Effects of Osmotic Stress and Cryoprotectant Toxicity on Mouse Oocyte Fertilization and Subsequent Embryonic Development In Vitro

Automation in vitrification and thawing of mouse oocytes and embryos

Vitrification is a common technique for cryopreserving oocytes or embryos. However, manual vitrification is tedious and labor-intensive, and can be subject to variations caused by human factors. To address these challenges, we developed an automated vitrification-thawing system (AVTS) based on a cryo-handle. Our study firstly assessed the efficiency of cryoprotectant exchange through comparing the osmolalities of fresh and collected solutions during automated vitrification and thawing, and evaluated the cooling and warming rates of the cryo-handle. We also compared mouse oocyte survival, fertilization and embryo development after thawing and ICSI, and the development of re-frozen cleavage embryos between manual operation and automated system. The results showed that the osmolalities of collected samples were within normal range and comparable to fresh solutions. Furthermore, the automated system could obtain the reliable cooling and warming rates. Particularly, there were no significant differences in oocyte survival rates, fertilization rates, and subsequent embryo development and its quality between two procedures. Our findings suggest that AVTS has no impact on osmolalities of vitrification and thawing solutions, ensuring the proper exchange of cryoprotectants. The cryo-handle also shows the ability to achieve reliable cooling and warming rates, which benefits for the cryopreservation and thawing process. Moreover, the results from mouse oocytes and embryos indicate that automated system has effectively maintained the survival and fertilization of frozen oocytes and supported subsequent embryo development. Therefore, the automated vitrification and thawing system will inevitably represent a superior alternative to manual operation.

Development of a new device for manipulating frozen mouse 2-cell embryos on the International Space Station

Whether mammalian embryos develop normally under microgravity remains to be determined. However, embryos are too small to be handled by inexperienced astronauts who orbit Earth on the International Space Station (ISS). Here we describe the development of a new device that allows astronauts to thaw and culture frozen mouse 2-cell embryos on the ISS without directly contacting the embryos. First, we developed several new devices using a hollow fiber tube that allows thawing embryo without practice and observations of embryonic development. The recovery rate of embryos was over 90%, and its developmental rate to the blastocyst were over 80%. However, the general vitrification method requires liquid nitrogen, which is not available on the ISS. Therefore, we developed another new device, Embryo Thawing and Culturing unit (ETC) employing a high osmolarity vitrification method, which preserves frozen embryos at −80°C for several months. Embryos flushed out of the ETC during thawing and washing were protected using a mesh sheet. Although the recovery rate of embryos after thawing were not high (24%-78%) and embryonic development in ETC could not be observed, thawed embryos formed blastocysts after 4 days of culture (29%-100%) without direct contact. Thus, this ETC could be used for untrained astronauts to thaw and culture frozen embryos on the ISS. In addition, this ETC will be an important advance in fields such as clinical infertility and animal biotechnology when recovery rate of embryos were improved nearly 100%.

Equilibrium vitrification of mouse embryos using low concentrations of cryoprotectants

Previously, we developed a method for vitrification of mouse embryos in a near-equilibrium state using EFS35c, PB1 medium containing 35% (v/v) ethylene glycol, and 0.98 M sucrose. This method has advantages in both slow freezing and vitrification. However, since the vitrification solution in this method contains high concentrations of cryoprotectants and thus has high osmolality, the solution would injure oocytes and embryos with high sensitivity to chemical toxicity and high osmolality. In this study, we examined whether embryos could be vitrified in a near-equilibrium state using a solution containing low concentrations of cryoprotectants and thus with low osmolality. To investigate whether embryos were vitrified in a near-equilibrium state, 2-cell mouse embryos were vitrified with EDFS10/10a, PB1 medium containing 10% (v/v) ethylene glycol, 10% (v/v) DMSO, and 0.4 M sucrose, in liquid nitrogen, stored at -80 °C for 4-28 days, and warmed in water at 25 °C. The viability of the embryos was evaluated by the appearance of embryos after warming and developmental ability. When embryos were vitrified in liquid nitrogen using EDFS10/10a, the survival and developmental ability into blastocysts after storage at -80 °C for 7 days were high, indicating that embryos were vitrified in a near-equilibrium state. A high proportion of embryos vitrified with EDFS10/10a developed to term after transportation with dry ice, re-cooling in liquid nitrogen, and transfer to recipients. Therefore, new equilibrium vitrification developed in this study may be useful for oocytes and embryos that are highly sensitive to the toxicity of cryoprotectants and high osmolality.

Microfluidic method reduces osmotic stress injury to oocytes during cryoprotectant addition and removal processes in porcine oocytes

Oocyte cryopreservation is an important technology in assisted reproduction and fertility preservation. However, the developmental potential of cryopreserved oocyte remains poor. Osmotic stress injury (OSI) during cryoprotectants (CPAs) loading and unloading steps has critical impact on successful cryopreservation. In order to minimize OSI to oocytes, a microfluidic device was designed and fabricated to achieve continuous CPA concentration change. MII porcine oocytes were loaded and unloaded CPAs with step-wise and microfluidic methods, oocyte volume changes were recorded and compared, loading and unloading duration of microfluidic methods were optimized. The survival and developmental rate of treated oocytes in step-wise and microfluidic linear methods were also evaluated. The results showed that oocyte volume changes with microfluidic method were obviously less than step-wise method, and the survival, cleavage and blastocyst rate of oocytes were 95.3%, 64.4%, and 19.4%, respectively, which were significantly higher than the traditional step-wise method (79.4%, 43.6%, and 9.7%) (p < 0.05). In conclusion, microfluidic device can effectively reduce the osmotic damage to oocytes and improve the survival rate and developmental rate of oocytes, which may provide a new path for oocyte cryopreservation.

Effect of type and concentration of cryoprotectant on post-thaw survivability of vitrified porcine follicular oocytes

A total of 950 and 510 oocytes with two or more cumulus cell layers adhered to zona pellucida obtained by aspiration from follicles (2–8 mm dia) of 315 and 135 abattoir porcine ovaries, respectively, were utilized in the study to find the effect of cryoprotectant and concentration on post-thaw survivability of porcine follicular oocytes. Vitrified post-thaw oocytes with intact zona pellucida and vitelline membrane, normal spherical shape and dark and evenly granulated cytoplasm under a stereo-zoom microscope were considered as viable. Out of the 10 cryoprotectant treatments i.e. ethylene glycol (EG), propylene glycol (PG), dimethyl sulphoxide (DMSO), glycerol (GL), EG + PG, EG + DMSO, EG + GL, PG + DMSO, PG + GL and DMSO + GL used for vitrification of oocytes at a concentration of 35%, the post-thaw survivability rate was the highest in EG +DMSO followed by EG + PG group which had significantly higher mean post-thaw survived oocytes as compared to GL, PG + DMSO, PG + GL and DMSO + GL groups. Exposing the oocytes to 30, 35 and 40% of EG + DMSO and EG + PG yielded no significant difference in post-thaw survivability rate of vitrified oocytes, although the highest value was obtained with 35%. It was concluded that 35 to 40% EG + DMSO yielded efficient vitrification of porcine oocytes.

Chemically Defined and Xeno-Free Cryopreservation of Human Adipose-Derived Stem Cells

The stromal compartment of adipose tissue harbors multipotent cells known as adipose-derived stem cells (ASCs). These cells can differentiate into various lineages including osteogenic, chrondrogenic, adipogenic, and neurogenic; this cellular fraction may be easily obtained in large quantities through a clinically safe liposuction procedure. Therefore, ASCs offer exceptional opportunities for tissue engineering and regenerative medicine. However, current practices involving ASCs typically use fetal bovine serum (FBS)-based cryopreservation solutions that are associated with risks of immunological reactions and of transmitting infectious diseases and prions. To realize clinical applications of ASCs, serum- and xeno-free defined cryopreservation methods are needed. To this end, an animal product-free chemically defined cryopreservation medium was formulated by adding two antioxidants (reduced glutathione and ascorbic acid 2-phosphate), two polymers (PVA and ficoll), two permeating cryoprotectants (ethylene glycol and dimethylsulfoxide), a disaccharide (trehalose), and a calcium chelator (EGTA) to HEPES-buffered DMEM/F12. To limit the number of experimental groups, the concentration of trehalose, both polymers, and EGTA was fixed while the presence of the permeating CPAs and antioxidants was varied. ASCs suspended either in different versions of the defined medium or in the conventional undefined cryopreservation medium (10% dimethylsulfoxide+10% DMEM/F12+80% serum) were cooled to -70°C at 1°C/min before being plunged into liquid nitrogen. Samples were thawed either in air or in a water bath at 37°C. The presence of antioxidants along with 3.5% concentration of each penetrating cryoprotectant improved the freezing outcome to the level of the undefined cryopreservation medium, but the plating efficiency was still lower than that of unfrozen controls. Subsequently, increasing the concentration of both permeating cryoprotectants to 5% further improved the plating efficiency to the level of unfrozen controls. Moreover, ASCs cryopreserved in this defined medium retained their multipotency and chromosomal normality. These results are of significance for tissue engineering and clinical applications of stem cells.

Autophagic activation in vitrified-warmed mouse oocytes

Vitrification involves the use of cryoprotectants (CPAs) and liquid nitrogen (LN2), which may cause osmotic damage and cryoinjury to oocytes. Autophagy is widely recognized as a survival or response mechanism elicited by various environmental and cellular stressors. However, the induction of autophagy in vitrified-warmed oocytes has not been examined. In this work, we investigated whether the vitrification-warming process induces autophagy in mouse oocytes. Metaphase II (MII) oocytes that were vitrified and stored in LN2 for at least 2 weeks were used in the study. In RT-PCR analyses, we observed that several Atg genes such as Atg5, Atg7, Atg12, LC3a (Map1lc3a), LC3b (Map1lc3b), and Beclin1 were expressed in MII mouse oocytes. Slight reduction in mRNA levels of Atg7 and Atg12 in vitrified-warmed oocytes was noted, and expression of these genes was not significantly influenced. Confocal live imaging analysis using oocytes from GFP-LC3 transgenic mice revealed that vitrified-warmed oocytes had a significantly higher number of GFP-LC3 puncta in comparison to fresh oocytes. The expression of BECLIN1 protein was also increased in vitrified-warmed oocytes. Treatment with 3-methyladenine, an inhibitor of autophagy, did not significantly affect the rates of oocyte survival, IVF, and embryonic development after warming and IVF. The results suggest that the observed autophagic activation in vitrified-warmed oocytes is a natural adaptive response to cold stress. Collectively, we show for the first time that vitrified-warmed mouse oocytes exhibit autophagic activation during warming and that this response is not induced by CPA-containing solutions. The induction of autophagy by cold temperature is first reported herein.

Oocyte vitrification: advances, progress and future goals

BACKGROUND

Recent advances in vitrification technology have markedly improved the efficacy of oocyte cryopreservation in terms of oocyte survival and pregnancy, as well as live birth rates. However, there still remains room for improvement in terms of vitrification techniques.

OBJECTIVE

The remaining challenges include the development of a less cytotoxic vitrification solution and of a safe vitrification device in order to have vitrification techniques considered as a standard clinical laboratory procedure.

METHODS

A systematic electronic literature search strategy has been conducted using PubMed (Medline) databases with the use of the following key words: oocyte, vitrification, cryoprotectant, preservation, pregnancy, and live birth. A list of published papers focused on the improvement of vitrification techniques to have the vitrification protocol standardized have been evaluated in full text for this review. Only key references were cited.

CONCLUSIONS

Vitrification technology has made significant advancements and holds great promise, but many issues remains to be addressed before it becomes a standardized procedure in clinical laboratories such as the fact that oocyte vitrification may not require a high concentration of cryoprotectant in the vitrification solution when it has a suitable cooling and warming rate. There is also no consistent evidence that indicates the absence of risk to the vitrified oocytes when they are stored for a prolonged period of time in direct-contact with liquid nitrogen. The long-term development of infants born as a result of this technology equally remains to be evaluated.

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 embryoid bodies to facilitate basic research and cell-based therapies

Pluripotent stem cells offer unique opportunities for curing debilitating diseases. However, further comprehensive research is needed to better understand cell signaling during the differentiation of pluripotent cells into different cell lineages and accordingly to develop clinically applicable protocols. One of the limiting steps for differentiation studies is proper culture and expansion of pluripotent stem cells, which is labor intensive, expensive, and requires a great deal of expertise. This limiting step can be overcome by successful banking and distribution of embryoid bodies (EBs), which are aggregates of pluripotent stem cells and typically the starting point of differentiation protocols. The objective of this study was to investigate the feasibility of EB banking by studying survival and functionality of cryopreserved EBs. To this end, EBs were formed by culturing mouse 129 embryonic stem (ES) cells in the absence of leukemia inhibitory factor (LIF) in hanging drops and then subjected to different cryopreservation protocols. In a series of experiments, we first tested the postthaw survival of EBs as a function of dimethylsulfoxide (DMSO) and extracellular trehalose concentrations and cooling rates. Next, we studied the functionality of cryopreserved EBs by assessing their postthaw attachment, growth, and differentiation into various cell types. Higher (≥5%) DMSO concentrations alone or in combination with trehalose (0.1 M and 0.2 M) yielded good postthaw survival rates of >80%, whereas cooling of EBs at 1°C/min in the presence of 5% DMSO +0.1 M trehalose gave the best attachment and growth rates, with differentiation into cell lineages of three germ layers. Taken together, our results suggest that EBs are tolerant to cryopreservation-associated stresses and retain their differentiation potential after freezing and thawing. Furthermore, our experiments with dissociated EB cells and nondissociated EBs suggest that the extracellular matrix may play a beneficial role in the cryotolerance of EBs. Overall, our data support the feasibility of EB banking, which would facilitate advancement of cell-based therapies.

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

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

Mouse oocyte vitrification: the effects of two methods on maturing germinal vesicle breakdown oocytes

PURPOSE

Evaluation of viability and subsequent developmental ability of mouse germinal vesicle breakdown oocytes vitrified in conventional straws.

METHODS

Oocytes with compact cumulus cells were cultured for 3 h in TCM199 medium GVBD and vitrified by two methods: the step-wise and single-step. After vitrification, the oocytes were thawed, and subjected to in vitro maturation and in vitro fertilization. Oocyte survival (post-thaw) was assessed by morphological appearance and staining, using propidium iodide (PI)/Hoechst 33342. The oocyte maturation and fertilization rates were examined in vitro.

RESULTS

In the single-step method the rates of post thaw survival, maturation to metaphase II and cleavage (2-cell embryos) were 58.68%, 56.41% and 38.63%, respectively. In the step-wise method, the corresponding rates were 81.75%, 68.59% and 51.80%, respectively.

CONCLUSION

Vitrification of mouse germinal vesicle breakdown oocytes by the step-wise method had the advantage of maintaining the viability and subsequent production of 2-cell embryos. In comparison with that in unvitrified control oocytes, the development of MII oocytes to 2-cell embryos was impaired following vitrification.

Application of intra- and extracellular sugars and dimethylsulfoxide to human oocyte cryopreservation

PURPOSE

Oocyte cryopreservation may avoid many complications of human embryo freezing and provide future fertility for women undergoing cancer therapy. The objective of this study was to explore the application of intra- and extracellular sugars in combination with small amounts of dimethylsulfoxide (DMSO) to human oocyte cryopreservation as an alternative approach.

METHODS

Discarded human oocytes that were obtained from IVF patients under informed consent and IRB approval, were cryopreserved by slow cooling to -196 degrees C after being randomly distributed into three groups: (i) DMSO control without intra- and extracellular sugar; (ii) extracellular sugar (raffinose) + DMSO; and (iii) intra- and extracellular sugar (trehalose and raffinose, respectively) + DMSO. Subsequently, all cryopreserved oocytes were thawed rapidly, and their survival was assessed by morphological criteria after 24 h of culture.

RESULTS

A total of 71 oocytes were evaluated in three groups with survival rates of 88.5% (23/26), 68.2% (15/22), and 52.2% (12/23) for intra- and extracellular sugar+DMSO, extracellular sugar+DMSO, and DMSO control groups, respectively.

CONCLUSION

These results support the use of intra- and extracellular sugars as an alternative approach for cryopreservation of human oocytes.

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

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

Cryopreservation of tissue-engineered dermal replacement in Me2SO: Toxicity study and effects of concentration and cooling rates on cell viability

Cryopreservation of tissue-engineered human dermal replacement plays an important role in skin tissue engineering and skin banking. With the inspection of electronic scanning microscope and viability evaluation by Trypan Blue staining assay and the tetrazolium salt, MTT (3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay, this study investigated the toxicity of Me(2)SO to dermal fibroblasts and effects of cryoprotectant concentration and cooling rate on the viability of dermal replacement. The results demonstrated that the Me(2)SO toxicity to fibroblasts was affected by the exposure time, temperature, and concentration. Furthermore adding cryoprotectant solution at low temperature of 4 degrees C significantly reduced the toxic effect on the tissue-engineered dermal equivalent. An optimal cryopreservation protocol consisting of cooling rate at 1 degrees Cmin(-1) in 10% (V/V) Me(2)SO was derived, with the viability of studied dermal equivalent treated by this protocol being 75% of that of fresh control. The micrograph obtained by electronic scanning microscope also confirmed this result.