepted on April 18, 1999.