Cryopreservation of mouse oocytes using a medium with low sodium content: effect of plunge temperature

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

Mouse oocyte vitrification with and without dimethyl sulfoxide: influence on cryo-survival, development, and maternal imprinted gene expression

PURPOSE

Oocytes and embryos can be vitrified with and without dimethyl sulfoxide (DMSO). Objectives were to compare no vitrification (No-Vitr), vitrification with DMSO (Vitr + DMSO), and vitrification without DMSO (Vitr - DMSO) on fresh/warmed oocyte survival, induced parthenogenetic activation, parthenogenetic embryo development, and embryonic maternal imprinted gene expression.

METHODS

In this prospective controlled laboratory study, mature B6C3F1 female mouse metaphase II oocytes were treated as: i) No-Vitr, ii) Vitr + DMSO/warmed, and iii) Vitr - DMSO/warmed with subsequent parthenogenetic activation and culture to the blastocyst stage. Oocyte cryo-survival, parthenogenetic activation and embryo development, parthenogenetic embryo maternal imprinted gene expression were outcome measures.

RESULTS

Oocyte cryo-survival was significantly improved in Vitr + DMSO versus Vitr - DMSO at initial warming and 2 h after warming. Induced parthenogenetic activation was similar between all three intervention groups. While early preimplantation parthenogenetic embryo development was similar between control, Vitr + DMSO, Vitr - DMSO oocytes, the development to blastocysts was significantly inferior in the Vitr - DMSO oocytes group compared to the control and Vitr + DMSO oocyte groups. Finally, maternal imprinted gene expression was similar between intervention groups at both the 2-cell and blastocyst parthenogenetic embryo stage.

CONCLUSION(S)

Inclusion of DMSO in oocyte vitrification solutions improved cryo-survival and developmental potential of parthenogenetic embryos to the blastocyst stage without significantly altering maternal imprinted gene expression.

Chapter 8 Human Oocytes Slow-Rate Freezing: Methodology

Cryopreservation of human oocytes is an important technique for the treatment of human infertility, as it deals successfully with legal, ethical, and moral issues related to embryo cryopreservation (Coticchio et al., Human Fertil (Camb) 4:152-157, 2001; Tucker et al., Eur J Obstet Gynecol Reprod Biol 113: 524-527, 2004) and maintains reproductive potential in women with diseases or conditions that may compromise reproductive capacity. Here, we describe an oocyte cryopreservation technique involving slow freezing-rapid thawing with differential sucrose concentration (0.2 M in the freezing stage-0.3 M in thawing stage) (Bianchi et al., Reprod Biomed Online. 14:64-71, 2007), describing the technique in detail, from the preparation of the solutions through to the performance of the technique and, finally, to a number of helpful hints for optimum results.

Freeze/thaw stress induces organelle remodeling and membrane recycling in cryopreserved human mature oocytes

PURPOSE

Our aim was to evaluate the ultrastructure of human metaphase II oocytes subjected to slow freezing and fixed after thawing at different intervals during post-thaw rehydration.

METHODS

Samples were studied by light and transmission electron microscopy.

RESULTS

We found that vacuolization was present in all cryopreserved oocytes, reaching a maximum in the intermediate stage of rehydration. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates decreased following thawing, particularly in the first and intermediate stages of rehydration, whereas mitochondria-vesicle (MV) complexes augmented in the same stages. At the end of rehydration, vacuoles and MV complexes both diminished and M-SER aggregates increased again. Cortical granules (CGs) were scarce in all cryopreserved oocytes, gradually diminishing as rehydration progressed.

CONCLUSIONS

This study also shows that such a membrane remodeling is mainly represented by a dynamic process of transition between M-SER aggregates and MV complexes, both able of transforming into each other. Vacuoles and CG membranes may take part in the membrane recycling mechanism.

Cryopreservation of embryos and oocytes in human assisted reproduction

Both sperm and embryo cryopreservation have become routine procedures in human assisted reproduction and oocyte cryopreservation is being introduced into clinical practice and is getting more and more widely used. Embryo cryopreservation has decreased the number of fresh embryo transfers and maximized the effectiveness of the IVF cycle. The data shows that women who had transfers of fresh and frozen embryos obtained 8% additional births by using their cryopreserved embryos. Oocyte cryopreservation offers more advantages compared to embryo freezing, such as fertility preservation in women at risk of losing fertility due to oncological treatment or chronic disease, egg donation, and postponing childbirth, and eliminates religious and/or other ethical, legal, and moral concerns of embryo freezing. In this review, the basic principles, methodology, and practical experiences as well as safety and other aspects concerning slow cooling and ultrarapid cooling (vitrification) of human embryos and oocytes are summarized.

Replacement of sodium with choline in slow-cooling media improves human ovarian tissue cryopreservation

Ovarian tissue cryopreservation is a promising technique for fertility preservation in young female cancer patients and efforts have been made to improve its effectiveness. During cooling and thawing, sodium ions significantly contribute to the 'solute effect' that plays a major role in disrupting cell membranes. Choline ions, which do not cross the cell membrane, should not contribute to the intracellular solute load. The present study assessed the effects of sodium substitution with choline in slow-cooling freezing media on human ovarian cortical strip cryopreservation. A total of 629 follicles (fresh control n=266; cryopreserved n=363), collected from ovarian biopsies of 11 women (22-40years) during laparoscopic surgery, were studied by light microscopy, immunohistochemistry and transmission electron microscopy to evaluate their morphology, apoptosis and ultrastructure. The results demonstrate that choline substitution leads to: (i) an improved preservation of oocytes and follicular cells; (ii) the recovery of a higher percentage of grade-1 follicles negative for p53, p21 and Apaf-1 apoptotic markers; (iii) a reduced mitochondrial damage as observed at an ultrastructural level; and (iv) a better preservation of ovarian tissue stroma. In conclusion, the use of choline-based media may represent a valuable tool to improve human ovarian tissue cryopreservation. Ovarian tissue cryopreservation is a promising fertility preservation approach for cancer patients before undergoing treatments that irreversibly reduce the ovarian reserve. Autotransplantation of ovarian cortical strips has resulted in viable offspring in animal models and human. Worldwide, 20 live births have been reported thus far following autotransplantation of frozen-thawed ovarian tissue. However, currently the success rate of this technology is far from being satisfactory. This could be due to inappropriate cryopreservation procedures that might impair the physiology of ovarian follicles. Sodium ions contained in freezing media significantly contribute to the 'solute effect' that plays a major role in disrupting cell membranes. Choline ions, which do not cross the cell membrane, would not be expected to contribute to the intracellular solute load. In the present study we assessed the effects of sodium substitution with choline in slow-cooling freezing media on human ovarian cortical strip cryopreservation. A total of 629 follicles, collected from ovarian biopsies of 11 women (aged 22-40years) during laparoscopic surgery, have been studied by light microscopy, immunohistochemistry and transmission electron microscopy to evaluate their morphology, apoptosis and ultrastructure. Results demonstrated that choline substitution allowed: (i) a better preservation of oocytes and follicular cells; (ii) the recovery of an higher percentage of healthy follicles negative for apoptotic markers; (iii) a lower mitochondria ultrastructural damage; and (iv) a better preservation of ovarian tissue stroma. In conclusion, the use of choline-based media could represent a valuable tool to cryopreserve human ovarian tissue for fertility preservation.

Microtubule organisation, pronuclear formation and embryonic development of mouse oocytes after intracytoplasmic sperm injection or parthenogenetic activation and then slow-freezing with 1,2-propanediol

The goal of this study was to investigate the effect of cryopreservation on oocytes at different times after intracytoplasmic sperm injection (ICSI) and parthenogenetic activation. The study was performed in mouse oocytes fertilised by ICSI, or in artificially-activated oocytes, which were cryopreserved immediately, one hour or five hours later through slow-freezing. After thawing, the rates of survival, fertilisation–activation, embryonic development of oocytes–zygotes and changes in the cytoskeleton and ploidy were observed. Our results reveal a significant difference in survival rates of 0-, 1- and 5-h cryopreserved oocytes following ICSI and artificial activation. Moreover, significant differences in two pronuclei (PN) development existed between the 0-, 1- and 5-h groups of oocytes frozen after ICSI, while the rates of two-PN development of activated oocytes were different between the 1-h and 5-h groups. Despite these initial differences, there was no difference in the rate of blastocyst formation from two-PN zygotes following ICSI or artificial activation. However, compared with ICSI or artificially-activated oocytes cryopreserved at 5 h, many oocytes from the 0- and 1-h cryopreservation groups developed to zygotes with abnormal ploidy; this suggests that too little time before cryopreservation can result in some activated oocytes forming abnormal ploidy. However, our results also demonstrate that spermatozoa can maintain normal fertilisation capacity in frozen ICSI oocytes and the procedure of freeze–thawing did not affect the later development of zygotes.

Mature oocyte cryopreservation: a guideline

There is good evidence that fertilization and pregnancy rates are similar to IVF/ICSI with fresh oocytes when vitrified/warmed oocytes are used as part of IVF/ICSI for young women. Although data are limited, no increase in chromosomal abnormalities, birth defects, and developmental deficits has been reported in the offspring born from cryopreserved oocytes when compared to pregnancies from conventional IVF/ICSI and the general population. Evidence indicates that oocyte vitrification and warming should no longer be considered experimental. This document replaces the document last published in 2008 titled, "Ovarian Tissue and Oocyte Cryopreservation," Fertil Steril 2008;90:S241-6.

Effects of various combinations of cryoprotectants and cooling speed on the survival and further development of mouse oocytes after vitrification

OBJECTIVE

The objectives of this study were to analyze efficacy of immature and mature mouse oocytes after vitrification and warming by applying various combinations of cryoprotectants (CPAs) and/or super-rapid cooling using slush nitrogen (SN(2)).

METHODS

Four-week old ICR female mice were superovulated for GV- and MII-stage oocytes. Experimental groups were divided into two groups. Ethylene glycol (EG) only group: pre-equilibrated with 1.5 M EG for 2.5 minutes and then equilibrated with 5.5 M EG and 1.0 M sucrose for 20 seconds. EG+dimethylsulfoxide (DMSO) group: pre-equilibrated with 1.3 M EG+1.1 M DMSO for 2.5 minutes and equilibrated with 2.7 M EG+2.1 M DMSO+0.5 M sucrose for 20 seconds. The oocytes were loaded onto grids and plunged into SN(2) or liquid nitrogen (LN(2)). Stored oocytes were warmed by a five-step method, and then their survival, maturation, cleavage, and developmental rates were observed.

RESULTS

The EG only and EG+DMSO groups showed no significant difference in survival of immature oocytes vitrified after warming. However, maturation and cleavage rates after conventional insemination were greater in the EG only group than in the EG+DMSO group. In mature oocytes, survival, cleavage, and blastocyst formation rates after warming showed no significant difference when EG only or EG+DMSO was applied. Furthermore, cleavage and blastocyst formation rates of MII oocytes vitrified using SN(2) were increased in both the EG only and EG+DMSO groups.

CONCLUSION

A combination of CPAs in oocyte cryopreservation could be formulated according to the oocyte stage. In addition, SN(2) may improve the efficiency of vitrification by reducing cryoinjury.

Cryopreservation of mammalian oocytes by using sugars: Intra- and extracellular raffinose with small amounts of dimethylsulfoxide yields high cryosurvival, fertilization, and development rates

Accumulation of intra- and extracellular sugars such as trehalose, glucose, and raffinose is central to survival strategies of a variety of organisms coping with extreme conditions including freezing and almost complete drying. The objective of the present study was to investigate the potential application of intra- and extracellular raffinose in combination with low concentrations of dimethylsulfoxide (Me(2)SO) to mammalian oocyte cryopreservation. To this end, the fertilization and embryonic development of cryopreserved metaphase II (M II) mouse oocytes were studied in comparison to unfrozen controls. For cryopreservation, M II oocytes were microinjected with 0.1M raffinose, and then cooled to -196 degrees C in the presence of either 0.3M raffinose and 0.5M Me(2)SO (cryopreservation group 1) or 0.3M raffinose and 1.0M Me(2)SO (cryopreservation group 2). The control groups included untreated oocytes (untreated control) and oocytes microinjected with raffinose, but not frozen (injection control). The post-thaw survival rates were 83.9% and 80.6% for the cryopreservation group 1 and 2, respectively. The fertilization and blastocyst rates in the cryopreservation group 1 (90.0% and 77.8%, respectively) and 2 (94.6% and 72.5%, respectively) were also high and similar to the ones of the injection controls (97.8% and 78.5%, respectively) and untreated controls (98.8% and 83.6%, respectively). These results are consistent with the findings of our earlier studies and support the use of sugars as intra- and extracellular cryoprotectants. Furthermore, the results of the present study indicate that the presence of intra- and extracellular sugars alleviates high concentrations of conventional penetrating cryoprotectants, and thus minimizes their toxicity.

Ovarian tissue and oocyte cryopreservation

Although currently investigational, ovarian tissue cryopreservation and oocyte cryopreservation hold promise for future female fertility preservation, particularly following aggressive chemotherapy and/or radiotherapy treatment protocols.

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.

Human oocyte and ovarian tissue cryopreservation and its application

PURPOSE

To review the recent progress in human oocyte and ovarian tissue cryopreservation, and in the application of these two technologies for preserving female fertility of patients who are undergoing cancer treatment.

DESIGN

The literature on human oocyte and ovarian tissue freezing was searched with PubMed. The scientific background, current developments and potential future applications of these two methods were reviewed.

RESULTS

Chemotherapy and/or radiotherapy can induce premature ovarian failure in most of female cancer patients. Consequently, there has been a greater need for options to preserve the reproductive potential of these individuals. However, options are somewhat limited currently, particularly following aggressive chemotherapy and/or radiotherapy treatment protocols. In recent years, there have been considerable advances in the cryopreservation of human oocytes and ovarian tissue. For women facing upcoming cancer therapies, cryopreservation of ovarian tissue and oocytes is a technology that holds promise for banking reproductive potential for the future. Recent laboratory modifications have resulted in improved oocyte survival, oocyte fertilization, and pregnancy rates from frozen-thawed oocytes in IVF. This suggests potential for clinical application.

CONCLUSIONS

In the case of patients who are facing infertility due to cancer therapy, oocyte cryopreservation may be one of the few options available. Ovarian tissue cryopreservation can only be recommended as an experimental protocol in carefully selected patients. In ovarian tissue transplantation, more research is needed in order to enhance the revascularization process with the goal of reducing the follicular loss that takes place after tissue grafting. These technologies are still investigational, although tremendous progress has been made. The availability of such treatment will potentially lead to its demand not only from patients with cancer but also from healthy women who chose to postpone childbearing until later in life and therefore wish to retain their fertility.

Cryopreservation of zebrafish (Danio rerio) oocytes using improved controlled slow cooling protocols

Cryopreservation of gametes provides a promising method to preserve fish genetic material. Previously we reported some preliminary results on cryopreservation of zebrafish (Danio rerio) oocytes using controlled slow cooling and determined the optimum cryoprotective medium and cooling rate for stage III zebrafish oocytes. In the present study, the effects of two different cryopreservation media, cryoprotectant removal method, final sample freezing temperature before LN(2) plunge, warming rate, and the post-thaw incubation time on oocyte viability were investigated. Commonly used cryoprotectant methanol and glucose were used in this study. Stage III zebrafish oocytes were frozen in standard culture medium 50% L-15 or in a sodium-free KCl buffer medium. Oocyte viability was assessed using trypan blue staining and ATP assay. The viability of oocytes frozen in KCl buffer was significantly higher than oocytes frozen in L-15 medium. The results also showed that fast thawing and stepwise removal of cryoprotectant improved oocyte survival significantly, with highest viability of 88.0+/-1.7% being obtained immediately after rapid thawing when assessed by trypan blue staining. However, after 2h incubation at 22 degrees C the viability of freeze-thawed oocytes decreased to 29.5+/-5.1%. Results also showed that the ATP level in oocytes decreased significantly immediately after thawing. All oocytes became translucent after freezing which complicated the use of GVBD test (in vitro maturation of oocytes followed by observation of germinal vesicle breakdown which results in oocytes becoming translucent). New oocyte viability assessment methods are urgently needed.

Truths and myths of oocyte sensitivity to controlled rate freezing

The mammalian oocyte is especially sensitive to cryopreservation. Because of its size and physiology, it can easily undergo cell death or sub-lethal damage as a consequence of intracellular ice formation, increase in the concentration of solutes and other undesired effects during the conversion of extracellular water into ice. This has generated the belief that oocyte storage cannot be achieved with the necessary efficiency and safety. However, many concerns raised by oocyte freezing are the result of unproven hypotheses or observations conducted under sometimes inappropriate conditions. For instance, spindle organization can undergo damage under certain freezing conditions but not with other protocols. The controversial suggestion that cryopreservation induces cortical granule discharge and zona pellucida hardening somehow questions the routine use of sperm microinjection. Damage to mouse oocytes caused by solute concentration is well documented but, in the human, there is no solid evidence that modifications of freezing mixtures, to prevent this problem, provide an actual advantage. The hope of developing oocyte cryopreservation as a major IVF option is becoming increasingly realistic, but major efforts are still required to clarify the authentic implications of oocyte cryopreservation at the cellular level and identify freezing conditions compatible with the preservation of viability and developmental ability.

Ovarian tissue and oocyte cryopreservation

Although currently investigational, ovarian tissue cryopreservation and oocyte cryopreservation hold promise for future female fertility preservation, particularly following aggressive chemotherapy and/or radiotherapy treatment protocols.

Cryopreservation of unfertilized human oocytes

Previous investigations revealed that choline-based freezing media developed in our laboratory were superior to conventional sodium-based media for storing mouse oocytes. This paper examines the ability of the choline-based medium CJ2 and a modified form of this medium, CJ3, to cryopreserve unfertilized human oocytes. Oocytes that were consented for research and matured overnight, as well as freshly collected, donor, mature metaphase II (MII) oocytes, were cryopreserved using choline-based media and an optimized slow-cooling protocol. The results showed higher survival and fertilization rates when CJ3 supplemented with 0.2 mmol/l sucrose was used as compared with CJ2 supplemented with either 0.1 mmol/l or 0.2 mmol/l sucrose. Freshly collected oocytes were more difficult to cryopreserve than those matured in vitro. Modification of the base medium proved to be one of the key factors in obtaining survival rates over 90%. Fertilization rates, embryo development, and genetic analysis of embryos resulting from control and frozen-thawed oocytes are provided. There appears to be a high correlation between chromosomal anomalies and abnormal morphology in embryos from thawed oocytes.

Fertility preservation options for female patients with malignancies

PURPOSE OF REVIEW

Preservation of fertility in female patients diagnosed with cancer has recently been an area of intensive investigation. This review summarizes available options and discusses recently published data concerning experimental methods. Specific strategies for fertility preservation in women with gynecologic malignancies are also presented.

RECENT FINDINGS

Success with ovarian stimulation protocols using tamoxifen or aromatase inhibitors has recently been reported for women with breast cancer who attempt embryo cryopreservation prior to chemotherapy. The first embryo transfer using oocytes retrieved from cryopreserved ovarian tissue implanted at a heterotopic location, the first pregnancy following orthotopic transplantation of cryopreserved ovarian tissue, and increasing success with oocyte cryopreservation were also reported.

SUMMARY

Fertility preservation in female patients with cancer has become an important health issue due to increasing survival rates and delayed childbearing especially in Western countries. Radical vaginal trachelectomy for cervical cancer, conservative surgery for ovarian tumors, and progestin treatment in endometrial cancers may be considered at early stages in order to preserve fertility. Embryo cryopreservation is an established technique that is available for fertility preservation, providing a delay in the initiation of chemotherapy or radiotherapy is acceptable, and a partner or donor sperm is available. Additional techniques that could be offered after counseling the patient about their experimental nature include oocyte cryopreservation, ovarian cryopreservation, and gonadotropin-releasing hormone agonist co-treatment with chemotherapy. Improvement of these techniques as well as better characterization of their success rates and risks await further investigation.

Cryopreservation of biopsied cleavage stage human embryos

The aim was to develop a method to optimize cryopreservation of biopsied multi-celled human embryos. Human day 3 embryos that were donated to research, along with those found to be chromosomally abnormal after blastomere biopsy and fluorescence in-situ hyridization (FISH), were cryopreserved using a slow-freezing protocol in either standard embryo cryopreservation solution [embryo transfer freezing medium (ETFM), a conventional sodium-based medium] or CJ3 (a choline-based, sodium-free medium). After thawing, the number of intact cells was recorded and the previously biopsied embryos were re-analysed using FISH. Biopsied embryos had a lower proportion of intact blastomeres after cryopreservation as compared with intact embryos. However, a significantly (P < 0.05) higher proportion of blastomeres from intact and biopsied embryos cryopreserved in CJ3 (84.1 and 80.1% respectively) survived after thaw than those in ETFM (73.6 and 50.5% respectively). The proportion of aneuploid and mosaic embryos was not statistically different between the two groups. In addition, the frequency of lost cells by aneuploid and mosaic embryos was similar. This study describes a new method that improves the survival of cryopreserved biopsied embryos, and shows that it may also be beneficial for the storage of intact human multi-celled embryos.

Ovarian tissue and oocyte cryopreservation

Although currently investigational, ovarian tissue cryopreservation and oocyte cryopreservation hold promise for future female fertility preservation, particularly following aggressive chemotherapy and/or radiotherapy treatment protocols.

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.

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.

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.

Recent progress in oocyte and ovarian tissue cryopreservation and transplantation

Ovarian tissue cryopreservation and transplantation is an experimental technique that has been developed to sustain the reproductive function of women and children who are faced with sterilizing chemotherapy, radiotherapy, or radical reproductive surgery. Oocyte cryopreservation, on the other hand, is less feasible in the context of cancer because there is usually inadequate time to complete an ovarian stimulation cycle. The main promise of oocyte cryopreservation is that it offers an alternative when embryo freezing is not possible for technical, regulatory, or religious reasons. Oocyte freezing is more suitable for a single woman when the concern is age-related decline in fecundity. There have been significant scientific advances in the field of cryopreservation of ovarian tissue and oocytes, especially within the past few years. Ovarian function has been reported after the first cases of ovarian transplantation, and the number of pregnancies from cryopreserved oocytes has grown. Ovarian tissue and oocyte freezing can now be recommended in a carefully selected group of patients, provided that these options are offered under protocols that are approved by an institutional review board.

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.