Vitrification of mouse oocytes results in aneuploid zygotes and malformed fetuses

Chromosome Segregation in the Oocyte: What Goes Wrong during Aging

Human female fertility and reproductive lifespan decrease significantly with age, resulting in an extended post-reproductive period. The central dogma in human female reproduction contains two important aspects. One is the pool of oocytes in the human ovary (the ovarian reserve; approximately 10⁶ at birth), which diminishes throughout life until menopause around the age of 50 (approximately 10³ oocytes) in women. The second is the quality of oocytes, including the correctness of meiotic divisions, among other factors. Notably, the increased rate of sub- and infertility, aneuploidy, miscarriages, and birth defects are associated with advanced maternal age, especially in women above 35 years of age. This postponement is also relevant for human evolution; decades ago, the female aging-related fertility drop was not as important as it is today because women were having their children at a younger age. Spindle assembly is crucial for chromosome segregation during each cell division and oocyte maturation, making it an important event for euploidy. Consequently, aberrations in this segregation process, especially during the first meiotic division in human eggs, can lead to implantation failure or spontaneous abortion. Today, human reproductive medicine is also facing a high prevalence of aneuploidy, even in young females. However, the shift in the reproductive phase of humans and the strong increase in errors make the problem much more dramatic at later stages of the female reproductive phase. Aneuploidy in human eggs could be the result of the non-disjunction of entire chromosomes or sister chromatids during oocyte meiosis, but partial or segmental aneuploidies are also relevant. In this review, we intend to describe the relevance of the spindle apparatus during oocyte maturation for proper chromosome segregation in the context of maternal aging and the female reproductive lifespan.

Vitrification-induced activation of lysosomal cathepsin B perturbs spindle assembly checkpoint function in mouse oocytes

As the age of child-bearing increases and correlates with infertility, cryopreservation of female gametes is becoming common-place in ART. However, the developmental competence of vitrified oocytes has remained low. The underlying mechanisms responsible for reduced oocyte quality post-vitrification are largely unknown. Mouse cumulus-oocyte complexes were vitrified using a cryoloop technique and a mixture of dimethylsulphoxide, ethylene glycol and trehalose as cryoprotectants. Fresh and vitrified/thawed oocytes were compared for chromosome alignment, spindle morphology, kinetochore-microtubule attachments, spindle assembly checkpoint (SAC) and aneuploidy. Although the majority of vitrified oocytes extruded the first polar body (PB), they had a significant increase of chromosome misalignment, abnormal spindle formation and aneuploidy at metaphase II. In contrast to controls, vitrified oocytes extruded the first PB in the presence of nocodazole and etoposide, which should induce metaphase I arrest in a SAC-dependent manner. The fluorescence intensity of mitotic arrest deficient 2 (MAD2), an essential SAC protein, at kinetochores was reduced in vitrified oocytes, indicating that the SAC is weakened after vitrification/thawing. Furthermore, we found that vitrification-associated stress disrupted lysosomal function and stimulated cathepsin B activity, with a subsequent activation of caspase 3. MAD2 localization and SAC function in vitrified oocytes were restored upon treatment with a cathepsin B or a caspase 3 inhibitor. This study was conducted using mouse oocytes, therefore confirming these results in human oocytes is a prerequisite before applying these findings in IVF clinics. Here, we uncovered underlying molecular pathways that contribute to an understanding of how vitrification compromises oocyte quality. Regulating these pathways will be a step toward improving oocyte quality post vitrification and potentially increasing the efficiency of the vitrification program.

Melatonin improves the first cleavage of parthenogenetic embryos from vitrified-warmed mouse oocytes potentially by promoting cell cycle progression

Background

This study investigated the effect of melatonin (MT) on cell cycle (G1/S/G2/M) of parthenogenetic zygotes developed from vitrified-warmed mouse metaphase II (MII) oocytes and elucidated the potential mechanism of MT action in the first cleavage of embryos.

Results

After vitrification and warming, oocytes were parthenogenetically activated (PA) and in vitro cultured (IVC). Then the spindle morphology and chromosome segregation in oocytes, the maternal mRNA levels of genes including Miss, Doc1r, Setd2 and Ythdf2 in activated oocytes, pronuclear formation, the S phase duration in zygotes, mitochondrial function at G1 phase, reactive oxygen species (ROS) level at S phase, DNA damage at G2 phase, early apoptosis in 2-cell embryos, cleavage and blastocyst formation rates were evaluated. The results indicated that the vitrification/warming procedures led to following perturbations 1) spindle abnormalities and chromosome misalignment, alteration of maternal mRNAs and delay in pronucleus formation, 2) decreased mitochondrial membrane potential (MMP) and lower adenosine triphosphate (ATP) levels, increased ROS production and DNA damage, G1/S and S/G2 phase transition delay, and delayed first cleavage, and 3) increased early apoptosis and lower levels of cleavage and blastocyst formation. Our results further revealed that such negative impacts of oocyte cryopreservation could be alleviated by supplementation of warming, recovery, PA and IVC media with 10^− 9 mol/L MT before the embryos moved into the 2-cell stage of development.

Conclusions

MT might promote cell cycle progression via regulation of MMP, ATP, ROS and maternal mRNA levels, potentially increasing the first cleavage of parthenogenetic zygotes developed from vitrified–warmed mouse oocytes and their subsequent development.

An improved method for vitrification of in vitro matured ovine oocytes; beneficial effects of Ethylene Glycol Tetraacetic acid, an intracellular calcium chelator

Vitrification affects fertilization ability and developmental competence of mammalian oocytes. This effect may be more closely associated with an intracellular calcium rise induced by cryoprotectants. The present study aimed to assess whether addition of Ethylene Glycol Tetraacetic acid (EGTA) to vitrification solution could improve quality and developmental competence of in vitro matured ovine oocytes. Vitrified groups were designed according to the presence or absence of EGTA and/or calcium in base media, including: mPB1+ (modified PBS with Ca²⁺), mPB1^- (modified PBS without Ca²⁺), mPB1⁺/EGTA (mPB1⁺ containing EGTA), mPB1^-/EGTA (mPB1^- containing EGTA). In vitro development, numerical chromosome abnormalities, hardening of zona pellucida, mitochondrial distribution and function of viable oocytes were evaluated and compared between groups. Quality of blastocysts was assessed by differential and TUNEL staining. Also, mRNA expression levels of six candidate genes (KIF11, KIF2C, CENP-E, KIF20A, KIF4A and KIF2A), were quantitatively evaluated by RT-PCR. Our results showed that calcium-free vitrification and EGTA supplementation can significantly increase the percentage of normal haploid oocytes and maintain normal distribution and function of mitochondria in vitrified ovine oocytes, consequently improving developmental rate after in vitro fertilization. qRT-PCR analysis showed no significant difference in mRNA expression levels of kinesin genes between vitrified and fresh oocytes. Also, the presence of calcium in vitrification solution significantly increased zona hardening. In conclusion, we have shown for the first time that supplementation of vitrification solution with EGTA, as a calcium chelator, improved the ability of vitrified ovine oocytes to preserve mitochondrial distribution and function, as well as normal chromosome segregation.

Pre-clinical validation of a closed surface system (Cryotop SC) for the vitrification of oocytes and embryos in the mouse model

Vitrification is currently a well-established technique for the cryopreservation of oocytes and embryos. It can be achieved either by direct (open systems) or indirect (closed systems) contact with liquid nitrogen. While there is not a direct evidence of disease transmission by transferred cryopreserved embryos, it was experimentally demonstrated that cross-contamination between liquid nitrogen and embryos may occur, and thus, the use of closed devices has been recommended to avoid the risk of contamination. Unfortunately, closed systems may result in lower cooling rates compared to open systems, due to the thermal insulation of the samples, which may cause ice crystal formation resulting in impaired results. In our study, we aimed to validate a newly developed vitrification device (Cryotop SC) that has been specifically designed for being used as a closed system. The cooling and warming rates calculated for the closed system were 5.254 °C/min and 43.522 °C/min, respectively. Results obtained with the closed system were equivalent to those with the classic Cryotop (open system), with survival rates in oocytes close to 100%. Similarly, the potential of the survived oocytes to develop up to good quality blastocysts after parthenogenetic activation between both groups was statistically equivalent. Assessment of the meiotic spindle and chromosome distribution by fluorescence microscopy in vitrified oocytes showed alike morphologies between the open and closed system. No differences were found either between the both systems in terms of survival rates of one-cell stage embryos or blastocysts, as well as, in the potential of the vitrified/warmed blastocysts to develop to full-term after transferred to surrogate females.

Chapter 1 Historical Background on Gamete and Embryo Cryopreservation

This chapter describes the development of the science of cryopreservation of gametes and embryos of various species including human. It attempts to record in brief the main contributions of workers in their attempts to cryopreserve gametes and embryos. The initial difficulties faced and subsequent developments and triumphs leading to present-day state of the art are given in a concise manner. The main players and their contributions are mentioned and the authors' aim is to do justice to them. This work also attempts to ensure that credit is correctly attributed for significant advances in gamete and embryo cryopreservation. In general this chapter has tried to describe the historical development of the science of cryopreservation of gametes and embryos as accurately as possible without bias or partiality.

Human embryo cryopreservation: one-step slow freezing does it all?

PURPOSE

Previous studies have shown that a modified one-step slow freezing method with higher sucrose concentration (0.2 M) can achieve higher embryo and blastomere survival rates that are comparable to vitrification. However, no study has evaluated the efficacy of a one-step method using commercial slow freezing kit without altering its composition. This retrospective study examines the effects of using 1.5 M PROH with 0.1 M sucrose (F2 medium) alone in a one-step slow freezing method compared to the conventional two-step method.

METHODS

Cleavage stage embryos from 526 thaw cycles previously cryopreserved by either the conventional two-step slow freezing method or the modified one-step method were studied. The embryo and blastomere survival rates, cleavage rate, clinical pregnancy rate and live birth rate were compared between the two groups.

RESULTS

The results showed that the embryo survival rate was significantly higher in the modified one-step method compared to the conventional two-step method (86.9 % and 83.1 %, respectively; p = 0.04). Total blastomere survival rate was also significantly increased as a result of the modification (81.0 % versus 76.5 %; p < 0.001). However, there was no statistical difference in the cleavage rates, clinical pregnancy rates (CPR/ET) and live birth rates between the two methods.

CONCLUSIONS

Slow freezing using the one-step method is superior to the conventional two-step method in terms of embryo and blastomere survival rates without affecting cleavage rate and clinical outcomes. It can be routinely applied to cleavage stage embryo cryopreservation in IVF centres for greater workflow efficiency.

Impact of vitrification on the meiotic spindle and components of the microtubule-organizing center in mouse mature oocytes

Cryopreservation of oocytes is becoming a valuable method for fertility preservation in women. However, various unphysiological alterations occur in the oocyte during the course of cryopreservation, one of which is the disappearance of the meiotic spindle. Fortunately, the meiotic spindle does regenerate after thawing the frozen oocytes, which enables completion of meiosis and further development after fertilization. Nonetheless, the mechanistic understanding of the meiotic spindle regeneration after cryopreservation is still scarce. Here, to gain insight into the mechanisms of the spindle disappearance and regeneration, we examined the status of spindle microtubules as well as the key components of the microtubule-organizing center (MTOC), specifically gamma-Tubulin, NEDD1, and Pericentrin, in mature (metaphase II) mouse oocytes at different steps of vitrification, a major cryopreservation technique. We found that the configuration of the spindle microtubules dynamically changed during the process of vitrification and that spindle regeneration was preceded by excessive microtubule polymerization, followed by reduction into the normal size and shape. Also, all three MTOC components exhibited disappearance and reappearance during the vitrification process, although Pericentrin appeared to regenerate in earlier steps compared to the other components. Furthermore, we found that the localization of the MTOC components to the spindle poles persisted even after depolymerization of spindle microtubules, suggesting that the MTOC components are impacted by vitrification independently from the integrity of the microtubules. The present study would set the stage for future investigations on the molecular mechanisms of the meiotic spindle regeneration, which may contribute to further improving protocols for oocyte cryopreservation.

A critical appraisal of cryopreservation (slow cooling versus vitrification) of human oocytes and embryos

BACKGROUND

Vitrification is now a commonly applied technique for cryopreservation in assisted reproductive technology (ART) replacing, in many cases, conventional slow cooling methodology. This review examines evidence relevant to comparison of the two approaches applied to human oocytes and embryos at different developmental stages.

METHODS

Critical review of the published literature using PubMed with particular emphasis on studies which include data on survival and implantation rates, data from fresh control groups and evaluation of the two approaches in a single setting.

RESULTS

Slow cooling is associated with lower survival rates and compromised development relative to vitrification when applied to metaphase II (MII) oocytes, although the vitrification results have predominantly been obtained using direct contact with liquid nitrogen and there is some evidence that optimal protocols for slow cooling of MII oocytes are yet to be established. There are no prospective randomized controlled trials (RCTs) which support the use of either technique with pronuclear oocytes although vitrification has become the method of choice. Optimal slow cooling, using modifications of traditional methodology, and vitrification can result in high survival rates of early embryos, which implant at the same rate as equivalent fresh counterparts. Many studies report high survival and implantation rates following vitrification of blastocysts. Although slow cooling of blastocysts has been reported to be inferior in some studies, others comparing the two approaches in the same clinical setting have demonstrated comparable results. The variation in the extent of embryo selection applied in studies can lead to apparent differences in clinical efficiency, which may not be significant if expressed on a 'per oocyte used' basis.

CONCLUSIONS

Available evidence suggests that vitrification is the current method of choice when cryopreserving MII oocytes. Early cleavage stage embryos can be cryopreserved with equal success using slow cooling and vitrification. Successful blastocyst cryopreservation may be more consistently achieved with vitrification but optimal slow cooling can produce similar results. There are key limitations associated with the available evidence base, including a paucity of RCTs, limited reporting of live birth outcomes and limited reporting of detail which would allow assessment of the impact of differences in female age. While vitrification has a clear role in ART, we support continued research to establish optimal slow cooling methods which may assist in alleviating concerns over safety issues, such as storage, transport and the use of very high cryoprotectant concentrations.

Cryopreservation of oocytes in experimental models

Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models has provided the opportunity for research that may not have been possible with human material. Today, results of these studies still continue to form the basis of oocyte cryobiology. This review discusses these studies, especially the physiological impacts of cryopreservation on oocyte biology. It will also focus on the role that animal models have played in improvement strategies, validation before translating new techniques into the human model and the advances made in the human in IVF because of these animal models. Finally, existing investigations and their potential impact in other areas of research will be discussed. Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models provided the opportunity for research that may not have been possible with human material. Today, animal models still continuously provide imperative data that facilitate further advancements in oocyte cryobiology. This review will focus on the physiological impacts, current improvement strategies and future applications of oocyte cryopreservation using animal models as they benefit not only human oocyte cryopreservation procedures, but also the human species through their usefulness in agriculture, medicine and conservation.

History of oocyte cryopreservation

The potential advantages of being able to cryopreserve oocytes have been apparent for many decades. Technical difficulties associated with the unique properties of the mammalian oocyte initially retarded rapid development in this area but recent advances have overcome many of the problems. A stage has now been reached where oocyte cryopreservation can be considered an important component of human assisted reproductive technology. The potential advantages of being able to cryopreserve oocytes have been apparent for many decades. Technical difficulties associated with the unique properties of the mammalian oocyte initially retarded rapid development in this area but recent advances have overcome many of the problems. A stage has now been reached where oocyte cryopreservation can be considered an important component of human assisted reproductive technology.

Vitrification in assisted reproduction: myths, mistakes, disbeliefs and confusion

The purpose of this work is to update embryologists and clinicians on different approaches in human oocyte and embryo cryopreservation, by clarifying some misunderstandings and explaining the underlying reasons for controversial opinions. The work is based on literature review and critical analysis of published papers or conference abstracts during the last 24 years, with special focus on the last 3 years. Due to the latest advancements in techniques, cryopreservation now offers new perspectives along with solutions to many demanding problems, and has developed from a backup procedure to a successful alternative that is an indispensable constituent of assisted reproductive techniques. However, this progress is not free from controversies, at some points is rather serendipitous, and many factors, including human ones, hamper the selection and widespread application of the most efficient technique for the given task. A better understanding of the basic features of the two rival approaches (slow-rate freezing and vitrification), a clarification of terms and technical details, and a balanced, pragmatic evaluation of possible risks and potential, or definite, gains are required to accelerate advancement. Alternatively, the increasing flow of patients to the few assisted reproduction clinics and countries that are highly successful in this field will enforce the required changes in methodology and mentality worldwide.

A critical evaluation of ovarian tissue cryopreservation and grafting as a strategy for preserving the human female germline

Ovarian tissue freezing has been used successfully in animals and it has recently begun to be offered clinically to young women who have medical conditions with a high risk of sterility. Although no frozen human ovarian grafts have yet been returned to the donor and resulted in a pregnancy, there are many indications that this procedure should be feasible. Although live young have been derived from frozen grafts in several species, research should aim to make further improvements to the cryopreservation and grafting procedures to optimize follicle survival, and hence minimize the amount of tissue that needs to be collected, stored and returned. Ovarian tissue freezing, particular if used in combination with egg and embryo freezing, should allow a patient to safeguard their chance of becoming a parent later (Table 1). In cases where the patient has a systemic cancer or infection and malignant cells or viruses may be present in the systemic circulation and the gonadal tissue, ovarian tissue could be collected and frozen, but grafting is not currently recommended (Table 3).

Nuclear transfer and oocyte cryopreservation

Somatic cells can be reprogrammed to a totipotent state through nuclear transfer or cloning, because it has been demonstrated that the oocyte has the ability to reprogramme an adult nucleus into an embryonic state that can initiate the development of a new organism. Therapeutic cloning, whereby nuclear transfer is used to derive patient-specific embryonic stem cells, embraces an entire new opportunity for regenerative medicine. However, a key obstacle for human therapeutic cloning is that the source of fresh human oocytes is extremely limited. In the present review, we propose prospective sources of human oocytes by using oocyte cryopreservation, such as an oocyte bank and immature oocytes. We also address some potential issues associated with nuclear transfer when using cryopreserved oocytes. In the future, if the efficacy and efficiency of cryopreserved oocytes are comparable to those of fresh oocytes in human therapeutic cloning, the use of cryopreserved oocytes would be invaluable and generate a great impact to regenerative medicine.

Blastocyst rate and live births from vitrification and slow-cooled two-cell mouse embryos

OBJECTIVE

The purpose of this study was to develop a closed vitrification system, compare vitrification to a slow-cooled cryopreservation method, and compare the pup rate between both methods using two-cell mouse embryos.

DESIGN

Randomized, prospective animal study.

SETTING

Hospital-based IVF practice.

ANIMAL(S)

B6C3F1 mouse embryos.

INTERVENTION(S)

Two-cell mouse embryos were cryopreserved using a slow-cooled or vitrification method and then thawed at a later date. The embryos were cultured and transferred to recipient females.

MAIN OUTCOME MEASURE(S)

Embryos were observed for blastocyst rate and pups were observed for phenotypic anomalies and weighed at 30, 60, and 90 days after birth.

RESULT(S)

Neither the blastocyst rate, pup rate, nor pup weights were significantly different when the two cryopreservation methods were compared.

CONCLUSION(S)

Because there were no differences in blastocyst rates, pup rates, or pup weights, we plan to further investigate the potential effects of vitrification on genotypic damage via the Comet Assay.

Human oocyte cryopreservation

The clinical role of oocyte cryopreservation in assisted reproduction, as an adjunct to sperm and embryo cryopreservation, has been comparatively slow to evolve as a consequence of theoretical concerns related to efficacy and safety. Basic biological studies in the 1990's alleviated many of these concerns leading to more widespread adoption of the technology. While a number of babies were born from the approach validated in the 1990's, its perceived clinical inefficiency led to the search for improved methods. Introduction of elevated dehydrating sucrose concentrations during cryopreservation increased survival and fertilization rates, but there is no well-controlled evidence of improved clinical outcome. Similarly, the use of sodium-depleted cryopreservation media has not been demonstrated to increase clinical efficiency. More recently, and in the absence of basic biological studies addressing safety issues, the application of vitrification techniques to human oocytes has resulted in reports of a number of live births. The small number of babies born from clinical oocyte cryopreservation and the paucity of well-controlled studies currently preclude valid comparisons between approaches. Legal restrictions on the ability to select embryos from cryopreserved oocytes in Italy, where many of the available reports originate, also obscure attempts to assess oocyte cryopreservation objectively.

Oocyte cryopreservation: oocyte assessment and strategies for improving survival

Despite significant progress in cryopreservation of mammalian oocytes and embryos, many of the molecular and biochemical events that underlie this technology are poorly understood. In recent years, researchers have focused on obtaining viable oocytes that are developmentally competent. Even under the most favourable conditions, experimental approaches have achieved only limited success compared with fresh oocytes used in routine in vitro embryo production. Chilling injuries and toxic effects of the cryoprotectants are the major adverse consequences following cryoprocedures. To overcome these problems, different strategies have been developed for improving cryopreservation results. These strategies include reducing container volumes, increasing the thermal gradient, changing the cell surface/volume ratio, enhancing cryotolerance by supplementation with various additives or modifying the lipid composition of the oocyte membrane. In order to develop new strategies for reducing the various forms of stress associated with oocyte cryopreservation, it is fundamental to gain a better understanding of the major changes responsible for poor post-thaw survival. With this knowledge, we hope that oocyte cryostorage will become a fully reliable reproductive technique in the near future.

[Ovarian tissue vitrification: Cortex and whole ovary in sheep]

OBJECTIVE

The aim of this study was to evaluate a cryopreservation technique by vitrification of cortex or whole ovaries in sheep, using two cryoprotectant solutions: VS1 and VS4 and to study their physical properties to avoid ice crystallisation by vitrification of whole sheep ovaries permeated with a cryoprotectant solution.

ANIMALS AND METHODS

From 6-month-old ewes, whole sheep ovaries with their vascular pedicles were collected at the slaughterhouse or at the veterinary school and prepared for cryoprotectant toxicity tests and freezing procedure. Follicle viability was measured by trypan blue test and histological examination of ovary. The hemi-ovarian cortex was stored in liquid nitrogen. Four to six weeks after the first laparotomy, the controlateral ovary was removed and the vitrified-warmed hemi-ovary was sutured. Thermal properties of a cryoprotectant solution called VS4 (critical cooling rates [Vccr], vitreous transition temperature [Tg], end of melting temperature [Tm]) were measured by differential scanning calorimetry.

RESULTS

No significant difference in follicle viability or normal follicle rates was observed between ovarian cortex exposed or non-exposed to cryoprotectant solutions. Nor was any significant difference observed before and after vitrification. Three pregnancies occurred, from which four lambs were born after autografts of vitrified ovarian cortex. With whole ovary, the decrease in the number of normal follicles was lower when frozen-thawed ovaries were treated with VS4 (P = 0.04). There were less nuclear anomalies (P = 0.02). The Vccr of VS4 has been estimated to be 14.3+/-1.1 degrees C/min and Tg was -125.0+/-0.2 degrees C. Because the penetration of cryoprotectants was very low, Vccr was very high and the cooling speed did not allow cortex to vitrify.

DISCUSSION AND CONCLUSIONS

Cryopreservation of cortex or whole ovary by vitrification seems a promising technique in reproductive medicine. The best histologic results were obtained with the VS4 cryoprotectant when whole ovary was vitrified.

Comparison of in-vitro outcomes from cryopreserved oocytes and sibling fresh oocytes

In Italy, the restrictive IVF law generalizes the indication for oocyte freezing for surplus oocytes in 78.5% of in-vitro assisted reproductive cycles. With a view to understanding better what the prospects for intracytoplasmic sperm injection (ICSI) on frozen-thawed oocytes might be, the consequences of freeze-thaw procedures on fertilization, cleavage rates and embryo quality obtained from frozen-thawed oocytes were studied and compared with the results obtained from sibling fresh oocytes. Eleven IVF and 29 ICSI on 76 and 169 fresh oocytes were performed and the corresponding 40 ICSI on 221 sibling frozen-thawed oocytes. There was no difference in terms of fertilization rate between fresh and sibling frozen-thawed oocytes. The cleavage rate (98.0 and 94.4% with fresh oocytes in IVF and ICSI; 77.3% with frozen-thawed oocytes in ICSI; P < 0.001) and embryo quality (grade I embryos over total embryos: 36.7 and 22.2% with fresh oocytes in IVF and ICSI; 12.1% with frozen-thawed oocytes in ICSI; respectively P < 0.001 and P < 0.05) were statistically lower after oocyte cryopreservation. The significant decrease in meiotic spindle retrieval rate before freezing (62.4%) and after thawing procedures (43.4%; P < 0.001) suggests that cryoconservation induces irreversible damage to microtubule repolymerization. The consequences of oocyte cryopreservation procedures on embryo development are reviewed.

Simplified EM grid vitrification is a convenient and efficient method for mouse mature oocyte cryopreservation

This study was performed to evaluate the efficiency of simplified EM grid vitrification, skipping the step of removing the cryoprotectant (5.5M EG + 1.0M sucrose) droplet on the grid after loading oocytes, compared to conventional cryopreservation protocols for mouse mature oocytes. Firstly, the recovery, survival, fertilization and hatching rates of simplified EM grid vitrification were compared with those of the slow freezing method using 1.5M DMSO. Then, conventional EM grid vitrification was compared with simplified EM grid vitrification. Simplified EM grid vitrification showed higher survival, fertilization and hatching rates than those of the slow freezing method (85.6% vs. 63.2%; 51.0% vs. 22.3%; 38.7% vs. 12.5%, p < 0.01, respectively). Moreover, simplified EM grid vitrification showed higher recovery, survival and fertilization rates than those of conventional EM grid vitrification (100% vs. 95.0%, p=0.024; 90.0% vs. 78.9%, p=0.033; 56.7% vs. 38.7%, p=0.021, respectively). Hatching rate tended to be higher for simplified EM grid vitrification compared to conventional EM grid vitrification (41.1% vs. 24.1%). In conclusion, simplified EM grid vitrification is a convenient and efficient method for cryopreservation of mouse mature oocytes, compared to conventional EM grid vitrification and slow freezing methods.

Oocytes cryopreservation: state of art

In the present review article we sought to analyze, on the basis of a systematic review, the indications, rationale of oocytes cryopreservation, as well as the techniques that improved the aforementioned procedure in order to higher the pregnancy rate in women undergoing that procedure. Moreover, we pointed out the importance of oocytes cryopreservation in the research field as oocyte banking may be of utmost importance to increase the availability of oocytes for research applications such as genetic engineering or embryo cloning. Oocyte freezing has 25 year of history alternating successes and setbacks. Human oocytes have a delicate architecture but are freezable. Clinical efficiency remains low, but healthy children have been born, indicating that chromosomally normal embryos can originate from frozen oocytes. Freezing protocols are not yet optimal and it is now desirable to combine empirical and theoretical knowledge.

Normal gestations and live births after orthotopic autograft of vitrified–warmed hemi-ovaries into ewes

BACKGROUND

The aim of this study was to evaluate the long-term outcome of autotransplantation of vitrified warmed hemi-ovaries into ewes.

METHODS

Six hemi-ovaries from six ewes aged 6 to 12 months were vitrified. After dissection of the medulla, the hemi-ovarian cortex was stored at -196 degrees C in liquid nitrogen. Four to six weeks after the first laparotomy, the left ovary was removed and the vitrified-warmed hemi-ovary was sutured.

RESULTS

Plasma progesterone concentration increased in a regular manner in all ewes. Three pregnancies occurred, from which four lambs were born. The first delivery of a normal lamb occurred in February 2003. The second delivery of two normal lambs occurred in March 2003 (a 2.5 kg male and a 2.8 kg female). The last lamb had a normal delivery but had a malformation of the left leg and the oesophagus. This lamb died two months after delivery from pneumariae. Histological examination of the grafted vitrified ovaries showed few primordial and antral follicles.

CONCLUSIONS

These three pregnancies in a ewe model may indicate that ovarian vitrification gives results as good as those from a slow cooling protocol in autograft. It is impossible to establish a link between the vitrification procedure and the malformation of the last lamb, and further studies are needed to evaluate the feasibility of ovarian vitrification.

Spindle organization after cryopreservation of mouse, human, and bovine oocytes

Oocyte cryopreservation would alleviate a number of ethical, social, and religious problems associated with human embryo storage. One potential problem is the effect of cryopreservation on the metaphase II spindle and chromosomes. The microtubules that make up the spindle tend to depolymerize at sub-physiological temperatures. Although there are numerous reports in the literature on this topic, discrepancies as to whether the spindle can or cannot reform persist. One of the confounding factors may be the low cryosurvival rates (around 50%) for mammalian oocytes. In recent years, a cryopreservation medium and protocol have been developed that allow oocytes of several species to be cryopreserved with high survival rates (>85%). Bovine, mouse, and human oocytes consistently reformed a morphologically normal spindle with chromosomes aligned along the metaphase plate (70% or higher) after first surviving cryopreservation (>87% survival for all species tested). Normal chromosome numbers were found in every second polar body tested by FISH (second polar bodies n = 4). It is concluded that the mammalian spindle, although depolymerized during cryopreservation, has the ability to reform, and in the mouse has been shown to function normally. Therefore, spindle reformation may not be a major cause for concern when storing mammalian MII oocytes.

Effect of cryoprotectants and their concentration on in vitro development of vitrified-warmed immature oocytes in buffalo (Bubalus bubalis)

Experiments were conducted to study the effect of cryoprotectants, dimethyl sulfoxide (DMSO), ethylene glycol (EG), 1,2-propanediol (PROH), and glycerol at different concentrations (3.5, 4, 5, 6, and 7 M each with 0.5 M sucrose and 0.4% BSA in DPBS) on survival, in vitro maturation, in vitro fertilization, and post-fertilization development of vitrified-thawed immature buffalo oocytes. The COCs were harvested from the ovaries by aspirating the visible follicles. The recovery of post-thaw morphologically normal oocytes was lower in 3.5 and 4 M DMSO, EG, and PROH compared to 5, 6, and 7 M. In all the concentrations of glycerol, an overall lower numbers of oocytes recovered were normal compared to other cryoprotectants. Less number of oocytes reached metaphase-II (M-II) stage from the oocytes cryopreserved in any of the concentrations of DMSO, EG, PROH, and glycerol compared to fresh oocytes. Among the vitrified groups, highest maturation was obtained in 7 M solutions of all the cryoprotectants. The cleavage rates of oocytes vitrified in different concentrations of DMSO, EG, PROH, and glycerol were lower than that of the fresh oocytes. The cleavage rates were higher in oocytes cryopreserved in 6 and 7 M DMSO, EG, PROH, and glycerol compared with oocytes cryopreserved in other concentrations. However, the percentage of morula and blastocyst formation from the cleaved embryos did not vary in fresh oocytes and vitrified oocytes. In conclusion, this report describes the first successful production of buffalo blastocysts from immature oocytes cryopreserved by vitrification.

An overview of oocyte cryopreservation

The ability to cryopreserve human oocytes and store them indefinitely would be beneficial for cancer patients at risk of becoming sterile after therapy, allow women to delay reproduction, and alleviate religious concerns associated with embryo storage. In 1986, Chen was the first to report a pregnancy originating from a frozen-thawed human oocyte. Although over 100 babies have been born from oocyte storage since then, pregnancy rates remain unacceptably low. Adapting embryo cryopreservation techniques to oocyte storage has had limited success and new reproducible methods are needed. Problem areas other than intracellular ice formation and osmotic effects need to be identified. A broad approach of critical analysis should be conducted regarding the entire cryopreservation process from pre-equilibration and cooling, to thawing and stepout. All established facets deserve reanalysis in order to assess which aspects can be optimized or changed so that cellular demise can be avoided and cellular viability enhanced. New methods, including the use of choline-based media and vitrification have proven useful in increasing survival and pregnancy rates in some clinics. Other methods yet untested, such as injection of complex carbohydrates into the oocyte, deserve further studies. Vitrification research has led to the formulation of new ideas and has demonstrated the flexibility of cells to survive cryopreservation. Although successful, vitrification protocols are potentially harmful and technically challenging, due to elevated cryoprotectant concentrations and rapid cooling rates. Bovine embryo vitrification methods have been used to store human oocytes and embryos, particularly blastocysts with some success. Vitrification solutions containing high molecular weight polymers have also proved beneficial by reducing solution toxicity. In general, further advances are needed to improve human oocyte storage before widespread routine clinical use.

High Survival Rate of Bovine Oocytes Matured In Vitro Following Vitrification

Improving pregnancy rates associated with the use of cryopreserved human oocytes would be an important advance in human assisted reproductive technology (ART). Vitrification allows glasslike solidification of a solution without ice crystal formation in the living cells. We have attempted to improve the survival rates of oocytes by a vitrification technique using bovine models. In vitro matured oocytes with or without cumulus cells were vitrified with either 15.0% (v/v) ethylene glycol (EG) + 15% (v/v) dimethylsulfoxide (DMSO) + 0.5 M sucrose or 15% (v/v) EG + 15% (v/v) 1,2-propanediol (PROH) + 0.5 M sucrose, using 'Cryotop' or 'thin plastic sticker', respectively. The oocyte survival rates after vitrifying-warming, and the capacity for fertilization and embryonic development were examined in vitro. The rate of embryonic development to blastocyst was significantly higher (P<0.05) in the oocytes vitrified with 15% (v/v) EG + 15% (v/v) PROH + 0.5 M sucrose than in the oocytes vitrified with 15% (v/v) EG + 15% (v/v) DMSO + 0.5 M sucrose (7.4% +/- 4.1 vs. 1.7% +/- 3.0, respectively). Oocytes vitrified without cumulus cells had a higher survival rate after thawing and a superior embryonic developmental capacity compared with oocytes vitrified with cumulus cells. Prolonged pre-incubation time after thawing adversely affected the rates of embryonic cleavage and development. These results indicate that in vitro matured bovine oocytes can be vitrified successfully with the mixture of the cryoprotectants, EG + PROH, the absence of cumulus cells for vitrification does not affect oocyte survival rate after warming, and vitrified and warmed oocytes do not require pre-incubation before in vitro fertilization.

Effects of cryopreservation on meiotic spindles of oocytes and its dynamics after thawing: clinical implications in oocyte freezing--a review article

Embryo freezing has been a successful practice, but oocyte cryopreservation formerly achieved poorer results. This was mainly due to low rates of survival, fertilization, and development. The major dissimilarities for oocytes to embryos are the character of the plasma membrane, the presence of cortical granules, at the metaphase of meiosis II with the spindle system. In addition, the oocytes must be fertilized by sperm at the appropriate time. To improve the survival rate, a refined slow freezing method with increased sucrose concentration would dehydrate oocytes more sufficiently. Vitrification is another approach to prevent ice crystal formation. Intracytoplasmic sperm injection is used to overcome possible zona hardening from the release of cortical granules. The microtubules of meiotic spindles are vulnerable to the thermal changes and would depolymerize. Cryopreserved oocytes exhibited serious disturbances of the microtubules immediately after thawing. Fertilization of oocytes with disorganized spindles could lead to chromosomal aneuploidy, digyny, and arrest of cleavage. After incubation, the microtubules would repolymerize in a time-dependent way. Normal fertilization and development of cryopreserved oocytes improved after appropriate incubation and timing of insemination, compatible with recovery of the spindles. With the improvement of survival, fertilization, and cleavage, oocyte cryopreservation would gain an imperative role.

Detection of DNA damage in bovine metaphase II oocytes resulting from cryopreservation

Developmental competence of mammalian oocytes is compromised by currently available oocyte cryopreservation protocols. Experiments were designed to examine the effect of three cryopreservation protocols on the integrity of bovine oocyte DNA. In vitro matured bovine oocytes were cryopreserved either by slow cooling, vitrification in 0.25 ml straws, or in open pulled straws. After thawing/warming, recovered oocytes were immediately subjected to morphological evaluation. Morphologically intact oocytes underwent comet assay to detect cryoinjury at DNA level. All cryopreservation protocols resulted in significant morphological damage as well as DNA damage compared to unfrozen control. Among the morphologically intact oocytes, there was no difference among protocols in the number of oocytes displaying DNA damage. However, oocytes that had been cryopreserved by slow cooling or by vitrification in open pulled straws exhibited more damage than those vitrified in 0.25 ml straws in the extent of DNA damage. If we combine the number of oocytes with morphological damage and oocytes with DNA damage, oocytes cooled by slow cooling resulted in the most damage. This experiment demonstrated that oocyte DNA is a target of cryoinjury and different protocols result in different degrees of damage.

Fetal development of mouse oocytes and zygotes cryopreserved in a nonconventional freezing medium

This study (1) analyzed fetal development of mouse embryos after oocyte cryopreservation in CJ2, a choline-based medium, (2) examined the effect of culture duration in vitro on subsequent fetal development, and (3) compared survival and fetal development of zygotes frozen in embryo transfer freeze medium (ETFM; sodium-based medium) or CJ2. Unfertilized oocytes and zygotes were cryopreserved using a slow-cooling protocol. After thawing, oocytes were inseminated after drilling a hole in their zona, cultured in vitro either to the two-cell or blastocyst stage, and transferred to the oviducts or uterine horns of recipient mice. In parallel experiments, frozen-thawed zygotes were similarly cultured and transferred. Implantation rates for transferred embryos were high (range 66-88%), regardless of whether they had been frozen as oocytes or zygotes and whether they had been transferred to the oviduct or uterus. However, fetal development was significantly higher when two-cell embryos were transferred. With blastocyst transfer, control embryos implanted and produced a greater proportion of fetuses than did oocytes frozen in CJ2, whereas transfer at the two-cell stage resulted in similar proportions of implantation sites and fetuses. Blastocyst transfer of zygotes cryopreserved in ETFM or CJ2 produced similar fetal development rates (23.6% vs 20.0%), but when frozen-thawed zygotes were transferred at the two-cell stage the fetal development rates were higher in the ETFM group (53.3%) than in the CJ2 group (32.0%). A high proportion (46.7%) of oocytes frozen in CJ2 in a nonprogrammable freezer and plunged at -20 degrees C developed into live offspring. This study shows that in the mouse (1) oocytes frozen in CJ2 can develop into viable fetuses, (2) prolonging culture in vitro has a detrimental effect on embryo transfer outcome, and (3) CJ2 offers no advantage for zygote cryopreservation.

Cryopreservation of oocyte and ovarian tissue

Cryopreservation of reproductive cells (i.e., oocytes, spermatozoa) and tissues (i.e., ovarian and testicular tissue) is a developing technology that has tremendous implications for rapid advancement of biomedical research in general. Since the early 1980s, advances have been made in establishing optimal conditions for in vitro oocyte maturation, fertilization, and culture of resulting embryos. These in vitro systems have contributed significantly to the utilization of these cells and tissues after thawing and have made it possible to evaluate protocols designed to cryopreserve such biomaterials more effectively. Although cryopreservation of preimplantation embryos from various species including mouse, human, and farm animals has been successful, cryopreservation of oocytes from most mammalian species has been more challenging due to their extreme sensitivity to suboptimal conditions during the cryopreservation process. Cryopreservation on mouse oocytes have been well documented and have resulted in greater success than studies with other mammalian species. Ovarian tissue cryopreservation and transplantation techniques have recently received much scientific and public attention due to their great potential use in human infertility treatment, in safeguarding the reproductive potential of the endangered species, and in genome banking of genetically important lab animal strains. A review of past and current research in the field of oocyte and ovarian tissue cryopreservation and transplantation and discussion of possible strategies for oocyte and ovarian tissue banking are provided.

The initial phase of embryonic patterning in mammals

Although specification of the antero-posterior axis is a critical intial step in development of the fetus, it is not known either how, or at what stage in development, this process begins. Such information is vital for understanding not only normal development in mammals but also monozygotic twinning, which, at least in man, is associated with a significantly increased incidence of birth defects. According to recent studies in the mouse, specification of the fetal anteroposterior axis begins well before gastrulation, and probably even before the conceptus implants. Moreover, evidence is accruing that the origin of relevant asymmetries depends on information that is already present in the zygote before it embarks on cleavage. Hence, early development in mammals does not differ as markedly from that in other animals as has generally been assumed. Consequently, at present, the possibility of adverse effects of techniques used to assist human reproduction cannot be disregarded.

Sugars exert a major influence on the vitrification properties of ethylene glycol-based solutions and have low toxicity to embryos and oocytes

A systematic approach was taken to assess the vitrification properties of ethylene glycol-based solutions supplemented with carbohydrates. Solutions were prepared by weight (gravimetrically) using ethylene glycol as the cryoprotectant, 0.9% NaCl in water, and six different sugars: d-glucose, d(-)-fructose, d-sorbitol, sucrose, d(+)-trehalose, and raffinose. Sugars were added on a molal basis (0. 1, 0.5, and 1 m). Characteristics of the solutions were measured during warming by differential scanning calorimetry using a cooling rate of 100 degrees C/min and a warming rate of 10 degrees C/min. In the absence of carbohydrates a 59 wt% EG-saline solution formed a stable glass. When EG was replaced by an equimolal concentration of glucose, fructose, or sorbitol (monosaccharides) at 0.1, 0.5, or 1.0 m there was no change in the total solute concentration at which vitrification occurred, but the glass transition (Tg) occurred at a higher temperature than in EG-saline alone. When EG was replaced by an equimolal concentration of sucrose or trehalose (disaccharides) both the Tg and the lowest total solute concentration required for vitrification became progressively higher as the molecular weight, or the ratio of sugar to EG in the solutions, increased. At the highest tested disaccharide concentration (1 m) vitrification was achieved at a total solute concentration of 65 wt% (sucrose) and 67 wt% (trehalose). The polysaccharide raffinose significantly modified the vitrification properties of ethylene glycol solutions. When 0.5 or 0.1 m raffinose replaced EG on an equimolal basis the glass transition point was raised more than with either the monosaccharides or the disaccharides. Raffinose allowed vitrification at a total solute concentration of 67 wt% (0.5 m) and 63 wt% (0.1 m). The maturation of immature mouse oocytes, and the development of embryos in media containing 5-7 mM of any sugar was comparable to controls, indicating that they are not toxic. Exposure of freshly collected GV or MII oocytes to sugar concentrations between 0.5 and 1.0 M, for up to 10 min had no significant effect on the proportion which subsequently formed two cells. We conclude that added sugars do contribute to a solutions overall vitrification properties, and their properties should be taken into consideration when vitrification solutions are being designed or modified.

Effect of cryoprotectants and their concentration on post-thaw survival and development of expanded mouse blastocysts frozen by a simple rapid-freezing procedure

Experiments were conducted to develop a simple rapid-freezing protocol for expanded mouse blastocyst-stage embryos. The effect of type of cryoprotectant (ethylene glycol and propylene glycol) and its concentrations (4.5, 6.0 and 7.0 mol/L each with 0.5 mol/L sucrose) on morphological survival and development in vitro were studied. The survival and development of embryos frozen with best concentration of each cryoprotectant pre-exposed to either a low concentration (1.5 mol/L with 0.25 mol/L sucrose) of the respective cryoprotectant or ascending concentrations of sucrose were also compared. The in vivo development of embryos frozen with best protocol (pre-exposure to 1.5 mol followed by 7.0 mol ethylene glycol) was compared with nonfrozen embryos. The rate of re-expansion and hatching was influenced by the type and concentration of the cryoprotectant. A significantly higher re-expansion and hatching rate was achieved at 7.0 mol of both cryoprotectants compared with 4.5 and 6.0 mol of the respective cryoprotectants. When comparing 2 cryoprotectants, a higher (P < 0.05) rate of hatching was obtained with ethylene glycol at 7.0 mol compared with a similar concentration of propylene glycol. The highest re-expansion (91%) and hatching (86%) of expanded blastocysts was achieved with pre-exposure of embryos to a low concentration of ethylene glycol followed by freezing in the same cryoprotectant at 7.0 mol. The transfer of embryos frozen using this protocol resulted in the development of live fetuses. The proportion of live fetuses in the pregnant recipients with frozen-thawed embryos were not different from those transferred nonfrozen embryos (49 vs 57%). It may be concluded that simple rapid-freezing with dehydration in ascending sucrose concentrations or pre-equilibration in a low concentration of ethylene glycol or propylene glycol followed by exposure to the respective cryoprotectant at 7.0 mol resulted in high survival and development of expanded blastocysts. Ethylene glycol at 7.0 mol with pre-equilibration is, however, most effective for cryopreservation of this stage in the mouse.

Vitrification of mature mouse oocytes in a 6 M Me2SO solution supplemented with antifreeze glycoproteins: the effect of temperature

Oocytes have been successfully cryopreserved using rapid and slow freezing procedures. However, variability in the success of replicates has limited its practical application. In the present study, mature mouse oocytes were vitrified in 6 M dimethyl sulfoxide supplemented with 1 mg/ml antifreeze glycoproteins (AFGP) (solution known as VSD + AFGP) from the blood of Antarctic notothenioid fish. Such AFGPs have been used to protect mammalian cells during hypothermia and cryopreservation. However, the degree of protection afforded is a contentious issue. Stepwise addition of cryoprotectant was performed either at room temperature (19-21 degreesC) or on ice (2-4 degreesC), at the final stage of which oocytes were pipetted into 0.25 ml plastic insemination straws and held in liquid nitrogen vapor at -140 degreesC for 3 min before being plunged into liquid nitrogen. Thawing involved holding the straw in the air for 10 s and then in water at 20 degreesC for 10 s before dilution of the VSD solution with 1 M sucrose. Viability was assessed by in vitro fertilization; results have been quoted as median (range). Statistical analyses were performed using Kruskall-Wallis and Mann-Whitney U tests (P < 0.05). Of the oocytes cryopreserved following exposure to VSD + AFGP at room temperature (n = 518, 15 experimental runs), 78% (0-94%) retained normal morphology and, of these, 53% (0-100%) cleaved to two cells. Of these two-cell embryos, 56% (0-100%) went on to develop to blastocyst. The overall percentage development to blastocyst, i.e., number of blastocysts/total number of oocytes treated x 100, was 20% (0-76%). Exposure of oocytes to the VSD + AFGP on ice prior to cryopreservation yielded significantly improved rates of fertilization (94%, 82-100%) and overall development to blastocyst (66%, 24-89%) when compared with oocytes cryopreserved following exposure to the VSD + AFGP at room temperature. Rates of normality (86%, 35-95%) and development to blastocyst (89%, 64-100%) were also improved. Cryopreservation in 6 M dimethyl sulfoxide supplemented with 1 mg/ml AFGP resulted in poor rates of survival which were highly variable when exposure to cryoprotective agent (CPA) was performed at room temperature. Lowering the temperature of exposure to CPA prior to cryopreservation resulted in improved viability.

Effects of the in vitro chemical environment during early embryogenesis on subsequent development

The development of the preimplantation embryo seems morphologically very simple, and embryologists previously assumed that an embryo that developed to the blastocyst stage was fully capable of normal development after transfer to the uterus of a recipient female. This complacency was disturbed by reports that exposure of early embryos to mutagens such as methylnitrosourea led to fetal abnormalities, decreased birth rates, and decreased life-span. Even more disturbing are recent reports that culture of early embryos in supposedly benign conditions can adversely affect their subsequent development. Techniques have been developed for the production of cattle and sheep embryos by in-vitro fertilization and by cloning. Such embryos must be cultured for several days before they can be transferred, and, in some cases, this has been related to abortion, very high birthweight, physical abnormalities and peri-natal mortality of the calves and lambs. This syndrome may result from an unbalanced development of the trophoblast relative to the inner-cell mass, possibly related to the presence of serum, glucose, or ammonium in the culture medium. An analogous phenomenon has been observed in human in-vitro fertilization where babies from single pregnancies have below-normal birth-weight. There is also evidence to suggest that the in-vitro environment of the gametes before fertilization can affect subsequent embryonal and fetal development. Exposure of mouse oocytes to vitrification solutions has been shown to lead to fetal malformations, and treatment of bull sperm with glutathione improves early embryo development. The common thread in these diverse observations is that development can be affected by events that occur long before any defect is apparent. Consequently, the production of a morphologically normal embryo is no guarantee that fetal development and post-natal life will be normal. This is of immediate concern in human reproductive medicine due to the increasing use of sperm injection for fertilization, and the emergence of in-vitro oocyte maturation. Further development and application of reproductive techniques would benefit from a toxicological evaluation of risk factors and exposure limits.

Cryopreserved immature mouse oocytes: a chromosomal and spindle study

PURPOSE

Cryopreservation of human oocytes might provide an alternative approach to freezing supernumerary embryos obtained during IVF. This process, performed on immature denuded prophase 1 mouse oocytes, was investigated.

METHODS

We first investigated the capacity of frozen, immature, murine oocytes to continue in vitro maturation after thawing. We then evaluated the risk to offspring from chromosomal damage by cytogenetical and cytological (spindle) analysis. Finally, we attempted to determine the reasons for and the stage of maturation failure.

RESULTS

A total of 700 immature oocytes was frozen, 629 (90%) were recovered intact after thawing, and 53% extruded the first polar body, versus 74% for the control group. Freezing was not accompanied by an increase in aneuploidy in maturing oocytes (18 and 15% for thawed and control oocytes, respectively). Consequently, the first meiotic division occurred normally, without an increase in nondisjunction. Spindle analysis demonstrated only a few abnormalities (15 and 2% for thawed and control oocytes, respectively) incompatible with further development. Oocytes arrested during in vitro maturation were mainly at the metaphase I stage (64 and 76% for thawed and control oocytes, respectively). Whereas 17% of thawed oocytes were blocked before the formation of the first meiotic spindle, this never occurred in the control group.

CONCLUSIONS

Immature murine oocytes can withstand cryo-preservation, which is encouraging for future human application of this technique.

Vitrification of mature mouse oocytes: improved results following addition of polyethylene glycol to a dimethyl sulfoxide solution

Oocytes have been successfully cryopreserved using rapid and slow freezing procedures; however, variability in the success of replicates has limited its practical application. We have evaluated the potentially beneficial effects of adding 1 mg/ml of the polymer polyethylene glycol (PEG) (M(r) 8000) to a 6 M dimethyl sulfoxide (Me2SO) vitrification solution. Stepwise addition of cryoprotectant, either with or without PEG, was performed at room temperature (19-21 degrees C). Oocytes were then loaded in plastic insemination straws and held in liquid nitrogen vapour at -140 degrees C for 3 min prior to storage in liquid nitrogen. Oocytes were warmed rapidly to room temperature and removal of cryoprotective agent was effected in the presence of 1 M sucrose solution. Viability was assessed by vitro fertilization. Oocytes cryopreserved after exposure to 6 M Me2SO in the absence of PEG showed 60% normality, 80% fertilization, and 55% development to blastocyst, median of 11 replicate experiments (191 oocytes). Individual replicates yield highly variable survival which ranged from 0 to 100%. The addition of PEG significantly improved oocyte normality to 95% (range 76-100%; median of 9 replicate experiments, 301 oocytes). Rates of fertilization (91%; 60-100%) and development of blastocyst (73%; 67-92) were also improved. The addition of 1 mg/ml PEG to a 6 M Me2SO solution resulted in greatly improved viability of oocytes following cryopreservation and vastly reduced the variability seen with Me2SO solution alone.