Technical approaches to correction of oocyte aneuploidy

Nuclear transfer improves the developmental potential of embryos derived from cytoplasmic deficient oocytes

Embryo development after fertilization is largely determined by the oocyte quality, which is in turn dependent on the competence of both the cytoplasm and nucleus. Here, to improve the efficiency of embryo development from developmentally incompetent oocytes, we performed spindle-chromosome complex transfer (ST) between in vitro matured (IVM) and in vivo matured (IVO) oocytes of the non-human primate rhesus monkey. We observed that the blastocyst rate of embryos derived from transferring the spindle-chromosome complex (SCC) of IVM oocytes into enucleated IVO oocytes was comparable with that of embryos derived from IVO oocytes. After transferring the reconstructed embryos into the uterus of surrogate mothers, two live rhesus monkeys were obtained, indicating that the nuclei of IVM oocytes support both the pre-and post-implantation embryo development of non-human primates.

Effects of intracytoplasmic sperm injection timing and fertilization methods on the development of bovine spindle transferred embryos

Cytoplasmic replacement by spindle transfer (ST) technique can be applied to improve the developmental competence of poor qualitied or aged oocytes. In cattle, ST technology has not been well established for producing embryos and the calves successfully using intracytoplasmic sperm injection (ICSI) or in vitro fertilization (IVF). The objective of this study was to develop a novel procedure for producing bovine ST embryos, which could be fundamental to applying ST technology in other mammals. In the present study, the efficacies of performing ICSI before (ICSI-ST) or after (ST-ICSI) and IVF on the development of ST bovine embryos were investigated. Results indicated that the blastocyst rate of ST embryos produced by ICSI-ST (24.7%) was higher (P < 0.05) than that produced by ST-ICSI (5.9%). On the other hand, ST-IVF had the highest fertilization rate (97.3%), polyspermy rate (24.7%), and lowest blastocyst rate (22.7%) when compared to denuded oocytes (DO), zona cut oocytes (ZC), and cumulus-oocyte complexes (COCs)-IVF groups. Finally, the in vitro development rates of ICSI-ST (24.5%) and ST-IVF (25.2%) were not significantly different (P > 0.05). However, the pregnancy rate (46.7%) and birth rate (33.3%) of ST-IVF were higher (P < 0.05) than those of ICSI-ST (6.3% and 0%, respectively). The percentage of mitochondrial DNA (mtDNA) heteroplasmy derived from donor karyoplasts of the 5 claves was between 2% and 18%. Taken together, performing ICSI prior to ST can improve the embryonic development of ST bovine embryos. Moreover, using IVF, instead of ICSI, for ST oocyte fertilization dramatically increased the pregnancy rate and birth rate of ST calves, in which mtDNA heteroplasmy derived from donor karyoplasts exists.

Thoughts on the popularity of ICSI

Purpose

Intracytoplasmic sperm injection (ICSI) is the most widely utilized assisted reproductive technique (ART) worldwide. In this feature, we review the early assisted fertilization attempts that eventually led to the development of ICSI, and discuss its current utilization in cases of male and non-male factor infertility.

Methods

We researched the literature related to the development, indications, and current use of ICSI, such as sperm structural abnormalities, male genetic indications, surgically retrieved sperm, high sperm chromatin fragmentation, oocyte dysmorphism, and preimplantation genetic testing (PGT). We also describe the potential future applications of ICSI.

Results

This review summarizes the early micromanipulation techniques that led to the inception of ICSI. We also explore its current indications, including non-male factor infertility, where its use is more controversial. Finally, we consider the benefits of future advancements in reproductive biology that may incorporate ICSI, such as in vitro spermatogenesis, neogametogenesis, and heritable genome editing.

Conclusion

The versatility, consistency, and reliability of ICSI have made it the most prevalently utilized ART procedure worldwide.

Evidence of Selection Against Damaged Mitochondria During Early Embryogenesis in the Mouse

There is evidence of a purifying filter acting in the female germline to prevent the expansion of deleterious mutations in the mitochondrial DNA (mtDNA). Given our poor understanding of this filter, here we investigate the competence of the mouse embryo to eliminate dysfunctional mitochondria. Toward that, mitochondria were damaged by photoirradiation of NZB/BINJ zygotes loaded with chloromethyl-X-rosamine (CMXRos). The resultant cytoplasm was then injected into C57BL/6J zygotes to track the levels of NZB/BINJ mtDNA during the preimplantation development. About 30% of NZB/BINJ mtDNA was present after injection, regardless of using photoirradiated or non-photoirradiated cytoplasmic donors. Moreover, injection of photoirradiated-derived cytoplasm did not impact development into blastocysts. However, lower levels of NZB/BINJ mtDNA were present in blastocysts when comparing injection of photoirradiated (24.7% ± 1.43) versus non-photoirradiated (31.4% ± 1.43) cytoplasm. Given that total mtDNA content remained stable between stages (zygotes vs. blastocysts) and treatments (photoirradiated vs. non-photoirradiated), these results indicate that the photoirradiated-derived mtDNA was replaced by recipient mtDNA in blastocysts. Unexpectedly, treatment with rapamycin prevented the drop in NZB/BINJ mtDNA levels associated with injection of photoirradiated cytoplasm. Additionally, analysis of mitochondria-autophagosome colocalization provided no evidence that photoirradiated mitochondria were eliminated by autophagy. In conclusion, our findings give evidence that the mouse embryo is competent to mitigate the levels of damaged mitochondria, which might have implications to the transmission of mtDNA-encoded disease.

Maternal spindle transfer overcomes embryo developmental arrest caused by ooplasmic defects in mice

The developmental potential of early embryos is mainly dictated by the quality of the oocyte. Here, we explore the utility of the maternal spindle transfer (MST) technique as a reproductive approach to enhance oocyte developmental competence. Our proof-of-concept experiments show that replacement of the entire cytoplasm of oocytes from a sensitive mouse strain overcomes massive embryo developmental arrest characteristic of non-manipulated oocytes. Genetic analysis confirmed minimal carryover of mtDNA following MST. Resulting mice showed low heteroplasmy levels in multiple organs at adult age, normal histology and fertility. Mice were followed for five generations (F5), revealing that heteroplasmy was reduced in F2 mice and was undetectable in the subsequent generations. This pre-clinical model demonstrates the high efficiency and potential of the MST technique, not only to prevent the transmission of mtDNA mutations, but also as a new potential treatment for patients with certain forms of infertility refractory to current clinical strategies.

Can cytoplasmic donation rescue aged oocytes?

BACKGROUND

The pregnancy and delivery rates following assisted reproductive technology (ART) start to decrease and that the miscarriage rate increases rapidly from 35 years old. The miscarriage rate exceeds 50% at 43 years old. The number of aneuploid fetuses in miscarriages increases according to female age, reaching more than 90% when women are over 40 years old.

METHODS

Different cytoplasmic donation technologies used to rescue aged oocytes with high percentage of aneuploidy were analyzed, and their efficacy compared.

MAIN FINDINGS RESULTS

Germinal vesicle transfer (GVT) might be superior to spindle chromosome transfer (ST) theoretically from the point of higher capability of rescuing the disjunction at meiosis I which cannot be helped by ST. However, actually, in vitro maturation (IVM) of oocyte after GVT has not yet been totally completed. ST among other nuclear donations showed the higher possibility to rescue them, due to the fact it does not require in vitro maturation and it has an ethical advantage over pronuclear transfer (PNT) which requires the destruction of an embryo.

CONCLUSION

Spindle chromosome transfer has the potential to rescue aged oocytes to some extent, but we have to continue the basic study further to establish the clinical application of cytoplasmic donation to rescue aged oocytes.

Development of ICSI

The first conception outside of the human body that led to the birth of Louise Brown was a tremendous accomplishment, which opened the door to the utilization of assisted reproductive techniques globally. This brought the understanding that accomplishing life in a dish required several steps, the most obvious being the timing and characteristics of fertilization. It soon became obvious in the 1980s that the most disappointing phenomenon was unexpected and complete fertilization failure. Among the approaches that were attempted to treat male factor infertility, ICSI surfaced as the technique that brought the ratio of the gametes to 1:1 and was also able to grant consistent fertilization and a higher pregnancy rate. ICSI has now been implemented for a quarter of a century, proving itself as the ultimate technique utilizing ejaculated spermatozoa independent of the semen parameters and is the sole insemination method to be used with surgically retrieved spermatozoa. There are currently various indications for ICSI that are widely adopted, rendering it the most popular insemination method worldwide. The reliability of ICSI ensures its employment in upcoming techniques involving in vitro spermatogenesis and neogametogenesis.

Molecular Mechanisms Responsible for Increased Vulnerability of the Ageing Oocyte to Oxidative Damage

In their midthirties, women experience a decline in fertility, coupled to a pronounced increase in the risk of aneuploidy, miscarriage, and birth defects. Although the aetiology of such pathologies are complex, a causative relationship between the age-related decline in oocyte quality and oxidative stress (OS) is now well established. What remains less certain are the molecular mechanisms governing the increased vulnerability of the aged oocyte to oxidative damage. In this review, we explore the reduced capacity of the ageing oocyte to mitigate macromolecular damage arising from oxidative insults and highlight the dramatic consequences for oocyte quality and female fertility. Indeed, while oocytes are typically endowed with a comprehensive suite of molecular mechanisms to moderate oxidative damage and thus ensure the fidelity of the germline, there is increasing recognition that the efficacy of such protective mechanisms undergoes an age-related decline. For instance, impaired reactive oxygen species metabolism, decreased DNA repair, reduced sensitivity of the spindle assembly checkpoint, and decreased capacity for protein repair and degradation collectively render the aged oocyte acutely vulnerable to OS and limits their capacity to recover from exposure to such insults. We also highlight the inadequacies of our current armoury of assisted reproductive technologies to combat age-related female infertility, emphasising the need for further research into mechanisms underpinning the functional deterioration of the ageing oocyte.

Advances in improving fertility in women through stem cell-based clinical platforms

INTRODUCTION

Due to their regenerative ability, stem cells are looked at as a promising tool for improving infertility treatments in women. As the main limiting factor in female fertility is represented by the decrease of ovarian reserve, the main goals of stem cell-based clinical platforms would be to obtain in vitro or in vivo neo-oogenesis. Refractory endometrial factor infertility also represents an obstacle for female reproduction for which stem cells might provide novel treatment strategies. Areas covered: A systematic search of the literature was performed on MEDLINE/PubMed database to identify relevant articles using stem-cell based clinical or research platforms in the field of female infertility. Expert opinion: In vitro oogenesis has not so far developed beyond the stage of oocyte-like cells whose normal progression to mature oocytes and ability to be fertilized was not proved. Extensive epigenetic programming of gamete precursors and the complex interactions between somatic and germ cells required for human oogenesis likely represent the main obstacles in stem-cell-based neo-oogenesis. Also resuming oogenesis in vivo in adulthood still appears a distant hypothesis, as there is still a lack of consensus about the existence and functionality of adult ovarian stem cells.

Artificial gametes from stem cells

The generation of artificial gametes is a real challenge for the scientific community today. In vitro development of human eggs and sperm will pave the way for the understanding of the complex process of human gametogenesis and will provide with human gametes for the study of infertility and the onset of some inherited disorders. However, the great promise of artificial gametes resides in their future application on reproductive treatments for all these people wishing to have genetically related children and for which gamete donation is now their unique option of parenthood. This is the case of infertile patients devoid of suitable gametes, same sex couples, singles and those fertile couples in a high risk of transmitting serious diseases to their progeny. In the search of the best method to obtain artificial gametes, many researchers have successfully obtained human germ cell-like cells from stem cells at different stages of differentiation. In the near future, this field will evolve to new methods providing not only viable but also functional and safe artificial germ cells. These artificial sperm and eggs should be able to recapitulate all the genetic and epigenetic processes needed for the correct gametogenesis, fertilization and embryogenesis leading to the birth of a healthy and fertile newborn.

The root of reduced fertility in aged women and possible therapentic options: current status and future perspects

It is well known that maternal ageing not only causes increased spontaneous abortion and reduced fertility, but it is also a high genetic disease risk. Although assisted reproductive technologies (ARTs) have been widely used to treat infertility, the overall success is still low. The main reasons for age-related changes include reduced follicle number, compromised oocyte quality especially aneuploidy, altered reproductive endocrinology, and increased reproductive tract defect. Various approaches for improving or treating infertility in aged women including controlled ovarian hyperstimulation with intrauterine insemination (IUI), IVF/ICSI-ET, ovarian reserve testing, preimplantation genetic diagnosis and screening (PGD/PGS), oocyte selection and donation, oocyte and ovary tissue cryopreservation before ageing, miscarriage prevention, and caloric restriction are summarized in this review. Future potential reproductive techniques for infertile older women including oocyte and zygote micromanipulations, derivation of oocytes from germ stem cells, ES cells, and iPS cells, as well as through bone marrow transplantation are discussed.

Effect of age, GV transfer and modified nucleocytoplasmic ratio on PKCα in mouse oocytes and early embryos

Protein kinase C (PKC) is a family of Ser/Thr protein kinases that can be activated by Ca2+, phospholipid and diacylglycerol. There is evidence that PKC plays key roles in the meiotic maturation and activation of mammalian oocytes. The present study aimed to monitor the effect of age, germinal vesicle (GV) transfer and modified nucleoplasmic ratio on the subcellular distribution profile of PKCα, an important isozyme of PKC, in mouse oocytes undergoing meiotic maturation and following egg activation. Germinal vesicle oocytes were collected from 6–8-week-old and 12-month-old mice. Germinal vesicle-reconstructed oocytes and GV oocytes with one-half or one-third of the original oocyte volume were created using micromanipulation and electrofusion. The subcellular localization of PKCα was detected by immunocytochemistry and laser confocal microscopy. Our study showed that PKCα had a similar location pattern in oocytes and early embryos from young and old mice. PKCα was localized evenly in ooplasm, with weak staining in GV at the GV stage, and present in the entire meiosis II (MII) spindle at the MII stage. In pronuclear and 2-cell embryos, PKCα was concentrated in the nucleus except for the nucleolus. After the GV oocytes were reconstructed, the resultant MII oocytes and embryos showed a similar distribution of PKCα between reconstructed and unreconstructed controls. After one-half or two-thirds of the cytoplasm was removed from the GV oocytes, PKCα still had a similar location pattern in MII oocytes and early embryos from the GV oocytes with modified nucleoplasmic ratio. Our study showed that age, GV transfer and modified nucleocytoplasmic ratio does not affect distribution of PKCα during mouse oocyte maturation, activation, and early embryonic mitosis.

Protocol for the production of viable bimaternal mouse embryos

A reliable nuclear transfer method was first reported in 1983; it provided definite evidence that parthenogenetic embryos are lethal at early postimplantation in mammals. Subsequently, nuclear transfer has been extensively used as an important and versatile tool for investigating embryo and somatic-cell cloning and nucleo-cytoplasmic interactions. Further development of this technique has enabled the generation of bimaternal embryos containing two haploid sets of maternal genomes from female germ cells of different origins. By using a 2-d nuclear transfer system for oocyte reconstruction, viable mice can be produced solely from maternal genomes, without the participation of the paternal genome. This oocyte reconstruction system, as described in this protocol, could provide valuable guidelines for exploring the potential endowments of gametes and for conferring novel properties to them.

Developmental competence of gametes reconstructed by germinal vesicle transplantation from fresh and cryopreserved bovine oocytes

OBJECTIVE

To evaluate the use of fresh or frozen bovine oocytes as an animal model for reconstructing artificial gametes by germinal vesicle transplantation (GVT), to study nucleocytoplasmic interaction and define clinical procedures for ooplasm donation in humans.

DESIGN

Prospective experimental study.

SETTING

University-based experimental laboratory.

ANIMAL(S)

Bovine oocytes from slaughterhouse ovaries.

INTERVENTION(S)

A total of 446 gametes were reconstructed from fresh immature oocytes; nuclear and cytoplasmic competencies were analyzed through the assessment of meiotic progression and cytoskeleton reorganization; embryonic developmental capability was evaluated after parthenogenetic activation of metaphase II (MII) reconstructed oocytes. Furthermore, the distribution of mitochondria in karyoplast and cytoplast in grafted oocytes was studied. Finally, meiotic and developmental competencies were determined in 199 gametes reconstructed from vitrified immature oocytes.

MAIN OUTCOME MEASURE(S)

Maturational and developmental rate of reconstructed oocytes, cytoskeleton organization, and mitochondrial distribution.

RESULT(S)

Gametes reconstructed from either fresh or cryopreserved immature oocytes showed similar meiotic competence (41.6% vs. 37.7%, respectively). All reconstituted oocytes that reached MII displayed a normal distribution of cytoskeletal elements. Embryonic developmental capability was higher in oocytes derived from fresh than from cryopreserved gametes (30.8% vs. 8.1%, respectively). Finally, oocyte centrifugation was effective in obtaining karyoplasts with <5% of mitochondria.

CONCLUSION(S)

Cows can provide a suitable organism model to develop GVT technique in both research and clinical settings as well as in fertility preservation programs.

Cryopreservation of immature bovine oocytes to reconstruct artificial gametes by germinal vesicle transplantation

Joining immature gamete cryopreservation and germinal vesicle transplantation (GVT) technique could greatly improve assisted reproductive technologies in animal breeding and human medicine. The present work was aimed to assess the most suitable cryopreservation protocol between slow freezing and vitrification for immature denuded bovine oocytes, able to preserve both nuclear and cytoplasmic competence after thawing. In addition, the outcome of germinal vesicle transfer procedure and gamete reconstruction was tested on the most effective cryopreservation system. Oocytes, isolated from slaughterhouse ovaries, were stored after cumulus cells removal either by slow freezing or by vitrification in open pulled straws. After thawing, oocytes were matured for 24 h in co-culture with an equal number of just isolated intact cumulus enclosed oocytes, and fixed in order to evaluate the stage of meiotic progression and cytoskeleton organization. Our results showed that after warming, vitrified oocytes reached metaphase II (MII) in a percentage significantly higher than oocytes cryopreserved by slow freezing (76.2% and 36.5% respectively, p < 0.05). Moreover, vitrification process preserved the organization of cytoskeleton elements in a higher proportion of oocytes than slow freezing procedure. Therefore vitrification has been identified as the elective method for denuded immature oocytes banking and it has been applied in the second part of the study. Our results showed that 38.3% of oocytes reconstructed from vitrified gametes reached the MII of meiotic division, with efficiency not different from oocytes reconstructed with fresh gametes. We conclude that vitrification represents a suitable method of GV stage denuded oocyte banking since both nuclear and cytoplasmic components derived from cryopreserved immature oocytes can be utilized for GVT.

How old is too old for fertility treatment?

Women are delaying starting their families. High-profile women such as Cherie Blair and Madonna are having successful pregnancies with healthy babies in their forties. Is this sending out messages to women that delaying conception during their twenties and thirties is a good idea? Fertility treatment can overcome certain age-related hurdles but is certainly not the magic wand some expect it to be.

The Perivitelline Space-Forming Capacity of Mouse Oocytes is Associated with Meiotic Competence

Although mouse oocytes progressively acquire meiotic competence during their growth in the ovaries, only half of full-grown oocytes can accomplish meiosis. Two types of full-grown oocytes have been reported on the basis of their chromatin configuration, the surrounded-nucleolus (SN) type and the non-surrounded-nucleolus (NSN) type. Therefore, full-grown oocytes collected from the ovaries of adult animals comprise a heterogeneous population; some oocytes are meiotically incompetent (NSN-type), and some are competent (SN-type). In the present study, we found that full-grown oocytes could be classified into two groups using the criterion of formation of the perivitelline space (PVS) after culture with 3-isobutyl-1-methylxanthine (IBMX) for 1 h. In oocytes with a PVS, actin-filled processes within zona pellucidae originating from cumulus cells were reduced, while they were rich in oocytes without a PVS, suggesting that a reduction in these processes contributes to PVS formation. PVS formation was highly correlated with meiotic competence and SN-type configuration. The results of this study demonstrate that PVS formation is a useful criterion for easily distinguishing between SN- and NSN-type oocytes, without injury to the cells.

Artificial gametes

In vitro fertilization (IVF) has been an efficient medical treatment for infertility in the past decades. However, conventional IVF approaches may be insufficient when gametes are lacking or non-viable thus precluding a significant number of patients from treatment. Ultimately, creation of artificial gametes may provide an universal solution for all indications. Somatic cell nuclear transfer (SCNT) has provided successful cloning in different animal species indicating that a derived technology may be applicable in infertility treatment procedures. Attempts to produce functional male or female gamete through nuclear transfer have been described through the process called haploidization. Initial successes have been observed, however, significant alterations at spindle construction and chromosomal segregation were also described. Stem cell technology may provide an alternative route to obtain fully functional gametes. Both sperm cells and oocytes were obtained using specific culture conditions for embryo originated stem cell. These two mainstream approaches are presented in the current review. Both of these techniques are involving sophisticated methods and consequently both of them demonstrate technical and ethical challenges. Related questions on (mitotic/meiotic) cell division, genetic/epigenetic alterations and cell renewal are needed to be addressed before clinical application.

The Use of Micromanipulation Methods as a Tool to Prevention of Transmission of Mutated Mitochondrial DNA

The introduction of different micromanipulation techniques into reproductive and developmental biology has helped us not only to answer many essential biological questions but it is now evident that these techniques also have wide practical applications. In human-assisted reproduction, the most commonly used approach is the injection of a donor sperm into the oocyte cytoplasm--intracytoplasmic sperm injection. It is, however, speculated that with these techniques it would be also possible to improve the oocyte developmental potential especially in those cases when the quality of the cytoplasm is rather poor and thus its function is compromised. Another important application would be the elimination of mutated mitochondrial DNA (mtDNA) by transferring the nuclear material from an abnormal oocyte into a healthy donor oocyte cytoplast. Some of these techniques were already successfully tested in experimental animals, but it is evident that before their introduction into human medicine many questions must be answered, and we must be sure that these approaches are absolutely or almost absolutely safe. In our contribution, we will be specifically oriented to the nuclear (nuclear material) replacement approaches that could be potentially used to prevent the transmission of mutated mtDNA from mother to offspring. Because these techniques are very delicate, some training with oocytes from other species other than human is strongly recommended.

Cytogenetic analysis and developmental assessment of mouse embryos derived from in vitro fertilization of oocytes reconstructed by meiosis-II chromosome transplantation

An electrofusion methodology for transferring meiosis-II chromosomes (M-II-t) has not been completely established. The present study compared the use of two temperatures (fusion at 37 C for Group A and 25 C for Group B) during an electrofusion procedure for mouse oocyte M-II-t and investigated the cytogenetic normality and developmental competence of embryos derived from in vitro fertilization using oocytes reconstructed by M-II-t. The M-II-t oocytes were fertilized in vitro and cultured to the blastocyst stage; the resultant embryos were analyzed cytogenetically. Subsequently, chromosomal normality of the resultant embryos at the prometaphase stage of first cleavage division and the integrity of the meiosis-II spindles of the reconstructed oocytes were analyzed. The success rate of electrofusion in Group B was 92.1%, which was significantly different from that in Group A (49.2%) (P<0.05). The fertilization rates (A, 80.7%; B, 77.2%) and development rates (A, 70.9%; B, 65.5%) in the M-II-t groups were significantly lower than those in the control group (95.0 and 92.2%, respectively) (P<0.05). The incidence of chromosomal abnormalities in the Group A embryos (20.5%) at the blastocyst stage was significantly higher than that in the control group embryos (8.5%) (P<0.05), but the incidence of chromosomal abnormalities in Group B (12.5%) was not significantly different compared with the other groups. A temperature of 25 C during the electrofusion procedure for M-II-t resulted in a good fusion rate, good development rate, and efficient production of chromosomally normal blastocysts. Furthermore, the incidence of chromosomal abnormalities in the first cleavage embryos at the prometaphase stage in Group B (9.6%) did not differ significantly from that in the control group (6.6%). The spindle morphology of the M-II-t oocytes in Group B was normal.

Transmission of mitochondrial DNA following assisted reproduction and nuclear transfer

Mitochondria are the organelles responsible for producing the majority of a cell's ATP and also play an essential role in gamete maturation and embryo development. ATP production within the mitochondria is dependent on proteins encoded by both the nuclear and the mitochondrial genomes, therefore co-ordination between the two genomes is vital for cell survival. To assist with this co-ordination, cells normally contain only one type of mitochondrial DNA (mtDNA) termed homoplasmy. Occasionally, however, two or more types of mtDNA are present termed heteroplasmy. This can result from a combination of mutant and wild-type mtDNA molecules or from a combination of wild-type mtDNA variants. As heteroplasmy can result in mitochondrial disease, various mechanisms exist in the natural fertilization process to ensure the maternal-only transmission of mtDNA and the maintenance of homoplasmy in future generations. However, there is now an increasing use of invasive oocyte reconstruction protocols, which tend to bypass mechanisms for the maintenance of homoplasmy, potentially resulting in the transmission of either form of mtDNA heteroplasmy. Indeed, heteroplasmy caused by combinations of wild-type variants has been reported following cytoplasmic transfer (CT) in the human and following nuclear transfer (NT) in various animal species. Other techniques, such as germinal vesicle transfer and pronuclei transfer, have been proposed as methods of preventing transmission of mitochondrial diseases to future generations. However, resulting embryos and offspring may contain mtDNA heteroplasmy, which itself could result in mitochondrial disease. It is therefore essential that uniparental transmission of mtDNA is ensured before these techniques are used therapeutically.

The kinase inhibitor indirubin-3′-oxime prevents germinal vesicle breakdown and reduces parthenogenetic development of pig oocytes

Oocytes undergo spontaneous germinal vesicle breakdown (GVBD) after being released from the follicular environment; this potentially prevents manipulation of the oocyte at the germinal vesicle (GV) stage. The objectives of this study were to investigate the effects of indirubin, a potent cdc2 kinase inhibitor, on GVBD and microtubular structure of porcine oocytes. Cumulus-oocyte-complexes (COCs) were collected from abattoir-derived ovaries and were randomly allocated to different concentrations of indirubin treatments (0, 10, 25, 50, and 100 microM in Experiment 1 and 0, 50, 75, and 100 microM in Experiment 2) during 44 h of IVM. The influences on the GVBD, microtubules, and maturation rates were evaluated using epifluorescence microscopy. The percentages of oocytes remaining at the GV stage were 0, 16, 26, 69, and 85% for oocytes treated with 0, 10, 25, 50, and 100 microM of indirubin, respectively, which differed among treatment groups (P<0.05). However, there were no significant differences between the oocytes treated with 75 and 100 microM (79 and 81%). The cytoplasmic microtubules were fragmented in oocytes maintained at the GV stage and the chromatin became condensed or aggregated. When COCs were incubated with indirubin (50-75 microM) for 22 h and then transferred to maturation medium for 44 h (Experiments 3-5), the percentages of oocytes reaching the metaphase II stage were generally higher than when the COCs were cultured in the presence of the drug for 44 h (62-65% versus 44-46%). However, the parthenogenetic development of the oocytes in Experiment 6 was reduced significantly in drug-treated oocytes. In summary, treatment with 50-75 microM of indirubin effectively prevented GVBD in porcine oocytes, but the developmental competence of the oocytes was compromised.

Construction and fertilization of reconstituted human oocytes

Construction of artificial gametes may be made possible by transferring somatic cells into enucleated oocytes and inducing chromosomal halving of their nuclei. This study examines the possibility of constructing viable human gametes, and their potential for participation in normal fertilization. Spare germinal vesicle-stage oocytes were donated by consenting patients undergoing intracytoplasmic sperm injection (ICSI). Approximately 62% of in-vitro matured oocytes survived enucleation and subsequent cumulus cell injection. Following micromanipulation and subsequent activation, about 40% of the reconstituted oocytes yielded two pronuclear-like entities. This was not accompanied by extrusion of a polar body, but resulted in the formation of two 'putative haploid' pronuclei. Therefore selective removal of a female pronucleus marker was required to restore a balanced ploidy. Male pronuclei were identified by association with sperm mitochondria. Additional pronuclei were then removed, allowing further cleavage. Zygotes derived were 'putatively haploid' in approximately 38% of cases with a limited number of chromosomes assessed. However, on karyotypic analysis, blastomeres isolated from cleaving embryos showed a chaotic distribution of chromosomes. Oocytes could induce 'putative haploidization' of transplanted somatic cell nuclei independently of donor cell gender. Fertilization of artificial oocytes was followed by embryonic cleavage despite blastocyst development and chromosomal content possibly being compromised.

Mammalian oocyte therapies

In assisted human reproduction, the cytoplasm of oocytes recovered from follicles is often abnormal. Its lower quality, especially in older patients, may be responsible for certain chromosomal abnormalities or developmental arrest. Thus, the deficiency of some vital molecules, which are necessary for oocyte maturation, can be the cause of infertility in women. Moreover, mutated mitochondrial DNA (mtDNA) that is located in the oocyte cytoplasm might be transmitted to offspring. With the advance of new micromanipulation techniques like the oocyte nucleus replacement or cytoplasmic transfer, some of these abnormalities could be theoretically eliminated. In this review, we briefly discuss some of these approaches and their potential use in assisted human reproduction.

Transfer of germinal vesicle to ooplasm of young mice could not rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice

BACKGROUND

[corrected] Transferring a germinal vesicle (GV) from an aged woman's oocyte into ooplasm from a younger woman has been proposed as a possible way to overcome the problem of age-related decline in female fertility. Here we assessed this possibility by determining whether ooplasts derived from young mice could rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice.

METHODS

Three groups of reconstructed oocytes, young GV-young cytoplast (group YY), aged GV-young cytoplast (group AY), and young GV-aged cytoplast (group YA), were created by micromanipulation and electrofusion.

RESULTS

Nuclear transplantation was successful in 89.8-94.4% of GV-ooplast complexes, and maturation rate of the reconstructed oocytes was 93.5-97.9%. Confocal microscopy analysis showed a significantly higher rate (49.2%) of chromosome misalignment in ageing mice than in young mice (16.9%), and 57.1% of oocytes in group AY exhibited chromosome misalignment, while the abnormality rate in groups YY and YA was 16.3 and 16.7% respectively. Calcium imaging showed that the three groups of reconstructed oocytes exhibited a similar pattern of calcium oscillations upon stimulation with bovine sperm extracts. Fertilization rate and developmental capacity to 2-cell embryos were also similar among the three groups of oocytes.

CONCLUSIONS

Our findings suggest that: (i) the ooplasm from young mice could not rescue ageing-associated chromosome misalignment in meiosis of GV from aged mice; and (ii) behaviour of chromosome alignment over metaphase spindle is predominantly determined by GV material.

Effect of Treating Induced Mitochondrial Damage on Embryonic Development and Epigenesis

Germinal vesicle transplantation (GVT) has been proposed as a possible treatment to correct age-related oocyte aneuploidy caused by dysfunctional ooplasm. How healthy ooplasm regulates normal meiosis and subsequent development has yet to be elucidated, but impaired mitochondrial metabolism may be attributable to incomplete segregation of the oocyte chromosomes. In the present study, after ooplasmic mitochondrial damage by photoirradiating chloromethyl-X-rosamine, examination of the oocyte nuclei's ability to survive after transfer into healthy ooplasts was performed. To assess their fertilizability and potential for development, GVT oocytes were fertilized by intracytoplasmic sperm injection (ICSI) and transferred to foster mice. Condition of the offspring at birth was assessed, and epigenetic analysis was performed. Photosensitization consistently inhibited oocyte maturation. However, after GVT of photosensitized nuclei into healthy ooplasts, 67.2% were reconstituted, and 76.2% of these matured normally, with an overall rate of 51.2%, much higher than that (6.0%) in the mitochondrially injured oocytes. After ICSI, 65.8% (52/79) of GVT oocytes were fertilized normally, and 21.1% (11/52) eventually reached the blastocyst stage. The transfer of 132 two-cell GVT embryos into the oviducts of pseudopregnant females resulted in 17 apparently healthy live offspring. For some key developmental genes, a high level of expression was identified in the GVT and "rescue"-derived fetal adnexa. Thus, one can induce in oocyte mitochondria a photosensitization-based type of damage, which consistently inhibits GV breakdown, meiotic spindle formation, chromosomal segregation, and polar body extrusion. Germinal vesicle transplanted and rescued oocytes were able to undergo maturation, fertilization, and embryonic cleavage and, ultimately, to develop to term. This approach may provide a model with which to study the age-related ooplasmic dysfunction seen in human oocytes.

Current advances in artificial gametes

The birth of Louise Brown, the first IVF baby, in 1978 marked a breakthrough in infertility treatment. In recent decades, several important new techniques have been introduced. One limiting factor has been the requirement to use reproductive cells (gametes) for fertilization and for embryonic development. Somatic cell nuclear transfer (cloning) has been successful in mammals, opening a potential new approach for the treatment of human infertility. In addition, nuclear transfer to achieve embryo development starting from somatic cells instead of gametes, and the creation of artificial oocytes/spermatozoa has been attempted. The present paper reviews the various alternative approaches to haploidization of somatic cells. It has been observed that chromosome segregation (of the donor somatic nucleus) may take place; however, this process is largely random, thus leading to major cytogenetic abnormalities. An alternative approach is related to stem cell technology, to be further explored in the future. Culture conditions may be adjusted so that the totipotent embryonic stem cells will differentiate to specific gametes, sperm cells or egg cells. Injecting spermatozoa produced in this manner into recipient oocytes has led to pronuclear formation and early cleavage stages in some embryos. Finally, the birth of parthenogenetic mice indicates that some of these epigenetic problems can be overcome, and that some of the embryos may survive to birth.

Cryopreservation of Isolated Mouse Germinal Vesicles

The storage of unfertilized oocytes, either immature, maturing or mature, is still unsatisfactory. Here we describe an approach in which germinal vesicles isolated as karyoplasts from immature oocytes are vitrified by open the pulled straws (OPS) method in evacuated porcine zonae pellucidae. After thawing, their survival was almost absolute. Moreover, when thawed GV-karyoplasts were fused to immature oocyte cytoplasts the maturation of reconstructed cells resulted in the production of secondary oocytes--metaphase II.

Cytogenetic analysis of human somatic cell haploidization

Despite recent interest in the derivation of female and male gametes through somatic cell nuclear transfer, there is still insufficient data on chromosomal analysis of these gametes resulting from haploidization, especially involving a human nuclear donor and recipient oocytes. The objective of this study was to investigate the fidelity of chromosomal separation during haploidization of human cumulus cells by in-vitro matured human enucleated MII oocytes. A total of 129 oocytes were tested 4-7, 8-14, or 15-21 h after nuclear transfer (NT) followed by electro-stimulation, resulting in 71.3% activation efficiency on average. Haploidization was documented by the formation of two separate groups of chromosomes, originating from either polar body/pronucleus (PB/PN), or only 2PN, which were tested by 5-colour FISH, or DNA analysis for copy number of chromosomes 13, 16, 18, 21, 22 and X. Two PN were formed more frequently than PB/PN, irrespective of incubation time. In agreement with recent reports on mouse oocytes, as many as 90.2% of the resulting haploid sets tested showed abnormal chromosome segregation, suggesting unsuitability of the resulting artificial gametes for practical application at the present time.

Nuclear Origin of Aging-Associated Meiotic Defects in Senescence-Accelerated Mice1

Factors of both cytoplasmic and nuclear origin regulate metaphase chromosome alignment and spindle checkpoint during mitosis. Most aneuploidies associated with maternal aging are believed to derive from nondisjunction and meiotic errors, such as aberrations in spindle formation and chromosome alignment at meiosis I. Senescence-accelerated mice (SAM) exhibit aging-associated meiotic defects, specifically chromosome misalignments at meiosis I and II that resemble those found in human female aging. How maternal aging disrupts meiosis remains largely unexplained. Using germinal vesicle nuclear transfer, we found that aging-associated misalignment of metaphase chromosomes is predominately associated with the nuclear factors in the SAM model. Cytoplasm of young hybrid B6C3F1 mouse oocytes could partly rescue aging-associated meiotic chromosome misalignment, whereas cytoplasm of young SAM was ineffective in preventing the meiotic defects of old SAM oocytes, which is indicative of a deficiency of SAM oocyte cytoplasm. Our results demonstrate that both nuclear and cytoplasmic factors contribute to the meiotic defects of the old SAM oocytes and that the nuclear compartment plays the predominant role in the etiology of aging-related meiotic defects.

The role of mitochondria in the establishment of oocyte functional competence

Mitochondria are maternally inherited, semi-autonomous organelles with their own genomes (mtDNA), largely responsible for the generation of energy in the form of cellular ATP. However, mitochondrial replication and transcription of mtDNA do not commence until well into embryonic differentiation. This means that the oocyte needs to contain sufficient stocks of functioning mitochondria to fuel the first few days of embryonic development. In this review, I examine how qualitative and quantitative aspects of mitochondria help us define the notion of functional competence.

Genetic and epigenetic modifications associated with human ooplasm donation and mitochondrial heteroplasmy – considerations for interpreting studies of heritability and reproductive outcome

The mitochondrial heteroplasmy present in offspring from IVF and human ooplasm donation is troublesome and merits further exploration in a debate that is already complex and controversial. Improving the understanding of mitochondrial genomics in this context is important because mitochondriopathies can impact crucial cellular processes in renal, cardiovascular, central nervous, and endocrine systems. Relevant epigenetic consequences of mitochondrial heteroplasmy include associated abnormalities in mitochondrial translation products. Furthermore, as transmission and inheritance patterns of mtDNA are species-specific, it remains to be proven if findings derived from animal studies are applicable to human offspring. As an alternative to gamete research and proteomics based on animal experimentation, continued molecular characterization of the de novo human mitochondriopathies is posed to offer further insights regarding mitochondrial heteroplasmy. In this context, because knowledge of human mitochondrial genetics remains limited and the risks associated with ooplasm donation cannot be quantified, we do not favor its use for our patients at present. However, the small number of infants already conceived from this experimental approach warrant careful longitudinal evaluation. In particular, observational study of the few children born after ooplasm donation could provide opportunities to assess human mtDNA transmission and inheritance. Such findings could help identify features distinguishing natural mtDNA heteroplasmy from heteroplasmy observed after ooplasm donation. Future investigations should also quantify the degree any such heteroplasmy can exist innocuously. Disclosure of mtDNA mutations potentially affecting children conceived from IVF and ooplasm donation must be included during patient education at centers contemplating such treatment.

Nuclear and Microtubule Dynamics of G2/M Somatic Nuclei During Haploidization in Germinal Vesicle-Stage Mouse Oocytes

During the haploidization process, it is expected that diploid chromosomes of somatic cells will be reduced to haploid for the generation of artificial gametes. In the present study, we aimed to use enucleated mouse oocytes at the germinal vesicle-stage (G2/M) as recipients for somatic cells that are also synchronized to the G2/M stage for haploidization. The reconstructed oocytes were then induced to undergo meiosis in vitro and observed for their nuclear morphology and microtubule network formation at various expected stages of the meiotic division. Following in vitro maturation, more than half (62/119, 52.1%) of the reconstructed oocytes completed the first round of meiosis-like division, as evidenced by the extrusion of pseudopolar bodies (PBs). However, accelerated PB extrusion, approximately 3-4 h earlier than that by control oocytes occurred. Furthermore, abnormally large pseudo-PBs, as large as four times the normal PB sizes, were observed. During the process of in vitro maturation at both the expected stages of metaphase I (MI) and metaphase II (MII), condensed chromosomes were observed in 38.7% and 55.2% of oocytes, respectively. However, two other types of nuclear configurations were also observed: 1) uneven distribution of chromatin and 2) an interphase-like nucleus, indicating deficiencies in chromosome condensation. Following oocyte activation, more than half (21/33, 63.6%) of the reconstructed oocytes with pseudo-PBs formed separated pseudopronuclei (PN), suggesting formation of functional spindles. The formation of bipolar spindle-like microtubule network at both the expected MI and MII stages during in vitro maturation was confirmed by immunohistochemistry. In summary, this study demonstrated that a high proportion of G2/M somatic nuclei appear to undergo meiosis-like division, in two successive steps, forming a pseudo-PB and two separate pseudo-PN upon in vitro maturation and activation treatment. Moreover, the enucleated geminal vesicle cytoplast retained its capacity for meiotic division following the introduction of a somatic G2/M nucleus.

Distribution of mitochondria in reconstructed mouse oocytes

It has been suggested that nucleus replacement (transfer) may be used as an efficient oocyte therapy in order to prevent transmission of mutated mitochondrial DNA from mother to offspring in humans. The essential and not yet answered question is how mitochondria surrounding the karyoplast will be distributed in the newly reconstructed oocytes. In our model experiments, we have evaluated the distribution of mitochondria in reconstructed immature mouse oocytes when germinal vesicle karyoplasts, with labeled mitochondria, were fused to unlabeled cytoplasts. The penetration of mitochondria from karyoplasts into cytoplasts can be detected almost immediately after the beginning of fusion. In immature reconstructed oocytes, mitochondria are first located in the oocyte center but they are homogenously distributed within the whole cytoplasm before the completion of maturation. Fusion of oocytes at different stages of maturation suggests that the speed of mitochondria distribution is cell cycle dependent.

Implications of cloning technique for reproductive medicine

The birth of Dolly following the transfer of mammary gland nuclei into enucleated eggs established cloning as a feasible technique in mammals, but the moral implications and high incidence of developmental abnormalities associated with cloning have induced the majority of countries to legislate against its use with human gametes. Because of such negative connotations, restrictive political reactions could jeopardize the therapeutic and scientific promise that certain types of cloning may present. For example, in addition to its proposed use as a way of generating stem cells, the basic technique of nuclear transplantation has proven useful in other ways, including its application to immature eggs as a new approach to the prevention of the aneuploidy common in older women, and for some recent advances in preimplantation genetic diagnosis. Thus, while attempts at reproductive cloning in man would seem premature and even dangerous at present, this field will require rational rather than emotional reactions as a basis for legislation if the therapeutic promise of stem cell research and the experimental potential of nuclear transplantation techniques are to be fully realized.