Oocyte cryopreservation: a biological perspective

Ultrastructural examination of cryodamage in Paracentrotus lividus eggs during cryopreservation

This study examinates the challenges of cryopreserving sea urchin (Paracentrotus lividus) eggs, a task hindered by factors like low membrane permeability and high sensitivity to cryoprotective agents (CPAs). While successful cryopreservation has been achieved for some marine invertebrates, eggs remain problematic due to their unique characteristics. The study explores the impact of various CPAs and cryopreservation techniques on sea urchin eggs, employing scanning and transmission electron microscopy to analyze cellular damage. The findings reveal that exposure to low CPA concentrations (0.5 M) did not induce significant damage to eggs. However, high concentrations (3 M) proved highly detrimental. Every cryopreservation approach investigated in this study resulted in irreversible damage to the sea urchin eggs, rendering them nonviable for future use. The research sheds light on the importance of understanding the structural alterations induced by CPAs and cryopreservation methods. This knowledge is essential for refining cryopreservation methods, potentially paving the way for successful preservation of these challenging cells.

Major challenges in cryopreservation of sea urchin eggs

Cryopreservation of gametes, embryos and larvae of marine invertebrates has been investigated in many studies throughout the years. There are many favorable studies on sperm cryopreservation but oocytes are still under research as no successful results have been sustainably obtained for this type of cells. The preservation of both maternal and paternal gametes separately would provide a reliable source of genetic material for their application to conservation, aquaculture and fundamental research. Unfortunately to date, it has not been possible to cryopreserve eggs from marine organisms. The aim of this review is to go over the factors that have been historically considered as obstacles for oocyte cryopreservation in aquatic organisms and discern those that may specifically apply to eggs of the sea urchin Paracentrotus lividus.

Vitrification has detrimental effects on maturation, viability, and subcellular quality of oocytes post IVM in cancerous women: An experimental study

Background

In vitro maturation (IVM) of immature oocytes retrieved from ovarian tissue has been considered as a valuable approach for fertility preservation in cancerous patients.

Objective

To evaluate the efficacy of vitrification on oocyte maturation, survival rates, as well as the subcellular oocyte quality post IVM.

Materials and Methods

The ovarian cortexes from 19 women with cervix and uterine malignancy aged 21-39 yr were collected. Cumulus-oocyte complexes were aspirated from all visible antral follicles. 102 immature oocytes were collected, and 43 oocytes were detected appropriately for IVM (control group). Also, 59 immature oocytes were vitrified, then matured in vitro (IVM) in two groups: with Growth/differentiation factor 9 (GDF9) (group 1) and without GDF9 (group 2) supplementation. Rates of oocytes viability, maturation, and survival along with meiotic spindle visualization and zona pellucida birefringence were assessed with Polyscope.

Results

The rate of maturation was significantly higher in controls (55.8%) compared to the other groups. Maturation rate was 23.3% in oocytes cultured in IVM medium enriched with GDF9, and 27.6% in those cultured in IVM medium lacking GDF9 (p = 0.86). Also, the meiotic spindle was present in 74.4% of control oocytes which was significantly higher than the other groups. The proportion of high zona pellucida birefringence was higher in the controls when compared with group 1 (51.2% vs. 23.3%, respectively, p = 0.04).

Conclusion

Vitrification had a detrimental effect on oocyte maturation, viability as well as the subcellular quality of the oocytes after IVM in cancerous women.

Caffeine and oocyte vitrification: Sheep as an animal model

Oocyte cryopreservation is valuable way of preserving the female germ line. Vitrification of immature ovine oocytes decreased the levels of both maturation promoting factor (MPF) and mitogen-activated protein kinase (MAPK) in metaphase II (MII) oocytes after IVM. Our aims were 1) to evaluate the effects of vitrification of ovine GV-oocytes on spindle assembly, MPF/MAP kinases activities, and preimplantation development following IVM and IVF, 2) to elucidate the impact of caffeine supplementation during IVM on the quality and development of vitrified/warmed ovine GV-oocytes. Cumulus-oocyte complexes (COCs) from mature ewes were divided into vitrified, toxicity and control groups. Oocytes from each group were matured in vitro for 18 h in caffeine free IVM medium and denuded oocytes were incubated in maturation medium supplemented with 10 mM (+) or without (−) caffeine for another 6 h. At 24 h.p.m., oocytes were evaluated for spindle configuration, MPF/MAP kinases activities or fertilized and cultured in vitro for 7 days. Caffeine supplementation did not significantly affect the percentages of oocytes with normal spindle assembly in all the groups. Caffeine supplementation during IVM did not increase the activities of both kinases in vitrified groups. Cleavage and blastocyst development were significantly lower in vitrified groups than in control. Caffeine supplementation during the last 6 h of IVM did not significantly improve the cleavage and blastocyst rates in vitrified group. In conclusion, caffeine treatment during in vitro maturation has no positive impact on the quality and development of vitrified/warmed ovine GV-oocytes after IVM/IVF and embryo culture.

Production of blastocysts following in vitro maturation and fertilization of dromedary camel oocytes vitrified at the germinal vesicle stage

Cryopreservation of oocytes would serve as an alternative to overcome the limited availability of dromedary camel oocytes and facilitate improvements in IVP techniques in this species. Our goal was to develop a protocol for the vitrification of camel oocytes at the germinal vesicle (GV) stage using different cryoprotectant combinations: 20% EG and 20% DMSO (VS1), 25% EG plus 25% DMSO (VS2) or 25% EG and 25% glycerol (VS3) and various cryo-carriers; straws or open pulled-straw (OPS) or solid surface vitrification (SSV); and Cryotop. Viable oocytes were cultured in vitro for 30 h. Matured oocytes were fertilized with epididymal spermatozoa and then cultured in vitro in modified KSOMaa medium for 7 days. Survival and nuclear maturation rates were significantly lower (P ≤ 0.05) in oocytes exposed to VS3 (44.8% and 34.0%, respectively) than those exposed to VS1 (68.2% and 48.0%, respectively) and VS2 (79.3% and 56.9%, respectively). Although recovery rates were significantly lower (P ≤ 0.05) in SSV and Cryotop vitrified oocytes (66.9% to 71.1%) than those vitrified by straws with VS1 or VS2 solutions (86.3% to 91.0%), survival rates were higher in the SSV and Cryotop groups (90.7% to 94.8%) than in the straw and OPS groups (68.2% to 86.5%). Among vitrified groups, maturation and fertilization rates were the highest in the Cryotop-VS2 group (51.8% and 39.2%, respectively). These values were comparable to those seen in the controls (59.2% and 44.6%, respectively). Cleavage (22.5% to 27.9%), morula (13.2% to 14.5%), and blastocyst (6.4% to 8.5%) rates were significantly higher (P ≤ 0.05) in SSV and Cryotop groups than in straws. No significant differences were observed in these parameters between the Cryotop and control groups. We report for the first time that dromedary oocytes vitrified at the GV-stage have the ability to be matured, fertilized and subsequently develop in vitro to produce blastocysts at frequencies comparable to those obtained using fresh oocytes.

Mesenchymal Stem Cell-Conditioned Medium Modulates Apoptotic and Stress-Related Gene Expression, Ameliorates Maturation and Allows for the Development of Immature Human Oocytes after Artificial Activation

The aim of the present study was to determine whether mesenchymal stem cell-conditioned medium (MSC-CM) modulates apoptotic and stress-related gene expression, and ameliorates maturation and developmental potential of immature human oocytes after artificial activation. A total of 247 surplus immature germinal vesicle (GV) oocytes obtained from infertile women were allocated into two in vitro maturation (IVM) groups: 1: GV oocytes (n = 116) matured in vitro (fIVM), and 2: GV oocytes (n = 131) that were vitrified, then in vitro matured (vIVM). Also, two maturation media were used: Alpha-minimum essential medium (α-MEM) and human umbilical cord-derived MSCs (hUCM). After 36 h of incubation, the IVM oocytes were examined for nuclear maturation. In IVM-matured oocytes, cytoplasmic maturation was evaluated after artificial activation through Ionomycin. Moreover, the quantitative expressions of B-cell CLL/lymphoma 2 (BCL2), BCL2-associated X protein (BAX), superoxide dismutase (SOD), and Heat shock proteins (HSP70) in matured oocytes were assessed by quantitative Real-time polymerase chain reaction (qRT-PCR) and compared with fresh and vitrified in vivo matured oocytes, which were used as fIVM and vIVM controls, respectively. The highest maturation rate was found in hUCM in fIVM, and the lowest maturation rate was found using α-MEM in vIVM (85.18% and 71.42%, respectively). The cleavage rate in fIVM was higher than that in vIVM (83.4% vs. 72.0%). In addition, the cleavage rate in α-MEM was lower than that in the hUCM (66.0% vs. 89.4%). Furthermore, the difference between parthenote embryo arrested in 4-8 cells (p < 0.04) and the quality of embryo arrested in 8-cell (p < 0.007) were significant. The developmental stages of parthenote embryos in hUCM versus α-MEM were as follows: 2-4 cell (89.45% vs. 66.00%, respectively), 4-8 cell (44.31% vs. 29.11%, respectively), morula (12.27% vs. 2.63%, respectively), and blastocysts (2.5% vs. 0%, respectively). The messenger RNA (mRNA) expression levels of BCL2, BAX and SOD were significantly different (p < 0.05) between the matured IVM oocytes. Overall, hUCM showed potential efficacy in terms of ameliorating oocyte maturation and in promoting the development and mRNA expression of BAX, BCL2, and SOD.

Effects of Re-Vitrification of Mouse Morula and Early Blastocyst Stages on Apoptotic Gene Expression and Developmental Potential

Objective

Re-vitrification of embryos immediately after thawing or after a culture period could be used to preserve the extra embryos after embryo transfer. This study aims to clarify the effect of re-vitrification on Bax and Bcl-2 gene expressions of compact and early blastocyst stage embryos.

Materials and Methods

This experimental study was performed on mouse embryos. We collected 8-cell stage embryos (n=400) from female mature mice, 60-62 hoursafter injection of human chorionic gonadotropin (hCG). The embryos were divided into 5 groups: fresh (n=80), vitrified at the 8-cell stage (n=80), vitrified at the blastocyst stage (n=80), vitrified at the 8-cell stage, and re-vitrified at the compact (n=80) and early blastocyst stages (n=80). Embryos were vitrified by cryolock. We analyzed the developmental rates of the vitrified-warmed embryos with the chi-square test. Quantitative polymerase chain reaction (qPCR) data were analyzed with SPSS version 16 using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test. P<0.05 were considered statistically significant.

Results

The expanded blastocyst formation rate showed a significant difference in re-vitrified embryos compared with fresh embryos (P<0.05). However, this result was similar between the two re-vitrified groups. Our data showed a significant difference in expression of the Bax and Bcl-2 genes between re-vitrified and fresh embryos (P<0.05). Expressions of the Bax and Bcl-2 genes showed no significant difference between the two re-vitrified groups.

Conclusion

Based on our study, re-vitrification could affect developmental rate and expressions of the Bax and Bcl2 genes.

FERTILIZATION OF BOVINE OOCYTES VITRIFIED PRE- AND POST IN VITRO MATURATION

The success rate of fertilization post save frozen oocytes is still very low, because the oocyte has distinctive features, namely the volume ratio and a lower surface to the limited penetration of water and cryoprotectants penetrate cells. Beside mature oocytes have a thread spindles are particularly vulnerable to the drop in temperature. Keep frozen oocytes is needed, especially in women who needed rescue fertility so their oosit can be fertilized. Maturation is done in TC 100 mL medium covered with mineral oil in a petri dish with a diameter of 36 mm. Oocyte vitrification begins with washing in PBS supplemented medium serum 20% for 1-2 minutes, followed by serum in the medium PBS + 20% + 10% ethylene glycol for 10-14 minutes. Then oocyte vitrification medium is transported in PBS + serum 20% + sucrose 0.5M ethylene glycol + 15% + 15% PROH for 25-30 seconds. Thawing oocytes is done by successive immersed in the media: 1). PBS + 20% serum + 0.5M sucrose, 2). PBS + 20% serum + 0.25M sucrose, and 3). PBS + 20% serum + 0.1 M sucrose. Insemination is done in rosset, and the number of fertilization was observed after 48 hours. Fertilization in the control group amounted to 42.97%, while the K1 and K2 there are no fertilization at all. The analysis showed that fertilization in the control and treatment groups significantly different at p <0.05 in both treatment groups K1 or K2 there are no fertilization at all. The conclusions of this study is there is no difference between the amount of fertilization of bovine oocytes were vitrified pre and post-maturation in vitro.

Strategies for Fertility Preservation in Female and Male Cancer Survivors

Improved survival rates and quality of life following modern cancer treatment have resulted in a growing number of patients requesting maintenance of reproductive capacity, both before and after completion of treatment. Several established options are currently available. In men, sperm banking should be offered as soon as the diagnosis of any malignant disease is established, irrespective of the expected cryosurvival rate. In such cases, conception can be achieved with frozen-thawed spermatozoa following either intrauterine insemination (IUI) or intracytoplasmic sperm injection (ICSI). In women, depending on the type of cancer, the presence of an adequate ovarian reserve, and time to delay cancer treatment, in vitro fertilization (IVF) with embryo cryopreservation constitutes a valid alternative. On the other hand, cryopreservation of mature oocytes following IVF/ICSI offers some advantages, but it is still limited by its low success rate. Emerging and exciting techniques of germ cell/gonadal tissue cryopreservation (banking) followed by autotransplantation have been clinically explored, particularly in women. Novel cryotechnologies of ovarian and testicular tissue have proven efficacious and new transplantation strategies promise improved results. However, only one live birth following autotransplanted frozen-thawed ovarian tissue has been established and there are still no pregnancies reported from autotransplanted cryopreserved testicular tissue in the human. Efficiency and safety of these techniques needs to be demonstrated. Methods for gamete in vitro maturation also need further development. Gonadal tissue cryopreservation and transplantation should be considered experimental in humans for the present time until greater evidence regarding efficacy and safety is accrued.

Morphometric and Ultrastructure studies of Primordial Follicles and Expression of Estrogen-dependent Genes in the Ovaries of Domestic Cats

The aim of this study was to compare and estimate the population of the primordial follicle morphometrically and ultrastructurally in the left and right side ovaries of 10 ovariohysterectomied healthy domestic shorthair cats. The ovaries were processed for light microscopy, electron microscopy, and estrogen-dependent gene expression for assessments. A total of 15,092 primordial follicles with and without a nucleus were examined and counted. A total of 6842 primordial follicles with a nucleus were examined and counted. The light-microscopy numerical data were collected from two histological sections per ovary for a total of 20 sections from the left ovary and 20 sections from the right ovary. The average surface area of the histological sections was 645.99 mm². The number of tertiary follicles was found to be higher in the left ovaries than in the right ovaries. The primordial follicles are under the tunica albuginea at various levels. Some are crowded or scattered in one or two rows, although at times, there were areas without any primordial follicles. The primordial follicles varied in size, and were surrounded by 4–10 squamous granulosa cells. Some primordial follicles shared their ooplasm with one or two neighboring primordial follicles, forming a giant primordial follicle with two or three nuclei. The ultrastructure of the primordial follicles showed rounded nuclei with distinct nucleoli, rounded and elongated mitochondria, and a considerably thick basement membrane under the granulosa cells. The squamous granulosa cells showed well-developed microvilli intermingled with the microvilli of the oocyte oolemma. Elongated mitochondria, coated pits, multicytoplasmic vesicles, ribosomes, and Golgi apparatuses were obvious in the oocyte ooplasm. Large vesicles contain small multivesicles and some scattered lipid globules in the ooplasm. There were estrogen-dependent gene-expression differences between the right and left ovaries. Further gene research is in the plan, using a larger pool of cats, with a focus on age differences.

Preservation of tissue microstructure and functionality during freezing by modulation of cytoskeletal structure

Cryopreservation is one of the key enabling technologies for tissue engineering and regenerative medicine, which can provide reliable long-term storage of engineered tissues (ETs) without losing their functionality. However, it is still extremely difficult to design and develop cryopreservation protocols guaranteeing the post-thaw tissue functionality. One of the major challenges in cryopreservation is associated with the difficulty of identifying effective and less toxic cryoprotective agents (CPAs) to guarantee the post-thaw tissue functionality. In this study, thus, a hypothesis was tested that the modulation of the cytoskeletal structure of cells embedded in the extracellular matrix (ECM) can mitigate the freezing-induced changes of the functionality and can reduce the amount of CPA necessary to preserve the functionality of ETs during cryopreservation. In order to test this hypothesis, we prepared dermal equivalents by seeding fibroblasts in type I collagen matrices resulting in three different cytoskeletal structures. These ETs were exposed to various freeze/thaw (F/T) conditions with and without CPAs. The freezing-induced cell-fluid-matrix interactions and subsequent functional properties of the ETs were assessed. The results showed that the cytoskeletal structure and the use of CPA were strongly correlated to the preservation of the post-thaw functional properties. As the cytoskeletal structure became stronger via stress fiber formation, the ET's functionality was preserved better. It also reduced the necessary CPA concentration to preserve the post-thaw functionality. However, if the extent of the freezing-induced cell-fluid-matrix interaction was too excessive, the cytoskeletal structure was completely destroyed and the beneficial effects became minimal.

Live birth from slow-frozen rabbit oocytes after in vivo fertilisation

In vivo fertilisation techniques such as intraoviductal oocyte transfer have been considered as alternatives to bypass the inadequacy of conventional in vitro fertilisation in rabbit. There is only one study in the literature, published in 1989, that reports live offspring from cryopreserved rabbit oocytes. The aim of the present study was to establish the in vivo fertilisation procedure to generate live offspring with frozen oocytes. First, the effect of two recipient models (i) ovariectomised or (ii) oviduct ligated immediately after transfer on the ability of fresh oocytes to fertilise were compared. Second, generation of live offspring from slow-frozen oocytes was carried out using the ligated oviduct recipient model. Throughout the experiment, recipients were artificially inseminated 9 hours prior to oocyte transfer. In the first experiment, two days after unilateral transfer of fresh oocytes, oviducts and uterine horns were flushed to assess embryo recovery rates. The embryo recovery rates were low compared to control in both ovariectomised and ligated oviduct groups. However, ligated oviduct recipient showed significantly (P<0.05) higher embryo recovery rates compared to ovariectomised and control-transferred. In the second experiment, using bilateral oviduct ligation model, all females that received slow-frozen oocytes became pregnant and delivered a total of 4 live young naturally. Thus, in vivo fertilisation is an effective technique to generate live offspring using slow-frozen oocytes in rabbits.

In-vitro maturation of human oocytes: before or after vitrification?

PURPOSE

This study aims to determine if in-vitro maturation (IVM) of human immature oocytes should be performed before or after vitrification.

METHODS

A total of 184 immature oocytes were randomly divided into two different groups: 100 were vitrified at metaphase II (MII) stage 24 h-48 h after IVM (group 1) and 84 were immediately vitrified at germinal vesicle (GV) or metaphase I (MI) stages and in vitro matured after warming (group 2).

RESULTS

Survival rate after warming was similar in both groups (86.9% versus 84.5%). However, oocyte maturation rate per collected oocyte was significantly higher for oocytes matured before vitrification (group 1, 46%) than for oocytes vitrified before IVM (group 2, 23.8%) (p < 0.01). Consequently, the number of MII oocytes inseminated per oocyte collected was significantly higher for group 1 (40%) than for group 2 (23.8%) (p < 0.05).

CONCLUSION

IVM procedure is more efficient when it is performed before oocyte vitrification.

Heterotopic autotransplantation of vitrified mouse ovary

Purpose

The aim of this study was to investigate the survival and development of premature follicles and oocytes from a vitrified-transplanted ovary in a murine experimental model.

Methods

The 14-day-old mice were unilaterally ovariectomized and the separated ovaries were vitrified by cryotop. After 2 weeks the ovaries were warmed and autotransplanted into the gluteus superfiscialis muscle. After 3 weeks, these ovaries (vit-trans), the ovaries from the opposite side (OPP), and 7-week fresh mouse ovaries as sham and control group (7 week-fresh), were recovered and examined histologically and by TUNEL test.

Results

All 4 vitrified-autotransplanted ovaries had developing follicles. Primordial, primary, preantral and antral follicles were found in all three groups (7 week-fresh, OPP and vit-trans). The rate of apoptosis by TUNEL test was similar in all groups and no significant difference was found between vitrified-transplanted ovarian tissue and controls.

Conclusions

These data demonstrate successful autotransplantation of vitrified whole mouse ovaries, manifested by the presence of all stages of folliculogenesis. According to the results of this experiment, heterotopic autotransplantation of whole cryopreserved ovary provides the opportunity for follicle development at all stages. However, further experiments are required to improve the efficiency of autotransplantation of cryopreserved ovaries to obtain better results.

John JC. The generation of live offspring from vitrified oocytes

Oocyte cryopreservation is extremely beneficial for assisted reproductive technologies, the treatment of infertility and biotechnology and offers a viable alternative to embryo freezing and ovarian grafting approaches for the generation of embryonic stem cells and live offspring. It also offers the potential to store oocytes to rescue endangered species by somatic cell nuclear transfer and for the generation of embryonic stem cells to study development in these species. We vitrified mouse oocytes using a range of concentrations of trehalose (0 to 0.3 M) and demonstrated that 0.1 and 0.3 M trehalose had similar developmental rates, which were significantly different to the 0.2 M cohort (P<0.05). As mitochondria are important for fertilisation outcome, we observed that the clustering and distribution of mitochondria of the 0.2 M cohort were more affected by vitifrication than the other groups. Nevertheless, all 3 cohorts were able to develop to blastocyst, following in vitro fertilisation, although developmental rates were better for the 0.1 and 0.3 M cohorts than the 0.2 M cohort (P<0.05). Whilst blastocysts gave rise to embryonic stem-like cells, it was apparent from immunocytochemistry and RT-PCR that these cells did not demonstrate true pluripotency and exhibited abnormal karyotypes. However, they gave rise to teratomas following injection into SCID mice and differentiated into cells of each of the germinal layers following in vitro differentiation. The transfer of 2-cell embryos from the 0.1 and 0.3 M cohorts resulted in the birth of live offspring that had normal karyotypes (9/10). When 2-cell embryos from vitrified oocytes underwent vitrification, and were thawed and transferred, live offspring were obtained that exhibited normal karyotypes, with the exception of one offspring who was larger and died at 7 months. We conclude that these studies highlight the importance of the endometrial environment for the maintenance of genetic stability and thus the propagation of specific genetic traits.

Structural, metabolic and developmental evaluation of ovulated rabbit oocytes before and after cryopreservation by vitrification and slow freezing

The cryopreservation of oocytes is valuable for the preservation of women's fertility and might also be an interesting tool to preserve animal genetic biodiversity but it is not often used because of the very poor fertility recovered after thawing, especially in rabbit species. The objective of our study was to evaluate the effect of slow-freezing and vitrification on the structural integrity of ovulated rabbit oocytes, their ATP contents, and their developmental competence. Results show that, whatever the method is used, cryopreservation has a dramatic effect on the metabolic integrity, the structural integrity, and the developmental ability of the oocytes. Vitrification and slow freezing both impair the rabbit oocytes viability after thawing but the processes act differently. Further studies are needed to improve the cryopreservation techniques in rabbit species. Moreover, we underlined that morphology and maintenance of the structural integrity of the oocytes are not suitable enough to assess the potential for further development of cryopreserved M(II) oocytes. The assessment of ATP metabolism allows efficient evaluation of the viability of the frozen or vitrified oocytes. It should be used in addition to parthenogenesis to better assess the potential for further development.

Co-transfer of embryos derived from cryopreserved and fresh natural cycle oocytes: a pilot study

Italian legislation regarding reproductive medicine prohibits embryo storage while allowing cryopreservation of supernumerary oocytes. This study evaluated the effect of fresh oocytes obtained from natural unstimulated cycles on the clinical success rates derived from the use of frozen-thawed (FR-TH) oocytes obtained following ovarian stimulation. For 36 women, intracytoplasmic sperm injection was performed on FR-TH oocytes supplemented by a fresh oocyte, if available, derived from a natural cycle in which gonadotrophin-releasing hormone-antagonist was used for premature LH surge control. The retrieval rate of fresh oocytes was 61.1% and survival rate of FR-TH oocytes was 43.6%. The fertilization rate of fresh and FR-TH oocytes was 70% and 52.5%, respectively. Fifty embryos were transferred, 14 of them developed from fresh oocytes and 36 from FR-TH oocytes. Six pregnancies occurred in 10 cycles in which the embryos developed from fresh and FR-TH oocytes (pregnancy rate 60.0%) and two in 12 patients in whom the embryos were obtained from only FR-TH oocytes (pregnancy rate 16.7%) (P < 0.05). In summary, the data demonstrate that the transfer of embryos derived from oocytes cryopreserved following a previous ovarian stimulation and an embryo developed from a fresh one retrieved in natural cycle ensures an excellent clinical outcome.

Follicle survival and growth to antral stages in short-term murine ovarian cortical transplants after Cryologic solid surface vitrification or slow-rate freezing

This study was designed to asses murine preantral follicle survival and growth, after cryopreservation of ovarian tissue by two different methodologies, solid-surface vitrification by the Cryologic vitrification method (CVM) and slow-rate freezing (SRF). Cryotreated tissue was stored in liquid nitrogen for 24h, and upon warming follicle viability was assessed by live/dead fluorescent probes, and by 7-day autotransplantation of both cryotreated tissue types to the left and right kidney capsule of the donor animals (n=16). The live/dead assay immediately upon tissue warming did not allow a distinction to be made in terms of follicle viability between the CVM and SRF cryoprocedure. In grafted tissue, follicular survival and growth was assessed by conventional histological examination and proliferating cell nuclear antigen immunohistochemistry. In each experimental group (control, CVM and SRF), follicles were classified according to developmental stage, and a comparison of the proportions of follicle stages between the three groups was executed by statistical analysis of variance. The fraction of primordial follicles in CVM and SRF grafts significantly decreased as compared to control tissue, whereas intermediary and primary follicles significantly increased. The proportion of secondary and antral follicles after SRF was significantly larger than after CVM, but did not differ significantly between CVM and control tissue. The observed massive follicle activation is a typical transplantation effect, but testifies to the survival of cryopreserved follicles. In both types of cryotreated tissue, growing follicles, including antral stage, were present in grafts from all recipient animals. The significantly more abundant further developed stages in SRF treated tissue, however, suggest that CVM treated tissue may have suffered a growth disadvantage. To our knowledge, this is the first time that the CVM technique has been utilized to vitrify preantral follicles.

Researching human oocyte cryopreservation: ethical issues

OBJECTIVE

To evaluate some ethical concerns related to the selection of participants for oocyte cryopreservation research.

DESIGN

Review of ethical issues related to human oocyte cryopreservation research.

SETTING

Academic medical center.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

A discussion of some ethical concerns surrounding selection of participants for oocyte cryopreservation research.

RESULT(S)

Human oocyte cryopreservation has the potential to expand reproductive options for infertile couples. Care needs to be taken, however, to carefully select participants in ways that maximizes benefits to them and to society and minimizes risks. Infertility programs can do so by trying to recruit women who stand to benefit the most from participating in oocyte cryopreservation protocols.

CONCLUSION(S)

Biomedical research is necessary to improve current medical therapies. Oocyte cryopreservation can increase the flexibility of assisted reproductive programs and offer hope to a significant number of infertile couples. Nonetheless, scientific research cannot be attentive only to the creation of new knowledge and new technologies; it also is inextricably tied to ethical considerations about the well-being of participants.

Immature bovine oocyte cryopreservation: Comparison of different associations with ethylene glycol, glycerol and dimethylsulfoxide

Research on different cryoprotectants and their associations is important for successful vitrification, since greater cryoprotectant concentration of vitrification solution may be toxic to oocytes. The aim of the present research was to compare the efficiency of immature bovine oocyte vitrification in different associations of ethylene glycol (EG), glycerol and dimethylsulfoxide (Me(2)SO). In the first experiment, oocytes were exposed to the cryoprotectant for either 30 or 60s in final solutions of EG+DMSO1 (20% EG+20% Me(2)SO) or EG+DMSO2 (25% EG+25% Me(2)SO) or EG+GLY (25% EG+25% glycerol). In the second experiment, the oocytes were vitrified in open pulled straws (OPS) using 30s exposure of final solutions of EG+DMSO1 or EG+DMSO2 or EG+GLY. Maturation rates of 30s exposure groups were not different from the control, but 60s cryoprotectant exposure was toxic, decreasing maturation rates. The vitrification with EG+DMSO2 resulted in enhanced maturation rate (29.2%) as compared with EG+DMSO1 (11.7%) and EG+GLY (4.3%) treatments. These data demonstrate that concentration and type of cryoprotectant have important effects on the developmental competence of vitrified oocytes.

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

BACKGROUND

We studied the ultrastructural characteristics of human mature oocytes frozen/thawed (F/T) using different concentrations of sucrose. Fresh human mature oocytes were used as controls.

METHODS

The oocytes (n = 48) were fixed in 1.5% glutaraldehyde at sampling (n = 16) or after freeze/thawing performed using a slow cooling method with propane-1,2-diol 1.5 mol/l and sucrose at either 0.1 mol/l (n = 16) or 0.3 mol/l (n = 16) in the freezing solution. The oocytes were then processed for electron microscopy observations.

RESULTS

Fresh and F/T oocytes belonging to both study groups were regularly rounded in sections, with a homogeneous cytoplasm and an intact zona pellucida (ZP). Organelles (mainly mitochondria-smooth endoplasmic reticulum aggregates and mitochondria-vesicle complexes) were abundant and uniformly dispersed in the ooplasm. The amount and density of cortical granules appeared to be abnormally reduced in some F/T samples, independently of the sucrose concentration in the freezing solution: this feature was frequently associated with an increased density of the inner ZP, possibly related to the occurrence of zona 'hardening'. Furthermore, slight to moderate microvacuolization was revealed in the ooplasm of some F/T oocytes, particularly in those treated with sucrose 0.3 mol/l.

CONCLUSIONS

Freeze/thawing procedures are associated with ultrastructural alterations in specific oocyte microdomains, presumably linked to the reduced developmental potential of mature cryopreserved oocytes. Further work is needed to determine whether or not a high concentration of sucrose plays a role, at least in part, in producing the above alterations.

Effects of cryopreservation on the developmental competence, ultrastructure and cytoskeletal structure of porcine oocytes

The purpose of this study was to determine ultrastructural and cytoskeletal changes that result from vitrification of porcine germinal vesicle- (GV-) and meiosis II- (MII-) stage oocytes. To investigate the effects of vitrification on developmental competence, oocytes were divided into three groups: fresh GV-oocytes (control), vitrified GV-oocytes, and vitrified MII-oocytes. In both GV- and MII-oocytes, vitrification resulted in a high proportion with normal morphology (92.4 vs. 94.2%, P > 0.05), while vitrified GV-oocytes yielded a higher survival rate than did vitrified MII-oocytes (56.8 vs. 41.9%, P < 0.05). In vitrified GV-oocytes, 12 of 154 oocytes underwent cleavage after fertilization in vitro, and 6 of these developed to the 8-cell stage, 3 developed to the 16-cell stage, and 3 developed into morulae. No cleavage was obtained from vitrified MII-oocytes. For ultrastructural analysis of oocytes, fresh and vitrified-warmed GV- and MII-oocytes were randomly selected for transmission electron microscopy (TEM). Results showed that vitrification caused various degrees of cryodamage in GV-oocytes. Cumulus cells of some oocytes were separated from the cumulus-oocyte complex (COC), and the zona pellucida adjacent to cumulus cells was fractured. The gap junctions between cumulus cells were ruptured, and many microvilli were disrupted or disappeared. Only homogeneous lipid droplets were observed. After vitrification, cortical granules still lined the oolemma of MII-oocytes. Only morphologically irregular, nonhomogeneous lipid droplets surrounding large vacuoles were found. To examine cytoskeletal structures, fresh and vitrified-warmed MII-oocytes were analyzed by laser-scanning confocal microscopy (LSCM); vitrified-warmed GV-oocytes were cultured for 42-44 hr before LSCM. Of 58 control oocytes, 79.5% displayed normal spindles with chromosomes aligned along the equatorial plate. In vitrified oocytes the percentage with normal spindle organization was decreased significantly in both vitrified GV-oocytes and MII-oocytes (10.1 and 12.9%, respectively, P < 0.05). The proportion of oocytes with normal distribution of F-actin was lower for vitrified GV- and MII-oocytes than for controls (16.9 and 37.2% vs. 72.3%). Results of this experiment suggest that irreversible damage to the cytoskeleton of porcine GV- and MII-oocytes after vitrification could be an important factor affecting developmental competence.

Calcium-free vitrification reduces cryoprotectant-induced zona pellucida hardening and increases fertilization rates in mouse oocytes

Despite the success of embryo cyropreservation, routine oocyte freezing has proved elusive with only around 200 children born since the first reported birth in 1986. The reason for the poor efficiency is unclear, but evidence of zona pellucida hardening following oocyte freezing indicates that current protocols affect oocyte physiology. Here we report that two cryoprotectants commonly used in vitrification procedures, dimethyl sulfoxide (DMSO) and ethylene glycol, cause a large transient increase in intracellular calcium concentration in mouse metaphase II (MII) oocytes comparable to the initial increase triggered at fertilization. Removal of extracellular calcium from the medium failed to affect the response exacted by DMSO challenge, but significantly reduced the ethylene glycol-induced calcium increase. These results suggest that the source of the DMSO-induced calcium increase is solely from the internal calcium pool, as opposed to ethylene glycol that causes an influx of calcium across the plasma membrane from the external medium. By carrying out vitrification in calcium-free media, it was found that zona hardening is significantly reduced and subsequent fertilization and development to the two-cell stage significantly increased. Furthermore, such calcium-free treatment appears not to affect the embryo adversely, as shown by development rates to the blastocyst stage and cell number/allocation. Since zona hardening is one of the early activation events normally triggered by the sperm-induced calcium increases observed at fertilization, it is possible that other processes are negatively affected by the calcium rise caused by cryoprotectants used during oocyte freezing, which might explain the current poor efficiency of this technique.

Human oocyte cryopreservation: past, present and future

Despite inferior results in the past compared with embryo freezing, oocyte cryopreservation has made great strides in recent years. In fact, it has become a necessity in assisted reproduction technology, providing alternatives to legal, moral and religious problems originating from embryo freezing. Recent advances in freezing technology, modifications of conventional protocols used and continuing optimization of vitrification have efficiently improved the method. A historical description of the method's progression over time, and a comparison of principles, procedures and results as reported in the literature are presented in this review.