Ultrastructure of human mature oocytes after slow cooling cryopreservation using different sucrose concentrations†

Ultrastructural Evaluation of Mouse Oocytes Exposed In Vitro to Different Concentrations of the Fungicide Mancozeb

Mancozeb is a widely used fungicide, considered to be an endocrine disruptor. In vivo and in vitro studies evidenced its reproductive toxicity on mouse oocytes by altering spindle morphology, impairing oocyte maturation, fertilization, and embryo implantation. Mancozeb also induces dose-dependent toxicity on the ultrastructure of mouse granulosa cells, including chromatin condensation, membrane blebbing, and vacuolization. We evaluated the effects on the ultrastructure of mouse oocytes isolated from cumulus-oocyte complexes (COCs), exposed in vitro to increasing concentrations of mancozeb. COCs were matured in vitro with or without (control) low fungicide concentrations (0.001-1 μg/mL). All mature oocytes were collected and prepared for light and transmission electron microscopy. Results showed a preserved ultrastructure at the lowest doses (0.001-0.01 μg/mL), with evident clusters of round-to-ovoid mitochondria, visible electron-dense round cortical granules, and thin microvilli. Mancozeb concentration of 1 μg/mL affected organelle density concerning controls, with a reduction of mitochondria, appearing moderately vacuolated, cortical granules, and microvilli, short and less abundant. In summary, ultrastructural data revealed changes mainly at the highest concentration of mancozeb on mouse oocytes. This could be responsible for the previously described impaired capability in oocyte maturation, fertilization, and embryo implantation, demonstrating its impact on the reproductive health and fertility.

Effects of Simulated Microgravity In Vitro on Human Metaphase II Oocytes: An Electron Microscopy-Based Study

The Gravity Force to which living beings are subjected on Earth rules the functionality of most biological processes in many tissues. It has been reported that a situation of Microgravity (such as that occurring in space) causes negative effects on living beings. Astronauts returning from space shuttle missions or from the International Space Station have been diagnosed with various health problems, such as bone demineralization, muscle atrophy, cardiovascular deconditioning, and vestibular and sensory imbalance, including impaired visual acuity, altered metabolic and nutritional status, and immune system dysregulation. Microgravity has profound effects also on reproductive functions. Female astronauts, in fact, suppress their cycles during space travels, and effects at the cellular level in the early embryo development and on female gamete maturation have also been observed. The opportunities to use space flights to study the effects of gravity variations are limited because of the high costs and lack of repeatability of the experiments. For these reasons, the use of microgravity simulators for studying, at the cellular level, the effects, such as those, obtained during/after a spatial trip, are developed to confirm that these models can be used in the study of body responses under conditions different from those found in a unitary Gravity environment (1 g). In view of this, this study aimed to investigate in vitro the effects of simulated microgravity on the ultrastructural features of human metaphase II oocytes using a Random Positioning Machine (RPM). We demonstrated for the first time, by Transmission Electron Microscopy analysis, that microgravity might compromise oocyte quality by affecting not only the localization of mitochondria and cortical granules due to a possible alteration of the cytoskeleton but also the function of mitochondria and endoplasmic reticulum since in RPM oocytes we observed a switch in the morphology of smooth endoplasmic reticulum (SER) and associated mitochondria from mitochondria-SER aggregates to mitochondria-vesicle complexes. We concluded that microgravity might negatively affect oocyte quality by interfering in vitro with the normal sequence of morphodynamic events essential for acquiring and maintaining a proper competence to fertilization in human oocytes.

Oocyte quality and aging

It is well known that female reproduction ability decreases during the forth decade of life due to age-related changes in oocyte quality and quantity; although the number of women trying to conceive has today increased remarkably between the ages of 36 to 44. The causes of reproductive aging and physiological aspects of this phenomenon are still elusive. With increase in the women's age, during Assisted Reproductive Technologies (ART) we have perceived a significant decline in the number and quality of retrieved oocytes, as well as in ovarian follicle reserves. This is because of increased aneuploidy due to factors such as spindle apparatus disruption; oxidative stress and mitochondrial damage. The aim of this review paper is to study data on the potential role of the aging process impacting oocyte quality and female reproductive ability. We present the current evidence that show the decreased oocyte quality with age, related to reductions in female reproductive outcome. The aging process is complicated and it is caused by many factors that control cellular and organism life span. Although the factors responsible for reduced oocyte quality remain unknown, the present review focuses on the potential role of ovarian follicle environment, oocyte structure and its organelles. To find a way to optimize oocyte quality and ameliorate clinical outcomes for women with aging-related causes of infertility.

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.

Ultrastructure of mitochondria of human oocytes in different clinical conditions during assisted reproduction

Infertility affects around 8% of couples with a slight change in percentage in the last years. Despite the significant efforts made in Assisted Reproductive Technologies (ARTs) in handling this disorder, oocyte quality remains a crucial factor for a positive outcome. A better understanding of the dynamics underlying oocyte maturation, fertilization, and embryo development remains one of the main areas for progress in the ARTs field. Mitochondria are believed to play an essential role in these processes. Mitochondria have a crucial part in producing energy for oocyte maturation and embryo development throughout precise cellular functions comprising Ca²⁺ homeostasis regulation, glycolysis, amino acid and fatty acid metabolism, and regulation of apoptosis. Recent studies suggest that mitochondrial structure, content, and function may be related to oocyte competence, embryo viability, and implantation success during ARTs. Their defects could lead to low fertilization rates and embryonic development failure. This review aimed to provide an overview of the available literature data surrounding the correlation between changes at ultrastructural level of mitochondria or correlated-mitochondrial aggregates and oocyte quality and ARTs treatments. Our reported data demonstrated that oocyte mitochondrial ultrastructural alterations could be partial or complete recovery during the early embryo stages. However, these changes could persist as quiescent during the pre-implantation embryo development, causing abnormalities that become evident only during fetal and postnatal life. These factors led to consider the mitochondria as a crucial marker of oocyte and embryo quality, as well as a strategic target for further prospective therapeutical approaches.

Association between Female Reproductive Health and Mancozeb: Systematic Review of Experimental Models

Mancozeb is a widely used fungicide approved for use in agriculture in many countries with long persistence in the environment and consequent bioaccumulation in tissues and biological fluids. Despite the large amount of studies published in recent years, the relationship between mancozeb exposure and female reproductive health is not fully elucidated. In order to summarize current evidence on mancozeb exposure and female reproductive disease, we performed a systematic review of literature. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used to make this review. An adapted version of the National Toxicology Program’s Office of Health and Assessment and Translation (OHAT) framework was used to evaluate the risk of bias. Electronic search on two databases (PubMed and Scopus) was used to find experimental studies (in vitro and in vivo) on mancozeb exposure. The database search identified 250 scientific articles, 20 of which met our inclusion criteria. Selected data were then reviewed and summarized in tables. Overall, mancozeb represents a hazard for female reproductive health, with different mechanisms of action. Undoubtedly more experimental and epidemiological studies are required to definitively validate mancozeb as reproductive toxicant.

Type of protein supplement in cryopreservation solutions impacts on the degree of ultrastructural damage in frozen-thawed human oocytes

Protein sources used as supplements of IVF culture media are known to have several implications for the function and stability of embryo culture environment. In fact, they i) transport biologically active molecules ii) chelate heavy metals, iii) regulate media pH, iii) scavenge reactive oxygen species (ROS) and iv) attenuate osmotic stress to which cells are exposed in sub-optimal culture conditions. Instead, their specific relevance to the formulation of cryopreservation solutions used for gamete and embryo cryopreservation remains uncertain. In the present work, we tested the hypothesis that different protein supplements present in cryopreservation solutions, serum or plasma protein solution (PPS), or different concentrations of the same supplement (serum), are associated with different types and/or magnitude of cryopreservation-derived cell damage. To this end, using cryopreservation solutions containing serum or PPS, donated supernumerary human mature oocytes were frozen-thawed by slow freezing and compared with fresh controls. Ultrastructural markers of oocyte quality were adopted as objective measure to assess possible damage from cryopreservation. The study results indicate that the adoption of serum minimises cell damage induced by cryopreservation. Indeed, typical hallmarks of cryodamage in human oocytes, i.e. loss of cortical granules, zona pellucida hardening and above all vacuolization, were largely reduced in oocytes cryopreserved with solutions containing serum, especially if used a higher concentration. This suggest that oocyte cryopreservation still has significant margins of improvement that may derive also from composition of cryopreservation media.

Revisiting embryo assisted hatching approaches: a systematic review of the current protocols

Zona pellucida (ZP) manipulation, termed "assisted hatching" (AH), has been introduced in order to favor embryo hatching and ultimately improve assisted reproductive technology success but with poor proofs of safety and biological plausibility. We herein provide a systematic review of clinical outcomes following the application of different methods of ZP manipulation on fresh or frozen/thawed embryos at different developmental stages in different groups of patients. Out of the 69 papers that compared the clinical outcomes deriving from hatched versus non-hatched embryos, only 11 considered blastocysts while the rest referred to cleavage stage embryos. The ZP thinning of fresh embryos either by chemical or laser approach was shown to provide very limited benefit in terms of clinical outcomes. Better results were observed with procedures implying a higher degree of zona manipulation, including zona removal. Studies comparing the mechanical or chemical procedures to those laser-mediated consistently reported a superiority of the latter ones over the former. Literature is consistent for a benefit of ZP breaching in thawed blastocysts. This review provides the current knowledge on the AH procedure in order to improve its efficacy in the appropriate context. Embryologists might benefit from the approaches presented herein in order to improve Assisted Reproduction Technologies (ART) outcomes.

State of the art on oocyte cryopreservation in female cancer patients: A critical review of the literature

During the last decades, important advances in therapeutic options have led to increased survival rates in cancer patients; however, cancer treatments are associated with several potential adverse effects including infertility in those diagnosed during their reproductive years. A proper discussion about fertility preservation options before the use of therapies with potential gonadotoxicity (i.e. oncofertility counseling) is standard of care and should be offered to all patients of childbearing age. Temporary ovarian suppression with LH-RH analogs, oocyte and embryo cryopreservation are standard strategies for fertility preservation in female cancer patients. Oocyte cryopreservation should be preferred to embryo cryopreservation when this latter is prohibited by law, avoided for ethical or religious issues and in single women refusing sperm donation. Despite the increasing use of this strategy, data are still lacking about the efficacy and safety of the procedure in female cancer patients, with most of the evidence on this regard deriving from infertile non-oncologic women. This article aims at critically review the available evidence about the success of oocyte cryopreservation in female cancer patients with the final goal to further improve the oncofertility counseling of these women.

Ultrastructural characteristics of human oocytes vitrified before and after in vitro maturation

The development of an effective program that combines in vitro maturation (IVM) and cryopreservation for immature oocytes would represent a novel advance for in vitro fertilization (IVF), especially as a means to preserve the fertility of women in unique situations. The aim of this study was to analyze the ultrastructural characteristics of human oocytes, obtained after controlled ovarian stimulation, to determine whether IVM is best performed before or after vitrification. To this end, we analyzed the following features in a total of 22 MII oocytes: size, zona pellucida and perivitelline space, mitochondria number, M-SER (mitochondria-smooth endoplasmic reticulum) aggregates and M-V (mitochondria-vesicle) complexes, the number of cortical granules and microvilli, and the presence of vacuolization using transmission electron microscopy (TEM). Each oocyte presented a rounded shape, with an intact oolemma, and was surrounded by a continuous zona pellucida and perivitelline space. Statistical analysis comparing oocytes vitrified before or after IVM indicated that there were no significant differences between examined characteristics.

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.

Ultrastructure of human oocytes after in vitro maturation

STUDY HYPOTHESIS

How does the ultrastructure of human oocytes matured in vitro compare with oocytes collected from women after full hormonal stimulation?

STUDY FINDING

The ultrastructure of human oocytes matured in vitro is largely, but not entirely, similar to those matured in vivo.

WHAT IS KNOWN ALREADY

Embryos derived from in vitro-matured oocytes often have limited developmental potential, possibly as an effect of inappropriate in vitro maturation (IVM) conditions. Transmission electron microscopy (TEM) is a valuable research tool to compare in vivo and in vitro matured oocytes. However, previous studies on the ultrastructure of human IVM oocytes were done with inadequate material or inappropriate IVM conditions, and have limited significance.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Immature cumulus cell-enclosed oocytes, retrieved from mid-sized antral follicles of women requiring IVM treatment, were matured in vitro for 30 h. No leftover germinal vesicle-stage oocytes collected from fully stimulated cycles were used. Control in vivo matured oocytes were obtained from age-matched women undergoing full ovarian stimulation. In vitro and in vivo matured oocytes were analysed by TEM and compared according to previously established morphometric criteria of oocyte quality.

MAIN RESULTS AND THE ROLE OF CHANCE

All oocytes had normal ooplasm showing uniform distribution of organelles. Mitochondrial morphology appeared similar between the maturation conditions. Cortical granules were found typically stratified in a single, mostly continuous row just beneath the ooplasm in all oocytes. Microvilli were well preserved after IVM. Vacuoles were only occasionally found in all oocytes and, if present, they were frequently associated with lysosomes. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates and mitochondria-vesicles (MV) complexes were commonly found in in vivo matured oocytes. However, large MV complexes partially replaced M-SER aggregates in IVM oocytes.

LIMITATIONS, REASONS FOR CAUTION

As a note of caution it should be noticed that, being laborious and technically demanding, TEM cannot be applied to a large number of samples in a single investigation. Therefore, our data require further independent confirmation.

WIDER IMPLICATIONS OF THE FINDINGS

Our data suggests the notion that TEM remains a valuable research tool that can also offer quantitative data if associated with morphometric criteria of evaluation. Therefore, it can be adopted to test pre-clinically the performance of novel in vitro systems that are demanded to make oocytes IVM more successful in the human.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This study was independently funded by Biogenesi Reproductive Medicine Centre, Monza, Italy. All authors declare that their participation in the study did not involve factual or potential conflicts of interests.

Ultrastructural markers of quality are impaired in human metaphase II aged oocytes: a comparison between reproductive and in vitro aging

PURPOSE

Childbearing delay contributes to the increase of subfertile couples that require assisted reproductive technology (ART). Subfertility relates with reproductive aging (RA). In vitro aging (IvA) (due to extended culture) may also impair oocyte competence. Aims of this study were to evaluate and compare the oocyte ultrastructure after RA and IvA.

METHODS

Cumulus-oocyte complexes (COCs) (n = 68), with metaphase II oocyte and expanded cumulus, from consenting patients (<35 years old and ≥35 years old, n = 36), were selected by phase contrast microscopy and fixed at pick up, or after 24 h culture. COCs (n = 44) were studied by light and qualitative/morphometric transmission electron microscopy. Two-way ANOVA, with age and culture as grouping factors, was applied for statistical analysis (p < 0.05). Metaphase II cumulus-free oocytes (n = 24) were selected for confocal microscopy observations.

RESULTS

Significant decrease of mitochondria-smooth endoplasmic reticulum aggregates, increase of mitochondria-vesicle complexes size and amount, decrease of cortical granules and microvilli, and alterations of the spindle structure characterized both RA and IvA oocytes. These changes were significantly more evident in the RA oocytes submitted to IvA. RA oocytes also showed changes of the zona pellucida and occurrence of vacuoles after culture. Cumuli appeared re-compacted after culture, irrespective of the age of the patients.

CONCLUSIONS

These data demonstrated that aging is related to decay of oocyte ultrastructural quality, and that oocytes from elder women are more sensitive to prolonged culture (IvA) than the oocytes from younger women. These morphological results should be considered when applying ART in aged patients, rescue ICSI, or artificial oocyte activation.

Fine morphological assessment of quality of human mature oocytes after slow freezing or vitrification with a closed device: a comparative analysis

BACKGROUND

Human mature oocytes are very susceptible to cryodamage. Several reports demonstrated that vitrification might preserve oocyte better than slow freezing. However, this is still controversial. Thus, larger clinical, biological and experimental trials to confirm this concept are necessary. The aim of the study was to evaluate and compare fine morphological features in human mature oocytes cryopreserved with either slow freezing or vitrification.

METHODS

We used 47 supernumerary human mature (metaphase II) oocytes donated by consenting patients, aged 27-32 years, enrolled in an IVF program. Thirtyfive oocytes were cryopreserved using slow freezing with 1.5 M propanediol +0.2 M sucrose concentration (20 oocytes) or a closed vitrification system (CryoTip Irvine Scientific CA) (15 oocytes). Twelve fresh oocytes were used as controls. All samples were prepared for light and transmission electron microscopy evaluation.

RESULTS

Control, slow frozen/thawed and vitrified/warmed oocytes (CO, SFO and VO, respectively) were rounded, 90-100 μm in diameter, with normal ooplasm showing uniform distribution of organelles. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates and small mitochondria-vesicle (MV) complexes were the most numerous structures found in all CO, SFO and VO cultured for 3-4 hours. M-SER aggregates decreased, and large MV complexes increased in those SFO and VO maintained in culture for a prolonged period of time (8-9 hours). A slight to moderate vacuolization was present in the cytoplasm of SFO. Only a slight vacuolization was present in VO, whereas vacuoles were almost completely absent in CO. Amount and density of cortical granules (CG) appeared abnormally reduced in SFO and VO, irrespective of the protocol applied.

CONCLUSIONS

Even though, both slow freezing and vitrification ensured a good overall preservation of the oocyte, we found that: 1) prolonged culture activates an intracellular membrane "recycling" that causes the abnormal transformation of the membranes of the small MV complexes and of SER into larger rounded vesicles; 2) vacuolization appears as a recurrent form of cell damage during slow freezing and, at a lesser extent, during vitrification using a closed device; 3) premature CG exocytosis was present in both SFO and VO and may cause zona pellucida hardening.

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.

Oocytes with a dark zona pellucida demonstrate lower fertilization, implantation and clinical pregnancy rates in IVF/ICSI cycles

The morphological assessment of oocytes is important for embryologists to identify and select MII oocytes in IVF/ICSI cycles. Dysmorphism of oocytes decreases viability and the developmental potential of oocytes as well as the clinical pregnancy rate. Several reports have suggested that oocytes with a dark zona pellucida (DZP) correlate with the outcome of IVF treatment. However, the effect of DZP on oocyte quality, fertilization, implantation, and pregnancy outcome were not investigated in detail. In this study, a retrospective analysis was performed in 268 infertile patients with fallopian tube obstruction and/or male factor infertility. In 204 of these patients, all oocytes were surrounded by a normal zona pellucida (NZP, control group), whereas 46 patients were found to have part of their retrieved oocytes enclosed by NZP and the other by DZP (Group A). In addition, all oocytes enclosed by DZP were retrieved from 18 patients (Group B). No differences were detected between the control and group A. Compared to the control group, the rates of fertilization, good quality embryos, implantation and clinical pregnancy were significantly decreased in group B. Furthermore, mitochondria in oocytes with a DZP in both of the two study groups (A and B) were severely damaged with several ultrastructural alterations, which were associated with an increased density of the zona pellucida and vacuolization. Briefly, oocytes with a DZP affected the clinical outcome in IVF/ICSI cycles and appeared to contain more ultrastructural alterations. Thus, DZP could be used as a potential selective marker for embryologists during daily laboratory work.

Oocyte cryopreservation: searching for novel improvement strategies

PURPOSE

To highlight emerging techniques aimed at improving oocyte cryopreservation.

METHODS

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

RESULTS

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

CONCLUSION

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

Effect of methanol on mitochondrial organization in zebrafish (Danio rerio) ovarian follicles

Successful cryopreservation is usually measured in terms of cell survival. However, there may also be more subtle effects within cells that survive. Previous studies on zebrafish have produced evidence of mitochondrial DNA (mtDNA) damage in cryopreserved embryonic blastomeres and, after exposure to cryoprotectants, alterations in mtDNA replication in embryos and decreased mitochondrial membrane potential, mtDNA and ATP production in ovarian follicles. This study shows that the decreased ATP levels previously observed in stage III zebrafish ovarian follicles exposed to ≥3 M methanol persisted in those follicles that subsequently developed to stage IV. However, the decreased mtDNA levels were restored in those follicles. In order to determine whether mitochondrial distribution and/or their transport network was affected by the methanol exposure, immunocytochemistry analysis of tubulin and mitochondrial cytochrome c oxidase I (COX-I) was performed, along with phalloidin staining of polymerized actin. Neat arrangements of all proteins were observed in control follicles, with COX-I and tubulin being colocalized near granulosa cell nuclei, while actin formed hexagonal and/or polygonal structures nearer granulosa cell membranes and projected into the oocyte surface. Exposure to methanol (2 to 4 M) disrupted the COX-I and tubulin arrangements and the hexagonal and/or polygonal actin distribution and actin projections into the oocyte. These effects were still observed in those follicles that developed to stage IV, although the severity was reduced. In summary, the disruption to function and distribution of mitochondria in ovarian follicles exposed to >2 M methanol may be mediated via disruption of the mitochondrial transport system. Some recovery of this disruption may take place after methanol removal and subsequent follicle maturation.

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.

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

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

Relevance of assisted hatching in an oocyte donation programme using egg cryobanking: a prospective randomised study

OBJECTIVE

The objective of this prospective randomised study was to evaluate if there is an improvement in clinical outcomes when assisted hatching (AH) is performed in embryos derived from vitrified/warmed oocytes in an ovum donation programme using egg cryobanking.

STUDY DESIGN

Sixty oocyte recipients in a donation programme using egg cryobanking were randomly allocated to the assisted hatched (AH, n=30) or control group (n=30). The pregnancy and implantation rates were compared between the groups.

RESULTS

A total of 288 vitrified oocytes were warmed for the 60 recipients. Of the 288 vitrified oocytes, 94.8% survived. All surviving oocytes were sperm injected, and 83.5% underwent fertilisation. There were 172 good-quality embryos selected for transfer. The total pregnancy rate was 40%. The pregnancy rate did not differ between the AH and control groups (43.3% and 33.3%, respectively, p=0.1967), but AH resulted in a higher implantation rate (31.6% vs. 18.4%, p=0.0206).

CONCLUSION

Our study demonstrates the effectiveness of AH in embryos derived from warmed oocytes. Our results also suggest that oocyte cryopreservation can be considered as a tool for providing highly successful outcomes in an egg donor programme.

A critical appraisal of cryopreservation (slow cooling versus vitrification) of human oocytes and embryos

BACKGROUND

Vitrification is now a commonly applied technique for cryopreservation in assisted reproductive technology (ART) replacing, in many cases, conventional slow cooling methodology. This review examines evidence relevant to comparison of the two approaches applied to human oocytes and embryos at different developmental stages.

METHODS

Critical review of the published literature using PubMed with particular emphasis on studies which include data on survival and implantation rates, data from fresh control groups and evaluation of the two approaches in a single setting.

RESULTS

Slow cooling is associated with lower survival rates and compromised development relative to vitrification when applied to metaphase II (MII) oocytes, although the vitrification results have predominantly been obtained using direct contact with liquid nitrogen and there is some evidence that optimal protocols for slow cooling of MII oocytes are yet to be established. There are no prospective randomized controlled trials (RCTs) which support the use of either technique with pronuclear oocytes although vitrification has become the method of choice. Optimal slow cooling, using modifications of traditional methodology, and vitrification can result in high survival rates of early embryos, which implant at the same rate as equivalent fresh counterparts. Many studies report high survival and implantation rates following vitrification of blastocysts. Although slow cooling of blastocysts has been reported to be inferior in some studies, others comparing the two approaches in the same clinical setting have demonstrated comparable results. The variation in the extent of embryo selection applied in studies can lead to apparent differences in clinical efficiency, which may not be significant if expressed on a 'per oocyte used' basis.

CONCLUSIONS

Available evidence suggests that vitrification is the current method of choice when cryopreserving MII oocytes. Early cleavage stage embryos can be cryopreserved with equal success using slow cooling and vitrification. Successful blastocyst cryopreservation may be more consistently achieved with vitrification but optimal slow cooling can produce similar results. There are key limitations associated with the available evidence base, including a paucity of RCTs, limited reporting of live birth outcomes and limited reporting of detail which would allow assessment of the impact of differences in female age. While vitrification has a clear role in ART, we support continued research to establish optimal slow cooling methods which may assist in alleviating concerns over safety issues, such as storage, transport and the use of very high cryoprotectant concentrations.

Oocyte slow freezing using a 0.2-0.3 M sucrose concentration protocol: is it really the time to trash the cryopreservation machine?

OBJECTIVE

To update results on outcomes with frozen/thawed oocytes using a differential sucrose concentration during dehydration (0.2 M) and rehydration (0.3 M), combined with a one-step propanediol exposure.

DESIGN

Retrospective cohort study.

SETTING

Private IVF centers.

PATIENT(S)

Infertile couples undergoing IVF treatment.

INTERVENTION(S)

Oocyte thawing cycles between May 2004 and December 2010.

MAIN OUTCOME MEASURE(S)

Survival, fertilization, and cleavage rates were reported to evaluate biological outcomes. Clinical pregnancy and implantation rates were analyzed as markers of efficiency.

RESULT(S)

Three hundred forty-two patients and 443 cycles were monitored; the survival was 71.8%, fertilization 77.9%, and of the embryos obtained 83.8% were classified as grade 1 and 2. Three hundred ninety-four transfers were performed, resulting in 90 pregnancies. The pregnancy rate per transfer was 22.8% and per patient was 26.3%, with 122 gestational sacs. The implantation rate per embryo was 13.5%. Patients were divided into three groups according to their age: ≤ 34 years (group A), 35-38 years (group B), and ≥ 39 years (group C). Biological outcomes were comparable in all three groups, whereas the pregnancy rate per transfer was higher in the first group (27.7% vs. 21.4% and 17.6%). The implantation rates per injected egg were 11.8%, 8.0%, and 7.5% for the three groups, respectively.

CONCLUSION(S)

The biological and clinical data obtained on 443 cycles are consistent with our previous results showing that slow freezing of oocytes can be a valid tool in IVF practice when performed with a suitable protocol.

Oocyte cryostorage to preserve fertility in oncological patients

Thanks to the progress in oncostatic treatments, young women affected by cancer have a fairly good chance of surviving the disease and leading a normal post-cancer life. Quite often, however, polychemiotherapy and/or radiotherapy can induce ovarian damage and significantly reduce the content of follicles and oocytes inside the ovary, thus predisposing the patient to menstrual disorders, infertility, and precocious menopause. Several techniques have been proposed to preserve fertility in these patients; among them oocyte collection and cryopreservation prior to the oncostatic treatment has been widely applied in the last decade. The proper indications, the permitting conditions, the available hormonal stimulation protocols, as well as the effectiveness and limits of this option will be discussed herein, with a comprehensive and up-to-date review of the two techniques commonly used to cryostore oocytes, the slow-freezing technique and the vitrification technique.

Oocyte morphology does not affect post-warming survival rate in an egg-cryobanking donation program

PURPOSE

To evaluate whether oocyte dysmorphisms affect oocyte survival rates in an egg-cryobanking donation program.

METHODS

This study included 54 patients undergoing intracytoplasmic sperm injection. A total of 415 metaphase II oocytes were vitrified using the Cryotop method. Oocyte morphology was assessed immediately prior to oocyte vitrification under 400× magnification. The influence of dysmorphisms on post-thaw survival rates was assessed using regression analysis. Results were considered to be significant at the 5% critical level.

RESULTS

Oocyte survival rate was not affected by the presence of the following analysed oocyte abnormalities: increased cytoplasmic granularity, vacuoles in the ooplasm, aggregates of smooth endoplasmic reticulum in the ooplasm, large perivitelline space size, perivitelline space granularity, fragmented first polar body and zona pellucida abnormalities.

CONCLUSIONS

Oocyte morphology, observed prior to vitrification, does not predict post-warming survival. The non-invasive identification of predictive markers for oocyte survival potential remains a difficult task.

Implantation rates of embryos generated from slow cooled human oocytes from young women are comparable to those of fresh and frozen embryos from the same age group

Previous reports of slow cooling of human mature oocytes have shown a reduced clinical efficiency relative to fresh oocytes. This study reports that equivalent fertilization and implantation rates to those obtained using fresh oocytes and cryopreserved embryos can be achieved with human mature oocytes dehydrated in 1.5 M propanediol and 0.2 M sucrose at 37°C and cryopreserved using slow cooling rates.

Reprint of: Theoretical and experimental basis of slow freezing

In human IVF, cryopreservation of oocytes has become an alternative to embryo storage. It has also shown enormous potential for oocyte donation, fertility preservation and animal biotechnology. Mouse oocytes have represented the elective model to develop oocyte cryopreservation in the human and over several decades their use has made possible the development of theoretical and empirical approaches. Progress in vitrification has overshadowed slow freezing to such an extent that it has been suggested that vitrification could soon become the exclusive cryopreservation choice in human IVF. However, recent studies have clearly indicated that human embryo slow freezing, a practice considered well established for decades, can be significantly improved by a simple empirical approach. Alternatively, recent and more advanced theoretical models can predict oocyte responses to the diverse factors characterizing an entire slow-freezing procedure, offering a global method for the improvement of current protocols. This gives credit to the notion that oocyte slow freezing still has considerable margins for improvement. In human IVF, cryopreservation of oocytes has become an alternative to embryo storage. It has also shown enormous potential for oocyte donation, fertility preservation and animal biotechnology. Mouse oocytes have represented the elective model to develop oocyte cryopreservation in the human and over several decades their use has made possible the development of theoretical and empirical approaches. Progress in vitrification has overshadowed slow freezing to such an extent that it has been suggested that vitrification could soon become the exclusive cryopreservation choice in human IVF. However, recent studies have clearly indicated that human embryo slow freezing, a practice considered well established for decades, can be significantly improved by a simple empirical approach. Alternatively, recent and more advanced theoretical models can predict oocyte responses to the diverse factors characterizing an entire slow freezing procedure, offering a global method for the improvement of current protocols. This gives credit to the notion that oocyte slow freezing still has considerable margins of improvement.

Current results with slow freezing and vitrification of the human oocyte

The past decade has witnessed renewed interest in human oocyte cryopreservation (OCP). This article reviews the two general methods used for OCP, slow freezing and vitrification, compares the outcomes associated with each technique and discusses the factors that might influence success with OCP (such as oocyte selection or day of transfer). Based on available data, OCP offers a reliable, reproducible method for preservation of the female gamete and will find increasing application in assisted reproductive technology. Oocyte cryopreservation can provide a number of advantages to couples undergoing assisted reproduction or to women interested in fertility preservation. Two methods, slow freezing and vitrification, have been used successfully for oocyte cryopreservation. This article reviews and compares these methods, and discusses various factors that can impact upon success of oocyte cryopreservation.

History of oocyte cryopreservation

The potential advantages of being able to cryopreserve oocytes have been apparent for many decades. Technical difficulties associated with the unique properties of the mammalian oocyte initially retarded rapid development in this area but recent advances have overcome many of the problems. A stage has now been reached where oocyte cryopreservation can be considered an important component of human assisted reproductive technology. The potential advantages of being able to cryopreserve oocytes have been apparent for many decades. Technical difficulties associated with the unique properties of the mammalian oocyte initially retarded rapid development in this area but recent advances have overcome many of the problems. A stage has now been reached where oocyte cryopreservation can be considered an important component of human assisted reproductive technology.

Impact of phase transition on the mouse oocyte spindle during vitrification

During vitrification, the glass-like solidification is the phase-transition process from liquid to solid. Phase transition is one of the major factors suspected to affect the physiology of the oocyte, such as the structure of the meiotic spindle. Therefore, it is very important to investigate the systematic and morphological alterations of the metaphase-II spindle and chromosome arrangement during complete course of a vitrification and warming process. B6D2F1 (C57BL/6 X DBA/2) mouse oocytes were cryopreserved by minimum volume cooling (MVC) method of vitrification in a solution with 15% ethylene glycol, 15% dimethylsulphoxide and 0.5 mol/l sucrose. To examine the spindle, oocytes were fixed before, during and after vitrification and were analysed by immunocytochemistry and confocal microscopy. It was shown that spindles in all oocytes could be maintained through the vitrification and warming process, even though they were exposed to extreme temperature and two rounds of phase transition. According to the sequential observations, chromosome alignment was maintained throughout the complete course of vitrification, warming and post-warming stage. The impact of phase transition was barely detectable when the oocyte was exposed to the vitrification and warming process. The oocyte spindle was able to recover immediately after warming.

Age related changes in mitochondrial function and new approaches to study redox regulation in mammalian oocytes in response to age or maturation conditions

Mammalian oocytes are long-lived cells in the human body. They initiate meiosis already in the embryonic ovary, arrest meiotically for long periods in dictyate stage, and resume meiosis only after extensive growth and a surge of luteinizing hormone which mediates signaling events that overcome meiotic arrest. Few mitochondria are initially present in the primordial germ cells while there are mitogenesis and structural and functional differentiation and stage-specific formation of functionally diverse domains of mitochondria during oogenesis. Mitochondria are most prominent cell organelles in oocytes and their activities appear essential for normal spindle formation and chromosome segregation, and they are one of the most important maternal contributions to early embryogenesis. Dysfunctional mitochondria are discussed as major factor in predisposition to chromosomal nondisjunction during first and second meiotic division and mitotic errors in embryos, and in reduced quality and developmental potential of aged oocytes and embryos. Several lines of evidence suggest that damage by oxidative stress/reactive oxygen species in dependence of age, altered antioxidative defence and/or altered environment and bi-directional signaling between oocyte and the somatic cells in the follicle contribute to reduced quality of oocytes and blocked or aberrant development of embryos after fertilization. The review provides an overview of mitogenesis during oogenesis and some recent data on oxidative defence systems in mammalian oocytes, and on age-related changes as well as novel approaches to study redox regulation in mitochondria and ooplasm. The latter may provide new insights into age-, environment- and cryopreservation-induced stress and mitochondrial dysfunction in oocytes and embryos.

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

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