Human oocyte cryopreservation: a valid alternative to embryo cryopreservation?

Vitrification within a nanoliter volume: oocyte and embryo cryopreservation within a 3D photopolymerized device

Purpose

Vitrification permits long-term banking of oocytes and embryos. It is a technically challenging procedure requiring direct handling and movement of cells between potentially cytotoxic cryoprotectant solutions. Variation in adherence to timing, and ability to trace cells during the procedure, affects survival post-warming. We hypothesized that minimizing direct handling will simplify the procedure and improve traceability. To address this, we present a novel photopolymerized device that houses the sample during vitrification.

Methods

The fabricated device consisted of two components: the Pod and Garage. Single mouse oocytes or embryos were housed in a Pod, with multiple Pods docked into a Garage. The suitability of the device for cryogenic application was assessed by repeated vitrification and warming cycles. Oocytes or early blastocyst-stage embryos were vitrified either using standard practice or within Pods and a Garage and compared to non-vitrified control groups. Post-warming, we assessed survival rate, oocyte developmental potential (fertilization and subsequent development) and metabolism (autofluorescence).

Results

Vitrification within the device occurred within ~ 3 nL of cryoprotectant: this volume being ~ 1000-fold lower than standard vitrification. Compared to standard practice, vitrification and warming within our device showed no differences in viability, developmental competency, or metabolism for oocytes and embryos. The device housed the sample during processing, which improved traceability and minimized handling. Interestingly, vitrification-warming itself, altered oocyte and embryo metabolism.

Conclusion

The Pod and Garage system minimized the volume of cryoprotectant at vitrification—by ~ 1000-fold—improved traceability and reduced direct handling of the sample. This is a major step in simplifying the procedure.

Supplementary information

The online version contains supplementary material available at 10.1007/s10815-022-02589-8.

Egg freezing experiences of women in Turkey: From the social context to the narratives of reproductive ageing and empowerment

This article explores egg freezing experiences of women in Turkey. Since 2014, it has been legal in Turkey to use egg freezing technology for ageing women, while it was previously allowed only for disease-related purposes. In cooperation with a private fertility clinic in Istanbul, the authors conducted 21 interviews with older, single women who held either professional or managerial positions and who were undergoing or had undergone the procedure. Drawing on a qualitative analysis of these interviews, the authors explore the social context in which women postpone motherhood and decide to freeze their eggs. The study also looks at the women’s emotional responses to ageing that were triggered by the experience of egg freezing and their narratives of empowerment as a result of the procedure.

Microfluidic Encapsulation of Ovarian Follicles for 3D Culture

The ovarian follicle that contains one single oocyte is the fundamental functional tissue unit of mammalian ovary. Therefore, isolation and in vitro culture of ovarian follicles to obtain fertilizable oocytes are regarded as a promising strategy for women to combat infertility. In this communication, we performed a brief survey of studies on microfluidic encapsulation of ovarian follicles in core-shell hydrogel microcapsules for biomimetic 3D culture. These studies highlighted that recapitulation of the mechanical heterogeneity of the extracellular matrix in ovary is crucial for in vitro culture to develop early pre-antral follicles to the antral stage, and for the release of cumulus-oocyte complex (COC) from antral follicles in vitro. The hydrogel encapsulation-based biomimetic culture system and the microfluidic technology may be invaluable to facilitate follicle culture as a viable option for restoring women's fertility in the clinic.

Evolution of human oocyte cryopreservation: slow freezing versus vitrification

PURPOSE OF REVIEW

The purpose is to determine the efficiency and efficacy of oocyte cryopreservation by slow freezing versus vitrification, recent data collected from the Italian National Assisted Reproductive Technology Register during the period 2009-2014 will be presented and reviewed. The data on oocyte cryopreservation were also compared with the results obtained with embryo cryopreservation and relative IVF with fresh oocytes.

RECENT FINDINGS

During the period 2009-2014 preservation of oocytes by vitrification had a significantly higher survival rate, implantation, and pregnancy rate than slow freezing; however, there are still large variations in success rates among centers in relation to the number of procedures performed.

SUMMARY

Vitrification has now become the method of choice for oocyte cryopreservation because of better results than slow freezing, but still requires a more standardized utilization. The transfer of fresh or cryopreserved embryo still shows a statistically significant better performance than transfers with embryos obtained with cryopreserved oocytes. Only in a few centers with much experience in cryopreservation are the results between transfers of frozen embryos or embryos obtained from oocyte cryopreservation comparable.

Fertility considerations and the pediatric oncology patient

Recent years have witnessed marked improvement in cytotoxic treatments with a parallel increase in patient survival. Despite efforts done to minimize long-term side effects of these treatment regimens, it is estimated that 40% of survivors of pediatric cancer will suffer from those. Some will be mild whereas others such as impaired fertility will be a heavy load on parents׳ expectations and patient׳s quality of life. Gonadal damage and severe loss of function is not a rare condition among children cured for cancer. Despite the young age of those patients, methods exist to try to reduce gonadal insult or to preserve gonadal function. Some of them are well studied and controlled; others are more experimental with encouraging results so far. This article aims to summarize all the procedures that can be offered to young patients treated for cancer in order to protect, as possible, their fertility potential.

Vitrification by Cryotop and the Maturation, Fertilization, and Developmental Rates of Mouse Oocytes

Background:

Oocyte cryopreservation is an important part of modern fertility treatment. The effect of vitrification on the fertilization and developmental rates of embryo is still a matter of debate.

Objectives:

This study aimed to investigate the effect of vitrification on the success of mouse oocyte maturation, fertilization, and preimplantation development in vitro.

Materials and Methods:

In this experimental study, a total of 200 germinal vesicle (GV) and 200 metaphase II (MII) oocytes were obtained from ovaries and fallopian tubes of NMRI mice, respectively and divided into two control and experimental (vitrified) groups. Oocytes in the experimental group were vitrified by Cryotop using vitrification medium (Origio, Denmark) and kept in liquid nitrogen for one month. Then, they were cultured in maturation medium for 24 hours. In vitro maturated metaphase 2 (IVM-MII) and ovulated metaphase 2 (OV-MII) oocytes were inseminated and the fertilized embryos assessed until the hatching blastocyst stage. Outcomes were assessed for statistical significance by Chi-square test using SPSS software.

Results:

Vitrification caused a significant reduction in the maturation rate of oocytes. Of those that matured, the fertilization rate of vitrified IVM-MII (44.1%) and OV-MII oocytes (50%) was not significantly different from each other but both were significantly lower than the control group (P < 0.05). There was no significant difference in developmental rates of both vitrified groups and the control group.

Conclusions:

The present study showed that vitrification using Cryotop and freezing medium can damage oocytes by reducing the maturation and fertilization rates in both developmental stages.

Improved low-CPA vitrification of mouse oocytes using quartz microcapillary

Cryopreservation by low-cryoprotectant (CPA) vitrification has the potential to combine all the advantages of the conventional high-CPA vitrification and slow-freezing approaches while avoiding their drawbacks. However, current low-CPA vitrification protocol for cryopreservation of oocytes requires a lengthy and multi-step procedure for unloading CPAs. In this study, we report a much-simplified procedure of using quartz microcapillary (QMC) for low-CPA vitrification of mouse oocytes with only one step for unloading CPAs. The immediate viability of oocytes after the improved low-CPA vitrification was determined to be more than 90%. Moreover, no significant difference was observed in terms of embryonic development from the two-cell to blastocyst stages between the fresh and vitrified oocytes after in vitro fertilization (IVF). This improved low-CPA vitrification technology has the potential for efficient cryopreservation of oocytes to preserve the fertility of mammals including humans for assisted reproductive medicine, maintenance of animal resource and endangered species, and livestock management.

Vitrification versus slow freezing for women undergoing oocyte cryopreservation

BACKGROUND

Oocyte cryopreservation is a technique with considerable potential in reproductive medicine, including  fertility preservation, as a way of delaying childbearing and as part of oocyte donation programs. Although the technique was relatively ineffective at first more recently numerous modifications have led to higher success rates.

OBJECTIVES

To compare the effectiveness and safety of vitrification and slow freezing as oocyte cryopreservation techniques for fertility outcomes in women undergoing assisted reproduction.

SEARCH METHODS

We searched electronic databases, trial registers and websites, including the Cochrane Menstrual Disorders and Subfertility Group Specialised Register of controlled trials, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and PsycINFO (date of search 3 March 2014).

SELECTION CRITERIA

Two review authors independently selected randomised controlled trials (RCTs) comparing vitrification and slow freezing for oocyte preservation in women undergoing assisted reproduction.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted the data from eligible studies and assessed their risk of bias. Any disagreements were resolved by discussion or by a third review author. Data extracted included study characteristics and outcome data. The overall quality of the evidence was assessed using GRADE methods.

MAIN RESULTS

Two RCTs were included in the review (106 participants). Neither study reported live birth rate. Vitrification was associated with an increased clinical pregnancy rate compared to slow freezing (RR 3.86, 95% CI 1.63 to 9.11, P = 0.002, 2 RCTs, 106 women, I(2) = 8%, moderate quality evidence). The effect of vitrification compared to slow freezing on ongoing pregnancy rates was only reported in one small study, with inconclusive findings (RR 6.07, 95% CI 0.86 to 43.04, P = 0.07, one RCT, 28 women, low quality evidence).No data were reported on adverse effects, nor were any other outcomes reported in the included trials. The evidence was limited by imprecision. We assessed the included studies as at low to unclear risk of bias as the methods were not well described.

AUTHORS' CONCLUSIONS

Oocyte vitrification compared to slow freezing probably increases clinical pregnancy rates in women undergoing assisted reproduction. However, the total number of women and pregnancies were low and the imprecision is high which limits applicability. The effect on ongoing pregnancy is uncertain as data were sparse. No data were available on live births or adverse effects.

Cumulative ongoing pregnancy rate achieved with oocyte vitrification and cleavage stage transfer without embryo selection in a standard infertility program

BACKGROUND

Recent advancement of minimum volume vitrification methods has resulted in a dramatic increase in the efficiency of the process. The aim of this study was to estimate the cumulative reproductive outcome of a cohort of infertile couples undergoing ICSI and oocyte vitrification in restrictive legal conditions, where only a limited number of oocytes could be inseminated per cycle and embryo selection and cryopreservation were forbidden.

METHODS

In this prospective longitudinal cohort study, the cumulative ongoing pregnancy rates obtained by the insemination of fresh and vitrified oocytes from the same cohort were calculated as primary outcome measures. Moreover, the effect of basal and cycle characteristics on clinical outcomes were assessed.

RESULTS

Between September 2008 and May 2009, 182 ICSI cycles were performed where oocyte vitrification was possible. A total of 104 first and 11 second oocyte warming cycles were then performed in non-pregnant patients of the same cohort. The overall ongoing pregnancy rates obtained in the fresh, and first and second warming cycles were 37.4, 25.0 and 27.3%, respectively. The overall cumulative ongoing clinical pregnancy rate observed per stimulation cycle was 53.3%. Maternal age was the only characteristic found to influence the reproductive outcome, with an inverse correlation between the age >40 and the ongoing pregnancy rates (P = 0.04, by Cox regression analysis).

CONCLUSIONS

High cumulative ongoing pregnancy rates can be obtained with transfers of embryos derived from fresh and cryopreserved oocytes in a typical infertile population. Female age significantly affects outcomes in this system.

Double activation improves rabbit freeze–thawed oocytes developmental potential

Objective: To investigate the effects of various activation methods on freeze–thawed rabbit oocytes developmental potential. Methods: Rabbit oocytes were vitrified by cryoleafs and cryoprotected with ethylene glycol and propanediol. After thawing, the oocytes were fertilized by intracytoplasmic sperm injection (ICSI). Surviving oocytes after ICSI were divided into five groups at random. Group 1: Oocytes (n = 30) activated 1 h after ICSI by calcium ionomycin (I0634); Group 2: Oocytes (n = 26) activated by strontium chloride an hour after ICSI; Group 3: Oocytes (n = 33) activated by I0634 twice; Group 4: Oocytes (n = 28) were activated by strontium chloride twice; Control Group: Inactivated oocytes (n = 39). Blastocysts derived from each group were transplanted to recipient rabbits. Results: Rates of fertilization, cleavage and blastocyst formation of Group 3 were higher than those of Group 1 and Group 2 (81.8% vs 33.3% vs 53.8%, 54.5% vs 16.7% vs 26.9%, p < 0.05; 15.2% vs 3.3% vs 7.7%, p > 0.05). The rabbit transplanted with embryos derived from Group 3 became pregnant. Embryos derived from double activation could implant into endometrium. Conclusion: Double activation may increase freeze–thawed oocytes developmental potential. After activation, oocytes cleavage velocity may be faster than that of oocytes without activation.

Medical and psychosocial aspects of fertility after cancer

Survival for cancer has improved significantly in the last 25 years with excellent overall 5- and 10-year survival rates. Hence, the majority of young women diagnosed with cancer can expect to live for decades, making quality of life issues such as fertility increasingly important. Risks of infertility vary based on the chemotherapy and treatments employed, as well as a woman's age. In general, younger women are less likely to experience permanent amenorrhea than older women, but even women who continue to menstruate have a greatly increased risk of premature menopause. Options for patients with cancer entering chemotherapy treatment, who wish to preserve fertility range from clinically well-established techniques such as embryo cryopreservation to more experimental techniques such as ovarian tissue cryopreservation. Pregnancy does not appear to increase the risk of cancer recurrence. Discussions of fertility issues in premenopausal women diagnosed with cancer present important challenges to the provider and to the patient. However, failure to discuss these options adequately can have lasting negative consequences on a woman's quality of life. Physician education interventions should seek to improve the knowledge of fertility preservations options, and of locally and nationally available resources.

Preservation of fertility in young cancer patients: contribution of transmission electron microscopy

During the last decade, new technologies in reproductive medicine have emerged to preserve the fertility of women whose gonadal function is threatened by premature menopause or gonadotoxic treatments. To offer an individualized approach to these patients, different experimental procedures are under investigation, including oocyte cryopreservation and cryopreservation and transplantation of ovarian tissue in the form of cortical fragments, whole ovary or isolated follicles. This review shows that transmission electron microscopy (TEM), combined with other in-vivo and in-vitro analysis techniques, is a valuable tool in the establishment of new experimental protocols to preserve female fertility. Ultrastructural studies allow in-depth evaluation of the oocyte's unique morpho-functional characteristics, which explain its low cryotolerance, and provide essential information on follicular, stromal and endothelial cell integrity, as well as cellular interactions crucial for normal folliculogenesis. In order to be able to offer appropriate and efficient options in every clinical situation, oocyte in-vitro maturation and ovarian tissue transplantation need to be optimized. Further development of new approaches, such as follicular isolation and whole ovary transplantation, should be encouraged. Fine ultrastructural details highlighted by TEM studies will be useful for the further optimization of these emerging technologies.

Oocyte cryopreservation: the birth of the first Hungarian babies from frozen oocytes

PURPOSE

To present data obtained with clinical application of oocyte cryopreservation.

METHODS

Slow freezing/rapid thawing in PBS based medium containing 1.5 M propanediol + 0.3 M sucrose.

RESULTS

A total of 127 embryos were transferred into 54 patients (1.9 embryo/cycle, 64 transfer cycles). Clinical pregnancy rate of 20% per cycle (13/64) and 24.0% per patient were achieved. Up-to-date, six patients delivered seven healthy babies; there are four ongoing pregnancies. Three abortions (23%) and one biochemical pregnancy (0.7%) was obtained. Implantation rates of 11% per transferred embryos (14/127) and 6.5% (14/215) per thawed eggs were found. In each case, normal karyotype was detected. No difference was found in the ratio of spindle positive oocytes at the polscope analyses done before and after freezing (75.8% vs. 82.5%).

CONCLUSION

Egg freezing is not a routine procedure yet, but there will certainly be a place for it in the future of assisted reproductive medicine.

Changes to the meiotic spindle and zona pellucida of mature mouse oocytes following different cryopreservation methods

This study is to investigate the change of morphology of the meiotic spindle and the extent of zona hardening relating to the morphological survival and developmental competence of thawed oocytes. Four- to 8-week-old female mice (C57BL/6) primed with an intraperitoneal injection of pregnant mare's serum gonadotropin and human chorionic gonadotropin. Cryopreserved oocytes using two protocols: vitrificaton using ethylene glycol (EG) and slow freezing using propanediol (PROH). The freezing oocytes were thawed and were fertilized and subsequently cultured in vitro. Spindle/chromosome imagery, dissolution of zona pellucida, and post-thawing survival and development were comparable between two groups. The vitrification cryopreservation method proved to be better than the slow-freezing protocol when comparing the frequency of normal-shaped spindle development post-thawing. The difference in the time required for the dissolution of the zona pellucida under treatment of pronase that was determined to exist between the two cryopreservation methods was statistically significant (P<0.005). The survival rate of post-thawed mature oocytes was significantly greater for the vitrification group than it was for the slow-freezing cryopreservation group (P=0.005). The vitrification cryopreservation of mature murine oocytes would appear to be more satisfactory than the slow controlled-rate freezing method as regards the post-thawing oocyte survival and also the incidence of the normal spindle apparatus in the ooplasm.

[Procedures for fertility preservation in female cancer patients]

Improving survival rates and quality of life following modern combined cancer treatments have resulted a growing number of patients requesting maintenance of reproductive functions. Several methods are currently available to maintain fertility during oncotherapy. Even though most of them are still experimental and their efficacy and safety have not been determined, the future for fertility preservation in women with cancer is promising. In vitro fertilization with embryo cryopreservation offers an established method, but time to delay cancer treatment could be risky regarding the progression of several cancer types. Moreover, exposure to a high oestrogen milieu during ovarian stimulation is undesirable when patients have estrogen-sensitive malignant tumours. Cryopreservation of mature oocytes following in vitro fertilization and intracytoplasmatic sperm injection offers advantages, but it is still limited due to its low success rate. Emerging techniques of ovarian tissue cryopreservation followed by autotransplantation have been clinically explored. Novel technologies of tissue freezing and thawing promise improving results. However, only one live birth following autotransplanted frozen-thawed ovarian tissue has been established. This procedure can be offered in the future for prepubertal girls before cancer treatment to maintain future fertility. Gonadal tissue cryopreservation and transplantation should be considered experimental in humans for the present time until greater evidence regarding efficacy and safety is accrued.

Delayed motherhood through oocyte and ovarian tissue cryopreservation - a perspective from Singapore

Recent advances in oocyte and ovarian tissue cryopreservation technology have brought hope not only to women facing premature loss of ovarian function, but also to healthy women seeking delayed motherhood. This is a major issue of contention in healthcare ethics. Proponents of this new technology argue that this enables women to pursue educational and career goals in their youth, so that they have greater financial security for children in later life. Nevertheless, this argument may be flawed by the reality that even if the cryopreservation of oocytes and ovarian tissue were optimized in the future, this would in no way be a guaranteed route for women to have biological children later in life. Moreover, because only a limited amount of autologous reproductive material can be cryopreserved and stored for a single healthy woman, there is a risk of material depletion before reproductive success is attained. Another prime consideration is the increased morbidity and mortality associated with clinical assisted reproduction in older women. Hence, it is imperative that delayed motherhood through the cryopreservation of oocytes and ovarian tissues be viewed with extreme caution, and mandatory counselling should be given to all patients if such a medical procedure ultimately finds approval for widespread application.

Progress with oocyte cryopreservation

PURPOSE OF REVIEW

This article reviews human oocyte cryopreservation, one of the most stimulating challenges of assisted reproduction technology. Since the first steps in assisted reproduction technology, researchers have pursued this goal, to greatly improve the management of infertility treatments. This present review depicts the present state of research and clinical applications of this methodology.

RECENT FINDINGS

Recent literature focuses on the possible mechanisms of oocyte damage caused by temperature and cryoprotectant injury and forecasts possible technological solutions. Several papers illustrate encouraging results in the increasing clinical application of this procedure.

SUMMARY

Findings give support to several indications of human female gamete cryostorage. Oocyte cryopreservation might replace embryo freezing. Egg freezing offers an alternative to women at risk of losing their reproductive function, caused by antineoplastic treatments, endometriosis, ovarian surgery or genetic premature ovarian failure. In addition, oocyte storage may contribute to an increase in in-vitro fertilization flexibility. Despite the early disappointing results, recent technical modifications have improved the clinical efficiency greatly, with the birth of several healthy children.

Efficiency of equilibrium cooling and vitrification procedures for the cryopreservation of ovarian tissue: comparative analysis between human and animal models

OBJECTIVE

To compare the efficiency of equilibrium cooling and vitrification for cryopreservation of human ovarian tissue and to determine the best experimental model for developing new protocols.

DESIGN

Experimental prospective study.

SETTING

An academic research environment.

PATIENT(S) AND ANIMAL(S)

Human ovarian biopsy specimens were obtained from three women undergoing operative laparoscopy for ovarian cyst enucleation. Adult cow and pig ovaries, collected at the abattoir.

INTERVENTION(S)

Ovarian tissue fragments of three individuals for each species were cryopreserved by using two protocols, either for equilibrium cooling or vitrification.

MAIN OUTCOME MEASURE(S)

Comparison between fresh and cryopreserved tissue of primordial, primary, and secondary follicle morphology, graded in three classes.

RESULT(S)

Human and bovine follicles responded in the same way to the two equilibrium cooling protocols, whereas pig tissue was more cryoresistant. Both vitrification protocols caused extensive damage to the tissue of all species. Human tissue showed a response to vitrification that was different from that of both animal species.

CONCLUSION(S)

Bovine is a good animal model for the development of human ovarian tissue cryopreservation protocols by equilibrium cooling procedures. Vitrification is less efficient than equilibrium cooling, and at present, neither bovine nor pig can be considered relevant animal models for human tissue.

Human oocyte cryopreservation: past, present and future

Despite inferior results in the past compared with embryo freezing, oocyte cryopreservation has made great strides in recent years. In fact, it has become a necessity in assisted reproduction technology, providing alternatives to legal, moral and religious problems originating from embryo freezing. Recent advances in freezing technology, modifications of conventional protocols used and continuing optimization of vitrification have efficiently improved the method. A historical description of the method's progression over time, and a comparison of principles, procedures and results as reported in the literature are presented in this review.

Fertility preservation for young patients with cancer: who is at risk and what can be offered?

Estimates suggest that by 2010, one in 715 people in the UK will have survived cancer during childhood. With increasing numbers of children cured, attention has focused on their quality of life. We discuss the causes of impaired fertility after cancer treatment in young people, and outline which patients are at risk and how their gonadal function should be assessed. With the report of a livebirth after orthotopic transplantation of cryopreserved ovarian tissue and the continued development of intracytoplasmic sperm injection for men with poor sperm quality, we assess established and experimental strategies to protect or restore fertility, and discuss the ethical and legal issues that arise.

The 2004 Italian legislation regulating assisted reproduction technology: a multicentre survey on the results of IVF cycles

BACKGROUND

The new Italian law, passed in 2004, regulating assisted reproduction technology imposes that no more than three oocytes can be fertilized at one time and that all embryos obtained must be transferred simultaneously. Oocyte cryopreservation is allowed while embryo cryostorage is banned. The aim of this study was to evaluate the clinical impact of these limitations.

METHODS

Seven Italian infertility centres were invited to collect data on IVF cycles performed over the first 4 months of application of the new legislation. As a control, all centres provided data on cycles performed in the same solar period, 1 year before.

RESULTS

Data from 1861 cycles were obtained, 961 in the pre-law period and 900 in the post-law period. Pregnancy rate per oocyte retrieval and rate of multiple pregnancies in the pre- and post-law periods were 27.0 and 24.2% (P=0.18) and 25.8 and 20.9% (P=0.11) respectively. However, the prohibition to freeze embryos does appear to have markedly reduced the cumulative rate of success.

CONCLUSIONS

The rate of success of IVF-ICSI cycles using fresh embryos is not significantly influenced by the new legislation while the prohibition to freeze embryos seems to result in a more relevant impact.

Observational clinical follow-up of oocyte cryopreservation using a slow-freezing method with 1,2-propanediol plus sucrose followed by ICSI

BACKGROUND

The value of oocyte cryopreservation remains controversial. Two major problems exist: poor survival and injury to the oocyte meiotic spindle after freezing and thawing.

METHODS

For slow oocyte cryopreservation, we used 1.5 mol/l 1,2-propanediol and 0.3 mol/l sucrose. We waited 3 h after thawing for possible recovery of the meiotic spindles before performing ICSI.

RESULTS

Forty-three women undergoing IVF or ICSI cycles cryopreserved some or all of their harvested oocytes; of these, 20 thawed their cryopreserved oocytes for personal use and one for donation. The survival rate of oocytes after thawing was 75%, with 67% of oocytes fertilizing normally after ICSI. All 21 cycles (100%) resulted in fertilization and embryo transfers. Seven pregnancies (33%) resulted. Four women delivered five babies with normal karyotypes. Three conceptions are ongoing. Compared to 38 cycles of frozen-thawed embryos at the pronuclear stage in the same period, the percentages of survival, pregnancy and implantation were similar. Additionally, four unmarried women with white blood cell diseases underwent oocyte freezing before preconditioning treatment for haematopoietic stem cell transplantation.

CONCLUSIONS

This protocol achieved reproducible success of survival, fertilization and pregnancy for freezing and thawing of human oocytes. The 3 h post-thaw incubation could permit restoration of the meiotic spindles, thus facilitating normal fertilization.

Recent developments in human oocyte, embryo and blastocyst vitrification: where are we now?

The target of any cryopreservation procedure should be to ensure high survival rates of living cells after thawing. Two important parameters determine the success of any cryopreservation protocol: the manner in which cells regain equilibrium in response to cooling, and the speed of freezing (cooling rate). Slow-rate freezing protocols result in the formation of ice crystals during cooling and warming. Vitrification, in which high cooling rates in combination with a high concentration of cryoprotectant are used, does not produce any ice crystals during cooling and warming. However, there is a practical limit to the attainable cooling speed, and also a biological limit to the concentration of cryoprotectant tolerated by the cells during vitrification. Although post-warming survival depends on the species, the developmental stage and the quality of the embryos being vitrified, it seems clear that vitrification methods are increasingly successful and might be a better method than slow cooling procedures in the field of cryobiology. Many of the potential problems and benefits underlying vitrification as a method of choice for embryo cryopreservation in clinical embryology will be discussed in this review.