Detrimental effects of sodium during mouse oocyte cryopreservation

Oocyte and embryo cryopreservation in assisted reproductive technology: past achievements and current challenges

Cryopreservation has revolutionized the treatment of infertility and fertility preservation. This review summarizes the milestones that paved the way to the current routinary clinical implementation of this game-changing practice in assisted reproductive technology. Still, evidence to support "the best practice" in cryopreservation is controversial and several protocol adaptations exist that were described and compared here, such as cumulus-intact vs. cumulus-free oocyte cryopreservation, artificial collapse, assisted hatching, closed vs. open carriers, and others. A last matter of concern is whether cryostorage duration may impact oocyte/embryo competence, but the current body of evidence in this regard is reassuring. From social and clinical perspectives, oocyte and embryo cryopreservation has evolved from an afterthought when assisted reproduction was intended for immediate pregnancy with supernumerary embryos of secondary interest to its current purpose, which primarily is to preserve fertility long-term and more comprehensively allow for family planning. However, the initial consenting process, which still is geared to short-term fertility care, may no longer be relevant when the individuals that initially preserved the tissues have completed their reproductive journey. A more encompassing counseling model is required to address changing patient values over time.

Ginger and echinacea extracts improve the quality and fertility potential of frozen-thawed ram epididymal spermatozoa

The current study examined the impact of the supplementation of ginger and echinacea extract, as natural antioxidant agents, in freezing extender on the quality and fertility potential of ram epididymal spermatozoa after cryopreservation. Epididymal spermatozoa isolated from Forty testicles, obtained from 20 rams, with motility >80% and total morphological abnormalities <10% were pooled, divided into 7 aliquots and used for cryopreservation. The semen samples were re-suspended with basic Tris egg yolk diluent containing ginger and echinacea extracts (5, 10 and 20 mg/l). The control diluent comprised of only extender and lacked any antioxidant agent. For the determination of sperm quality, frozen straws were thawed after 7-10 days, and then the sperm characteristics were assessed. The supplementation of ginger at a concentration of 10 mg/l, as well as the addition of 10 and 20 mg/l echinacea extract significantly improved total motility and velocity parameters. The status of acrosome integrity and lipid peroxidation significantly improved in spermatozoa when supplemented with 10 mg/l ginger and 20 mg/l echinacea extract. Also, 5 mg/l ginger extract and 20 mg/l echinacea extract significantly improved mitochondrial activity. The highest ratio of the dispersion of sperm chromatin was observed in spermatozoa treated with 10 mg/l ginger extract. The cleavage rate was markedly higher in matured oocytes that were fertilized with frozen spermatozoa treated with 20 mg/l ginger extract and 10 mg/l echinacea. The application of ginger and echinacea extract resulted in improvement in the quality and fertility of frozen-thawed spermatozoa. However, future studies are wanted to elucidate how the active components in these extracts prevent cryo-damages in spermatozoa.

A brief history of oocyte cryopreservation: Arguments and facts

The term "cryopreservation" refers to the process of cooling cells and tissues and storing them at subzero temperatures in order to stop all biological activity and preserve their viability and physiological competences for future use. Cooling to subzero temperatures is not a physiological condition for human cells; this is probably due to the high content of water in the living matter, whose conversion to ice crystals may be associated with severe and irreversible damage. Among reproductive cells and tissues, metaphase II oocytes are notably vulnerable to cryopreservation, mainly because of their large size, low surface area to volume ratio, relatively high water content and presence of the meiotic spindle. As human biological systems lack efficient internal defense mechanisms against chilling injuries, it is of the utmost importance to supply adequate external support, in terms of cryoprotectant additives, appropriate cooling/warming rates, and suitable long-term storage. Over the years, scientists have proposed different cryopreservation strategies in the effort to achieve an optimized recipe ensuring cell survival and, at the same time, maintenance of the physiological functions and abilities necessary to continue life. However, despite the first success obtained in the 1980s with frozen oocytes, it was not until recently that notable improvements in the cryopreservation technique, thanks to the advent of vitrification, allowed a breakthrough of this fine procedure.

Oocyte cryopreservation: searching for novel improvement strategies

PURPOSE

To highlight emerging techniques aimed at improving oocyte cryopreservation.

METHODS

Review of available and relevant literature through Pubmed and Medline searches.

RESULTS

Oocyte cryopreservation is an increasingly common procedure utilized for assisted reproduction and may benefit several patient populations. Therefore, improving efficiency is paramount in realizing the tremendous promise of this approach. However, in addition to numerous studies looking to improve oocyte cryopreservation efficacy via examination of variables involved with protocol methodology, such as type/concentration of cryoprotectant (CPA), type of storage device, or cooling/warming rates, there are more novel approaches for improvement. These alternate approaches include utilizing different the stages of oocytes, examining alteration of basal media and buffer composition, optimizing CPA exchange protocols and device loading through use of automated technology, as well as examination/manipulation of oocyte cellular composition to improve cryotolerance. Finally, elucidating more accurate or insightful indicators of "success" is crucial for continued improvement of oocyte cryopreservation.

CONCLUSION

Oocyte cryopreservation has improved dramatically in recent years and is receiving widespread clinical use. Novel approaches to further improve success, as well as improved methods to assess this success will aid in continued improvement.

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.

Is it best to cryopreserve human cumulus-free immature oocytes before or after in vitro maturation?

Freezing unfertilized oocytes is an option for females without a partner, either to preserve their fertility prior to sterilizing cancer treatment or for social reasons. Our study considered whether it is best to freeze immature human oocytes at the germinal vesicle (GV) stage, prior to in vitro maturation (IVM) or at metaphase-II (M-II), after IVM. Sibling GV-stage oocytes from stimulated ICSI cycles were allocated to freezing either prior to (n=109) or after (n=107) IVM. Cumulus-free oocytes were cryopreserved using a choline-substituted slow-freezing protocol and matured in a defined medium, with analysis of chromatin, microtubules, and microfilaments by three-dimensional imaging. Cryopreserved oocytes were compared with oocytes matured in vitro but never frozen (n=114). Survival was similar between oocytes frozen before or after IVM (69.7% vs. 70.5%). Polar body extrusion after IVM was lower in oocytes frozen at the GV stage versus those matured and then frozen (51.3% vs. 75.7%) or not frozen (75.4%). Stratification by patient age (<36 and ⩾36year) showed no difference in oocyte survival or maturation. Oocytes frozen as GVs showed elevated proportions of spontaneous activation (with or without polar body), an effect augmented by patient age. Spindle and chromosome configurations were disrupted to similar extents in both groups of frozen oocytes, with no further detrimental effect of patient age. The length, width, and volume of bipolar M-II spindles were comparable in all three groups. When frozen as GVs, oocytes exhibited decreased maturation and increased spontaneous activation, suggesting that it is best to freeze oocytes at M-II.

Cryopreservation of immature buffalo oocytes: effects of cytochalasin B pretreatment on the efficiency of cryotop and solid surface vitrification methods

The aim of the present study was to compare the efficiency of the solid surface (SSV), cryotop (CT) vitrification methods and cytochalasin B (CB) pretreatment for cryopreservation of immature buffalo oocytes. Cumulus-oocyte complexes (COCs) were placed for 1 min in TCM199 containing 10% dimethylsulfoxide (DMSO), 10% ethylene glycol (EG), and 20% fetal bovine serum, and then transferred for 30 s to base medium containing 20% DMSO, 20% EG and 0.5 mol/L sucrose. CB pretreated ((+)CB) or non-pretreated ((-)CB) COCs were vitrified either by SSV or CT. Surviving vitrified COCs were selected for in vitro maturation (IVM) and in vitro fertilization (IVF). The rate of viable oocytes after vitrification in CT groups (82%) was significantly lower (P < 0.05) than that in a fresh control group (100%), but significantly higher (P < 0.05) than those in SSV groups (71-72%). Among vitrified groups, the highest maturation rate was obtained in the CT (-)CB group (32%). After IVF, the cleavage and blastocyst formation rates were similar among vitrified groups but significantly lower than those of the control group. In conclusion, a higher survival rate of oocytes after vitrification and IVM was obtained in the CT group compared with that in the SSV group, indicating the superiority of the CT method. Pretreatment with CB did not increase the viability, maturation or embryo development of vitrified oocytes.

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.

Cryopreservation of human failed maturation oocytes shows that vitrification gives superior outcomes to slow cooling

This study investigated whether failed maturation oocytes could be used to evaluate different cryopreservation procedures. A total of 289 failed maturation oocytes (GV and MI stages), obtained from 169 patients undergoing IVF treatment (mean age 33.84±5.0) were divided into two different slow-cooling groups (1.5 mol/l 1,2-propanediol+0.2 mol/l sucrose in either NaCl (group A) or choline chloride (ChCl) (group B) based cryopreservation solutions) and one vitrification group (15% ethylene glycol+15% dimethyl sulphoxide). Survival rate, in vitro maturation (IVM) rate, fertilization and developmental rate of cryopreserved oocytes were assessed. Regardless of the stage at which cryopreservation was performed (GV+MI), the slow cooling with ChCl based medium always gave significantly lower survival rate than the slow cooling in NaCl based medium (p=0.01) and vitrification (p<0.001). An extended study also showed statistically reduced survival rate between slow-cooling NaCl based medium and vitrification (p<0.05). Global results of in vitro maturation and fertilization showed worse results between both slow-cooling NaCl and ChCl based media versus vitrification. In conclusion, for oocytes that had failed to mature, vitrification gave better survival, maturation, fertilization and also cleavage rates than the slow-cooling protocols. Four cells embryos were obtained only from vitrified in vitro matured MI oocytes.

Effect of dehydration prior to cryopreservation of large equine embryos

Cryopreservation of equine embryos>300microm in diameter results in low survival rates using protocols that work well for smaller equine embryos. These experiments tested the potential benefit of incorporating a dehydration step prior to standard cryopreservation procedures. Forty-six, day 7-8, grade 1, equine embryos 300-1350microm in diameter were subjected to one of the following treatments: (A) 2 min in 0.6M galactose, 10min in 1.5M glycerol, slow freeze (n=21); (B) 10min in 1.5M glycerol, slow freeze (n=15); (C) 2min in 0.6M galactose, 10min in 1.5M glycerol, followed by exposure to thaw solutions, then culture medium (n=5); (D) transferred directly to culture medium (n=5). Frozen embryos were thawed and subjected to a three-step cryoprotectant removal. Five embryos from each treatment were evaluated morphologically after 24 and 48h culture (1=excellent, 5=degenerate/dead). All treatments had at least 4/5 embryos with a quality score >or=3 at these time points except treatment B (2/5 at 24h, 1/5 at 48h). Subsequent embryos from treatment A (n=16) or B (n=10) were matched in sets of two for size and treatment, thawed, and immediately transferred in pairs to 13 recipients. Only two recipient mares were pregnant; one received two 400microm embryos from treatment A, and the other one 400 and one 415microm embryo from treatment B. There was no advantage of incorporating a 2min dehydration step into the cryopreservation protocol for large equine embryos.

Effect of droplet vitrification on development competence, actin cytoskeletal integrity and gene expression in in vitro cultured mouse embryos

The effect of modified droplet vitrification was assessed on cellular actin filament organization, apoptosis related gene expression and development competence in mouse embryos cultured in vitro. Mouse zygotes, 2-cell embryos and morulae were vitrified in ethylene glycol (VS-1) and ethylene glycol plus DMSO (VS-2) and thawed by directly placing the vitrified drop into 0.3M sucrose solution at 37 degrees C. High recovery (93-99%) of morphologically normal embryos was evident following vitrification and thawing. No detectable actin filament disruption was observed in the embryos at any development stage following vitrification and thawing and/or in vitro culture. The expression pattern of Bax, Bcl2 and p53 genes was altered (P<0.05) in vitrified zygotes and 2-cell embryos, but not in morulae. Although a large proportion of the vitrified zygotes (59.5+/-4.4% in VS-1 and 57.9+/-4.5% in VS-2; mean+/-S.E.M.) and 2-cell embryos (63.1+/-4.4% in VS-1 and 59.2+/-4.3% in VS-2) developed into blastocysts, development of control embryos (70.2+/-5.0% of zygotes and 75.5+/-4.4% of 2-cell embryos) into blastocysts was higher (P<0.05). In contrast, development of the control and vitrified morulae into blastocysts (more than 85%) was similar. We concluded that the modified droplet vitrification procedure supported better survival of morula stage compared to zygotes and 2-cell mouse embryos.

Development of cryopreservation protocols for early stage zebrafish (Danio rerio) ovarian follicles using controlled slow cooling

Cryopreservation of germplasm of aquatic species offers many benefits to the fields of aquaculture, conservation and biomedicine. Although successful fish sperm cryopreservation has been achieved with many species, there has been no report of successful cryopreservation of fish embryos and late stage oocytes which are large, chilling sensitive and have low membrane permeability. In the present study, cryopreservation of early stage zebrafish ovarian follicles was studied for the first time using controlled slow freezing. The effect of cryoprotectant, freezing medium, cooling rate, method for cryoprotectant removal, post-thaw incubation time and ovarian follicle developmental stage were investigated. Stages I and II ovarian follicles were frozen in 4M methanol and 3M DMSO in either L-15 medium or KCl buffer. Ovarian follicle viability was assessed using trypan blue, FDA+PI staining and ADP/ATP assay. The results showed that KCl buffer was more beneficial than L-15 medium, methanol was more effective than DMSO, optimum cooling rates were 2-4 degrees C/min, stepwise removal of cryoprotectant improved ovarian follicle viability significantly and stage I ovarian follicles were more sensitive to freezing. The results also showed that FDA+PI staining and ADP/ATP assay were more sensitive than TB staining. The highest follicle viabilities after post-thaw incubation for 2h obtained with FDA+PI staining were 50.7+/-4.0% although ADP/ATP ratios of the cryopreserved follicles were significantly increased indicating increased cell damage. Studies are currently being carried out on in vitro maturation of these cryopreserved ovarian follicles.

Impact of anisosmotic conditions on structural and functional integrity of cumulus-oocyte complexes at the germinal vesicle stage in the domestic cat

During cryopreservation, the immature oocyte is subjected to anisosmotic conditions potentially impairing subsequent nuclear and cytoplasmic maturation in vitro. In preparation for cryopreservation protocols and to characterize osmotic tolerance, cat cumulus-oocyte complexes (COC) at the germinal vesicle (GV) stage were exposed for 15 min to sucrose solutions ranging from 100 to 2,000 mOsm and then examined for structural integrity and developmental competence in vitro. Osmolarities > or =200 and < or =750 mOsm had no effect on incidence of oocyte nuclear maturation, fertilization success, and blastocyst formation compared to control COC (exposed to 290 mOsm). This relatively high osmotic tolerance of the immature cat oocyte appeared to arise from a remarkable stability of the GV chromatin structure as well as plasticity in mitochondrial distribution, membrane integrity, and ability to maintain cumulus-oocyte communications. Osmolarities <200 mOsm only damaged cumulus cell membrane integrity, which contributed to poor nuclear maturation but ultimately had no adverse effect on blastocyst formation in vitro. Osmolarities >750 mOsm compromised nuclear maturation and blastocyst formation in vitro via disruption of cumulus-oocyte communications, an effect that could be mitigated through 1,500 mOsm by adding cytochalasin B to the hyperosmotic solutions. These results (1) demonstrate, for the first time, the expansive osmotic tolerance of the immature cat oocyte, (2) characterize the fundamental role of cumulus-oocyte communications when tolerance limits are exceeded, and (3) reveal an interesting hyperosmotic tolerance of the immature oocyte that can be increased two-fold by supplementation with cytochalasin B.

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.

Improved cryopreservation of bovine preimplantation embryos cultured in chemically defined medium

The aim of this study was to examine the effects of modifications to a standard slow freezing protocol on the viability of in vitro produced bovine embryos. Bovine oocytes were matured, fertilized with frozen-thawed semen, and presumptive zygotes cultured in defined two-step culture media. The standard freezing medium was 1.5M ethylene glycol (EG), 0.1M sucrose, 10% fetal bovine serum (FBS) in Dulbecco's phosphate buffered saline (D-PBS). A preliminary trial showed that in vitro produced embryos cryopreserved in this medium had a survival rate of 74.6% at 24h and 53.5% at 48 h post-thaw. Experiment 1 studied the effects of omitting the sucrose supplement or replacing it with 0.1M xylose. In Experiment 2, the effects of partial (0%, 25% or 50%) or total (100%) replacement of sodium chloride with choline chloride in the cryopreservation medium were examined (the medium with 100% replacement was designated CJ1). The effects of replacing the 10% FBS with 0.4% BSA or 0.4% lipid-rich BSA (Albumax I) in CJ1 was studied in Experiment 3. In Experiment 4, pregnancy/calving rates following the post-thaw transfer of in vitro produced embryos cryopreserved in the standard freezing medium were compared with those of in vitro and in vivo produced embryos cryopreserved in the improved medium (Albumax I in CJ1). Supplementation of the cryopreservation medium with 0.1M sucrose resulted in higher post-thaw survival rates at 24 h (71.3% versus 53.5 and 51.7%; P<0.05), 48 h (51.1% versus 45.3 and 40.2%), and 72 h (34.0% versus 24.4 and 23.0%) than 0.1M xylose or no supplement, respectively, in Experiment 1. Experiment 2 showed that embryos cryopreserved in the standard medium had poorer survival rates at 24 h (72.8% versus 86.5%; P<0.05), 48 h (53.1% versus 66.3%) or 72 h (28.4% versus 44.9%) than those frozen in CJ1. The post-thaw survival rate of embryos frozen in medium supplemented with Albumax I was better than that for the FBS or BSA supplements at 24h (92.0% versus 90.7 and 87.3%), 48 h (87.3% versus 76.9 and 70.9%; P<0.05), and 72 h (70.4% versus 49.1 and 46 4%; P<0.05; Experiment 3). In Experiment 4, in vitro produced embryos cryopreserved in CJ1 medium supplemented with Albumax I resulted in higher pregnancy rates at Day 35 (31.9% versus 22.9%) and Day 60 (24.1% versus 14.3%) of gestation, and calving rates (22.6% versus 10.0%; P<0.05) than similar embryos frozen in the standard medium. However, in vivo produced embryos cryopreserved in Albumax I in CJ1 resulted in higher pregnancy rates at Day 35 (50.7%; P<0.05) and Day 60 (45.1%; P<0.05) of gestation, and calving rate (43.7%; P<0.05). It was concluded that modification of the freezing medium by addition of lipid-rich BSA and replacing sodium chloride with choline chloride improves the post-thaw survival of in vitro produced embryos, and their viability post-transfer.

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.

Effects of vitrification procedures on subsequent development and ultrastructure of in vitro-matured swamp buffalo (Bubalus bubalis) oocytes

The purpose of the present study was to investigate the effects of two vitrification procedures on developmental capacity and ultrastructural changes of matured swamp buffalo oocytes. In vitro-matured oocytes were vitrified by using 35 and 40% ethylene glycol as vitrification solution for solid surface vitrification (SSV) and in-straw vitrification (ISV), respectively. Survival rate of vitrified-warmed oocytes, evaluated on the basis of ooplasm homogeneity, oolemma integrity and zona pellucida intactness, as well as parthenogenetic blastocyst rates of vitrified-warmed oocytes were significantly higher with SSV (89.3 and 13.6%, respectively) than ISV (81.8 and 5.5%, respectively). However, they were still significantly lower than that of control oocytes (100 and 34.2%, respectively). For examining the ultrastructural changes, fresh, VS-exposed (ISV and SSV), and vitrified-warmed (ISV and SSV) oocytes were processed for transmission electron microscopy. In VS-exposed oocytes, reduction of microvilli abundance and damage of mitochondrial membrane were found only in the ISV group. In vitrified-warmed oocytes, however, it was clear that both methods of vitrification induced profound ultrastructural modifications to microvilli, mitochondria, oolemma and cortical granules as well as to the size and position of vesicles. Damaged mitochondria were, however, more abundant in ISV vitrified oocytes than in SSV vitrified oocytes, which correlated with the developmental data, showing the superiority of the SSV method. The present study demonstrated the feasibility of vitrification of in vitro-matured swamp buffalo oocytes.

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.

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.

Vitrification of Immature Bovine Oocytes by the Microdrop Method

This study was conducted to determine the optimal vitrification conditions for immature bovine oocytes using the microdrop method. In experiment 1, the optimal pre-equilibration period for microdrop vitrification was examined. The maturation rate of vitrified oocytes with a 3 min first pre-equilibration period (41.1%) was higher than that of vitrified oocytes with a 0 min first pre-equilibration period (21.4%), and the values of those with a 1 (33.9%) or 5 min (27.4%) first pre-equilibration period were intermediate. The value for a 1 min second pre-equilibration period (44.4%) was significantly higher (P<0.05) than those for a 0.5 (28.6%) and 2 min (21.4%) second pre-equilibration period. In experiment 2, the distribution of microtubules in matured oocytes was investigated. There was no difference among the first pre-equilibration times in terms of the rates of normal spindles in vitrified oocytes. However, this value was significantly higher (P<0.05) in the 1 min group (52.8%) compared with the 0.5 (16.7%) and 2 min groups (12.3%). In experiment 3, we investigated the developmental capacity of immature bovine oocytes vitrified under optimal pre-equilibration conditions (3 min and 1 min for the first and second pre-equilibrations, respectively). Although the total fertilization rates were significantly lower (P<0.05) in the vitrified oocytes (65.6%) compared with the control oocytes (92.4%), there was no difference in the rate of normal fertilization (2PN) between the vitrified (78.6%) and control (82.0%) oocytes. Cleavage and blastocyst rates were significantly lower (P<0.05) in vitrified oocytes (55.7 and 2.3%) than in control oocytes (84.4 and 34.7%). Thus, these results indicated that immature bovine oocytes can survive after microdrop vitrification and subsequently can be cultured to mature oocytes capable of undergoing fertilization in vitro and developing into blastocysts.

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.

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.

1,2-propanediol and the type of cryopreservation procedure adversely affect mouse oocyte physiology

BACKGROUND

The aim of this work was to examine the effect of 1,2-propanediol (PrOH) and type of cryopreservation procedure (slow freezing and vitrification) on oocyte physiology.

METHODS

Intracellular calcium of mouse metaphase II (MII) oocytes was quantified by fluorescence microscopy. The effect of PrOH on cell physiology was further assessed through analysis of zona pellucida hardening and cellular integrity. Protein profiles of cryopreserved oocytes were generated by time-of-flight mass spectrometry (TOF-MS).

RESULTS

PrOH caused a protracted increase in calcium, which was sufficient to induce zona pellucida hardening and cellular degeneration. Using 'nominally calcium free' media during PrOH exposure significantly reduced the detrimental effects. Proteomic analysis identified numerous up- and down-regulated proteins after slow freezing when compared with control and vitrified oocytes.

CONCLUSIONS

Using such approaches to assess effects on cellular physiology is fundamental to improving assisted reproduction techniques (ART). This study demonstrates that PrOH causes a significant rise in intracellular calcium. Using calcium-free media significantly reduced the increase in calcium and the associated detrimental physiological effects, suggesting that calcium-free media should be used with PrOH. In addition, analysis of the oocyte proteome following cryopreservation revealed that slow freezing has a significant effect on protein expression. In contrast, vitrification had a minimal impact, indicating that it has a fundamental advantage for the cryopreservation of oocytes.

Fertility preservation in cancer patients

Fertility preservation is an important but often neglected need of cancer patients. There are several options available but many are considered experimental or are unproven. Most require interventions that can postpone the initiation of cancer treatment. The options include surgical procedures to move the ovaries out of the path of a radiation field, prophylactic medical therapy with gonadotropin-releasing hormone agonists and storage of gametes, embryos or gonadal tissue. Embryo freezing after conventional in vitro fertilization has a high success rate. Experimental fertility preservation procedures such as oocyte freezing and ovarian tissue cryopreservation/transplantation should be offered to cancer patients only under institutional review board oversight. Laparoscopic ovarian transposition is a viable option if radiotherapy is to be used alone. Oncologists, reproductive endocrinologists and other healthcare professionals should be part of a multidisciplinary team that offers cancer patients options to preserve their reproductive potential if they desire to do so.

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.

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.

Successful cryopreservation of Pacific oyster (Crassostrea gigas) oocytes

Protocols for cryopreservation of sperm and oocytes would provide the ultimate control over parental crosses in selective breeding programmes. Sperm freezing is routine for many species, but oocyte freezing remains problematic, with virtually zero success in aquatic species to date. This paper describes the development of a successful protocol for cryopreserving high concentrations of Pacific oyster (Crassostrea gigas) oocytes. Ethylene glycol (10%) and dimethyl sulfoxide (15%) were found to be the most effective cryoprotectants resulting in post-thaw fertilization rates of 51.0+/-8.0 and 45.1+/-8.3%, respectively. Propylene glycol was less effective and methanol resulted in zero fertilization post-thaw. The use of Milli-Q water rather than seawater as a base medium significantly improved fertilization (20.4+/-3.0 and 8.7+/-2.2%, respectively) as did the inclusion of a 5 min isothermal hold at -10 or -12 degrees C (35.9+/-5.0 and 31.9+/-4.6%, respectively). The optimal cooling rate post-hold was 0.3 degrees C min(-1), with virtually zero post-thaw fertilization with cooling rates of 3 and 6 degrees C min(-1). Using an optimized protocol, post-thaw fertilization rates for oocytes from eight individual females ranged from 0.8 to 74.5% and D-larval yields from 0.1 to 30.1%. For three individuals, larvae were reared through to spat. Development of D-larvae to eyed larvae and spat was similar for larvae produced from unfrozen (24.8+/-4.1% developed to eyed larvae and 16.5+/-3.2% to spat) and cryopreserved (28.4+/-0.6 and 18.7+/-0.5%, respectively) oocytes. The ability to cryopreserve large quantities of oyster oocytes represents a major advance in cryobiology and selective breeding.

Extrusion and removal of lipid from the cytoplasm of porcine oocytes at the germinal vesicle stage: Centrifugation under hypertonic conditions influences vitrification

In the present study, we examined a novel lipid removal method, centrifugation in solutions made hypertonic by adding 0.27 M sugar. This allowed the lipid to be extruded and removed without the loss of active mitochondria or extra cytoplasm. The type of sugar influenced the proportion of oocytes that could be stratified by centrifugation. Glucose induced the highest extrusion rate of lipid droplets. After vitrification the rates of survival, germinal vesicle breakdown and metaphase II were 30, 26, and 7%, respectively, for lipid-removed GV oocytes; this was significantly higher (P<0.05) than for corresponding vitrified lipid-intact oocytes (2, 0, and 0%, respectively). These results indicated that this method is useful to remove whole lipid droplets without losing mitochondria and improves cryotolerance of porcine GV oocytes.

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.

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.

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

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

Ovarian function preservation in the cancer patient

OBJECTIVE

To review the clinical modalities that are available to women receiving potentially sterilizing cancer therapy.

DESIGN

The MEDLINE database was reviewed for all publications on medication, surgery, or assisted reproductive technology that could potentially preserve fertility in women who are receiving cancer therapy.

CONCLUSION(S)

There are many options available to a patient undergoing a treatment that will negatively impact her fertility. Many procedures and medical interventions have been proven successful both in terms of ovarian function and pregnancy rates. Other techniques have great potential but do not have long-term clinical data. It is important that the patient's primary care physician understand the methods available to preserve fertility in cancer patients and communicate this information to the patient.

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.

Fundamentals of cryobiology in reproductive medicine

The aim of this review will be to provide a basic understanding of the biophysical processes that accompany the application of cryopreservation in reproductive medicine. The ability to store cells in 'suspended animation' outside the body has become a keystone practice in the development of many modern clinical therapies, and, in fact, the sciences of cryobiology and IVF have developed in parallel over the past 50 years. During this time, some of the underlying principles of the quantitative biophysical aspects of cryobiology have been clarified. Water is the universal biocompatible solvent, but also possesses unique properties for stability of living cells. Whilst low temperatures themselves have defined effects on cell structure and function, it is the phase transition of water to ice that is the most profound challenge for survival. The thermodynamics of dilute aqueous solutions dictate how cells and tissues respond to the freezing process. Current concepts of nucleation, ice crystal growth and solute exclusion from the ice lattice will be discussed to illustrate what cells must negotiate to avoid lethal damage, and the role of cryoprotectants in enhancing recovery. Quantitative formalisms now exist to model and predict how water and solutes move across cell membranes before and during freezing, or how nucleation events will proceed, and these will be outlined. Cryoprotectants have both positive and negative effects on cell function depending on the kinetics of exposure. The concept of tolerable osmotic excursion of cell volume will be discussed, along with the evidence for a 'pseudo-glassy' state for cells during traditional cryopreservation. This will be compared with the recent interest in promoting glassy states in the whole sample using vitrification protocols, outlining the advantages and drawbacks of each approach. Additional methods for controlling ice nucleation have a role to play here, and a brief outline of current technologies will be given. Finally, issues of safety and stability of cryopreserved samples will be set out.

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

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

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

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