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

At what age can human oocytes be obtained?

OBJECTIVE

To determine whether oocyte retrieval and in vitro maturation (IVM) is effective in girls undergoing fertility preservation before cancer treatment.

DESIGN

Cohort study.

SETTING

Tertiary university medical center.

PATIENT(S)

Patients <or=20 years old before gonadotoxic chemotherapy undergoing ovarian cortex cryopreservation.

INTERVENTION(S)

Before ovarian cortex cryopreservation, oocytes in all observed follicles were aspirated, matured in vitro, and cryopreserved.

MAIN OUTCOME MEASURE(S)

Maturation of oocytes.

RESULT(S)

One hundred seventy-nine oocytes were detected in 17/19 patients (89%) aged 5-20 years. We found 7, 8, and 17 oocytes in patients 5, 8, and 10 years old, respectively. The median number of oocytes per patient was 9 (0-37). Maturation rate was 45/133 oocytes (34%). In total, 81 oocytes were cryopreserved. We cryopreserved 4 of 12 detected, 4 of 9 detected, 1 of 8 detected, and 4 of 9 detected IVM oocytes for patients aged 5-10, 11-14, 15-17, and 18-20 years old, respectively.

CONCLUSION(S)

Patients undergoing ovarian cryopreservation could benefit from supplementary oocyte aspiration from the cortex. Surprisingly, oocytes were detected even in young premenarcheal girls. The number of oocytes detected, matured, and cryopreserved was not age dependent. Retrieved oocytes can be matured in vitro and cryopreserved. Because no pregnancy has yet resulted from this procedure it should be considered to be experimental. We describe the youngest patients to undergo ovum collection, IVM, and oocyte cryopreservation.

Comparison of survival and embryonic development in human oocytes cryopreserved by slow-freezing and vitrification

OBJECTIVE

To compare the survival, fertilization, early embryonic development, and meiotic spindle assembly and chromosome alignment in frozen-thawed human oocytes after slow-freezing and vitrification.

DESIGN

A randomized study.

SETTING

A university-affiliated assisted reproductive center.

PATIENT(S)

Donated extra eggs from women undergoing assisted reproduction treatment.

INTERVENTION(S)

A total of 605 mature oocytes were divided into a slow-freezing group and a vitrification group for cryopreservation.

MAIN OUTCOME MEASURE(S)

After frozen-thawing, the oocyte survival rate, spindle assembly, and chromosome alignment were compared. The surviving oocytes were inseminated by intracytoplasmic sperm injection, and the rate of fertilization and embryo development were also compared in two groups.

RESULT(S)

The oocyte survival rate was statistically significantly lower in the slow-freezing group (75 out of 123, 61.0%) than the vitrification group (268 out of 292, 91.8%). The fertilization rate was the same for both groups, but the cleavage rate of zygotes was statistically significantly different between two groups: (slow-freezing, 25/46 (54.4%) versus vitrification, 142 out of 182 (78.0%). There was a considerable difference in the percentage of high-quality embryos between slow-freezing and vitrification groups: 6 out of 25 (24.0%) versus 60 out of 142 (42.3%), respectively. The percentage of blastocyst development was statistically significantly higher in the vitrification group (47 out of 60, 33.1%) than in the slow-freezing group (3 out of 25, 12.0%). There was a much higher percentage of oocyte abnormalities in terms of spindle assembly and chromosome alignment in the slow-freezing group (25 out of 64, 39.1%) compared with the vitrification group (11 out of 62, 17.7%).

CONCLUSION(S)

Vitrification is superior to the slow-freezing method, leading to improved oocyte survival rate, fertilization, and embryonic development in vitro. These results may be related to vitrified human oocytes incurring less damage to spindle integrity and chromosome alignment.

Co-transfer of embryos derived from cryopreserved and fresh natural cycle oocytes: a pilot study

Italian legislation regarding reproductive medicine prohibits embryo storage while allowing cryopreservation of supernumerary oocytes. This study evaluated the effect of fresh oocytes obtained from natural unstimulated cycles on the clinical success rates derived from the use of frozen-thawed (FR-TH) oocytes obtained following ovarian stimulation. For 36 women, intracytoplasmic sperm injection was performed on FR-TH oocytes supplemented by a fresh oocyte, if available, derived from a natural cycle in which gonadotrophin-releasing hormone-antagonist was used for premature LH surge control. The retrieval rate of fresh oocytes was 61.1% and survival rate of FR-TH oocytes was 43.6%. The fertilization rate of fresh and FR-TH oocytes was 70% and 52.5%, respectively. Fifty embryos were transferred, 14 of them developed from fresh oocytes and 36 from FR-TH oocytes. Six pregnancies occurred in 10 cycles in which the embryos developed from fresh and FR-TH oocytes (pregnancy rate 60.0%) and two in 12 patients in whom the embryos were obtained from only FR-TH oocytes (pregnancy rate 16.7%) (P < 0.05). In summary, the data demonstrate that the transfer of embryos derived from oocytes cryopreserved following a previous ovarian stimulation and an embryo developed from a fresh one retrieved in natural cycle ensures an excellent clinical outcome.

Successful cryopreservation of mouse oocytes by using low concentrations of trehalose and dimethylsulfoxide

Sugars such as trehalose, sucrose, and glucose are effectively used by a variety of animals (e.g., brine shrimp, tardigrades, some frogs, and insects), as well as by bacteria, yeasts, and plant seeds to survive freezing and extreme drying. The objective of this study was to examine the potential application of sugars to mammalian oocyte cryopreservation. To this end, we used trehalose, a nonreducing disaccharide, and mouse metaphase II oocytes as models. Our experiments show that extracellular trehalose alone affords some protection at high subzero temperatures (e.g., -15 degrees C), which diminishes with further cooling of the oocytes to -30 degrees C and below. When present both intracellularly and extracellularly, trehalose dramatically improves the cryosurvival with increasing extracellular concentrations to 0.5 M, even after cooling to -196 degrees C. Furthermore, the combination of intracellular and extracellular trehalose with small amounts of a conventional penetrating cryoprotectant (i.e., 0.5 M dimethylsulfoxide) provide high survival, fertilization, and embryonic development rates statistically similar to untreated controls. When transferred to foster mothers, cryopreserved oocytes give rise to healthy offspring showing the proof of principle. Our experiments with differential scanning calorimetry indicate that when cooled using the same cryopreservation protocol, the mixture of 0.5 M trehalose and cryopreservation medium undergoes glass transition at high subzero temperatures, which further substantiates the use of sugars as intracellular and extracellular cryoprotectants. Taken together, our results are in agreement with the survival schemes in nature and demonstrate the successful use of sugars in cryopreservation of mammalian oocytes.

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.

Clinical efficiency of oocyte and embryo cryopreservation

The assessment of the standard for success of in vitro fertilization (IVF) treatment is an arduous task. Clinical efficiency and safety of a given procedure should represent fundamental tools for objective comparison. However, differences in patient populations, laboratory protocols, and expression of clinical data make the analysis of different studies and strategies very difficult. Formulation of the standard for success through the cumulative delivery rate per cycle of stimulation is a very attractive option because it includes the essential contribution of frozen embryos, which can represent 30-40% of all deliveries, while taking into account the need to minimize the proportion of pharmacological and surgical treatments. Embryo cryopreservation may be applied at different postinsemination stages. Larger and more detailed sets of data are available for day 2 embryo freezing, which allows cumulative delivery rates of 50-60% in good prognosis patients. In the last few years, novel freezing methods have improved the overall efficiency of oocyte cryopreservation. Especially in contexts afflicted by legal restrictions to embryo cryo- preservation, this form of preservation has started to have an impact on the IVF standard of success, generating cumulative pregnancy rates approaching 50%. Despite having been applied systematically by some IVF programs for only a few years, oocyte freezing already competes in efficiency with pronuclear-stage cryopreservation, and it does not appear unrealistic to predict that in the future it will challenge the dominance of embryo cryopreservation as the preferred form of conservation.

Human oocyte vitrification: the permeability of metaphase II oocytes to water and ethylene glycol and the appliance toward vitrification

OBJECTIVE

To determine the permeability of human metaphase II oocytes to ethylene glycol and water in the presence of ethylene glycol, and to use this information to develop a method to vitrify human oocytes.

DESIGN

An incomplete randomized block design.

SETTING

A university-affiliated assisted reproductive center.

PATIENT(S)

Women undergoing assisted reproduction in the Center for Reproductive Medicine at Shandong University.

INTERVENTION(S)

Oocytes were exposed to 1.0 molar ethylene glycol in a single step and photographed during subsequent volume excursions.

MAIN OUTCOME MEASURE(S)

A two-parameter model was employed to estimate the permeability to water and ethylene glycol.

RESULT(S)

Water permeability ranged from 0.15 to 1.17 microm/(min.atm), and ethylene glycol permeability ranged from 1.5 to 30 microm/min between 7 degrees C at 36 degrees C. The activation energies for water and ethylene glycol permeability were 14.42 Kcal/mol and 21.20 Kcal/mol, respectively.

CONCLUSION(S)

Despite the lower permeability of human metaphase II oocytes to ethylene glycol compared with previously published values for propylene glycol and dimethylsulfoxide, methods to add and remove human oocytes with a vitrifiable concentration of ethylene glycol can be designed that prevent excessive osmotic stress and minimize exposure to high concentrations of this compound.

Researching human oocyte cryopreservation: ethical issues

OBJECTIVE

To evaluate some ethical concerns related to the selection of participants for oocyte cryopreservation research.

DESIGN

Review of ethical issues related to human oocyte cryopreservation research.

SETTING

Academic medical center.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

A discussion of some ethical concerns surrounding selection of participants for oocyte cryopreservation research.

RESULT(S)

Human oocyte cryopreservation has the potential to expand reproductive options for infertile couples. Care needs to be taken, however, to carefully select participants in ways that maximizes benefits to them and to society and minimizes risks. Infertility programs can do so by trying to recruit women who stand to benefit the most from participating in oocyte cryopreservation protocols.

CONCLUSION(S)

Biomedical research is necessary to improve current medical therapies. Oocyte cryopreservation can increase the flexibility of assisted reproductive programs and offer hope to a significant number of infertile couples. Nonetheless, scientific research cannot be attentive only to the creation of new knowledge and new technologies; it also is inextricably tied to ethical considerations about the well-being of participants.

Therapy Insight: fertility in women after cancer treatment

Cancer is the second-commonest cause of death in women under 40 years of age in Western Europe and the US. The survival of cancer patients has, nevertheless, improved during the past two decades. During this period, and especially during the last decade, there have been ground-breaking advances in the optimization of the quality of life of patients treated for cancer, in particular by the development of fertility-enhancing and fertility-preserving procedures in young patients treated for cancer. Surgery, chemotherapy and radiation therapy affect the fertility potential of women in different ways. Surgery to remove the uterus and ovaries has a direct impact on fertility. Radiation therapy (external or brachytherapy) can affect ovarian and also uterine function. Different drugs used in chemotherapy can directly influence ovarian function. Some markers have now been evaluated that are predictive of the potential toxic injury to the gonads and uterus. Various procedures have been proposed to preserve the fertility potential in women before anticancer treatment begins or after the tumor is treated; however, such optimization of management should only be undertaken if it does not have a deleterious effect on the survival of the patient.

Evidence-based clinical outcome of oocyte slow cooling

In the last few years, there has been a significant improvement in oocyte cryopreservation techniques. To investigate the clinical significance of oocyte freezing, an assessment of the cumulative pregnancy rate per started cycle derived from the use of fresh and frozen-thawed oocytes was performed. Between 2004 and 2006, 749 cycles were carried out, in which no more than three fresh oocytes were inseminated either by standard IVF or microinjection. Supernumerary mature oocytes were cryopreserved by slow cooling. Cryopreservation of fresh embryos was performed in rare cases to prevent the risk of ovarian hyperstimulation syndrome using a standard embryo freezing protocol. Fresh embryo transfer cycles totalled 680, 257 of which resulted in pregnancy. The pregnancy rates per patient and per transfer were 34.3% and 37.8% respectively. When frozen-thawed oocytes were used, following 660 thawing cycles, 590 embryo transfers were performed in 510 patients. Eighty-eight pregnancies were achieved with embryos from frozen oocytes, with a success rate of 17.2% per cycle. When fresh and frozen-thawed cycles were combined, the number of pregnancies was 355, giving a cumulative pregnancy rate of 47.4%. Oocyte cryopreservation can contribute considerably to the overall clinical success, ensuring a cumulative rate approaching that achievable with embryo storage.

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.

Live birth after the transfer of human embryos developed from cryopreserved oocytes harvested before cancer treatment

OBJECTIVE

To document the first successful fertility preservation with freezing mature oocytes for a cancer patient.

DESIGN

Case report.

SETTING

Private IVF center.

PATIENT(S)

A woman with Hodgkin's disease and her gestational carrier.

INTERVENTION(S)

Metaphase II oocytes cryopreserved before cancer treatments were thawed and fertilized for transfer to a gestational carrier.

MAIN OUTCOME MEASURE(S)

Post-thaw survival and pregnancy.

RESULT(S)

Ten of the 12 metaphase II oocytes survived the thawing process. Nine oocytes were fertilized after microinjection with her husband's sperm. All fertilized oocytes developed into good quality embryos and were transferred to a gestational carrier in three cycles. A biochemical pregnancy was achieved after the second transfer. A singleton pregnancy was achieved after the last transfer of three embryos, resulting in the birth of a healthy male.

CONCLUSION(S)

Our findings suggest that oocyte cryopreservation should be considered as a preferred strategy for the cancer patient who has time to cryopreserve oocytes before cancer treatment.

Reconstruction of ooplasm recipient oocytes with frozen-thawed donor microcytoplasts and influence on the microtubular spindle

OBJECTIVE(S)

To cryopreserve micromanipulated ooplast segments (microcytoplasts) from mouse oocytes, compare microcytoplast and parent or recipient oocyte fusion performed within or without the zona pellucida, compare electrofusion of fresh or frozen oocyte with frozen-thawed microcytoplasts, and assess spindle integrity after reconstruction of oocytes.

DESIGN

Prospective experimental study.

SETTING

University-based experimental laboratory.

ANIMAL(S)

Mouse (MII) oocytes obtained after superovulation (n = 363).

INTERVENTION(S)

Micromanipulation of oocytes (n = 363) into microcytoplasts (n = 181), cryopreservation of microcytoplasts along with parent and sibling control oocytes (n = 182), reconstruction by electrofusion of microcytoplast and parent or recipient oocyte performed with (group A, n = 35) or without a zona pellucida (group B, n = 32), comparison of electrofusion of fresh oocyte (group C, n = 40) or frozen oocyte (group D, n = 36) with frozen-thawed microcytoplasts fused within zona, and assessment of spindle morphology of reconstructed oocyte.

MAIN OUTCOME MEASURE(S)

Post-thaw survival, success of fusion, and spindle integrity as assessed by immunostaining.

RESULT(S)

Higher success of post-thaw fusion was seen in group A (91.4%) compared with group B (56.2%). The post-thaw fusion of microcytoplasts with either fresh or frozen oocytes was not significantly different. Spindle integrity was 82.5% in group C as compared with 47.2% in group D.

CONCLUSION(S)

Microcytoplasts created from oocytes can be successfully cryopreserved, thawed, and used to reconstruct oocytes with intact spindles.

Live birth following transfer of a cryopreserved embryo generated from a cryopreserved oocyte and a cryopreserved sperm: case report

As is the case with non-frozen oocytes, the efficient and successful use of cryopreserved oocytes in human assisted reproduction is, in part, dependent on the ability to apply selection criteria when choosing the 'best' embryos for transfer from a cohort. In many cases this, in turn, will necessitate the cryopreservation of non-transferred embryos to minimise the risk of multiple pregnancy. It is therefore important to establish that an embryo, generated by fertilization of a frozen-thawed oocyte, can be capable of surviving subsequent cryopreservation while retaining the potential for normal development. In this case report, we document the delivery of a normal male infant following transfer of a frozen-thawed embryo, generated by the fertilization of a frozen-thawed oocyte by a frozen-thawed sperm.

Theoretical considerations for oocyte cryopreservation by freezing

Attempts to cryopreserve oocytes by freezing have, to date, been based mostly on empirical approaches rather than on basic principles, and perhaps in part for this reason have not been very successful. Theoretical considerations suggest some fairly 'heretical' conclusions. The concentrations of permeating cryoprotectants employed in past studies have probably been inadequate, and the choice of propylene glycol (PG) as a protective agent is questionable. The use of non-penetrating agents, such as sucrose to preshrink oocytes prior to freezing and which, therefore, exacerbate osmotic stress during freezing, may be inappropriate, yet may protect in part by reducing the concentration of PG during freezing. The methods used to add and remove cryoprotectant may be suboptimal, and may be based on an inadequate understanding of the cryobiological constraints for oocyte survival. Given these concerns, it is not surprising that fully satisfactory results have been elusive, but there is every reason to believe that greater success is possible using a more theoretically appropriate approach.

Differential sucrose concentration during dehydration (0.2 mol/l) and rehydration (0.3 mol/l) increases the implantation rate of frozen human oocytes

Novel protocols have increased survival and fertilization rates of cryopreserved oocytes. Nevertheless, in most cases clinical experiences have been disappointing or contradictory. Human oocytes of 141 patients were cryopreserved using a modified slow-cooling protocol involving 1.5 mol/l propane-1,2-diol (PrOH) and 0.2 mol/l sucrose during dehydration, while rehydration was conducted applying decreasing concentrations of PrOH and 0.3 mol/l sucrose. One thousand and eighty-three oocytes were frozen and 403 were thawed, with a survival rate of 75.9%. Among the 306 surviving oocytes, 252 were microinjected and 192 (76.2%) showed two pronuclei. One hundred and eighty zygotes (93.8%) cleaved. The proportion of good quality embryos (grade I and II) was 86.2%. All embryos were transferred and 17 clinical pregnancies were obtained. Pregnancy rates were 21.3% per transfer, 21.8% per patient, and 18.9% per thawing cycle. The implantation rate was 13.5% while the miscarriage rate was 11.8%. To date, four babies have been delivered, while the remaining pregnancies are ongoing. Increased oocyte survival rates can be achieved by moderately high sucrose concentrations in the freezing and thawing solutions. This also ensures elevated success rates in terms of fertilization, embryo development and clinical outcome.

Objective evaluation of the viability of cryopreserved oocytes

Recent studies of fundamental cryobiology, empirical observations and more systematic clinical experiences have generated a renewed interest in oocyte cryopreservation. Poor survival rate has long been the limiting factor which has prevented widespread adoption of oocyte storage. Slow-cooling and vitrification protocols developed in the last few years have apparently solved this problem, ensuring high recovery of viable oocytes from liquid nitrogen storage. However, the definition of oocyte viability appears rather vague. In fact, post-storage survival as assessed on morphological criteria, indicated by the absence of overt cell degeneration, is not necessarily synonymous with viability. Despite its sensitivity to low temperatures, the meiotic spindle can be preserved after cryopreservation and its constitution after thawing can be monitored non-invasively through polarized light microscopy. Assessment of oocyte cryopreservation via clinical parameters is a daunting task. Most studies are small and difficult to interpret because of confounding factors, such as age, patient selection and quality and strategy of use of the cryopreserved material. Some progress has been made, however, as suggested by recent experiences in which the implantation efficiency of embryos produced from thawed oocytes approaches that reported using cryopreserved embryos directly.

How should the clinical efficiency of oocyte cryopreservation be measured?

Clinical application of oocyte cryopreservation may be in the context of fertility preservation for women about to undergo cytotoxic therapies or may be as an alternative to embryo cryopreservation in routine assisted reproduction. The clinical efficiency of oocyte cryopreservation will be a consequence of the cumulative impact of pre-freeze oocyte quality, postthaw survival and subcellular effects of cryopreservation protocols, which impact on early embryo quality and post-transfer viability, together with the degree of selection which is applied to the available biological material. Any valid assessment must include reference to all the above aspects, particularly when comparing cryopreserved oocytes with non-frozen controls or cryopreserved embryos. Cumulative pregnancy rates from oocyte collections may provide the most relevant index of success. Survival of human oocytes cryopreserved using current methodology is similar to that achieved with early-cleavage-stage embryos. Although evidence suggests that developmental potential may be compromised when current oocyte cryopreservation protocols are applied, there is a paucity of rigorously controlled studies in the literature.

Oocyte cryopreservation: clinical outcome of slow-cooling protocols differing in sucrose concentration

Oocyte cryopreservation represents an important option for management of female fertility, avoiding the ethical concerns associated with embryo storage. This retrospective study evaluated the clinical outcome of two alternative slow freezing protocols involving different sucrose concentrations. From January 2004 to March 2006, spare oocytes from selected couples undergoing IVF or intracytoplasmic sperm injection were frozen using a slow-cooling protocol and thawed at a later stage. Patients were divided into two groups: group A (n = 65), whose oocytes were frozen with propane-1,2-diol (PrOH) and 0.1 mol/l sucrose; and group B (n = 66) whose oocytes were frozen with 0.3 mol/l sucrose. A total of 543 oocytes were thawed in group A and 601 in group B, achieving a survival rate of 24.3 and 71.2% respectively. Whilst fertilization rate (53.5 and 80.4% respectively) was higher in group B, enhanced results for group A were achieved over all (implantation rate per transferred embryos 12.2 versus 5.7%; pregnancy rate per transfer 16.7 versus 9.5%). Normal births and ongoing pregnancies have occurred in both groups. Although in slow-cooling methods higher sucrose concentration in the freezing mixture allows higher post-thaw survival and fertilization rates, overall this did not coincide with an improved clinical outcome.

Does oocyte cryopreservation have a future in Hungary?

The Committee of Human Reproduction established by the Hungarian Ministry of Health is currently working on a proposition that has the intention to ban oocyte cryopreservation in Hungary temporarily. According to the notion of the committee, oocyte cryopreservation and the utilization of frozen oocytes entail enormous risks for future generations. They argue that the safety of the method is unproven. Consequently, their standpoint maintains that cryopreservation invariably remains a grave risk factor with regards to the genetic material of the oocyte by significantly increasing the risk to the offspring. Therefore,the Committee is requesting additional animal experiments to clarify the probability of permanent lesions to the spindle and alterations to the genetic material. According to their point of view, the suspension of the method is further justified since the efficiency of the procedure has yet not reached the level required for clinical adaptation. With the proposed forthcoming step,this commentary takes the opportunity to make critical comments and clarify necessary questions.

Ultrastructure of human mature oocytes after slow cooling cryopreservation using different sucrose concentrations†

BACKGROUND

We studied the ultrastructural characteristics of human mature oocytes frozen/thawed (F/T) using different concentrations of sucrose. Fresh human mature oocytes were used as controls.

METHODS

The oocytes (n = 48) were fixed in 1.5% glutaraldehyde at sampling (n = 16) or after freeze/thawing performed using a slow cooling method with propane-1,2-diol 1.5 mol/l and sucrose at either 0.1 mol/l (n = 16) or 0.3 mol/l (n = 16) in the freezing solution. The oocytes were then processed for electron microscopy observations.

RESULTS

Fresh and F/T oocytes belonging to both study groups were regularly rounded in sections, with a homogeneous cytoplasm and an intact zona pellucida (ZP). Organelles (mainly mitochondria-smooth endoplasmic reticulum aggregates and mitochondria-vesicle complexes) were abundant and uniformly dispersed in the ooplasm. The amount and density of cortical granules appeared to be abnormally reduced in some F/T samples, independently of the sucrose concentration in the freezing solution: this feature was frequently associated with an increased density of the inner ZP, possibly related to the occurrence of zona 'hardening'. Furthermore, slight to moderate microvacuolization was revealed in the ooplasm of some F/T oocytes, particularly in those treated with sucrose 0.3 mol/l.

CONCLUSIONS

Freeze/thawing procedures are associated with ultrastructural alterations in specific oocyte microdomains, presumably linked to the reduced developmental potential of mature cryopreserved oocytes. Further work is needed to determine whether or not a high concentration of sucrose plays a role, at least in part, in producing the above alterations.

Oocyte cryopreservation

OBJECTIVE

To review historical and contemporary advances in oocyte-cryopreservation techniques and outcomes.

DESIGN

Publications related to oocyte cryopreservation were identified through MEDLINE and other bibliographic databases.

CONCLUSION(S)

Oocyte cryopreservation can be used as an adjunct to conventional IVF and as an option for fertile women to electively cryopreserve their gametes. Recent reports indicate pregnancy rates comparable to those for cryopreserved embryos by either slow-freeze or vitrification methods. Larger prospective trials are needed to determine the true efficacy and safety of oocyte cryopreservation. Until a sufficient number of births is reached and adequate outcome data are collected, oocyte cryopreservation should continue to be considered experimental and to be performed under the oversight of an institutional review board.

Novel direct cover vitrification for cryopreservation of ovarian tissues increases follicle viability and pregnancy capability in mice

BACKGROUND

Cryopreservation of ovarian tissue is valuable for fertility preservation. We develop an innovative vitrification method using less concentrated cryoprotectants and direct application of liquid nitrogen to the ovarian tissue (direct cover vitrification, DCV) to improve its efficiency.

METHODS

Ovaries of 5- to 6-week-old C57BL/6J mice were randomly allocated to four groups: DCV, conventional vitrification, slow-freezing and non-frozen controls. Experiment 1: observing the follicle morphology. Experiment 2: assessing viability. Experiment 3: investigating the ultrastructure. Experiment 4: examining the follicle number after grafting. Experiment 5: ascertaining pregnancy potential by allogeneic orthotopic transplantation.

RESULTS

The percentages of morphologically normal or viable follicles from DCV were significantly greater than those achieved from conventional vitrification and slow freezing (P < 0.01). The ultrastructure of primordial follicles from DCV appeared better than that achieved from conventional vitrification and slow freezing. After grafting, the follicle number from DCV was greater than conventional vitrification (P = 0.001) and slow freezing (P = 0.021). The pregnancy rate of DCV was higher than conventional vitrification (P < 0.01). The litter size from DCV was comparable with that from non-frozen graft and was significantly greater than that achieved from conventional vitrification and slow freezing (P < 0.01).

CONCLUSIONS

DCV is highly efficient for cryopreservation of ovarian tissue. Using less concentrated cryoprotectants appears to reduce toxicity. Direct cover by liquid nitrogen maximizes cooling that could facilitate vitrification and prevent ice crystal injury.

Successful delivery after the transfer of twice-vitrified embryos derived from in vitro matured oocytes: a case report

We report here the first case of successful pregnancy and delivery after the blastocyst transfer of twice-vitrified embryos produced following in vitro maturation (IVM) and ICSI. The patient received 5000 IU hCG on day 12 of the treatment cycle, and oocyte retrieval was carried out 36 h after hCG injection. A total of 22 immature oocytes were obtained. Following incubation for 26 h in IVM medium, 15 oocytes (68.2%) reached metaphase II stage. In total, 13 oocytes (86.7%) were fertilized after ICSI with the husband's sperm, and 11 embryos at the pronuclear stage and two cleaved embryos on day 2 were vitrified because of thin endometrial thickness. Eight cryopreserved embryos at the pronuclear stage were warmed and cultured until the day 3 stage. Three embryos were transferred, and three embryos were twice vitrified. Unfortunately, these transferred embryos did not implant. Three twice-vitrified embryos were rewarmed and cultured until the day 5 stage, and two embryos were transferred. The second transfer attempt of twice-vitrified embryos resulted in the full-term delivery of a healthy infant. This case report demonstrates that twice-vitrified embryos, developed using an IVM protocol, retain the developmental competence for full-term, healthy infants.

Human oocyte cryopreservation: 5-year experience with a sodium-depleted slow freezing method

A slow freezing/rapid thawing method for the cryopreservation of human oocytes has been employed using a sodium-depleted culture media. In 53 frozen egg-embryo transfer (FEET) cycles, a 60.4% survival rate post-thaw was obtained and a 62.0% fertilization rate following intracytoplasmic sperm injection. Overall pregnancy rates were 26.4% per thaw attempt, 30.4% per patient, and 32.6% per embryo transfer. Pregnancy rates using sodium-depleted phosphate-buffered saline (PBS) as the base medium were 20.0% per thaw, 21.7% per patient, and 26.3% per transfer. With sodium-depleted modified human tubal fluid (mHTF) as the base for the cryopreservation medium, rates were 32.1% per thaw attempt, 39.1% per patient, 37.5% per transfer. The overall implantation rates were 4.2% per thawed oocyte and 13.6% per embryo, (PBS: 3.0% per egg, 10.6% per embryo; mHTF:5.3% per oocyte; 15.9% per embryo). These data indicate that the use of a sodium-depleted media with slow freezing and rapid thawing can yield acceptable pregnancy rates after FEET.

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.

Efficiency of oocyte cryopreservation: a meta-analysis

OBJECTIVE

To determine the efficiency of oocyte cryopreservation relative to IVF with unfrozen oocytes.

DESIGN

Meta-analysis.

SETTING

Academic assisted reproduction center.

PATIENT(S)

Results of all reports from January 1997 to June 2005 with the patients undergoing IVF-intracytoplasmic sperm injection (ICSI) with cryopreserved cycles between 1996 and 2004 were compared with those of patients who underwent IVF-ICSI with unfrozen oocytes in 2002 and 2003 in our program.

INTERVENTION(S)

Mean age and number of ET cycles originating from unfrozen oocytes was matched with those for thaw cycles originating from oocytes cryopreserved with a slow-freezing (SF) protocol. Vitrification (VF) reports were not included in the comparative analysis because of a small number of pregnancies (10) before June 2005.

MAIN OUTCOME MEASURE(S)

The comparison of fertilization rate, clinical pregnancy, and live-birth rates per injected oocyte, clinical pregnancy and live-birth rates per transfer, and implantation rate between IVF-ICSI cycles with frozen and unfrozen oocytes.

RESULT(S)

Live-birth rates per oocyte thawed were 1.9% and 2.0% for SF and VF, respectively, before June 2005. Live-birth rates per injected oocyte and ET, respectively, were 3.4% and 21.6% for SF and were 6.6% and 60.4% for IVF with unfrozen oocytes. Compared to women who underwent IVF after SF, IVF with unfrozen oocytes resulted in significantly better rates of fertilization (odds ratio [95% confidence interval]); 2.22 (1.80, 2.74), of live birth per injected oocyte; 1.5 (1.26, 1.79), and of implantation; 4.66 (3.93, 5.52). These odds ratios were lower when oocyte cryopreservation success rates from 2002-2004 were compared with those for IVF with unfrozen oocytes. When the reports after June 2005 were considered, this trend did not appear to continue. With the consideration of VF reports after June 2005, however, higher pregnancy rates were achieved.

CONCLUSION(S)

In vitro fertilization success rates with slow-frozen oocytes are significantly lower when compared with the case of IVF with unfrozen oocytes. Although oocyte cryopreservation with the SF method appears to be justified for preserving fertility when a medical indication exists, its value for elective applications remains to be determined. Pregnancy rates with VF appear to have improved, but further studies will be needed to determine the efficiency and safety of this technique.

Cumulative pregnancy rates resulting from the use of fresh and frozen oocytes: 7 years' experience

Storing supernumerary embryos and transferring them later fully utilizes the reproductive potential of retrieved oocytes, allowing a significant increase in the overall number of pregnancies achieved from a single cycle of ovarian stimulation treatment. As an alternative to embryo cryopreservation, preservation of unfertilized oocytes has been proposed to maximize clinical outcome. This paper presents data concerning the cumulative pregnancy rate after use of fresh and cryopreserved oocytes. In 80 treatment cycles in which patients chose to have only a few fresh oocytes inseminated, 24 pregnancies were obtained (30.0%), with an implantation rate of 22.6%. After cryopreservation with the standard slow-cooling protocol, the survival, fertilization and cleavage rates of 918 frozen oocytes were 43.4, 51.5 and 86.0% respectively. A total of 14 frozen pregnancies were achieved, with pregnancy rate 19.2% per transfer and implantation rate 12.3%. The cumulative pregnancy rate was 47.5% per patient. Therefore, despite a low rate of oocyte post-thaw survival, it appears that oocyte storage appreciably improves the number of pregnancies per treatment cycle in cases in which only a minority of oocytes are destined for the fresh treatment. This outcome provides valuable information for appraising the chances of clinical success when the option of embryo cryopreservation is not available.

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.

Preservation of female germ cells from ovaries of cat species

This review provides an overview on recent knowledge on female germ cell population within cat ovaries; on isolation, culture and cryopreservation of feline preantral follicles and on ovarian tissue preservation.

Sucrose concentration influences the rate of human oocytes with normal spindle and chromosome configurations after slow-cooling cryopreservation

BACKGROUND

Recently described slow-cooling cryopreservation protocols involving elevated sucrose concentration have improved survival frequencies of human oocytes, potentially overcoming a major hurdle that has limited the adoption of oocyte storage. Because implantation rates of embryos from frozen oocytes remain generally low, it is still debated whether, irrespective of survival rates, this form of cryopreservation leads inevitably to the disruption or complete loss of the metaphase II (MII) spindle.

METHODS

Human oocytes with an extruded polar body I (PBI) were cryopreserved using a slow-cooling method including 1.5 mol/l propane-1,2-diol (PrOH) and alternative sucrose concentrations (either 0.1 or 0.3 mol/l) in the freezing solution. Fresh control and frozen-thawed survived oocytes were analysed by confocal microscopy to evaluate MII spindle and chromosome organizations.

RESULTS

Of the 104 oocytes included in the unfrozen group, 76 (73.1%) displayed normal bipolar spindles with equatorially aligned chromosomes. Spindle and chromatin organizations were significantly affected (50.8%) after cryopreservation involving lower sucrose concentration (61 oocytes), whereas these parameters were unchanged (69.7%) using the 0.3 mol/l sucrose protocol (152 oocytes).

CONCLUSIONS

Partial disruption of the MII spindle and associated chromosomes accompanies inadequate cryopreservation during slow cooling. However, protocols adopting higher sucrose concentration in the freezing solution promote the retention of an intact chromosome segregation apparatus comparable in incidence to freshly collected oocytes.

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.

Clinical outcome of oocyte cryopreservation after slow cooling with a protocol utilizing a high sucrose concentration

BACKGROUND

Recently, interest in oocyte cryopreservation has steadily increased. Newly developed protocols have dramatically improved survival rates, removing perhaps the major hurdle that has prevented this approach from becoming a fully established form of treatment. However, the clinical efficiency of these protocols has not been exhaustively explored and therefore remains controversial.

METHODS

Morphologically normal oocytes displaying the first polar body were frozen-thawed with a slow cooling protocol that utilized 1.5 mol/l propane-1,2-diol (PrOH) and 0.3 mol/l sucrose.

RESULTS

A total of 927 oocytes from 146 patients were frozen-thawed, achieving a 74.1% survival rate. Over 76% of microinjected oocytes displayed two pronuclei 16 h post-insemination, while the proportion of embryos at 44-46 h post-insemination was 90.2%. At this time point, the majority (68.3%) of embryos were at the two-cell stage, showing in most cases (78.7%) minimal or moderate fragmentation. Eighteen clinical pregnancies, three of which were twin, were observed, giving rise to rates of 12.3 and 9.7%, calculated per patient and per embryo transfer, respectively. The implantation rate was 5.2%. To date, four children have been born and three pregnancies resulted in spontaneous abortions, while the remaining pregnancies are ongoing.

CONCLUSIONS

Our data indicate that although the combination of slow cooling and high sucrose concentration ensures high rates of oocyte survival, it is not sufficient to guarantee a high standard of clinical efficiency.

Cryopreserved oocytes of infertile couples undergoing assisted reproductive technology could be an important source of oocyte donation: a clinical report of successful pregnancies

BACKGROUND

Surplus oocytes in assisted reproduction treatment cycles could be saved and donated to other couples. ICSI is usually performed for oocytes that have been stored frozen, considering possible exocytosis of cortical granules (CG). The unavoidability of ICSI merits further study.

METHODS

We used a slow method to freeze excess oocytes from infertile couples. After thawing, oocytes were fertilized by either IVF or ICSI according to semen parameters. Some oocytes were examined for CG.

RESULTS

Twenty-eight infertile couples cryopreserved a proportion of their oocytes and 12 thawed their oocytes. Three couples used their own oocytes, whereas nine donated their oocytes to nine other couples for 12 cycles. The survival rate from thawing was 90% (73/81). The fertilization rate using IVF (83%) was similar to ICSI (82%). Seven pregnancies (47% per cycle) were achieved; one used her own oocytes and six received donated oocytes. Five women delivered six babies including one set of twins. Two pregnancies aborted. The frozen-thawed oocytes (15/15) revealed no exocytosis of CG.

CONCLUSIONS

To freeze oocytes of infertile couples undergoing assisted reproduction treatment may help other couples. Our successful experience may facilitate oocyte banks to become a reality. Both IVF and ICSI are valuable for frozen oocytes.

Criteria to assess human oocyte quality after cryopreservation

Oocyte cryopreservation certainly represents one of the most attractive developments in the field of assisted reproduction, with the aim of preserving female fertility and circumventing the ethical and legal drawbacks associated with embryo freezing. Despite the achievement of the first pregnancy from frozen oocytes dating back as early as 1987, since then fewer than 150 pregnancies have been reported. Over a long period of time, application of oocyte storage on a large scale has been prevented by various factors, namely poor post-thaw survival. Fertilization rates remained low even after the introduction of intracytoplasmic sperm injection. Modifications of slow-freezing protocols, mainly based on the increase of the concentration of sucrose used as non-penetrating cryoprotectant (CPA) and the replacement of sodium with choline, appear to have decisively improved survival rates to over 80%. Investigations at the cellular level on thawed oocytes are largely lacking. Fertilization rates have also benefited from protocol modifications, reaching values indistinguishable from those normally obtained with fresh material. Vitrification protocols have also been tested, giving rise to improvements whose reproducibility is still uncertain. Data on the dynamics of fertilization and preimplantation development of embryos derived from frozen oocytes are extremely scarce. At the moment, clinical efficiency of oocyte cryopreservation cannot be precisely assessed because of the lack of controlled studies, although it appears to be considerably lower than that achieved with embryo freezing. In summary, encouraging advances have been made in the field of oocyte cryopreservation, but presently no protocol can ensure standards of success and safety comparable to those guaranteed by embryo storage.