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

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.

How developments in cryobiology, reproductive technologies and conservation genomics could shape gene banking strategies for (farm) animals

Many local breeds are currently at risk because of replacement by a limited number of specialized commercial breeds. Concurrently, for many breeds, allelic diversity within breeds declines because of inbreeding. Gene banking of germplasm may serve to secure the breeds and the alleles for any future use, for instance to recover a lost breed, to address new breeding goals, to support breeding schemes in small populations to minimize inbreeding, and for conservation genetics and genomics research. Developments in cryobiology and reproductive technology have generated several possibilities for preserving germplasm in farm animals. Furthermore, in some mammalian and bird species, gene banking of material is difficult or impossible, requiring development of new alternative methods or improvement of existing methods. Depending on the species, there are interesting possibilities or research developments in the use of epididymal spermatozoa, oocytes and embryos, ovarian and testicular tissue, primordial germ cells, and somatic cells for the conservation of genetic diversity in farm- and other animal species. Rapid developments in genomics research also provide new opportunities to optimize conservation and sampling strategies and to characterize genome-wide genetic variation. With regard to gene banks for farm animals, collaboration between European countries is being developed through a number of organizations, aimed at sharing knowledge and expertise between national programmes. It would be useful to explore further collaboration between countries, within the framework of a European gene banking strategy that should minimize costs of conservation and maximize opportunities for exploitation and sustainable use of genetic diversity.

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.

The presence of 1 mM glycine in vitrification solutions protects oocyte mitochondrial homeostasis and improves blastocyst development

PURPOSE

Embryos generated from oocytes which have been vitrified have lower blastocyst development rates than embryos generated from fresh oocytes. This is indicative of a level of irreversible damage to the oocyte possibly due to exposure to high cryoprotectant levels and osmotic stress. This study aimed to assess the effects of vitrification on the mitochondria of mature mouse oocytes while also examining the ability of the osmolyte glycine, to maintain cell function after vitrification.

METHODS

Oocytes were cryopreserved via vitrification with or without 1 mM Glycine and compared to fresh oocyte controls. Oocytes were assessed for mitochondrial distribution and membrane potential as well as their ability to fertilise. Blastocyst development and gene expression was also examined.

RESULTS

Vitrification altered mitochondrial distribution and membrane potential, which did not recover after 2 h of culture. Addition of 1 mM glycine to the vitrification media prevented these perturbations. Furthermore, blastocyst development from oocytes that were vitrified with glycine was significantly higher compared to those vitrified without glycine (83.9 % vs. 76.5 % respectively; p<0.05) and blastocysts derived from oocytes that were vitrified without glycine had significantly decreased levels of IGF2 and Glut3 compared to control blastocysts however those derived from oocytes vitrified with glycine had comparable levels of these genes compared to fresh controls.

CONCLUSION

Addition of 1 mM glycine to the vitrification solutions improved the ability of the oocyte to maintain its mitochondrial physiology and subsequent development and therefore could be considered for routine inclusion in cryopreservation solutions.

Oocyte cryostorage to preserve fertility in oncological patients

Thanks to the progress in oncostatic treatments, young women affected by cancer have a fairly good chance of surviving the disease and leading a normal post-cancer life. Quite often, however, polychemiotherapy and/or radiotherapy can induce ovarian damage and significantly reduce the content of follicles and oocytes inside the ovary, thus predisposing the patient to menstrual disorders, infertility, and precocious menopause. Several techniques have been proposed to preserve fertility in these patients; among them oocyte collection and cryopreservation prior to the oncostatic treatment has been widely applied in the last decade. The proper indications, the permitting conditions, the available hormonal stimulation protocols, as well as the effectiveness and limits of this option will be discussed herein, with a comprehensive and up-to-date review of the two techniques commonly used to cryostore oocytes, the slow-freezing technique and the vitrification technique.

John JC. The generation of live offspring from vitrified oocytes

Oocyte cryopreservation is extremely beneficial for assisted reproductive technologies, the treatment of infertility and biotechnology and offers a viable alternative to embryo freezing and ovarian grafting approaches for the generation of embryonic stem cells and live offspring. It also offers the potential to store oocytes to rescue endangered species by somatic cell nuclear transfer and for the generation of embryonic stem cells to study development in these species. We vitrified mouse oocytes using a range of concentrations of trehalose (0 to 0.3 M) and demonstrated that 0.1 and 0.3 M trehalose had similar developmental rates, which were significantly different to the 0.2 M cohort (P<0.05). As mitochondria are important for fertilisation outcome, we observed that the clustering and distribution of mitochondria of the 0.2 M cohort were more affected by vitifrication than the other groups. Nevertheless, all 3 cohorts were able to develop to blastocyst, following in vitro fertilisation, although developmental rates were better for the 0.1 and 0.3 M cohorts than the 0.2 M cohort (P<0.05). Whilst blastocysts gave rise to embryonic stem-like cells, it was apparent from immunocytochemistry and RT-PCR that these cells did not demonstrate true pluripotency and exhibited abnormal karyotypes. However, they gave rise to teratomas following injection into SCID mice and differentiated into cells of each of the germinal layers following in vitro differentiation. The transfer of 2-cell embryos from the 0.1 and 0.3 M cohorts resulted in the birth of live offspring that had normal karyotypes (9/10). When 2-cell embryos from vitrified oocytes underwent vitrification, and were thawed and transferred, live offspring were obtained that exhibited normal karyotypes, with the exception of one offspring who was larger and died at 7 months. We conclude that these studies highlight the importance of the endometrial environment for the maintenance of genetic stability and thus the propagation of specific genetic traits.

Cryopreservation of oocytes in experimental models

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

History of oocyte cryopreservation

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

Solid-surface vitrification is an appropriate and convenient method for cryopreservation of isolated rat follicles

BACKGROUND

Cryopreservation of isolated follicles may be a potential option to restore fertility in young women with cancer, because it can prevent the risks of cancer transmission. Several freezing protocols are available, including slow-rate freezing, open-pulled straws vitrification (OPS) and solid-surface vitrification (SSV, a new freezing technique). The purpose of our study was to investigate the effects of these freezing procedures on viability, ultrastructure and developmental capacity of isolated rat follicles.

METHODS

Isolated follicles from female Sprague-Dawley rats were randomly assigned to SSV, OPS and slow-rate freezing groups for cryopreservation. Follicle viability assessment and ultrastructural examination were performed after thawing. In order to study the developmental capacity of thawed follicles, we performed in vitro culture with a three-dimensional (3D) system by alginate hydrogels.

RESULTS

Our results showed that the totally viable rate of follicles vitrified by SSV (64.76%) was slightly higher than that of the OPS group (62.38%) and significantly higher than that of the slow-rate freezing group (52.65%; P < 0.05). The ultrastructural examination revealed that morphological alterations were relatively low in the SSV group compared to the OPS and slow-rate freezing groups. After in vitro culture within a 3D system using alginate hydrogels, we found the highest increase (28.90 +/- 2.21 microm) in follicle diameter in follicles from the SSV group. The estradiol level in the SSV group was significantly higher than those in the OPS and slow-rate freezing groups at the end of a 72-hr culture period (P < 0.05).

CONCLUSIONS

Our results suggest that the SSV method is an appropriate and convenient method for cryopreservation of isolated rat follicles compared with the conventional slow-rate freezing method and the OPS method.

Effect of choline-supplemented sodium-depleted slow freezing versus vitrification on mouse oocyte meiotic spindles and chromosome abnormalities

OBJECTIVE

To evaluate and compare vitrification and choline-supplemented sodium-depleted slow freezing of mouse oocytes.

DESIGN

Animal study.

SETTING

University-affiliated hospital.

ANIMAL(S)

CD-1 mice.

INTERVENTION(S)

Oocyte cryopreservation by vitrification or choline-supplemented sodium-depleted slow freezing.

MAIN OUTCOME MEASURE(S)

Survival rate, fertilization and embryonic development in vitro, meiotic spindle and chromosome configuration, and aneuploidy screening after parthenogenetic activation.

RESULT(S)

A total of 564 oocytes were vitrified, and 791 oocytes were cryopreserved using the slow freezing. The survival rates were 91.8% (518/564) and 73.3% (579/791), respectively. After IVF, the cleavage and blastocyst formation rates of vitrified oocytes were significantly higher than those of slow-frozen oocytes (63.6% vs. 39.9% and 30.50% vs. 20.2%, respectively). Vitrified oocytes were more likely than slow-frozen oocytes to maintain normal meiotic spindles and chromosome alignment (86.9% vs. 70.1%). However, the incidence of aneuploidy was similar in vitrified oocytes and slow-frozen oocytes (9.30% vs. 8.7%).

CONCLUSION(S)

Vitrification is superior to choline-supplemented sodium-depleted slow freezing, leading to improved survival, fertilization, and embryonic development in vitro. Analysis of meiotic spindle integrity and chromosome alignment indicates that less damage was detected in vitrified oocytes. However, the incidence of aneuploidy is similar in both vitrified and slow-frozen oocytes.

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.

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.

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.

Oocytes cryopreservation: state of art

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

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.

Comparison of embryo quality between sibling embryos originating from frozen or fresh oocytes

Human embryo cryopreservation techniques allow storage of surplus embryos created during assisted reproduction procedures; however, the existence of these same surplus embryos has sparked further debate. What can be their fate once they are no longer desired by their parents, or if the parents are deceased? Thus, the level of interest in the cryopreservation of oocytes has increased, as has the necessity for further scientific study. This study had the objective of comparing embryo quality from 16 women who underwent intracytoplasmic sperm injection, where approximately half of the retrieved oocytes per cycle were inseminated fresh after collection, and the remainder cryopreserved for subsequent fertilization. Normal fertilization rates were not significantly different between the two oocyte-treatment groups. There was no significant difference in the frequency of good quality embryos (morphology grades I and II) on the second day of laboratory evaluation between embryos derived from the two oocyte-treatment groups. It is interesting to note that embryo transfer from fresh oocytes produced no pregnancies, which shows that even embryos derived from frozen oocytes that are fragmented or have a slower cleavage rate are viable embryos, capable of producing healthy babies.

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.

Cryopreservation of Isolated Mouse Germinal Vesicles

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

Successful birth after injection of frozen human oocytes with frozen epididymal spermatozoa

A couple (female 31, male 42 years old) with infertility due to obstructive azoospermy returned to the clinic in order to attempt pregnancy using their frozen oocytes and epididymal sperm cells, which had been cryopreserved at the time of a previous IVF attempt. Two days before the scheduled transfer, eight oocytes were thawed; 5/8 (63%) oocytes survived and 4/5 (80%) oocytes fertilized after intracytoplasmic sperm injection (ICSI) with the previously frozen epididymal spermatozoa. All four fertilized ova cleaved (100%). On day 2 after thawing, four embryos were transferred; three with two cells (grade II) and one with three cells (grade III). Hormonal support for the established pregnancy was maintained with oestradiol and progesterone orally until 12 weeks of gestation, and the patient was delivered by Caesarean section at 40 weeks of gestation; the baby boy weighed 3025 g, and measured 51 cm, with Apgar of 10 in the 1st and 5th min. The cryopreservation and warming protocol used for this study yielded very favourable results, comparing well with reports in the literature. This case report demonstrates that it is possible to obtain high rates of oocyte survival following thawing and high rates of fertilization after ICSI, with viable development of the resulting embryos.

Vitrification of Mouse Embryos at Various Stages by Open-Pulled Straw (OPS) Method

This study was performed to pursue the optimal condition for the cryopreservation of mouse morulae by a two-step OPS method and to investigate the feasibility of the optimal condition for vitrification of embryos at other developmental stages. First, the mouse morulae were vitrified in OPS using one-step procedure-that is, embryos were vitrified after direct exposure to EDFS30 (15% ethylene glycol (EG), 15% dimethyl sulfoxide (DMSO), Ficoll and sucrose), or two-step method-that is, embryos were first pretreated in 10%E + 10%D (10% EG and 10% DMSO in mPBS) for 30 sec, then exposed to EDFS30 for 15 to 60 sec, respectively. After vitrification and warming, the embryos were morphologically evaluated and assessed by their development to blastocysts, expanded/hatched blastocysts, or to term after transfer. The result showed that all the vitrified-warmed morulae had similar blastocyst rate compared to that of control (91.7% vs. 100%), and the highest developmental rate to expanded blastocysts (100%) or hatched blastocysts (62.3%) was observed when the morulae were pretreated with 10%E + 10%D for 0.5 min, exposed to EDFS30for 25 sec before vitrification and warming in 0.5 M sucrose for 5 min. After transfer, the survival rate (33.1%) in vivo of the vitrified morulae was higher (P > 0.05) than that of the fresh embryos (24.6%). Secondly, embryos at different stages were cryopreserved and thawed following the above program. Most (93.4 to 100%) of the embryos recovered after vitrification were morphologically normal at all the developmental stages. The blastocyst rates of the vitrified one-cell (52.5 to 66.7%) and the two-cell (63.3 to 68.9%) embryos were lower (P < 0.05) than those of the vitrified four-cell embryos (81.7 to 86.4%), the eight-cell embryos (90.0 to 93.3%), morulae (96.7 to 100%), and the expanded blastocysts rate (98.3 to 100.0%) of the vitrified early blastocysts. The highest survival rate in vivo of vitrified embryos were from the early blastocysts (40.4%), which was similar to that of fresh embryos (48.6%). The data demonstrate that the optimal protocol for the cryopreservation of morulae was suitable for the four-cell embryos to early blastocyst stages and that the early blastocyst stage is the most feasible stage for mouse embryo cryopreservation under our experimental conditions.

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.

Use of sugars in cryopreserving human oocytes

In the last 20 years, a worldwide effort to cryopreserve oocytes has resulted in 40 infants and approximately 50 ongoing pregnancies being reported. While the ability to freeze human embryos has become a standard of practice in assisted reproductive technologies, obtaining reliable techniques for oocyte cryopreservation has been more difficult. The unique properties of the mature oocyte, such as the meiotic stage with sensitive spindle structure as well as the large cell volume, are responsible for the limited success obtained to date. There have been two approaches to cryopreserving the oocyte: (i) slow freeze-rapid thaw, and (ii) vitrification protocols with rapid cooling-rapid warming. Both methods have incorporated sugars (sucrose) as a beneficial non-permeating extracellular cryoprotectant. Studies of organisms that survive extreme conditions of freezing/dehydration have demonstrated the ability to accumulate intracellular sugars to afford protection and survival. A novel technique using microinjection of sugars into the oocyte for cryopreservation has been developed as an alternative approach to external addition of sugars. Freezing the human oocyte has been a challenging goal; however, developing research and efforts will, in the near future, provide women with an important option for their reproductive health.