Cryopreservation of mature and immature oocytes

New Insights on In Vitro Maturation of Oocytes for Fertility Preservation

In the last decade, the evolution of oncofertility has sparked a resurgence of interest in in vitro maturation (IVM) due to its suitability in certain oncological scenarios where controlled ovarian hyperstimulation may not be feasible. The retrieval of immature cumulus-oocyte complexes from small antral follicles, regardless of the menstrual cycle phase, presents a swift opportunity to vitrify mature oocytes or embryos post-IVM in urgent situations or when stimulation is not advisable. Harvesting immature cumulus-oocyte complexes and immature oocytes can be achieved transvaginally or directly in the laboratory from extracorporeal ovarian tissue. Although IVM has transitioned from an experimental status due to safety validations, it relies on the intricate process of oocyte maturation. Despite successful live births resulting from IVM in fertility preservation contexts, the comparatively lower developmental competence of in vitro matured oocytes highlights the necessity to enhance IVM culture systems. Recent advancements in IVM systems hold promise in bolstering oocyte competence post-IVM, thereby narrowing the gap between IVM and outcomes from ovarian stimulation. Additionally, for optimizing the chances of conception in cancer survivors, the combination of IVM and ovarian tissue cryopreservation stands as the favored choice when ovarian stimulation is unfeasible.

Vitrification induces a focused spindle pole in mouse MI oocytes

Immature oocyte (germinal vesicle stage, GV) vitrification can avoid a cycle of ovarian stimulation, which is friendly to patients with hormone-sensitive tumors. However, the in vitro maturation of vitrification-thawed GV oocyte usually results in aneuploidy, and the underlying mechanism remains unclear. Stable spindle poles are important for accurate chromosome segregation. Acentriolar microtubule-organizing centers (aMTOCs) undergo fragmentation and reaggregation to form spindle poles. Microtubule nucleation is facilitated via the perichromosome Ran after GVBD, which plays an important role in aMTOCs fragmentation. This study showed that vitrification may reduce microtubule density by decreasing perichromosomal Ran levels, which reduced the localization of pKIF11, thereby decreased the fragmentation of aMTOCs and formed a more focused spindle pole, ultimately resulted in aneuploidy. This study revealed the mechanism of abnormal spindle pole formation in vitrified oocytes and offered a theoretical support to further improve the quality of vitrified oocytes.

Comparing the effects of vitrification, before and after mouse oocyte in vitro maturation on developmental competence, changes in epigenetic regulators and stress oxidative response

Since the developmental stage of oocyte is a challenging issue in the success of vitrification, this study investigated the effects of vitrification, before and after in vitro maturation, on the survival and maturation rates, developmental competence and the expression levels of genes involved in apoptosis, oxidative stress and epigenetic modifications. Mouse germinal vesicle (GV) oocytes were divided into four groups: fresh in vitro matured oocytes without vitrification (fIVM), in vitro matured oocytes after vitrification (vIVM), in vitro matured oocytes before vitrification (IVMv). In addition, in vivo matured oocytes (MII) were used as control. After oocytes collection, maturation and survival rates as well as the intracellular reactive oxygen species (ROS) level were evaluated. Also, the expression level of various genes was analyzed by qRT-PCR. In addition, following artificial activation (parthenogenesis), the developmental competence of oocytes to the blastocyst stage was evaluated. A significant decrease in maturation rate and survival of vIVM oocytes was observed compared to fIVM and IVMv oocytes. Intracellular ROS levels were significantly increased in both vitrified groups compared to the fIVM group, and no significant difference between vitrified groups. Pro-apoptotic genes; BAX and Bcl2 as well as genes related to oxidative stress response Hsp1a, Hsp1b and SOD1were significantly increased in the vIVM group compared to the IVMv group. Interestingly, epigenetic regulators genes DNMT1, DNMT3a and DNMT3b were highly expressed in IVMv oocytes along with a decrease in the artificial activation rate compared to the vIVM oocytes. Our results indicated that despite observing more negative effects of vitrification before IVM on the survival rate and maturation as well as apoptosis status, less epigenetic changes in vIVM oocytes can make this process a better option in the treatment of infertility than IVM of oocytes followed by vitrification, a hypothesis that needs to be investigation in human oocytes.

Alterations in the expression pattern of some epigenetic-related genes and microRNAs subsequent to oocyte cryopreservation

MicroRNAs (miRNAs) are small non-encoding RNAs that actively regulate biological and physiological processes, and play an important role in regulating gene expression in all cells, especially in most animal cells, including oocytes and embryos. The expression of miRNAs at the right time and place is crucial for the oocyte's maturation and the embryo's subsequent development. Although assisted reproductive techniques (ART) have helped to solve many infertility problems, they cause changes in the expression of miRNA and genes in oocytes and preimplantation embryos, and the effect of these changes on the future of offspring is unknown, and has caused concerns. The relevant genomic alterations commonly imposed on embryos during cryopreservation may have potential epigenetic risks. Understanding the biological functions of miRNAs in frozen maturated oocytes may provide a better understanding of embryonic development and a comparison of fertility conservation in female mammals. With the development of new techniques for genomic evaluation of preimplantation embryos, it has been possible to better understand the effects of ART. The results of various articles have shown that freezing of oocytes and the cryopreservation method are effective for the expression of miRNAs and, in some cases, cause changes in the expression of miRNAs and epigenetic changes in the resulting embryo. This literature review study aimed to investigate the effects of oocyte cryopreservation in both pre-maturation and post-maturation stages, the cryopreservation method and the type of cryoprotectants (CPA) used on the expression of some epigenetic-related genes and miRNAs.

Effects of Ovarian Tissue Cryopreservation on Primary Ovarian Insufficiency in Girls Undergoing Bone Marrow Transplantation

OBJECTIVE

To determine if removing an ovary for ovarian tissue cryopreservation (OTC) increased rates of primary ovarian insufficiency (POI) in girls undergoing bone marrow transplantation (BMT). Institutional review board approval was obtained from all 3 clinical sites.

DESIGN

Multicenter retrospective cohort study SETTING: Academic children's hospitals PATIENTS: Females aged 2-21 who underwent BMT with or without OTC from 2010 to 2017.

INTERVENTIONS

None MAIN OUTCOME MEASURES: Rates of POI in girls who underwent OTC vs those who underwent BMT alone as determined by serum markers, presence of menses, or clinical diagnosis.

RESULTS

A total of 142 patients were identified, 43 who had OTC and 99 with BMT alone. The rate of POI in girls undergoing OTC was 65% vs 41.8% in those who underwent BMT alone (P = .26).

CONCLUSIONS

Although this study was not powered to detect a lack of difference, it is reassuring that there does not seem to be a clinically significant increase in POI in patients undergoing OTC.

Cryopreservation of Fetal Porcine Kidneys for Xenogeneic Regenerative Medicine

Kidney xenotransplantation has been attracting attention as a treatment option for end-stage renal disease. Fetal porcine kidneys are particularly promising grafts because they can reduce rejection through vascularization from host vessels. We are proposing xenogeneic regenerative medicine using fetal porcine kidneys injected with human nephron progenitor cells. For clinical application, it is desirable to establish reliable methods for the preservation and quality assessment of grafts. We evaluated the differentiation potency of vitrified porcine fetal kidneys compared with nonfrozen kidneys, using an in vivo differentiation model. Fetal porcine kidneys connected to the bladder were frozen via vitrification and stored in liquid nitrogen. Several days later, they were thawed and transplanted under the retroperitoneum of immunocompromised mice. After 14 days, the frozen kidneys grew and differentiated into mature nephrons, and the findings were comparable to those of nonfrozen kidneys. In conclusion, we demonstrated that the differentiation potency of vitrified fetal porcine kidneys could be evaluated using this model, thereby providing a practical protocol to assess the quality of individual lots.

IVM of human immature oocytes for infertility treatment and fertility preservation

Background

Thousands of healthy babies are born from in vitro maturation (IVM) procedures, but the rate of efficiency differs with the source of immature oocytes obtained. Recently, there are different IVM protocols proposed for infertility treatment and fertility preservation.

Methods

Based on the literature, the clinical application for IVM of immature oocytes was summarized.

Main findings Results

Immature oocytes may be retrieved from women after priming with or without the use of follicular stimulation hormone (FSH), human chorionic gonadotrophin (hCG) or a combination of both FSH and hCG. Successful pregnancy rates with IVM technology seem to be correlated with the number of immature oocytes obtained. With the source and culture course of immature oocytes, there are various IVM protocols. IVM of immature oocytes is profoundly affected by the culture conditions, but no breakthrough has been made by improving the IVM medium itself. Thus, the clinical application of IVM technology continues to evolve.

Conclusion

IVM technology is a useful technique for infertile women and fertility preservation. Mild stimulation IVF combined with IVM of immature oocytes is a viable alternative to the conventional stimulation IVF cycle treatment as it may prove to be an optimal first-line treatment approach.

Germinal stage vitrification is superior to MII stage vitrification in prepubertal mouse oocytes

This study investigated if in vitro maturation (IVM) before or after vitrification would be more successful for prepubertal oocytes. To mimic prepubertal conditions in an experimental setup, oocytes were collected from healthy 14, 21 and 28day old Swiss albino mice. The germinal vesicle (GV) stage oocytes and in vitro matured MII oocytes were subjected to vitrification-warming. Both structural (meiotic spindle morphology, mitochondrial integrity, cortical granules) and functional (sperm zona binding, fertilization) characteristics were assessed in oocytes after warming. This study demonstrated that IVM was more detrimental to prepubertal oocytes than to young adults. Further, vitrification of the IVM oocytes resulted in an increase in the number of abnormal meiotic spindles, a change in the cortical distribution pattern, a reduction in sperm zona binding and the fertilization rate. Importantly, oocyte integrity was better when prepubertal oocytes were vitrified before, rather than after, IVM. The above observations support GV stage vitrification for prepubertal oocytes requiring fertility preservation. Understanding the mechanisms behind the differing outcomes for oocytes from immature females will help in refining current protocol, thereby retaining the oocytes' maximum structural and functional integrity Further investigation is necessary to determine whether human prepubertal oocytes also behave in a similar way. It is to be noted here, with great emphasis, that a major limitation of this study is that the oocytes' abilities were tested only until fertilisation, as a consequence of which the study cannot reveal the developmental potentials of the embryos beyond fertilisation.

Vitrification of Human Germinal Vesicle Oocytes: before or after In Vitro Maturation?

BACKGROUND

The use of immature oocytes derived from stimulated cycles could be of great importance, particularly for urgent fertility preservation cases. The current study aimed to determine whether in vitro maturation (IVM) was more successful before or after vitrification of these oocytes.

MATERIALS AND METHODS

This prospective study was performed in a private in vitro fertilization (IVF) center. We collected 318 germinal vesicle (GV) oocytes from 104 stimulated oocyte donation cycles. Oocytes were divided into two groups according to whether vitrification was applied at the GV stage (group 1) or in vitro matured to the metaphase II (MII) stage and then vitrified (group 2). In the control group (group 3), oocytes were in vitro matured without vitrification. In all three groups, we assessed survival rate after warming, maturation rate, and MII-spindle/chromosome configurations. The chi-square test was used to compare rates between the three groups. Statistical significance was defined at P<0.05 and we used Bonferroni criterion to assess statistical significance regarding the various pairs of groups. The Statistical Package for the Social Sciences version 17.0 was used to perform statistical analysis.

RESULTS

There was no significant difference in the survival rate after vitrification and warming of GV (93.5%) and MII oocytes (90.8%). A significantly higher maturation rate occurred when IVM was performed before vitrification (82.9%) compared to after vitrification (51%). There was no significant difference in the incidence of normal spindle/ chromosome configurations among warmed oocytes matured in vitro before (50.0%) or after (41.2%) vitrification. However, a higher incidence of normal spindle/chromosome configurations existed in the in vitro matured oocytes which were not subjected to vitrification (fresh oocytes, 77.9%).

CONCLUSION

In stimulated cycles, vitrification of in vitro matured MII oocytes rather than GV oocytes seems to be more efficient. This approach needs to be verified in nonstimulated fertility preservation cases.

In vitro maturation of human immature oocytes for fertility preservation and research material

Aim

In recent years, the importance of fertility preservation (FP) has increased. In vitro maturation (IVM), an important technique in FP, has started to be used in the clinic, but controversies persist regarding this technique. Here, a survey of IVM for FP is provided.

Methods

Based on a literature review, the applications of FP, methods of FP, IVM of oocytes that had been collected in vivo and ex vivo, maturation of oocytes after IVM for FP, cryopreservation of oocytes for FP, explanation of the procedures to patients, and recent research on FP using IVM were investigated.

Results

Although IVM for FP remains controversial, the application of FP is expected to expand. Depending on the age and disease status of the patient, various methods of oocyte collection and ovarian stimulation, as well as various needle types and aspiration pressures, have been reported. The maturation rate of IVM in FP ranges widely and requires optimization in the future. In regard to cryopreservation for matured oocytes, the vitrification method is currently recommended.

Conclusion

Regarding FP for patients with cancer, the treatment of cancer is prioritized; thus, the time and use of medicines are often constrained. As several key points regarding IVM remain unclear, well‐designed and specific counseling for patients is necessary.

Cryopreservation and its clinical applications

Cryopreservation is a process that preserves organelles, cells, tissues, or any other biological constructs by cooling the samples to very low temperatures. The responses of living cells to ice formation are of theoretical interest and practical relevance. Stem cells and other viable tissues, which have great potential for use in basic research as well as for many medical applications, cannot be stored with simple cooling or freezing for a long time because ice crystal formation, osmotic shock, and membrane damage during freezing and thawing will cause cell death. The successful cryopreservation of cells and tissues has been gradually increasing in recent years, with the use of cryoprotective agents and temperature control equipment. Continuous understanding of the physical and chemical properties that occur in the freezing and thawing cycle will be necessary for the successful cryopreservation of cells or tissues and their clinical applications. In this review, we briefly address representative cryopreservation processes, such as slow freezing and vitrification, and the available cryoprotective agents. In addition, some adverse effects of cryopreservation are mentioned.

Developmental competence of mature yak vitrified-warmed oocytes is enhanced by IGF-I via modulation of CIRP during in vitro maturation

The objective of this study was to investigate whether developmental competence of mature vitrified-warmed yak (Bos grunniens) oocytes can be enhanced by supplemented insulin-like growth factor I (IGF-1) during in vitro maturation (IVM), and its relationship with the expression of cold-inducible RNA-binding protein (CIRP). In experiment 1, immature yak oocytes were divided into four groups, and IVM supplemented with 0, 50, 100 and 200 ng/mL IGF-1 was evaluated; the mRNA and protein expression levels of CIRP in mature oocytes in the four groups were evaluated using quantitative real-time PCR and western blotting analyses. In experiment 2, the mature yak oocytes in the four groups were cryopreserved using the Cryotop (CT) method, followed by chemical activation and in vitro culture for two days and eight days to determine cleavage, blastocyst rates, and total cell number in the blastocysts. Mature yak oocytes without vitrification served as a control group. The outcomes were as following: (1) the expression of CIRP in the matured oocytes was up-regulated in the IGF-1 groups and was highest expression was observed in the 100 ng/mL IGF-1 treatment group. (2) In the vitrified-warmed groups, the rates of cleavage and blastocyst were also highest in the 100 ng/mL IGF-1 treatment group (81.04 ± 1.06%% and 32.16 ± 1.01%), which were close to the rates observed in groups without vitrification (83.25 ± 0.85% and 32.54 ± 0.34%). The rates of cleavage and blastocyst in the other vitrified-warmed groups were 70.92 ± 1.32% and 27.33 ± 1.31% (0 ng/mL); 72.73 ± 0.74% and 29.41 ± 0.84% (50 ng/mL); 72.43 ± 0.61% and 27.61 ± 0.59% (200 ng/mL), respectively. There was no significant difference in the total cell number per blastocysts between the vitrified-warmed groups and group without vitrification. Thus, we conclude that the enhancement in developmental competence of mature yak vitrified-warmed oocytes after the addition of IGF-1 during IVM might result from the regulation of CIRP expression in mature yak oocytes prior to vitrification.

Female fertility: is it safe to "freeze?"

Objective:

To evaluate the safety and risk of cryopreservation in female fertility preservation.

Data sources:

The data analyzed in this review were the English articles from 1980 to 2013 from journal databases, primarily PubMed and Google scholar. The criteria used in the literature search show as following: (1) human; embryo; cryopreservation/freezing/vitrification, (2) human; oocyte/immature oocyte; cryopreservation/ freezing/vitrification, (3) human; ovarian tissue transplantation; cryopreservation/freezing/vitrification, (4) human; aneuploidy/DNA damage/epigenetic; cryopreservation/freezing/vitrification, and (5) human; fertility preservation; maternal age.

Study selection:

The risk ratios based on survival rate, maturation rate, fertilization rate, cleavage rate, implantation rate, pregnancy rate, and clinical risk rate were acquired from relevant meta-analysis studies. These studies included randomized controlled trials or studies with one of the primary outcome measures covering cryopreservation of human mature oocytes, embryos, and ovarian tissues within the last 7 years (from 2006 to 2013, since the pregnancy rates of oocyte vitrification were significantly increased due to the improved techniques). The data involving immature oocyte cryopreservation obtained from individual studies was also reviewed by the authors.

Results:

Vitrifications of mature oocytes and embryos obtained better clinical outcomes and did not increase the risks of DNA damage, spindle configuration, embryonic aneuploidy, and genomic imprinting as compared with fresh and slow-freezing procedures, respectively.

Conclusions:

Both embryo and oocyte vitrifications are safe applications in female fertility preservation.

Developmental Competence of Vitrified-Warmed Bovine Oocytes at the Germinal-Vesicle Stage is Improved by Cyclic Adenosine Monophosphate Modulators during In Vitro Maturation

Cryopreservation of mature oocytes and embryos has provided numerous benefits in reproductive medicine. Although successful cryopreservation of germinal-vesicle stage (GV) oocytes holds promise for further advances in reproductive biology and clinical embryology fields, reports regarding cryopreservation of immature oocytes are limited. Oocyte survival and maturation rates have improved since vitrification is being performed at the GV stage, but the subsequent developmental competence of GV oocytes is still low. The purpose of this study was to evaluate the effects of supplementation of the maturation medium with cyclic adenosine monophosphate (cAMP) modulators on the developmental competence of vitrified-warmed GV bovine oocytes. GV oocytes were vitrified-warmed and cultured to allow for oocyte maturation, and then parthenogenetically activated or fertilized in vitro. Our results indicate that addition of a cAMP modulator forskolin (FSK) or 3-isobutyl-1-methylxanthine (IBMX) to the maturation medium significantly improved the developmental competence of vitrified-warmed GV oocytes. We also demonstrated that vitrification of GV oocytes led to a decline in cAMP levels and maturation-promoting factor (MPF) activity in the oocytes during the initial and final phases of maturation, respectively. Nevertheless, the addition of FSK or IBMX to the maturation medium significantly elevated cAMP levels and MPF activity during IVM. Taken together, our results suggest that the cryopreservation-associated meiotic and developmental abnormalities observed in GV oocytes may be ameliorated by an artificial increase in cAMP levels during maturation culture after warming.

Ultrastructure of immature and mature human oocytes after cryotop vitrification

In vitro maturation of vitrified immature germinal vesicle (GV) oocytes is a promising fertility preservation option. We analyzed the ultrastructure of human GV oocytes after Cryotop vitrification (GVv) and compared it with fresh GV (GVc), fresh mature metaphase II (MIIc) and Cryotop-vitrified mature (MIIv) oocytes. By phase contrast microscopy and light microscopy, the oolemmal and cytoplasmic organization of fresh and vitrified oocytes did not show significant changes. GVv oocytes showed significant ultrastructural alterations of the microvilli in 40% of the samples; small vacuoles and occasional large/isolated vacuoles were abnormally present in the ooplasm periphery of 50% of samples. The ultrastructure of nuclei and mitochondria-vesicle (MV) complexes, as well as the distribution and characteristics of cortical granules (CGs), were comparable with those of GVc oocytes. MIIv oocytes showed an abnormal ultrastructure of microvilli in 30% of the samples and isolated large vacuoles in 70% of the samples. MV complexes were normal, but mitochondria-smooth endoplasmic reticulum aggregates appeared to be of reduced size. CGs were normally located under the oolemma but presented abnormalities in distribution and matrix electron density. In conclusion, Cryotop vitrification preserved main oocyte characteristics in the GV and MII stages, even if peculiar ultrastructural alterations appeared in both stages. This study also showed that the GV stage appears more suitable for vitrification than the MII stage, as indicated by the good ultrastructural preservation of important structures that are present only in immature oocytes, like the nucleus and migrating CGs.

In vitro maturation of human oocytes: Its role in infertility treatment and new possibilities

IVM refers to the maturation of immature oocytes in culture after their recovery from small antral follicles at the stage prior to selection and dominance. IVM requires little or no FSH in vivo and has been proposed as an alternative to conventional IVF, since it reduces the primary adverse effects caused by controlled ovarian stimulation, including the ovarian hyperstimulation syndrome. Moreover, IVM is a promising option for cases for which no standard protocol is suitable, such as FSH resistance, contraindications for ovarian stimulatory drugs, and the need for urgent fertility preservation. Recently, IVM has been used in women with regular cycles and normal ovaries. However, the pregnancy rate following IVM is suboptimal compared with that of conventional IVF, indicating that further studies to optimize the protocol and the culture conditions are warranted.

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.

Fertility preservation for patients with cancer: American Society of Clinical Oncology clinical practice guideline update

PURPOSE

To update guidance for health care providers about fertility preservation for adults and children with cancer.

METHODS

A systematic review of the literature published from March 2006 through January 2013 was completed using MEDLINE and the Cochrane Collaboration Library. An Update Panel reviewed the evidence and updated the recommendation language.

RESULTS

There were 222 new publications that met inclusion criteria. A majority were observational studies, cohort studies, and case series or reports, with few randomized clinical trials. After review of the new evidence, the Update Panel concluded that no major, substantive revisions to the 2006 American Society of Clinical Oncology recommendations were warranted, but clarifications were added.

RECOMMENDATIONS

As part of education and informed consent before cancer therapy, health care providers (including medical oncologists, radiation oncologists, gynecologic oncologists, urologists, hematologists, pediatric oncologists, and surgeons) should address the possibility of infertility with patients treated during their reproductive years (or with parents or guardians of children) and be prepared to discuss fertility preservation options and/or to refer all potential patients to appropriate reproductive specialists. Although patients may be focused initially on their cancer diagnosis, the Update Panel encourages providers to advise patients regarding potential threats to fertility as early as possible in the treatment process so as to allow for the widest array of options for fertility preservation. The discussion should be documented. Sperm and embryo cryopreservation as well as oocyte cryopreservation are considered standard practice and are widely available. Other fertility preservation methods should be considered investigational and should be performed by providers with the necessary expertise.

Ultrastructure of human mature oocytes after vitrification

Since the introduction of human assisted reproduction, oocyte cryopreservation has been regarded as an attractive option to capitalize the reproductive potential of surplus oocytes and preserve female fertility. However, for two decades the endeavor to store oocytes has been limited by the not yet optimized methodologies, with the consequence of poor clinical outcome or of uncertain reproducibility. Vitrification has been developed as the promising technology of cryopreservation even if slow freezing remains a suitable choice. Nevertheless, the insufficiency of clinical and correlated multidisciplinary data is still stirring controversy on the impact of this technique on oocyte integrity. Morphological studies may actually provide a great insight in this debate. Phase contrast microscopy and other light microscopy techniques, including cytochemistry, provided substantial morpho-functional data on cryopreserved oocyte, but are unable to unraveling fine structural changes. The ultrastructural damage is one of the most adverse events associated with cryopreservation, as an effect of cryo-protectant toxicity, ice crystal formation and osmotic stress. Surprisingly, transmission electron microsco py has attracted only limited attention in the field of cryopreservation. In this review, the subcellular structure of human mature oocytes following vitrification is discussed at the light of most relevant ultrastructural studies.

Diminished ovarian reserve and infertility

Over the past several decades, social and demographic trends have led to an increased tendency for women to delay childbearing. Owing primarily to abnormalities in the oocyte and resulting embryonic aneuploidy, implantation, clinical pregnancy, and live birth rates decline sharply by the end of the fourth decade. As a result, the incidence of age-related infertility has increased. Improved awareness of the effects of aging on fertility combined with ovarian reserve assessment, patient education, and early infertility evaluation and intervention are important elements in appropriate family planning and prevention of age-related infertility.

Cryopreservation of human failed-matured oocytes followed by in vitro maturation: vitrification is superior to the slow freezing method

BACKGROUND

Oocyte cryopreservation is an important method used in a number of human fertility circumstances. Here, we compared the survival, in vitro maturation, fertilization, and early embryonic development rates of frozen-thawed human immature oocytes using two different cryopreservation methods.

METHODS

A total of 454 failed-matured oocytes [germinal vesicle (GV) and metaphase I (MI) stages] were collected from 135 patients (mean age 33.84 +/- 5.0 y) who underwent intracytoplasmic sperm injection (ICSI) cycles between February 2009 and December 2009 and randomly divided into a slow freezing group [1.5 mol/L-1, 2-propanediol (PROH) + 0.2 mol/l sucrose] and vitrification group [20% PROH + 20% ethylene glycol (EG) + 0.5 mol/l sucrose].

RESULTS

The vitrification protocol yielded a better survival rate than the slow freezing protocol at each maturation stage assessed. Regardless of the maturation stage (GV + MI), the slow freezing protocol had a significantly lower survival rate than the vitrification protocol (p < 0.001). In addition, a significant difference was found in the survival rates between GV and MI oocytes regardless of the protocol used (90.1 vs. 64.7%, respectively; p < 0.01). We also found that the maturation rates of GV and MI oocytes from the slow freezing and vitrification groups were 16.7 vs. 24.4% and 50.8 vs. 55.4%, respectively. Regardless of the protocol used, the GV oocytes had significantly lower viability than MI oocytes after 36 h of in vitro maturation (21.2 vs. 54.0%, respectively; p < 0.01). In addition, the GV and MI oocytes from the slow freezing group had a markedly lower maturation rate than those from the vitrification group (33.6 vs. 43.1%, respectively), but no statistical difference was found between the two groups (P > 0.05). For the GV-matured oocytes, no fertilized eggs were obtained in the slow-freezing group, while a 19.0% (4/21) fertilization rate was observed in the vitrification group. For the MI-matured oocytes, fertilization rates for the slow freezing and vitrified groups were 36% and 61.1%, respectively, but no significant difference was found between the two groups (PIn the Methods section in the MS, all procedures were compliant with ethical guidelines, i.e. approved by the Ethical Committee of our university and Informed Consent signed by each patient. > 0.05). In the GV vitrification group, no embryo formed; however, in the MI slow freezing group, 12 oocytes were fertilized, but only two achieved cleavage and were subsequently blocked at the 2-cell stage. In the MI vitrification group, a total of 22 embryos were obtained, five of which developed to the blastocyst stage.

CONCLUSIONS

Vitrification is superior to the slow freezing method in terms of the survival and developmental rates for the cryopreservation of human failed-matured oocytes. In addition, GV oocytes appeared to be more resistant than MI oocytes to the low temperature and cryoprotectant used during cryopreservation.