Effect of osmotic stress on the developmental competence of germinal vesicle and metaphase II stage bovine cumulus oocyte complexes and its relevance to cryopreservation

Meta-analysis of the Boyle van 't Hoff relation: Turgor and leak models explain non-ideal volume equilibrium

There has been much recent attention paid to the interaction of cell volume, its regulation, and the molecular biology of the cell. Cells are generally assumed to behave as linear osmometers, with their water volume linearly proportionate to the inverse of osmotic pressure as described by the Boyle van 't Hoff (BvH) relation. This study evaluates the generality of this and other long-standing assumptions about cell responses to anisotonic conditions. We present alternative models that account for osmoregulation including mechanical resistance to volumetric expansion (the turgor model) and ion-osmolyte leakage (the leak model). To evaluate the generality of the BvH relation and determine the suitability of alternative models, we performed a comprehensive survey of the literature and a careful analysis of the resulting data, and then we used these data to compare among models. We identified 137 articles published from 1964 to 2019 spanning 14 animal species and 26 cell types and determined the BvH relation is not an appropriate general model but is adequate when restricted to the hypertonic region. In contrast, models that account for either mechanical resistance or ion-osmolyte leakage fit well to almost all collected data. The leak model has fitted parameters that are in the same range as the current literature estimate, while the turgor model typically requires an elastic modulus value of one or multiple orders of magnitude larger than literature values. However, confirmation of the underlying mechanism of osmotic regulation is required at the cell-specific level and cannot be assumed a priori.

The Role of Aquaporin 7 in the Movement of Water and Cryoprotectants in Bovine In Vitro Matured Oocytes

Simple Summary

The permeability of the plasma membrane to water and cryoprotectants is a critical factor in the effective vitrification of oocytes. The goal of this study is to better understand the pathways used to transport water and other cryoprotectants through the plasma membrane of bovine in vitro matured oocytes, with a focus on the role of aquaporin 7 (AQP7). We demonstrated that cryoprotectants stimulated AQP3 and AQP7 but not AQP9 expression in mature bovine oocytes. Dimethyl sulfoxide upregulates AQP3 expression, while ethylene glycol upregulates AQP7 expression in oocytes in a CPA-dependent fashion. We also demonstrated that exogenous expression of aquaglyceroporins such as AQP7 is possible in in vitro matured oocytes. When permeability values for membrane transport of dimethyl sulfoxide, ethylene glycol and sucrose were assessed, we observed that AQP7 overexpressed oocytes are more permeable to water in the presence of dimethyl sulfoxide solution. These biophysical characteristics, together with the use of membrane transport modeling, will allow re-evaluation and possibly improvement of previously described protocols for bovine oocyte cryopreservation.

Abstract

Aquaglyceroporins are known as channel proteins, and are able to transport water and small neutral solutes. In this study, we evaluate the effect of exposure of in vitro matured bovine oocytes to hyperosmotic solutions containing ethylene glycol (EG), dimethyl sulfoxide (Me₂SO) or sucrose on the expression levels of AQP3, AQP7 and AQP9. Moreover, we studied whether artificial protein expression of AQP7 in bovine oocytes increases their permeability to water and cryoprotectants. Exposure to hyperosmotic solutions stimulated AQP3 and AQP7 but not AQP9 expression. Oocytes exposed to hyperosmotic Me₂SO solution exhibited upregulated AQP3 expression, while AQP7 expression was upregulated by EG hyperosmotic exposure. Microinjection of oocytes at the germinal vesicle stage with enhanced green fluorescent protein (EGFP) or EGFP+AQP7 cRNAs resulted in the expression of the corresponding proteins in ≈86% of the metaphase-II stage oocytes. AQP7 facilitated water diffusion when bovine MII oocytes were in presence of Me₂SO solution but not EG or sucrose solution. However, the overexpression of this aquaporin did not increase membrane permeability to Me₂SO or EG. In summary, cryoprotectant-induced increase of AQP3 and AQP7 expression could be one of the mechanisms underlying oocyte tolerance to hyperosmotic stress. Water diffusion appears to be improved when AQP7 overexpressed oocytes are exposed to Me₂SO, shortening the time required for oocytes to achieve osmotic balance with cryoprotectant solutions.

Effects of the preservation medium and storage duration of domestic cat ovaries on the maturational and developmental competence of oocytes in vitro

We examined the effectiveness of saline, Euro-Collins solution (EC), and ET-Kyoto solution (ET-K) as preservation media for the cold storage of feline ovaries. Ovaries were maintained in these media at 4°C for 24, 48, or 72 h until oocyte retrieval. The ET-K group exhibited a higher oocyte maturation rate than the saline group after 72 h of storage. Moreover, ET-K could sustain the competence of the feline oocytes to cleave after 48 h, and the morula formation rate of the ET-K group was higher than that of the other groups after 24 and 48 h. Furthermore, the ET-K group exhibited a higher blastocyst formation rate than the other groups after storage for 24 h, and only ET-K retained the developmental competence in blastocysts after 48 h of storage. In addition, regarding the cell numbers of the blastocysts, there was no significant difference among the tested groups. In conclusion, our results indicate that ET-K is a suitable preservation medium for feline ovaries.

Treatment of mouse cumulus-oocyte complexes with L-carnitine during vitrification and in vitro maturation affects maturation and embryonic developmental rate after parthenogenetic activation

The technique of oocyte vitrification remains a challenge in most animal species. The present study aimed to evaluate the effects of cumulus cell presence and L-carnitine (LC) treatment during vitrification of selected immature oocytes by brilliant cresyl blue (BCB) staining on maturation and embryonic developmental rate after parthenogenetic activation. Immature oocytes were obtained from C57BL/6 female mice ovaries and stained with BCB. The BCB⁺ cumulus-oocyte complexes (COCs) were then selected and random parts of COCs were denuded from cumulus cells (denuded oocytes: DOs). COCs and DOs were treated with/out LC (0.6 mg/ml) during vitrification and in vitro maturation (IVM) procedures. A number of non-vitrified COCs were also treated with LC during the IVM process (fresh group). Maturation rate, intracellular glutathione (GSH) contents, and developmental competence of oocytes were also examined. The GSH levels in vitrified DOs+LC and vitrified COCs+LC groups were significantly higher (p < 0.01) than untreated vitrified-warmed COCs and DOs. Maturation rate and blastocyst developmental rate were reduced after the vitrification-warming procedure compared with the fresh group. The vitrified COCs+LC group showed a higher percentage of mature oocytes and the ability to develop to blastocyst stage than the vitrified-warmed DOs group (p < 0.01). These data indicated that the presence of cumulus cells around the competent oocyte and LC treatment during vitrification and IVM procedure could improve parthenogenetic developmental competence of vitrified-warmed oocytes by increasing GSH levels and accelerating oocyte maturation.

Oocyte Cryopreservation in Domestic Animals and Humans: Principles, Techniques and Updated Outcomes

Oocyte cryopreservation plays important roles in basic research and the application of models for genetic preservation and in clinical situations. This technology provides long-term storage of gametes for genetic banking and subsequent use with other assisted reproductive technologies. Until recently, oocytes have remained the most difficult cell type to freeze, as the oocytes per se are large with limited surface area to cytoplasm ratio. They are also highly sensitive to damage during cryopreservation, and therefore the success rate of oocyte cryopreservation is generally poor when compared to noncryopreserved oocytes. Although advancement in oocyte cryopreservation has progressed rapidly for decades, the improvement of cryosurvival and clinical outcomes is still required. This review focuses on the principles, techniques, outcomes and prospects of oocyte cryopreservation in domestic animals and humans.

Effect of cryoprotectant concentration on bovine oocyte permeability and comparison of two membrane permeability modelling approaches

The plasma membrane permeability to water and cryoprotectant (CPA) significantly impacts vitrification efficiency of bovine oocytes. Our study was designed to determine the concentration-dependent permeability characteristics for immature (GV) and mature (MII) bovine oocytes in the presence of ethylene glycol (EG) and dimethyl sulphoxide (Me₂SO), and to compare two different modeling approaches: the two parameter (2P) model and a nondilute transport model. Membrane permeability parameters were determined by consecutively exposing oocytes to increasing concentrations of Me₂SO or EG. Higher water permeability was observed for MII oocytes than GV oocytes in the presence of both Me₂SO and EG, and in all cases the water permeability was observed to decrease as CPA concentration increased. At high CPA concentrations, the CPA permeability was similar for Me₂SO and EG, for both MII and GV oocytes, but at low concentrations the EG permeability of GV oocytes was substantially higher. Predictions of cell volume changes during CPA addition and removal indicate that accounting for the concentration dependence of permeability only has a modest effect, but there were substantial differences between the 2P model and the nondilute model during CPA removal, which may have implications for design of improved methods for bovine oocyte vitrification.

Cryopreservation of equine oocytes: looking into the crystal ball

Invitro embryo production has evolved rapidly in the horse over the past decade, but blastocyst rates from vitrified equine oocytes remain quite poor and further research is needed to warrant application. Oocyte vitrification is affected by several technical and biological factors. In the horse, short exposure of immature oocytes to the combination of permeating and non-permeating cryoprotective agents has been associated with the best results so far. High cooling and warming rates are also crucial and can be obtained by using minimal volumes and open cryodevices. Vitrification of invivo-matured oocytes has yielded better results, but is less practical. The presence of the corona radiata seems to partially protect those factors that are necessary for the construction of the normal spindle and for chromosome alignment, but multiple layers of cumulus cells may impair permeation of cryoprotective agents. In addition to the spindle, the oolemma and mitochondria are also particularly sensitive to vitrification damage, which should be minimised in future vitrification procedures. This review presents promising protocols and novel strategies in equine oocyte vitrification, with a focus on blastocyst development and foal production as most reliable outcome parameters.

Transport processes in equine oocytes and ovarian tissue during loading with cryoprotective solutions

BACKGROUND

Rational design of cryopreservation strategies for oocytes and ovarian cortex tissue requires insights in the rate at which cryoprotective agents (CPA) permeate and concomitant water transport takes place. The aim of the current study was to investigate possible differences in permeation kinetics of different CPAs (i.e., glycerol/GLY, ethylene glycol/EG, dimethyl sulfoxide/DMSO, and propylene glycol/PG), in equine oocytes as well as ovarian tissue.

METHODS

Membrane permeability of oocytes to water (Lp) and to CPAs (Ps) was inferred from video microscopic imaging of oocyte volume responses during perfusion with anisotonic and CPA solutions. CPA diffusion into ovarian tissue and tissue dehydration was monitored during incubation, using osmometer and weight measurements, to derive CPA diffusion coefficients (D).

RESULTS

Membrane permeability of oocytes towards CPAs was found to increase in the order GLY < EG < DMSO<PG. Permeability towards water in anisotonic solutions was determined to be higher than in CPA solutions, indicating CPAs alter membrane permeability properties. CPA diffusion in ovarian tissue increased in the order GLY,PG < EG,DMSO. Tissue dehydration was found to increase with exposure to increasing CPA concentrations, which inversely correlated with CPA diffusivity.

CONCLUSIONS

In conclusion, it is shown here that the rate of CPA movement across membrane bilayers is determined by different physical barrier factors than those determining CPA movement in tissues.

GENERAL SIGNIFICANCE

The parameters presented in this study can be applied in models describing solute and water transport in cells and tissues, as well as in cryopreservation protocols.

Methodological approaches for vitrification of bovine oocytes

Numerous factors affect vitrification success and post-thaw development of oocytes after in vitro fertilization. Therefore, elaboration of an optimal methodology ensuring higher cryotolerance of oocytes and subsequent blastocyst yield is still of great interest. This paper describes and evaluates critical factors affecting the success of oocyte vitrification. In particular, an appropriate oocyte stage such as maturation status (germinal vesicle stage, metaphase II stage), presence/absence of cumulus cells before vitrification, and the effect of follicle size, as well as different culture systems and media for in vitro production of embryos, the types and concentrations of cryoprotectants, and cooling and warming rates at vitrification are considered. Special attention is paid to various cryocarriers used for low-volume vitrification, which ensures safe storage of oocytes/embryos in liquid nitrogen and their successful post-thaw recovery. At the end, we focussed on how age of oocyte donors (heifers, cows) influences post-thaw development. This review summarizes results of recently published studies describing different methodologies of cryopreservation and post-thaw oocyte development with the main focus on vitrification of bovine oocytes.

Probing lasting cryoinjuries to oocyte-embryo transcriptome

Clinical applications of oocytes cryopreservation include preservation of future fertility of young cancer patients, substitution of embryo freezing to avoid associated legal and ethical issues, and delaying childbearing years. While the outcome of oocyte cryopreservation has recently been improved, currently used vitrification method still suffer from increased biosafety risk and handling issues while slow freezing techniques yield overall low success. Understanding better the mechanism of cryopreservation-induced injuries may lead to development of more reliable and safe methods for oocyte cryopreservation. Using the mouse model, a microarray study was conducted on oocyte cryopreservation to identify cryoinjuries to transcriptionally active genome. To this end, metaphase II (MII) oocytes were subjected to standard slow freezing, and then analyzed at the four-cell stage after embryonic genome activation. Non-frozen four-cell embryos served as controls. Differentially expressed genes were identified and validated using RT-PCR. Embryos produced from the cryopreserved oocytes displayed 200 upregulated and 105 downregulated genes, associated with the regulation of mitochondrial function, protein ubiquitination and maintenance, cellular response to stress and oxidative states, fatty acid and lipid regulation/metabolism, and cell cycle maintenance. These findings reveal previously unrecognized effects of standard slow oocyte freezing on embryonic gene expression, which can be used to guide improvement of oocyte cryopreservation methods.

Human oocytes and zygotes are ready for ultra-fast vitrification after 2 minutes of exposure to standard CPA solutions

Vitrification of human oocytes and embryos in different stages of development is a key element of daily clinical practice of in vitro fertilization treatments. Despite the cooling and warming of the cells is ultra-fast, the procedure as a whole is time consuming. Most of the duration is employed in a long (8–15 minutes), gradual or direct exposure to a non-vitrifying cryoprotectant solution, which is followed by a short exposure to a more concentrated vitrifying solution. A reduction in the duration of the protocols is desirable to improve the workflow in the IVF setting and reduce the time of exposure to suboptimal temperature and osmolarity, as well as potentially toxic cryoprotectants. In this work it is shown that this reduction is feasible. In silico (MatLab program using two-parameter permeability model) and in vitro observations of the oocytes’ osmotic behaviour indicate that the dehydration upon exposure to standard cryoprotectant solutions occurs very fast: the point of minimum volume of the shrink-swell curve is reached within 60 seconds. At that point, intracellular water ejection is complete, which coupled with the permeation of low molecular weight cryoprotectants results in similar intracellular and extracellular solute concentrations. This shows that prolonging the exposure to the cryoprotectant solutions does not improve the cytosolic glass forming tendency and could be avoided. To test this finding, human oocytes and zygotes that were donated for research were subjected to a shortened, dehydration-based protocol, consisting of two consecutive exposures of one-minute to two standard cryoprotectant solutions, containing ethylene glycol, dimethyl sulfoxide and sucrose. At the end of this two-minute dehydration protocol, the critical intracellular solute concentration necessary for successful vitrification was attained, confirmed by the post-warming survival and ability to resume cytokinesis of the cells. Further studies of the developmental competency of oocytes and embryos would be necessary to determine the suitability of this specific dehydration protocol for clinical practice, but based on our results, short times of exposure to increasingly hypertonic solutions could be a more time-efficient strategy to prepare human oocytes and embryos for vitrification.

Extracellular vesicles of follicular fluid from heat-stressed cows modify the gene expression of in vitro-matured oocytes

The effect of heat stress (HS) on cattle reproduction is deleterious with respect to ovarian follicular development and oocyte quality. The objective of this study was to investigate the effect of follicular fluid extracellular vesicles (EVs) obtained from cows maintained in thermoneutral (TN) or HS conditions on in vitro oocyte maturation. Nonlactating cows were estrous synchronized. Immediately after ovulation day (D1), the cows were randomly assigned to TN or HS environments. Follicular fluid from all follicles from each treatment was pooled, and EVs were obtained. Pools of 20 cumulus oocyte-complexes (COCs), were allocated to the following treatments: Control (n = 4 COC pools): matured in base medium; TN (n = 4 COC pools): matured in base medium supplemented with TN EV suspension; and HS (n = 4 COC pools): matured in base medium that was supplemented with the HS EV suspension. All treatments were conducted at 38.5 °C for 24 h in a humid atmosphere with 5% CO2. After maturation, the COCs were evaluated for meiotic progression, DNA integrity and oocyte quality-related gene expression. When the experimental groups were compared with the control group, a treatment effect was not observed for meiotic progression and DNA integrity. In the cumulus cells of TN group, there was relatively lesser expression of the IGFBP4 gene. In the oocytes of the TN as compared with the HS group, the IGFBP2, BMP15, GDF9, CDCA8, HAS2, RPL15, STAT3 and PFKP genes were expressed to a lesser extent. The findings indicated that oocytes matured in the presence of EVs from the follicular fluid of cows collected when there were TN conditions, however, there was a lesser expression of genes related to oocyte quality.

Application of microfluidic technologies to human assisted reproduction

Microfluidics can be considered both a science and a technology. It is defined as the study of fluid behavior at a sub-microliter level and the investigation into its application to cell biology, chemistry, genetics, molecular biology and medicine. There are at least two characteristics of microfluidics, mechanical and biochemical, which can be influential in the field of mammalian gamete and preimplantation embryo biology. These microfluidic characteristics can assist in basic biological studies on sperm, oocyte and preimplantation embryo structure, function and environment. The mechanical and biochemical characteristics of microfluidics may also have practical and/or technical application(s) to assisted reproductive technologies (ART) in rodents, domestic species, endangered species and humans. This review will consider data in mammals, and when available humans, addressing the potential application(s) of microfluidics to assisted reproduction. There are numerous sequential steps in the clinical assisted reproductive laboratory process that work, yet could be improved. Cause and effect relations of procedural inefficiencies can be difficult to identify and/or remedy. Data will be presented that consider microfluidic applications to sperm isolation, oocyte cumulus complex isolation, oocyte denuding, oocyte mechanical manipulation, conventional insemination, intracytoplasmic sperm injection, embryo culture, embryo analysis and oocyte and embryo cryopreservation. While these studies have progressed in animal models, data with human gametes and embryos are significantly lacking. These data from clinical trials are requisite for making future evidence-based decisions regarding the application of microfluidics in human ART.

Importance of oil overlay for production of porcine embryos in vitro

Technologies to edit the zygote genome have revolutionized biomedical research not only for the creation of animal models for the study of human disease but also for the generation of functional human cells and tissues through interspecies blastocyst complementation technology. The pig is the ideal species for these purposes due to its great similarity in anatomy and physiology to humans. Emerging biotechnologies require the use of oocytes and/or embryos of good quality, which might be obtained using in vitro production (IVP) techniques. However, the current porcine embryo IVP systems are still suboptimal and result in low monospermic fertilization and blastocyst formation rates and poor embryo quality. During recent years, intensive investigations have been performed to evaluate the influence of specific compounds on gametes and embryos and to avoid the use of undefined supplements (serum and serum derivate) in the incubation media. However, little consideration has been given to the use of the mineral oil (MO) to overlay incubation droplets, which, albeit being a routine component of the IVP systems, is a totally undefined and thus problematic product for the safety of gametes and embryos. In this review, we provide an overview on the advantages and disadvantages of using MO to cover the incubation media. We also review one important concern in IVP laboratories: the use of oils containing undetected contamination. Finally, we discuss the effects of different types of oils on the in vitro embryo production outcomes and the transfer of compounds from oil into the culture media.

Cholesterol added prior to vitrification on the cryotolerance of immature and in vitro matured bovine oocytes

This study examines whether incorporating cholesterol-loaded methyl-β-cyclodextrin (CLC) in the bovine oocyte plasma membrane improves oocyte tolerance to vitrification. In vitro matured oocytes were incubated with 2 mg/ml BODIPY-labeled CLC for different time intervals in FCS or PVA supplemented medium or exposed to different CLC concentrations to examine the subcellular localization of cholesterol by confocal microscopy live-cell imaging. Subsequently, the effects of optimized CLC concentrations and incubation times prior to vitrification on early embryo development were assessed. Then, we evaluated the effects of pretreatment with 2 mg/ml CLC for 30 min before the vitrification of immature (GV) and in vitro matured (MII) oocytes on developmental competence and gene expression. Our results indicate a high plasma membrane labeling intensity after 30 min of incubation with 2 mg/ml CLC for 30 min, regardless of the holding medium used. When oocytes were incubated with 1 mg/ml, 2 mg/ml and 3 mg/ml of CLC, intense labeling was observed at the plasma membrane after 40, 30 and 20 min, respectively. CLC pre-treatment before the vitrification of bovine oocytes did not affect subsequent cleavage and embryo development rates irrespective of CLC concentrations, incubation times or meiotic stage. However, pretreatment seems to improve the quality of embryos derived from vitrified oocytes, mainly when oocytes were vitrified at the GV stage.

In Vitro Activation: A Possible New Frontier for Treatment of Primary Ovarian Insufficiency

In vitro activation (IVA) represents a new frontier in the treatment of women with primary ovarian insufficiency as well as patients with cancer desiring fertility preservation. Here, we review the biological basis of IVA and the recent translation of IVA to humans by targeting Hippo and Akt-signaling pathways. We then provide a new integrated viewpoint on IVA, highlighting basic science research on the aspects of follicular development and ovarian tissue transplantation which may potentially optimize future translational research on IVA. Specific topics discussed include cryopreservation techniques, additional IVA pathway targets, the roles of actin polymerization, paracrine and endocrine factors, and the role of mechanical signaling and associated tissue rigidity in controlling ovarian follicular activation. Further research and improved understanding is needed to optimize success of IVA.

Effect of different rehydration temperatures on the survival of human vitrified-warmed oocytes

PURPOSE

To evaluate the effect of different exposure temperatures during the dilution process on the survival rate of vitrified oocytes and following development.

METHODS

Patients were divided at random into two groups for different dilution temperature (20-22 °C, RT group; 37 °C,37 °C group) according to computer-generated random numbers on the day of oocyte warming. The survival and fertilization rates of vitrified oocytes as well as the implantation and clinical pregnancy rates of the resulting embryos were recorded.

RESULTS

A total of 662 and 676 oocytes were warmed in the 37 °C group and RT group, respectively, and significant difference was observed in the survival rate between 37 °C group (88.37%) and RT group (79.88%) (P = 0.0000). There was significant difference between the survival rate of 37 °C group (87.27%) and RT group (75.64%) in nondonor patients (P = 0.0001). Multiple linear regression analysis showed that dilution temperature (β = 0.079, P = 0.017) and clinical outcomes of fresh cycles (β = 0.063, P = 0.001) were significantly and independently associated with survival rate. No significant difference was found between the 37 °C group and RT group in: fertilization rate (66.67 versus 65.37%), implantation rate (20.0 versus 19.46%), clinical pregnancy rate (37.5 versus 35.0%).

CONCLUSIONS

In conclusion, the results of this study give supportive evidence of the application of 37 °C in the dilution process, especially for oocytes of poor quality. Further studies with well-controlled experimental groups are needed to optimize protocols for human oocyte vitrification.

Spindle configuration and developmental competence of in vitro-matured bovine oocytes exposed to NaCl or sucrose prior to Cryotop vitrification

In the present study we examined whether exposure to high concentrations of NaCl or sucrose before vitrification improves the cryotolerance of in vitro-matured bovine oocytes. In Experiment 1, oocytes were exposed to different concentrations of NaCl (375-1517 mOsm) or sucrose (375-812 mOsm) for 1h. On the basis of the results of this experiment, in Experiment 2 oocytes were exposed to 0.25% NaCl (375 mOsmol) or 2.77% sucrose (375 mOsmol) solution, vitrified and warmed. Microtubule and chromosome configurations were examined by immunocytochemistry. In Experiment 3, in vitro embryo development was assessed after vitrification of oocytes with or without 2.77% sucrose (375 mOsmol) pretreatment. There was a similar percentage of oocytes showing normal spindle configurations in the sucrose-pretreated and control groups. Higher rates of abnormal spindles were found in groups treated with NaCl or sucrose solutions with >375 mOsmol. After vitrification and warming, a significantly higher percentage of oocytes with normal chromosome configurations was recorded for oocytes exposed to 375 mOsmol sucrose solution before vitrification compared with the control vitrified oocytes. However, these percentages were significantly lower than those recorded in untreated controls. Cleavage and blastocyst rates were higher in non-vitrified than vitrified oocytes. In conclusion, pretreatment with 375 mOsmol NaCl or sucrose solution had no adverse effects on the spindle status of vitrified-warmed cow oocytes. However, sucrose pretreatment offered no benefits for embryo development.

The Nuclear Maturation and Embryo Development of Mice Germinal Vesicle Oocytes with and without Cumulus Cell after Vitrification

BACKGROUND

Cryobiology is an essential tool in assisted reproductive technology. Research in this area focuses on the possibility of restoring fertility in women with reproductive problems or after cancer treatments.

AIM

The purpose of this study was to evaluate viability of oocytes, In vitro maturation and embryo development in vitrified germinal vesicle oocytes with and without cumulus cell after single and stepwise vitrification procedure.

MATERIALS AND METHODS

Germinal vesicle oocytes with or without cumulus cells were obtained from 4 weeks old female mice 48h after intraperitoneal injection of 7.5 IU pregnant mare serum gonadotropin (PMSG). For vitrification collected oocytes vitrification were exposed to cryoprotectant, which was composed of 30% (v/v) ethylene glycol, 18% (w/v) Ficoll-70, and 0.3 M sucrose, either by single step or in a step-wise way. After exposure to cryoprotectant and immerged in liquid nitrogen, the oocytes were thawed and washed in medium TCM199 two times. Then the oocytes transferred to IVM medium for maturation and embryo development to blastocyst.

RESULTS

The oocytes survival rates after vitrifying-warming, maturation rate, the capacity of fertilization and embryonic development to blastocyst were examined in vitro. The oocytes survival, maturation to MII, fertilization developmental rate in the step-wise exposure and with cumulus cell was significantly higher (p<0.05) as compared with corresponding rate in the single step procedure without cumulus cell.

CONCLUSION

The results of present study indicated that oocytes vitrified with cumulus cells and stepwise procedure had positive effect on maturation and developmental rate to blastocyst than oocytes without cumulus cell and single step procedure.

Slow and steady cell shrinkage reduces osmotic stress in bovine and murine oocyte and zygote vitrification

STUDY QUESTION

Does the use of a new cryoprotectant agent (CPA) exchange protocol designed to minimize osmotic stress improve oocyte or zygote vitrification by reducing sublethal cryodamage?

SUMMARY ANSWER

The use of a new CPA exchange protocol made possible by automated microfluidics improved oocyte and zygote vitrification with superior morphology as indicated by a smoother cell surface, higher sphericity, higher cytoplasmic lipid retention, less cytoplasmic leakage and higher developmental competence compared with conventional methods.

WHAT IS KNOWN ALREADY

The use of more 'steps' of CPA exposure during the vitrification protocol increases cryosurvival and development in the bovine model. However, such an attempt to eliminate osmotic stress is limited by the practicality of performing numerous precise pipetting steps in a short amount of time.

STUDY DESIGN, SIZE, DURATION

Murine meiotically competent germinal vesicle intact oocytes and zygotes were harvested from the antral follicles in ovaries and ampulla, respectively. Bovine ovaries were obtained from a local abattoir at random stages of the estrous cycle. A total of 110 murine oocytes, 802 murine zygotes and 52 bovine oocytes were used in this study.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Microfluidic devices were fabricated using conventional photo- and soft-lithography. CPAs used were 7.5% ethylene glycol (EG) and 7.5% dimethyl sulfoxide (DMSO) for equilibration solution and 15% EG, 15% DMSO and 0.5 M sucrose for vitrification solution. End-point analyses include mathematical modeling using Kedem-Katchalsky equations, morphometrics assessed by conventional and confocal microscopy, cytoplasmic lipid quantification by nile red staining, cytoplasmic leakage quantification by fluorescent dextran intercalation and developmental competence analysis by 96 h embryo culture and blastomere quantification.

MAIN RESULTS AND THE ROLE OF CHANCE

The automated microfluidics protocol decreased the shrinkage rate of the oocyte and zygote by 13.8 times over its manual pipetting alternative. Oocytes and zygotes with a lower shrinkage rate during CPA exposure experienced less osmotic stress resulting in better morphology, higher cell quality and improved developmental competence. This microfluidic procedure resulted in murine zygotes with a significantly smoother cell surface (P < 0.001), more spherical cellular morphology (P < 0.001), increased cytoplasmic lipid retention in vitrified and warmed bovine oocytes (P < 0.01), decreased membrane perforations and cytoplasmic leakage in CPA-exposed murine zygotes (P < 0.05) and improved developmental competence of vitrified and warmed murine zygotes (P < 0.05) than CPA exposure using the current clinically used manual pipetting method.

LIMITATIONS, REASONS FOR CAUTION

It is necessary to design the microfluidic device to be more user-friendly for widespread use.

WIDER IMPLICATIONS OF THE FINDINGS

The theory and approach of eliminating osmotic stress by decreasing shrinkage rate is complementary to the prevalent osmotic stress theory in cryobiology which focuses on a minimum cell volume at which the cells shrink. The auto-microfluidic protocol described here has immediate applications for improving animal and human oocyte, zygote and embryo cryopreservation. On a fundamental level, the clear demonstration that at the same minimum cell volume, cell shrinkage rate affects sublethal damage should be broadly useful for cryobiology.

STUDY FUNDING/COMPETING INTERESTS

This project was funded by the National Institutes of Health and the University of Michigan Reproductive Sciences Program. The authors declare no conflicts of interest.

Optimization of cryoprotectant loading into murine and human oocytes

Loading of cryoprotectants into oocytes is an important step of the cryopreservation process, in which the cells are exposed to potentially damaging osmotic stresses and chemical toxicity. Thus, we investigated the use of physics-based mathematical optimization to guide design of cryoprotectant loading methods for mouse and human oocytes. We first examined loading of 1.5 M dimethyl sulfoxide (Me(2)SO) into mouse oocytes at 23°C. Conventional one-step loading resulted in rates of fertilization (34%) and embryonic development (60%) that were significantly lower than those of untreated controls (95% and 94%, respectively). In contrast, the mathematically optimized two-step method yielded much higher rates of fertilization (85%) and development (87%). To examine the causes for oocyte damage, we performed experiments to separate the effects of cell shrinkage and Me(2)SO exposure time, revealing that neither shrinkage nor Me(2)SO exposure single-handedly impairs the fertilization and development rates. Thus, damage during one-step Me(2)SO addition appears to result from interactions between the effects of Me(2)SO toxicity and osmotic stress. We also investigated Me(2)SO loading into mouse oocytes at 30°C. At this temperature, fertilization rates were again lower after one-step loading (8%) in comparison to mathematically optimized two-step loading (86%) and untreated controls (96%). Furthermore, our computer algorithm generated an effective strategy for reducing Me(2)SO exposure time, using hypotonic diluents for cryoprotectant solutions. With this technique, 1.5 M Me(2)SO was successfully loaded in only 2.5 min, with 92% fertilizability. Based on these promising results, we propose new methods to load cryoprotectants into human oocytes, designed using our mathematical optimization approach.

Cryobanking of farm animal gametes and embryos as a means of conserving livestock genetics

In the last few decades, farm animal genetic diversity has rapidly declined, mainly due to changing market demands and intensification of agriculture. But, since the removal of single species can affect the functioning of global ecosystems, it is in the interest of international community to conserve the livestock genetics and to maintain biodiversity. Increasing awareness on the reduction of breed diversity has prompted global efforts for conservation of farm animal breeds. The goals of conservation are to keep genetic variation as gene combinations in a reversible form and to keep specific genes of interest. For this purpose two types of strategies are usually proposed: in situ and ex situ conservation. In situ conservation is the breed maintaining within the livestock production system, in its environment through the enhancement of its production characteristics. Ex situ in vivo conservation is the safeguard of live animals in zoos, wildlife parks, experimental farms or other specialized centres. Ex situ in vitro conservation is the preservation of genetic material in haploid form (semen and oocytes), diploid (embryos) or DNA sequences. In the last few years, ex situ in vitro conservation programs of livestock genetic resources have focused interest on cryopreservation of gametes, embryos and somatic cells as well as testis and ovarian tissues, effectively lengthening the genetic lifespan of individuals in a breeding program even after the death. However, although significant progress has been made in semen, oocytes and embryo cryopreservation of several domestic species, a standardized procedure has not been established yet. The aim of the present review is to describe the cryobanking purposes, the collection goals, the type of genetic material to store and the reproductive biotechnologies utilized for the cryopreservation of farm animal gametes and embryos.

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.

Cells under pressure: how sublethal hydrostatic pressure stress treatment increases gametes' and embryos' performance

The principal approach in in vitro embryo culture and manipulation has been a defensive one: procedures aim to satisfy passively the supposed or real physiological needs of gametes and embryos. Similarly, during cryopreservation the aim is to cause minimal damage to cells whilst attempting to obtain the highest achievable cell survival. However, carefully chosen and precisely controlled sublethal stress treatment of cells has been described to improve embryos’ and gametes’ performance, and, as a consequence, subsequent morphological survival, fertilisation, in vitro development, pregnancy and farrowing rates improved considerably compared with untreated controls. This review summarises studies that open up a new approach: instead of – and besides – trying to passively reduce the harm to cells during in vitro manipulations and culture, procedures may also prepare the cells themselves to ward off or reduce the damage by turning up the cells’ own, inner capacities.

Cryopreservation of Mammalian oocyte for conservation of animal genetics

The preservation of the female portion of livestock genetics has become an international priority; however, in situ conservation strategies are extremely expensive. Therefore, efforts are increasingly focusing on the development of a reliable cryopreservation method for oocytes, in order to establish ova banks. Slow freezing, a common method for cryopreservation of oocytes, causes osmotic shock (solution effect) and intracellular ice crystallization leading to cell damage. Vitrification is an alternative method for cryopreservation in which cells are exposed to a higher concentration of cryoprotectants and frozen with an ultra rapid freezing velocity, resulting in an ice crystal free, solid glass-like structure. Presently, vitrification is a popular method for cryopreservation of embryos. However, vitrification of oocytes is still challenging due to their complex structure and sensitivity to chilling.

Membrane permeability characteristics of bovine oocytes and development of a step-wise cryoprotectant adding and diluting protocol

Membrane permeability is very helpful for the optimization of effective cryopreservation protocols. In this study, experiments were performed to determine these characteristics for immature (germinal vesicle (GV)) and in vitro matured (metaphase II (MII)) bovine oocytes within 4-37 degrees C, and a new step-wise adding and diluting protocol for ethylene glycol (EG) was developed and verified. Osmotically inactive volumes (V(b)) of GV and MII oocytes were calculated to be 16.1% and 26.1%. The membrane permeability of the oocytes to water (L(p)) in the presence of EG were between 0.08-0.18 and 0.14-0.28 microm/min/atm, and the membrane permeability of the oocytes to solutes (Ps) were between 0.0011-0.0038 and 0.0029-0.0061 cm/min for GV and MII oocytes, respectively. The activation energies (E(a)) for L(p) and P(s) in the presence of EG were 3.68 and 6.84 kcal/mol for GV oocyte, while 3.62 and 0.83-9.08 kcal/mol for MII oocyte. The data indicated that L(p) and P(s) varied significantly between developmental stages and among temperatures evaluated. Based on these results, different protocols for EG adding and diluting from oocytes were developed and tested. The assessment of cleavage rate and embryonic development in vitro confirmed that the designed 4-step adding 2-step diluting protocol indicated a better outcome. The present study is helpful for better understanding of cryobiological properties and the design of cryopreservation protocols for bovine oocytes.

Cryopreservation of immature bovine oocytes to reconstruct artificial gametes by germinal vesicle transplantation

Joining immature gamete cryopreservation and germinal vesicle transplantation (GVT) technique could greatly improve assisted reproductive technologies in animal breeding and human medicine. The present work was aimed to assess the most suitable cryopreservation protocol between slow freezing and vitrification for immature denuded bovine oocytes, able to preserve both nuclear and cytoplasmic competence after thawing. In addition, the outcome of germinal vesicle transfer procedure and gamete reconstruction was tested on the most effective cryopreservation system. Oocytes, isolated from slaughterhouse ovaries, were stored after cumulus cells removal either by slow freezing or by vitrification in open pulled straws. After thawing, oocytes were matured for 24 h in co-culture with an equal number of just isolated intact cumulus enclosed oocytes, and fixed in order to evaluate the stage of meiotic progression and cytoskeleton organization. Our results showed that after warming, vitrified oocytes reached metaphase II (MII) in a percentage significantly higher than oocytes cryopreserved by slow freezing (76.2% and 36.5% respectively, p < 0.05). Moreover, vitrification process preserved the organization of cytoskeleton elements in a higher proportion of oocytes than slow freezing procedure. Therefore vitrification has been identified as the elective method for denuded immature oocytes banking and it has been applied in the second part of the study. Our results showed that 38.3% of oocytes reconstructed from vitrified gametes reached the MII of meiotic division, with efficiency not different from oocytes reconstructed with fresh gametes. We conclude that vitrification represents a suitable method of GV stage denuded oocyte banking since both nuclear and cytoplasmic components derived from cryopreserved immature oocytes can be utilized for GVT.

Osmotic tolerance and freezability of isolated caprine early-staged follicles

Isolated caprine early-staged follicles were submitted to osmotic tolerance tests in the presence of sucrose, ethylene glycol (EG), or NaCl solutions and were exposed to and cryopreserved (by slow or rapid cooling) in MEM alone or MEM supplemented with sucrose, EG (1.0 or 4.0 M), or both. When follicles were exposed to 1.5 M NaCl, only 2% of the follicles were viable, whereas 87% of the follicles were viable after exposure to 4.0 M EG. Regarding exposure time, the highest percentage of viable follicles was obtained when follicles were exposed for 10 min to 1.0 M EG + 0.5 M sucrose; exposure for 60 s to 4.0 M EG + 0.5 M sucrose also maintained high percentage viability in follicles. Slow cooling in the presence of 1.0 M EG + 0.5 M sucrose (75%) or rapid cooling in the presence of 4.0 M EG + 0.5 M sucrose (71%) resulted in a significantly higher proportion of viable follicles than all other treatments (P < 0.05). A 24-h culture of frozen-thawed follicles was used to assess survival; only slow-frozen follicles showed viability rates similar to control follicles (64% vs. 69% respectively; P > 0.05). Interestingly, the percentage of viable rapid-cooled follicles (59%) was similar to that obtained after in vitro culture of conventional slow-cooled follicles but was significantly lower than that in controls. Thus, in addition to determining improved procedures for the exposure of follicles to EG and sucrose before and after freezing of caprine early-staged follicles, we report the development of rapid- and slow-cooling protocols.

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

OBJECTIVE

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

DESIGN

An incomplete randomized block design.

SETTING

A university-affiliated assisted reproductive center.

PATIENT(S)

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

INTERVENTION(S)

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

MAIN OUTCOME MEASURE(S)

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

RESULT(S)

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

CONCLUSION(S)

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

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.

Effect of slow freezing on morphology and developmental competence of buffalo (Bubalus bubalis) immature oocytes

The present study was conducted to evaluate the effects of three cryoprotectants, dimethyl sulphoxide (DMSO), ethylene glycol (EG) and 1,2-propanediol (PROH), each used at two concentrations (1.0 and 1.5 M) on the morphology, maturation rate and developmental capacity of usable quality immature buffalo oocytes subjected to slow freezing. The addition of the cryoprotectant before freezing and its dilution after thawing were carried out in a two- (for 1.0 M) or three-step manner (for 1.5 M). The incidence of damage was found to be significantly higher (P<0.05) with the lower concentration of 1.0 M, compared to that with 1.5 M for all the three cryoprotectants examined. The proportion of immature oocytes recovered in a morphologically normal state was significantly higher (P<0.05) for DMSO than those for EG or PROH at both 1.0 and 1.5 M concentrations. Among the six combinations evaluated, that of DMSO at 1.5 M concentration was found to be superior to others. Irrespective of the type or concentration of the cryoprotectant, partial or complete loss of the cumulus mass was the most prevalent damage. Following in vitro maturation, the nuclear maturation rate was significantly higher (P<0.05) for DMSO than those for EG or PROH at both 1.0 and 1.5 M concentrations. When the in vitro matured oocytes were subjected to in vitro fertilization after slow freezing, using 1.5 M DMSO as cryoprotectant, 4.5% and 0.6% of them were able to develop to morulae and blastocysts, respectively, on Day 9 post insemination, compared to 19.2% and 10.6%, respectively, for the controls. In conclusion, DMSO was more effective than EG or PROH for the slow freezing of immature buffalo oocytes and blastocysts could be produced from immature buffalo oocytes subjected to slow freezing in 1.5 M DMSO.

The effect of osmotic stress on the cell volume, metaphase II spindle and developmental potential of in vitro matured porcine oocytes

Porcine animal models are used to advance our understanding of human physiology. Current research is also directed at methods to produce transgenic pigs. Cryobanking gametes and embryos can facilitate the preservation of valuable genotypes, yet cryopreserving oocytes from pigs has proven very challenging. The current study was designed to understand the effects of anisotonic solutions on in vitro matured porcine oocytes as a first step toward designing improved cryopreservation procedures. We hypothesized that the proportion of oocytes demonstrating a normal spindle apparatus and in vitro developmental potential would be proportional to the solution osmolality. Oocytes were incubated for 10 min at 38 degrees C in various hypo- or hypertonic solutions, and an isotonic control solution and then assessed for these two parameters. Our results support the hypothesis, with an increasing proportion of spindles showing a disrupted structure as the levels of anisotonic exposure diverge from isotonic. Only about half of the oocytes maintained developmental potential after exposure to anisotonic solutions compared to untreated controls. Oocyte volume displayed a linear response to anisotonic solutions as expected, with an estimated relative osmotically inactive cell volume of 0.178. The results from this study provide initial biophysical data to characterize porcine oocytes. The results from future experiments designed to determine the membrane permeability to various cryoprotectants will allow predictive modeling of optimal cryopreservation parameters and provide a basis for designing improved cryopreservation procedures.

Vitrification devices affect structural and molecular status of in vitro matured ovine oocytes

We evaluated the effect of three different cryodevices on membrane integrity, tubulin polymerization, maturation promoting factor (MPF) activity and developmental competence of in vitro matured (IVM) ovine oocytes. IVM oocytes were exposed during 3 min to 7.5% DMSO and 7.5% ethylene glycol (EG) in TCM199 and 25 sec to 0.5 M sucrose, 16.5% DMSO and 16.5% EG, loaded in open pulled straws (OPS), cryoloops (CL) or cryotops (CT) and immersed into liquid nitrogen. Untreated (CTR) or exposed to vitrification solutions but not cryopreserved (EXP) oocytes were used as controls. After warming, double fluorescent staining evidenced a lower membrane integrity in vitrified groups compared to the controls (P < 0.01). After in vitro fertilization and culture OPS and CL groups evidenced a lower cleavage rate than CT and controls (P < 0.01) while blastocysts were obtained only in CL and EXP, at a lower rate than CTR (P < 0.01). All vitrified groups showed alterations in spindle conformation, which were partially recovered in OPS and CT groups. MPF activity was lower in treated compared to CTR and CT showed the lowest value (P < 0.01). After 2 hr culture MPF activity was restored in all groups except CT. Parthenogenetic activation was higher in treated compared to CTR and CT evidenced the highest value. Our results indicate that cryodevice influences not only the ability to survive cryopreservation but is also associated with molecular alterations which affect developmental competence.

Oocyte cryopreservation: oocyte assessment and strategies for improving survival

Despite significant progress in cryopreservation of mammalian oocytes and embryos, many of the molecular and biochemical events that underlie this technology are poorly understood. In recent years, researchers have focused on obtaining viable oocytes that are developmentally competent. Even under the most favourable conditions, experimental approaches have achieved only limited success compared with fresh oocytes used in routine in vitro embryo production. Chilling injuries and toxic effects of the cryoprotectants are the major adverse consequences following cryoprocedures. To overcome these problems, different strategies have been developed for improving cryopreservation results. These strategies include reducing container volumes, increasing the thermal gradient, changing the cell surface/volume ratio, enhancing cryotolerance by supplementation with various additives or modifying the lipid composition of the oocyte membrane. In order to develop new strategies for reducing the various forms of stress associated with oocyte cryopreservation, it is fundamental to gain a better understanding of the major changes responsible for poor post-thaw survival. With this knowledge, we hope that oocyte cryostorage will become a fully reliable reproductive technique in the near future.

Osmotic tolerance of in vitro produced porcine blastocysts assessed by their morphological integrity and cellular actin filament organization

This experiment investigated the osmotic tolerance limits of the morphology and the cellular actin filament organization of porcine blastocysts. In vitro produced Day 6 blastocysts were subjected to osmotic treatments with sucrose solutions of different osmolalities (75, 150, 210, 600, 1200, and 2400 mOsm) and one isotonic solution (NCSU-23, 285 mOsm). Blastocysts were then either fixed immediately, or cultured for 18 h and subsequently fixed with formalin. The morphology of the treated blastocysts was examined under a stereomicroscope and the integrity of the cellular actin filaments of the blastocysts was examined by confocal microscopy after staining with Alexa Fluor 488 phalloidin. The results indicated that there was a significant relationship between the osmotic levels and the probability of blastocysts exhibiting disrupted cellular actin filaments. In addition, blastocysts also collapsed in proportion to the levels of osmotic treatments. The osmotic tolerance limits which would maintain 70% of the blastocysts with their original morphology immediately after the treatment were 90 and 170%, respectively, of isotonicity. After 18 h of culture, the osmotic tolerance limits were 61 and 163%, respectively, of isotonicity. Similarly, the osmotic conditions relative to isotonicity which would maintain the integrity of cellular actin filaments in 70% of treated blastocysts had to be within the range of 87 and 147% immediately after the treatment and 87 and 169% after 18 h of culture. Collectively, these data indicate that in vitro produced porcine blastocysts are very sensitive to osmotic stress. This information can be used to optimize cryopreservation procedures for porcine embryos.