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

Effects of multi-gradient equilibration during vitrification on oocyte survival and embryo development in mice

Vitrification has been widely used for oocyte cryopreservation, but there is still a need for optimization to improve clinical outcomes. In this study, we compared the routine droplet merge protocol with modified multi-gradient equilibration vitrification for cryopreservation of mouse oocytes at metaphase II. Subsequently, the oocytes were thawed and subjected to intracytoplasmic sperm injection (ICSI). Oocyte survival and spindle status were evaluated by morphology and immunofluorescence staining. Moreover, the fertilization rates and blastocyst development were examined in vitro. The results showed that multi-gradient equilibration vitrification outperformed droplet merge vitrification in terms of oocyte survival, spindle morphology, blastocyst formation, and embryo quality. In contrast, droplet merge vitrification exhibited decreasing survival rates, a reduced proportion of oocytes with normal spindle morphology, and lower blastocyst rates as the number of loaded oocytes increased. Notably, when more than six oocytes were loaded, reduced oocyte survival rates, abnormal oocyte spindle morphology, and poor embryo quality were observed. These findings highlight that the vitrification of mouse metaphase II oocytes by the modified multi-gradient equilibration vitrification has the advantage of maintaining oocyte survival, spindle morphology, and subsequent embryonic development.

Efficiency and safety of vitrification of surplus oocytes following superovulation: a comparison of different clinical indications of oocyte cryopreservation in IVF/ICSI cycles

Objective

To evaluate the effectiveness and safety of utilizing the small number of remaining vitrified oocytes after the failure of adequate fresh sibling oocytes. The outcome of present study would provide more comprehensive information about possible benefits or disadvantage to cryopreserve supernumerary oocytes for patients who have plenty oocytes retrieved.

Methods

This retrospective cohort study included 791 IVF/ICSI cycles using 6344 oocytes that had been vitrified in the Reproductive Hospital affiliated to Shandong University between January 2013 and December 2019.They were divided into three groups: SOC group (supernumerary oocytes cryopreservation), relative-MOC group (relative male factor-oocyte cryopreservation), and absolute-MOC group (absolute male factor-oocyte cryopreservation). Laboratory and clinical outcomes were analysed, and multivariate regression analysis was used to study the effect of different indications of vitrification on CLBR.

Results

The CLBR was highest in absolute-MOC, and lowest in SOC (39.0% vs 28.9%, P=0.006); however, after adjusting for confounding factors, the difference was not statistically significant. Multivariable regression analysis showed no impact of indications of vitrified oocytes on CLBR according to controlled age, BMI, preservation duration, use of donor sperm or not, use of PESA/TESA or not, number of oocytes retrieved, number of oocytes thawed, and oocyte survival rate. The preliminary data of safety showed no significant differences in the perinatal and neonatal outcoms after ET and FET between the SOC and MOC groups.

Conclusion

Different indications of vitrification did not affect CLBR. The CLBR of vitrified oocytes for different indications was correlated with age and number of warmed oocytes. For women who have plenty oocytes retrieved, the strategy of cryopreserving a small number of oocytes is a valuable option and might benefit them in the future. Additional data from autologous oocyte vitrification research employing a large-scale and variable-controlled methodology with extending follow-up will complement and clarify the current results.

An Eight Year Experience of Autologous Oocyte Vitrification for Infertile Patients Owing to Unavailability of Sperm on Oocyte Retrieval Day

Objective: The objective of this study was to provide a descriptive analysis of the clinical outcomes achieved in oocyte vitrification in cases where sperm was unavailable on oocyte retrieval day, and to identify predictors of oocyte survival. Methods: This retrospective cohort study used data from a university-affiliated reproductive medical center. There were 321 cycles in which some of, or all oocytes were vitrified owing to the unavailability of sperm between March 2009 and October 2017. A descriptive analysis of the clinical outcomes including both fresh embryo transfers and cryopreserved embryo transfers was provided. The ability of an individual parameter to forecast oocyte survival per thawing cycle was assessed by binary logistic regression analysis. The cumulative probability of live birth (CPLB) was estimated by using the Kaplan-Meier method according to the total number of oocytes thawed in consecutive procedures. Results: The average survival rate was 83.13%. High-quality embryo rate and blastocyst rate decreased significantly decreased significantly in vitrification oocyte group compared to fresh control oocytes. The comparison of sibling oocytes in part-oocyte-vitrified cycles shows fewer high-quality embryos developed in the vitrified group. The live birth rate per warmed-oocyte was 4.3%. Reasons for lack of sperm availability on oocyte retrieval day and serum cholesterol levels were found to be associated with oocyte survival rate in the present study. Kaplan-Meier analysis showed no significant difference in CPLB between patients ≤35 vs. >35 years. Conclusions: Oocyte vitrification is an indispensable and effective alternative when sperm are not available on oocyte retrieval day. The present study provided evidence that oocytes from infertile couples were more likely to suffer oocyte/embryo vitrification injury. Clinicians need to take this into account when advising patients in similar situations. Further studies will be necessary to clarify the correlation between serum metabolism parameters and human oocyte survival after vitrification.

A modified vitrification method reduces spindle and chromosome abnormalities

Development of an effective system for oocyte-cryopreservation is of clinical relevance in reproductive medicine. However, oocyte-preservation is not as effective as embryo preservation. In this study, we used a 37°C pre-equilibrium temperature as part of a modified vitrification method for human oocyte cryopreservation. The effect of the new method on spindle configuration, chromosomal arrangement, and mitochondrial distribution was investigated in in vitro-matured human oocytes. A total of 101 in vitro-matured oocytes were randomly assigned for vitrification at pre-equilibrium temperature of 37°C (37°C Group, n=50) or at room temperature (RT Group, 22-24°C, n=51). The time needed for vitrification in the 37°C group was significantly shorter than that in the RT group. Defective spindles were found in 45.5% and 69.0% oocytes in the 37°C group and RT group, respectively (p < 0.05). Abnormal chromosomes were found in 47.7% and 71.4% oocytes, respectively (p < 0.05). There were no significant differences with respect to oocyte survival rate and mitochondrial distribution pattern between the two groups. These results indicate that vitrification at a pre-equilibrium temperature of 37°C may reduce the incidence of defective spindle configuration and chromosomal abnormalities in in-vitro-matured human oocytes.

ABBREVIATIONS

ICSI: intracytoplasmic sperm injection; FSH: follicle-stimulating hormone; MII: metaphase II; EG: ethylene glycol; PROH: 1,2-propanediol.

Vitrification of porcine immature oocytes: Association of equilibration manners with warming procedures, and permeating cryoprotectants effects under two temperatures

The aim of this study was to evaluate the association of equilibration manners with warming procedures, and the different permeating cryoprotectants (pCPAs) effects under two temperatures, in terms of survival, maturation and subsequent parthenogenetic development of porcine immature oocytes after Cryotop vitrification. In Experiment 1, oocytes were equilibrated by exposure to 5% (v/v) ethylene glycol (EG) for 10 min (EM1) or stepwise to 7.5% (v/v) and 15% (v/v) EG for 2.5 min respectively (EM2). Warming procedures were performed in 1.0 M sucrose for 1 min, then in 0.5 and 0.25 M sucrose for 2.5 min respectively (WP1), or in 0.5, 0.25 and 0.125 M sucrose each step for 2 min (WP2), or in 0.25, 0.125 and 0.063 M sucrose each step for 2 min (WP3). After 2 h of warming, the survival rate of oocytes treated by EM1 and WP1 was significantly higher (P < 0.05) than that of the other groups. Moreover, a similar proportion of survival and nuclear maturation in all vitrified groups was obtained after completion of the IVM. No significant difference in blastocyst development was observed among vitrified groups except the group treated by EM2 and WP3. In Experiment 2, oocytes were vitrified by using EG alone, EG combined with dimethyl sulphoxide (EG + DMSO) or propylene glycol (EG + PROH) as pCPAs under 25 °C and 39 °C. The percentages of cryosurvival and nuclear maturation were similar in all vitrified groups. Under 25 °C, the embryo development and total cell numbers of blastocysts were not significantly different among EG, EG + DMSO and EG + PROH groups. However, the application of EG + PROH at 39 °C resulted in significantly decreased both cleavage and blastocyst formation rates. In conclusion, our data showed that equilibration manner and warming procedure affect the cryosurvival of porcine immature oocytes, and the combination of pCPAs cannot give a better cryopreservation outcome whether 25 °C or 39 °C. Notably, the Cryotop vitrification accompanied by our modified strategy for porcine immature oocytes could achieve high survival and respectable blastocyst production.

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

PURPOSE

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

METHODS

Samples were studied by light and transmission electron microscopy.

RESULTS

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

CONCLUSIONS

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

High survival of mouse oocytes using an optimized vitrification protocol

The method of vitrification has been widely used for cryopreservation. However, the effectiveness of this method for mammalian oocytes could be improved by optimizing each step of the process. In the present study, we tested the effects of varying several key parameters to determine the most effective protocol for mouse oocyte vitrification. We found that cryoprotectant containing ethylene glycol and dimethylsulfoxide plus 20% fetal calf serum produced the highest rates of oocyte survival, fertilization, and blastocyst formation. The duration and temperature of oocyte exposure to vitrification and thawing solutions influenced survival rate. The presence of cumulus cells surrounding oocytes and the incubation of thawed oocytes in Toyoda-Yokoyama-Hosoki medium also increased oocyte survival. Open pulled straw and nylon loop methods were more effective than the mini-drop method. Finally, the combination of these improved methods resulted in better spindle morphology when compared to the unimproved methods. These results demonstrate that the outcomes of mouse oocyte vitrification can be improved by a suitable combination of cryopreservation methods, which could be applied to future clinical research with human oocytes.