Effects of 1,2-propanediol and freezing-thawing on the in vitro developmental capacity of human immature oocytes

In vitro maturation (IVM) of human immature oocytes: is it still relevant?

In vitro maturation (IVM) of human immature oocytes has been shown to be a viable option for patients at risk of ovarian hyperstimulation syndrome (OHSS), those seeking urgent fertility preservation and in circumstances where controlled ovarian stimulation is not feasible. Moreover, IVM techniques can be combined with ovarian tissue cryobanking to increase the chances of conception in cancer survivors. The clinical applications of IVM in the field of reproductive medicine are rapidly expanding and the technique is now classified as non-experimental. In contrast to conventional IVF (in vitro fertilization), IVM offers several advantages, such as reduced gonadotropin stimulation, minimal risk of ovarian hyperstimulation syndrome (OHSS), reduced treatment times and lower costs. However, the technical expertise involved in performing IVM and its lower success rates compared to traditional IVF cycles, still pose significant challenges. Despite recent advances, such as innovative biphasic IVM systems, IVM is still an evolving technique and research is ongoing to refine protocols and identify techniques to improve its efficiency and effectiveness. A comprehensive understanding of the distinct mechanisms of oocyte maturation is crucial for obtaining more viable oocytes through in vitro methods, which will in turn lead to significantly improved success rates. In this review, the present state of human IVM programs and future research directions will be discussed, aiming to promote a better understanding of IVM and identify potential strategies to improve the overall efficiency and success rates of IVM programs, which will in turn lead to better clinical outcomes.

Microenvironment factors promoting the quality of vitrified cat oocytes

In oocyte cryopreservation programs, vitrification has overthrown conventional slow freezing both in veterinary and human medicine. In animals, its feasibility in field conditions makes it the preferred technique for the safeguard of genetic resources from zoo or wild animals, including threatened felids, for which the domestic cat is an excellent model. However, many cellular injuries, such as cytoskeleton, mitochondria and meiotic spindle alterations, DNA damage, zona pellucida hardening and cumulus cell loss, might occur following vitrification. After warming, although the exact mechanisms are still unclear, degeneration is a frequent outcome for cat vitrified oocytes. For immature (germinal vesicle) gametes, in vitro maturation after warming is a challenge, and cleavage after fertilization barely reaches 15-30%, while for mature (metaphase II) cryopreserved gametes it can get to 30-50%. Anyway, the progression to late embryos stages is often impaired, and improvements are needed. Standard cryopreservation protocol and the use of conventional in vitro culture systems after warming may not be enough for vitrified oocytes to recover and demonstrate their full developmental potential. Physical or chemical factors applied to oocytes undergoing vitrification, as an enrichment to the vitrification step, or to the culture microenvironment, could create more favorable conditions and promote vitrified oocyte survival and development. From the use of three-dimensional culture systems to the regulation of metabolic activities and cellular pathways, this review aims to explore all the possibilities employed so far, including the studies performed by our own lab, and the future perspectives, to present the most effective strategies for cat oocyte vitrification and the best time for their application (i.e., before, during, or after vitrification-warming).

Immature Oocyte for Fertility Preservation

In vitro maturation (IVM) of human immature oocytes has been offered to women who are at risk of developing ovarian hyperstimulation syndrome (OHSS) caused by gonadotropin stimulation, such as PCO(S) patients or who have poor ovarian reserve. Cryopreservation of oocytes matured in vivo obtained in IVF cycles has improved after implementing the vitrification method and many successful results have been reported. Now, this procedure can be successfully offered to fertility preservation programs for patients who are in danger of losing their ovarian function due to medical or social reasons, and to oocyte donation programs. This vitrification technique has also been applied to cryopreserve oocytes obtained from IVM program. Some advantages of oocytes vitrification related with IVM are: (1) eliminating costly drugs and frequent monitoring; (2) completing treatment within 2 to 10 days (3) avoiding the use of hormones in cancer patients with hormone-sensitive tumors; and (4) retrieving oocytes at any point in menstrual cycle, even in the luteal phase. In addition, immature oocytes can also be collected from extracorporeal ovarian biopsy specimens or ovaries during caesarian section. Theoretically, there are two possible approaches for preserving immature oocytes: oocyte cryopreservation at the mature stage (after IVM) and oocyte cryopreservation at the Germinal Vesicle (GV)-stage (before IVM). Both vitrification of immature oocyte before/after IVM is not currently satisfactory. Nevertheless, many IVF centers worldwide are doing IVM oocyte cryopreservation as one of the options to preserve fertility for female cancer. Therefore, more studies are urgently required to improve IVM- and vitrification method to successfully preserve oocytes collected from cancer patients. In this review, present oocyte maturation mechanisms and recent progress of human IVM cycles will be discussed first, followed by some studies of the vitrification of human IVM oocyte.

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

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Chapter 3 Current Challenges in Immature Oocyte Cryopreservation

Current freezing technology, especially the vitrification method, has markedly improved oocyte survival rate after warming, and the pregnancy rate is comparable to that achieved with fresh oocytes. However, most groups report using oocytes matured in vivo for vitrification. Although immature oocytes can be vitrified successfully, clinical outcomes do not reach that of vitrification of matured oocytes. The current literature suggests that oocytes should be vitrified at mature metaphase II (M-II) stage following IVM rather than at the immature germinal vesicle (GV) stage, because the potential for oocyte maturation is reduced when vitrification is performed on immature oocytes at the GV stage.

Toward precision medicine for preserving fertility in cancer patients: existing and emerging fertility preservation options for women

As the number of young cancer survivors increases, quality of life after cancer treatment is becoming an ever more important consideration. According to a report from the American Cancer Society, approximately 810,170 women were diagnosed with cancer in 2015 in the United States. Among female cancer survivors, 1 in 250 are of reproductive age. Anticancer therapies can result in infertility or sterility and can have long-term negative effects on bone health, cardiovascular health as a result of reproductive endocrine function. Fertility preservation has been identified by many young patients diagnosed with cancer as second only to survival in terms of importance. The development of fertility preservation technologies aims to help patients diagnosed with cancer to preserve or protect their fertility prior to exposure to chemo- or radiation therapy, thus improving their chances of having a family and enhancing their quality of life as a cancer survivor. Currently, sperm, egg, and embryo banking are standard of care for preserving fertility for reproductive-age cancer patients; ovarian tissue cryopreservation is still considered experimental. Adoption and surrogate may also need to be considered. All patients should receive information about the fertility risks associated with their cancer treatment and the fertility preservation options available in a timely manner, whether or not they decide to ultimately pursue fertility preservation. Because of the ever expanding number of options for treating cancer and preserving fertility, there is now an opportunity to take a precision medicine approach to informing patients about the fertility risks associated with their cancer treatment and the fertility preservation options that are available to them.

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

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

Exposing mouse oocytes to necrostatin 1 during in vitro maturation improves maturation, survival after vitrification, mitochondrial preservation, and developmental competence

Necrostatin 1 (Nec1) is widely used in disease models to examine the contribution of receptor-interacting protein kinase 1 in cell death. The biological actions of Nec1 are blocking necrotic cell death. The purpose of this study was to investigate whether adding Nec1 into in vitro maturation (IVM) media, followed by vitrification procedures, could enhance the survival and developmental competency of oocytes. Germinal vesicle oocytes were matured in IVM medium containing 2 different doses of Nec1 (0.5 and 1 μmol/L). After IVM, the oocytes were vitrified using a 2-step exposure to equilibrium and vitrification solutions. After warming, the rates of survival, fertilization, embryonic development up to blastocyst in vitro, morphology of spindle and chromosome, membrane integrity, mitochondria integrity, and several gene expressions were evaluated. The survival and developmental competency of oocytes were higher in the 1 μmol/L Nec1-treated group than control. The proportion with intact spindles/chromosomes and stable membranes was similar in all the groups. The mitochondrial integrity of all Nec1-treated groups showed a higher score with strong staining. The 1 μmol/L Nec1 showed significantly increased expressions of Mad2, Gdf9, and Bcl2. The Cirp level had a tendency to be downregulated in the 0.5 µmol/L Nec1 but upregulated in the 1 μmol/L Nec1, compared with the control. The Mtgenome expressions were significantly decreased in both Nec1 groups. The supplementation of 1 μmol/L Nec1 into the IVM medium could be beneficial for the survival and development of immature oocytes after vitrification.

Oocyte vitrification: advances, progress and future goals

BACKGROUND

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

OBJECTIVE

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

METHODS

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

CONCLUSIONS

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

The current challenges to efficient immature oocyte cryopreservation

Oocyte cryopreservation represents an important tool for assisted reproductive technology. It offers the opportunity to preserve fertility in women at risk of loss of the ovarian function for various pathologies. It also represents a treatment alternative for couples that cannot benefit from embryo cryopreservation because of moral, religious, or legal constrains. On the other hand, in vitro oocyte maturation has a range of applications. It can be applied in patients with a contraindication to ovarian stimulation to prevent ovarian hyperstimulation syndrome or to eliminate the risk of stimulation of hormone-sensitive tumours in cancer patients. However, while mature oocyte cryopreservation has found wide-spread application and oocyte in vitro maturation has a place for the treatment of specific clinical conditions, data on the efficiency of freezing of immature or in vitro matured oocytes are poorer. In this review we will focus on the combination of oocyte in vitro maturation with oocyte cryopreservation with particular emphasis on the biological implications of the cryopreservation of immature or in vitro matured oocytes. The two cryopreservation approaches, slow freezing and vitrification, will be discussed in relation to possible cryodamage occurring to subcellular structures of the oocyte and the functional interaction between oocyte and cumulus cells.

Optimizing human oocyte cryopreservation for fertility preservation patients: should we mature then freeze or freeze then mature?

OBJECTIVE

To evaluate the maturation and post-thaw survival rates of immature oocytes to determine whether in vitro maturation (IVM) should be attempted prior to or after cryopreservation.

DESIGN

Nonrandomized observational study.

SETTING

Private academic and clinical reproductive center.

PATIENT(S)

Patients (n = 71) who donated immature unusable oocytes after vaginal oocyte retrieval (VOR) after undergoing controlled ovarian hyperstimulation using a standard GnRH antagonist protocol.

INTERVENTION(S)

Germinal vesicle (GV), metaphase I (MI), and metaphase II (MII) oocytes (n = 175) were obtained from consenting IVF patients for fresh IVM, post-thaw IVM, or control group. In the fresh IVM group, GV- and MI- stage oocytes (n = 69) were cultured for 24 hours, matured in vitro (IVM-MII), cryopreserved, thawed, and evaluated for survival. In the post-thaw IVM group, GV- and MI- stage oocytes (n = 27) were frozen on day 0, thawed, evaluated for survival, and cultured for 24-hour IVM. MII donor oocytes (n = 79) were cryopreserved and thawed as a control.

MAIN OUTCOME MEASURE(S)

Survival postfreeze and oocyte development to the MII stage was analyzed using a χ(2) analysis.

RESULT(S)

Fresh IVM had a significantly higher maturation rate than post-thaw IVM.

CONCLUSION(S)

Oocyte cryopreservation is important for patients at risk of ovarian cancer, elective fertility preservation, and, potentially, for ovum donation. The superior maturation rate of GV and MI oocytes in the fresh versus post-thaw groups provides strong evidence for maturing oocytes to the MII stage before cryopreservation.

New trends in female fertility preservation: in vitro maturation of oocytes

As antineoplastic treatments have become more successful, an increasing number of women with cancer survive to endure the long-term consequences of chemotherapy. One of the most important of these consequences in young females is premature ovarian failure and infertility. Owing to increasing survival rates, many of these young women are seeking methods to preserve their fertility. Currently, embryo/oocyte cryopreservation obtained after controlled ovarian stimulation appears to provide the best fertility preservation option. However, patients may not have enough time to undergo ovarian stimulation prior to chemotherapy and/or have contraindications to exogenous gonadotropin administration owing to estrogen-dependant tumors. In vitro maturation of oocytes is an attractive alternative for fertility preservation in cancer patients because it does not require ovarian stimulation and it can be performed at any time of the menstrual cycle. In addition, this technique can be combined with ovarian tissue cryobanking. In this review, we discuss the position of in vitro maturation of oocytes in the fertility preservation strategy in young women.

The beneficial effects of antifreeze proteins in the vitrification of immature mouse oocytes

Antifreeze proteins (AFPs) are a class of polypeptides that permit organismal survival in sub-freezing environments. The purpose of this study was to investigate the effect of AFP supplementation on immature mouse oocyte vitrification. Germinal vesicle-stage oocytes were vitrified using a two-step exposure to equilibrium and vitrification solution in the presence or absence of 500 ng/mL of AFP III. After warming, oocyte survival, in vitro maturation, fertilization, and embryonic development up to the blastocyst stage were assessed. Spindle and chromosome morphology, membrane integrity, and the expression levels of several genes were assessed in in vitro matured oocytes. The rate of blastocyst formation was significantly higher and the number of caspase-positive blastomeres was significantly lower in the AFP-treated group compared with the untreated group. The proportion of oocytes with intact spindles/chromosomes and stable membranes was also significantly higher in the AFP group. The AFP group showed increased Mad2, Hook-1, Zar1, Zp1, and Bcl2 expression and lower Eg5, Zp2, Caspase6, and Rbm3 expression compared with the untreated group. Supplementation of the vitrification medium with AFP has a protective effect on immature mouse oocytes, promoting their resistance to chilling injury. AFPs may preserve spindle forming ability and membrane integrity at GV stage. The fertilization and subsequent developmental competence of oocytes may be associated with the modulation of Zar1, Zp1/Zp2, Bcl2, Caspase6, and Rbm3.

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.

Maturation outcomes are improved following Cryoleaf vitrification of immature human oocytes when compared to choline-based slow-freezing

PURPOSE

The cryopreservation of immature oocytes permits oocyte banking for patients at risk of losing their fertility. However, the optimum protocol for such fertility preservation remains uncertain.

METHODS

The present study investigated the survival, maturation, cytoskeletal and chromosome organization of sibling immature oocytes leftover from controlled ovarian stimulation cycles, that were either slow-frozen (with choline-substitution) or vitrified. A comparison group included oocytes that were never cryopreserved.

RESULTS

Among the three groups, comparable rates were observed for both survival (67-70%) and polar body extrusion (59-79%). Significantly more oocytes underwent spontaneous activation after IVM following slow-freezing compared with either vitrification or no cryopreservation. Likewise, the incidence of spindle abnormalities was greatest in the slow-frozen group, with no differences in spindle morphometrics or chromosome organization.

CONCLUSIONS

While the overall incidence of mature oocytes with normal bipolar spindles from warmed immature oocytes was low, the yield using Cryoleaf vitrification was slightly superior to choline-based slow-freezing.

In vitro maturation and fertilization of vitrified immature human oocytes, subsequent vitrification of produced embryos, and embryo transfer after thawing

OBJECTIVE

To report on the capability of vitrified immature human oocytes to undergo in vitro maturation, fertilization, and embryo development, and to report on the post-thaw survival of vitrified embryos.

DESIGN

Case report.

SETTING

University-based IVF unit.

PATIENT(S)

Two women (34 and 36 years old) undergoing IVF therapy.

INTERVENTION(S)

Immature oocytes, retrieved after ovarian stimulation, were vitrified. Post-thaw, they underwent in vitro maturation and fertilization by intracytoplasmic sperm injection (ICSI). The produced embryos were vitrified at day 2. In one case, the embryos were thawed and transferred.

MAIN OUTCOME MEASURE(S)

Post-thaw survival of vitrified immature oocytes, capability for in vitro maturation, fertilization, embryo development; post-thaw survival of vitrified embryos.

RESULT(S)

In the first case, six immature oocytes survived after thawing and matured after in vitro maturation. Five of them underwent ICSI, four fertilized, and three cleaved. The embryos were vitrified for future use. In the second case, two out of three immature oocytes survived after thawing. They were normally fertilized and cleaved, and the embryos were vitrified. Later, the embryos were thawed and transferred to the patient.

CONCLUSION(S)

These cases demonstrate that vitrified immature oocytes can undergo post-thaw in vitro maturation and fertilization. The produced embryos are capable to undergo vitrification and thawing.

Cryopreservation of immature and in-vitro matured human oocytes by vitrification

The objective of this study was to determine the efficacy of vitrification of human oocytes before and after in-vitro maturation (IVM). The immature oocytes recovered (n = 472) were divided into two groups: (i) immature oocytes (n = 219) vitrified at the germinal vesicle (GV) stage; and (ii) immature GV-stage oocytes (n = 253) that were firstly matured in vitro (MII-stage oocytes; n = 178), then vitrified (n = 79). The remaining oocytes (n = 99), which were not vitrified, were processed as controls. After warming, the oocyte survival, maturation and fertilization rates, as well as embryonic development, were compared. The results showed no significant difference between the survival rates of the oocytes vitrified at GV stage and those vitrified at MII stage (85.4% versus 86.1%). However, oocyte maturation rates were significantly reduced (P < 0.05) when oocytes were vitrified at immature GV stage followed by IVM (50.8%) in comparison with the control group (70.4%). Following insemination by intracytoplasmic sperm injection, there was no difference in the fertilization (62.1% versus 58.8%), cleavage (69.5% versus 67.5%) and blastocyst development (0.0% versus 0.0%) rates between these two groups. However, these results were significantly lower (P < 0.05) than those achieved in the control group. This suggests that better results can be achieved by vitrifying mature oocytes rather than immature oocytes.

A critical evaluation of ovarian tissue cryopreservation and grafting as a strategy for preserving the human female germline

Ovarian tissue freezing has been used successfully in animals and it has recently begun to be offered clinically to young women who have medical conditions with a high risk of sterility. Although no frozen human ovarian grafts have yet been returned to the donor and resulted in a pregnancy, there are many indications that this procedure should be feasible. Although live young have been derived from frozen grafts in several species, research should aim to make further improvements to the cryopreservation and grafting procedures to optimize follicle survival, and hence minimize the amount of tissue that needs to be collected, stored and returned. Ovarian tissue freezing, particular if used in combination with egg and embryo freezing, should allow a patient to safeguard their chance of becoming a parent later (Table 1). In cases where the patient has a systemic cancer or infection and malignant cells or viruses may be present in the systemic circulation and the gonadal tissue, ovarian tissue could be collected and frozen, but grafting is not currently recommended (Table 3).

At what age can human oocytes be obtained?

OBJECTIVE

To determine whether oocyte retrieval and in vitro maturation (IVM) is effective in girls undergoing fertility preservation before cancer treatment.

DESIGN

Cohort study.

SETTING

Tertiary university medical center.

PATIENT(S)

Patients <or=20 years old before gonadotoxic chemotherapy undergoing ovarian cortex cryopreservation.

INTERVENTION(S)

Before ovarian cortex cryopreservation, oocytes in all observed follicles were aspirated, matured in vitro, and cryopreserved.

MAIN OUTCOME MEASURE(S)

Maturation of oocytes.

RESULT(S)

One hundred seventy-nine oocytes were detected in 17/19 patients (89%) aged 5-20 years. We found 7, 8, and 17 oocytes in patients 5, 8, and 10 years old, respectively. The median number of oocytes per patient was 9 (0-37). Maturation rate was 45/133 oocytes (34%). In total, 81 oocytes were cryopreserved. We cryopreserved 4 of 12 detected, 4 of 9 detected, 1 of 8 detected, and 4 of 9 detected IVM oocytes for patients aged 5-10, 11-14, 15-17, and 18-20 years old, respectively.

CONCLUSION(S)

Patients undergoing ovarian cryopreservation could benefit from supplementary oocyte aspiration from the cortex. Surprisingly, oocytes were detected even in young premenarcheal girls. The number of oocytes detected, matured, and cryopreserved was not age dependent. Retrieved oocytes can be matured in vitro and cryopreserved. Because no pregnancy has yet resulted from this procedure it should be considered to be experimental. We describe the youngest patients to undergo ovum collection, IVM, and oocyte cryopreservation.

Human oocyte cryopreservation

The clinical role of oocyte cryopreservation in assisted reproduction, as an adjunct to sperm and embryo cryopreservation, has been comparatively slow to evolve as a consequence of theoretical concerns related to efficacy and safety. Basic biological studies in the 1990's alleviated many of these concerns leading to more widespread adoption of the technology. While a number of babies were born from the approach validated in the 1990's, its perceived clinical inefficiency led to the search for improved methods. Introduction of elevated dehydrating sucrose concentrations during cryopreservation increased survival and fertilization rates, but there is no well-controlled evidence of improved clinical outcome. Similarly, the use of sodium-depleted cryopreservation media has not been demonstrated to increase clinical efficiency. More recently, and in the absence of basic biological studies addressing safety issues, the application of vitrification techniques to human oocytes has resulted in reports of a number of live births. The small number of babies born from clinical oocyte cryopreservation and the paucity of well-controlled studies currently preclude valid comparisons between approaches. Legal restrictions on the ability to select embryos from cryopreserved oocytes in Italy, where many of the available reports originate, also obscure attempts to assess oocyte cryopreservation objectively.

Reconstruction of ooplasm recipient oocytes with frozen-thawed donor microcytoplasts and influence on the microtubular spindle

OBJECTIVE(S)

To cryopreserve micromanipulated ooplast segments (microcytoplasts) from mouse oocytes, compare microcytoplast and parent or recipient oocyte fusion performed within or without the zona pellucida, compare electrofusion of fresh or frozen oocyte with frozen-thawed microcytoplasts, and assess spindle integrity after reconstruction of oocytes.

DESIGN

Prospective experimental study.

SETTING

University-based experimental laboratory.

ANIMAL(S)

Mouse (MII) oocytes obtained after superovulation (n = 363).

INTERVENTION(S)

Micromanipulation of oocytes (n = 363) into microcytoplasts (n = 181), cryopreservation of microcytoplasts along with parent and sibling control oocytes (n = 182), reconstruction by electrofusion of microcytoplast and parent or recipient oocyte performed with (group A, n = 35) or without a zona pellucida (group B, n = 32), comparison of electrofusion of fresh oocyte (group C, n = 40) or frozen oocyte (group D, n = 36) with frozen-thawed microcytoplasts fused within zona, and assessment of spindle morphology of reconstructed oocyte.

MAIN OUTCOME MEASURE(S)

Post-thaw survival, success of fusion, and spindle integrity as assessed by immunostaining.

RESULT(S)

Higher success of post-thaw fusion was seen in group A (91.4%) compared with group B (56.2%). The post-thaw fusion of microcytoplasts with either fresh or frozen oocytes was not significantly different. Spindle integrity was 82.5% in group C as compared with 47.2% in group D.

CONCLUSION(S)

Microcytoplasts created from oocytes can be successfully cryopreserved, thawed, and used to reconstruct oocytes with intact spindles.

Novel gamete storage

The aim of this review is to outline recent advances in gamete storage that are beneficial for rescuing endangered species or for the breeding of companion animals. Much more information is available on the technical resolutions and practical applications of sperm cryopreservation in various species than of female gametes, reproductive tissues or organs. Mammalian sperm cryopreservation often works relatively efficiently; however, the ability of female gametes to be cryopreserved and still be viable for fertilisation is also essential for rescuing endangered species. For a proper evaluation of gamete cryopreservation possibilities in a given species, it is essential to understand the basic mechanism affecting the survival of cryopreserved cells, the technical and physical limitations, the available techniques and the new avenues to resolve the specific problems in that species. This paper is aimed to provide some help for this process. The limited length of this paper resulted in the omission of information on many important areas, including most data on teleosts, amphibian and insect cryopreservation.

Oocytes cryopreservation: state of art

In the present review article we sought to analyze, on the basis of a systematic review, the indications, rationale of oocytes cryopreservation, as well as the techniques that improved the aforementioned procedure in order to higher the pregnancy rate in women undergoing that procedure. Moreover, we pointed out the importance of oocytes cryopreservation in the research field as oocyte banking may be of utmost importance to increase the availability of oocytes for research applications such as genetic engineering or embryo cloning. Oocyte freezing has 25 year of history alternating successes and setbacks. Human oocytes have a delicate architecture but are freezable. Clinical efficiency remains low, but healthy children have been born, indicating that chromosomally normal embryos can originate from frozen oocytes. Freezing protocols are not yet optimal and it is now desirable to combine empirical and theoretical knowledge.

Membrane integrity and development of immature murine cumulus–oocyte complexes following slow cooling to −60°C: The effect of immediate rewarming, plunging into LN2 and two-controlled-rate-stage cooling

Cryopreservation of murine germinal vesicle (GV) stage cumulus-oocyte complexes (COCs) has been shown to result in poor development and cumulus cell damage. In an attempt to determine the stage of the cryopreservation protocol at which damage occurs, three cooling profiles were compared: slow-cooling (0.3 degrees C/min) to -60 degrees C (protocol A); slow-cooling to -60 degrees C and plunging to -196 degrees C (protocol B); or slow-cooling to -60 degrees C followed by further cooling at 10 degrees C/min to -150 degrees C, then plunging to -196 degrees C (protocol C). GV-stage COCs were collected from hormone-primed mice by repeated puncturing of ovarian follicles. COCs were exposed to 1.5 M Me(2)SO prior to cooling to -60 or -196 degrees C. Membrane integrity was assessed immediately after thawing using carboxy fluorescein and propidium iodide. A greater proportion of cumulus cells were damaged following protocol B than protocol A. Damage was less extensive following protocol C than following protocol B. For assessment of development, COCs were matured and fertilised in vitro. Morphological normality was significantly reduced following cooling to -60 or -196 degrees C compared with non-cryopreserved controls. Fertilisation of oocytes assessed as normal post-treatment was not significantly different between any of the groups. Development to blastocyst was least from oocytes exposed to protocol B, being significantly worse than for oocytes exposed to protocol A, but not significantly different to protocol C. A protocol comprising two stages of controlled-rate cooling decreased damage to the membranes of cumulus cells but did not significantly improve embryo development.

Effects of Partial Removal of Cytoplasmic Lipid on Survival of Vitrified Germinal Vesicle Stage Pig Oocytes

This study was designed to investigate whether the partial removal of cytoplasmic lipid from immature pig oocytes prior to vitrification had any positive effects on subsequent maturation, fertilization and early development. Oocytes at the germinal vesicle stage were partially freed from cumulus cells and centrifuged, and then polarized cytoplasmic lipid was removed by micromanipulation. When cultured for 44-48 h, significantly fewer of the centrifuged oocytes reached metaphase II (M-II) than did the non-centrifuged oocytes (approximately 53% vs approximately 68%, respectively); however, no further reduction in the M-II rate was observed when centrifuged oocytes were then delipated prior to culture (approximately 47%). To evaluate their sensitivity to the equilibration and vitrification solutions containing ethylene glycol, non-centrifuged, centrifuged, and delipated oocytes were cultured continuously for several minutes in those solutions, then washed and cultured further; no significant differences in the M-II rates (approximately 20-27%) were observed among the three treatment groups. When oocytes were vitrified and then warmed, significantly more delipated oocytes reached M-II in culture (approximately 15%) than did the non-delipated oocytes, whether centrifuged or not (approximately 4% in each group). When delipated, vitrified and matured oocytes were microsurgically injected with frozen-thawed spermatozoa, approximately 39% were activated and male pronucleus formation was observed in approximately 40% of activated oocytes; none developed beyond the 4-cell stage. These results show that maturation in vitro of vitrified pig oocytes can be promoted by partial removal of cytoplasmic lipid prior to vitrification and that the vitrified oocytes can be fertilized, although the embryonic development obtained in this study was limited.

In-vitro maturation of germinal-vesicle oocytes and cryopreservation in metaphase I/II: a possible additional option to preserve fertility during ovarian tissue cryopreservation

This study describes the possibility of combining two options in order to preserve female fertility: cryopreservation of human ovarian tissue and in-vitro matured germinal vesicle (GV) oocytes retrieved during tissue dissection. In contrast to ovarian tissue cryopreservation, the cryostorage of in-vitro matured unfertilized metaphaseI/II oocytes could be a more realistic option. This concept of preserving fertility before chemotherapy and/or radiotherapy without a long time delay could be an additional reason for favouring ovarian tissue cryopreservation. This concept is discussed in regard to two cases.

Effects of cryopreservation on the meiotic spindle of human oocytes

The microtubular meiotic spindle of most mammals, including humans, is very sensitive to cooling [Hum. Reprod. 16 (2001) 2374; Fertil. Steril. 54 (1990) 102; Fertil. Steril. 75 (2001) 769; Zygote 3 (1995) 357] and is rapidly depolymerised even after a slight reduction in temperature to 33 degrees C. Spindle disassembly is dependent on the extent of temperature decrease and its duration. After rewarming, the recovery is far from complete. Cryoprotectants themselves may alter the spindle structure, depending on the duration and temperature of exposure, the duration of recovery at 37 degrees C and the species [Hum. Reprod. Update 2 (1996) 193]. Damage to the meiotic spindle is considered to be the cause of aneuploid embryos, by inducing chromatid non-disjunction and chromosome scattering and by disturbing the sequence of events leading to the completion of meiosis and fertilisation. Nevertheless, a consensus arose from all the studies: appropriate exposure to cryoprotectants and appropriate rates of cooling and thawing allow the cryopreservation of mature oocytes without any significant changes in their second meiotic spindle organisation and without any increase in the rate of aneuploid embryos [Mol. Hum. Reprod. 2 (1996) 445; Hum. Reprod. 8 (1993) 1101; Hum. Reprod. 9 (1994) 684; Microsc. Res. Technol. 27 (1994) 165; Fertil. Steril. 75 (2001) 354]. These fundamental studies in humans, showing good preservation of cell structures after freeze-thaw procedures opened the way to new successful clinical trials with embryos derived from cryopreserved mature oocytes [Fertil. Steril. 68 (1997) 724]. Considering immature oocyte freezing at prophase I (germinal vesicle (GV) stage), a stage which was thought to be less sensitive to cryoinjury, pooled data from the literature showed no advantage in terms of survival rates, fertilisation rates of in vitro matured oocytes and developmental ability of the resulting embryos, especially in unstimulated cycles. Moreover, conflicting results are reported on the effects of freezing on the spindle-chromosome configuration of immature oocytes or in vitro matured oocytes, highlighting the need for large scale studies [Hum. Reprod. 10 (1995) 1816; Hum. Reprod. 13 (Suppl. 3) (1998) 161; Hum. Reprod. 17 (2002) 1885; Microsc. Res. Technol. 27 (1994) 165; Fertil. Steril. 68 (1997) 920]. One child has been born after the use of cryopreserved immature oocytes at GV stage, matured in vitro and fertilised by ICSI [Hum. Reprod. 13 (1998) 3156], demonstrating at least the feasibility of this technique. Improvements are required so as to make mature and immature oocyte cryopreservation an established and safe technique for ART.

Cryopreservation of caprine ovarian tissue using dimethylsulphoxide and propanediol

The caprine ovary is a rich source of potentially viable immature oocytes enclosed in preantral follicles (PF). Previous experiments showed that these oocytes can be successfully cryopreserved in ovarian tissue of several species. However, until now, no information about the caprine PF cryopreservation is available in the literature. The aim of the present research was to evaluate the structural and ultrastructural characteristics of caprine PF after treatment and cryopreservation of ovarian tissue with 1.5 and 3 M dimethylsulphoxide (DMSO) and propanediol (PROH). One fragment of ovarian tissue was immediately fixed for histological examination and ultrastructural analysis, after slaughter (control). Four fragments were equilibrated at 20 degrees C/20 min in 1.8 ml of minimum essential medium (MEM) containing 1.5 or 3 M DMSO or PROH for the toxicity test, and the other four fragments were slowly frozen in each cryoprotectant at the concentrations previously described. After toxicity test and freezing/thawing procedures, the ovarian fragments were fixed for histological examination. The results showed that after toxicity test and cryopreservation of ovarian tissue using both cryoprotectants, the percentage of normal PF was less (P < 0.05) as compared with the control group. The present study revealed that the percentage of normal PF after toxicity test and cryopreservation in 1.5 M DSMO was significantly greater (P < 0.05) as compared with results obtained with 3 M DMSO or 1.5 and 3 M PROH. This result was confirmed by transmission electron microscopy, which showed that the PF were preserved in a higher quality state with 1.5 M DMSO. In conclusion, the present study demonstrated that caprine PF can be cryopreserved in ovarian tissue using 1.5 M DMSO.

Cryopreservation of caprine ovarian tissue using glycerol and ethylene glycol

Cryopreservation of ovarian tissue may be a potential alternative for the conservation of genetically superior animals, including high milk- and meat-producing goat breeds. However, until now, no information was available concerning the cryopreservation of preantral follicles (PF) enclosed in caprine ovarian tissue. The objective of the present study was to evaluate the structural and ultrastructural characteristics of caprine PF after exposure to and cryopreservation of ovarian tissue in 1.5 and 3M glycerol (GLY) and ethylene glycol (EG). At the slaughterhouse, each ovarian pair from five adult mixed breed goats was divided into nine fragments and randomly distributed into treatment groups. One fragment was immediately fixed for histological examination and ultrastructural analysis, after slaughter (control). Four of the ovarian fragments were equilibrated at 20 degrees C for 20 min in 1.8 ml of MEM containing 1.5 or 3M GLY or EG for a toxicity test and the final four fragments were slowly frozen using these cryoprotectants at the concentrations above. After toxicity testing and freezing/thawing, the ovarian fragments were fixed for histological examination. Histological analysis showed that after toxicity testing and cryopreservation of the ovarian tissue in GLY or EG at both concentrations, the percentage of normal PF was significantly lower than controls. Ultrastructural analysis of PF frozen in 1.5 and 3M GLY, as well as 3M EG demonstrated that these follicles remained morphologically normal. In conclusion, we demonstrated cryopreservation of caprine PF in ovarian tissue.

Ethical aspects of cryobiology: responsible applications in biomedicine and in clinical practice

This paper focuses on ethical issues in applications of cryobiology to humans, more particularly in the field of human reproduction and cryosurgery. The paper also provides essential ingredients for the interface of bioethics and cryobiology. For instance, since the 1970s bioethicists have developed four principles to guide the moral evaluation of the 'new medicine.' These are: respect for persons, non-maleficence, beneficence, and justice. In the field of human reproduction major progress was made by the cryopreservation of reproductive material. Still, ethical issues arise whenever partnerships cease to exist (death) or deteriorate (divorce), and decisions have to be made about the disposition of frozen gametes and embryos. Policy-making becomes, then, a prime concern. Examples of regulation in the United States of America, in the United Kingdom, and across Europe are being offered. Cryosurgery remains a field where cryobiologists struggle in their quest for an optimal technique, thus illustrating the need for assessment of safety, efficacy, and benefit to patients. Increasingly, cryobiologists have been joining in the ethical reflection on the use of cryo-technologies. They may further do so by perfecting their ability to identify ethical aspects, by analysing the norms and values at stake, by learning the skill of making the appropriate choices, and by showing their willingness to justify the choices made be it in the inner circle of pairs or publicly.

Vitrification can be more favorable than slow cooling

OBJECTIVE

Cryopreservation of embryos and oocytes has become an essential service for infertility treatment. The clinical application of this technology should ensure optimal survival of the embryos and oocytes that are stored and subsequently thawed for transfer. The aim of this review is to compare the widely employed slow cooling procedures with vitrification to evaluate and recommend the more effective and safer procedure.

DESIGN

The review is mainly based on a comparison of the principles, procedures, and results reported in the literature. A historical description of vitrification and personal experiences with this technology are also included.

SETTING

University-based hospitals and private clinics that treat infertility and have published information on cryopreservation.

PATIENT(S)

Women being treated for infertility and reproductive technology clinics.

INTERVENTION(S)

The application of slow cooling involving a range of cooling rates is compared with vitrification using rapid and ultrarapid cooling in simple containers. The purpose of both techniques is the induction of a glasslike state in cells to protect them from damage by ice crystals. The early development of vitrification involved the use of long pre-equilibration procedures. Improved methods resulted from the use of mixtures of penetrating and nonpenetrating solutes that are not toxic and a range of cooling rates.

MAIN OUTCOME MEASURE(S)

Reported number of pregnancies established after transfer of embryos that were cryopreserved by vitrification, or transfer of embryos derived from vitrified oocytes.

RESULT(S)

Both slow cooling and vitrification procedures have resulted in the successful cryopreservation of human embryos and oocytes. Both procedures have resulted in healthy births, although the slow cooling of oocytes gives very low success rates. Vitrification is a promising novel technique in assisted reproductive technology, but comparative success rates are yet to be established.

CONCLUSION(S)

Vitrification is a simple procedure that requires less time and is likely to become safer and more cost effective than slow cooling.

A confocal microscopy analysis of the spindle and chromosome configurations of human oocytes cryopreserved at the germinal vesicle and metaphase II stage

BACKGROUND

Routine oocyte cryopreservation remains an elusive technique in the wide range of assisted reproductive technologies available. This study examines the effect of a cryopreservation protocol on the spindle and chromosome configurations of human oocytes cryopreserved at the germinal vesicle (GV) and metaphase II (MII) stage.

METHODS

GV oocytes were randomly assigned to one of three groups: (i) control oocytes matured in vitro to MII stage (n = 156); (ii) oocytes cryopreserved at the GV stage and then matured in vitro (n = 90); (iii) oocytes cryopreserved at the MII stage (n = 147). Following cryopreservation and in-vitro maturation, immunostaining of tubulin and chromatin was performed, before visualization using confocal microscopy.

RESULTS

A statistically significant increase was observed in the survival rate in group 2 (73.3%, 66/90) compared to group 3 (55.7%, 82/147) (P < 0.007). Exposure of oocytes to the cryoprotective solutions without freezing had no effect on the structure of their second meiotic spindle. However, statistically significant differences were observed on both spindle and chromosome configurations of oocytes from group 2 (5.2 and 5.2% respectively) and group 3 (16.2 and 18.8% respectively) compared with group 1 oocytes (71.6 and 82.0% respectively) (P < 0.001 in all cases).

CONCLUSIONS

The protocol followed results in high rates of survival and potential for in-vitro maturation, but has a deleterious effect on the organization of the meiotic spindle of human oocytes cryopreserved at both the GV and MII stages.

Pregnancies resulting from in vitro matured oocytes collected from women with regular menstrual cycle

PURPOSE

To demonstrate that human immature oocytes retrieved from women with regular menstrual cycles can undergo maturation and fertilization, and that the resulting embryos can establish pregnancies.

METHODS

Immature oocytes (n = 568) were retrieved from women with regular menstrual cycle. The intact immature oocytes (n = 506) were allowed to mature in YS medium supplemented with 70% human follicular fluid (hFF); the matured oocytes were fertilized with husband sperm. Two pronuclei oocytes were cocultured with cumulus cells in YS medium supplemented with 10% hFF until 2 or 3 days after insemination. The cleaved embryos were transferred in uteri.

RESULTS

Follicles were aspirated on Day 9.2 +/- 5.3 of 63 natural cycles from 51 patients (mean age = 34.8 +/- 4.0 years). The average number of retrieved immature oocytes was 9.0. The maturation rate was 74.3% (376/506). The two PN and cleavage rates were 72.6% (273/376) and 89.0% (243/273), respectively. Embryo transfer was achieved in 51 cycles and clinical pregnancy rate was 17.6% (9/51).

CONCLUSIONS

The results suggest that in vitro matured oocytes can undergo fertilization and the resulting embryos may successfully lead to pregnancies. However, further research is needed to improve IVM technique to achieve success rate comparable to gonadotrophin stimulated cycles.

A novel system for in vitro maturation of human oocytes

OBJECTIVE

To compare in vitro maturation of cumulus-free oocytes in glucose-free medium (P1) and standard medium (TC199).

DESIGN

Prospective, cohort study.

SETTING

Assisted reproductive technology program.

PATIENT(S)

One hundred eight patients undergoing ICSI.

INTERVENTION(S)

Germinal vesicle-stage or metaphase I--stage oocytes were allocated to culture with P1 or TC199. Metaphase II oocytes were fixed for immunofluorescence analysis or fluorescence in situ hybridization at 24 or 48 hours (or both). Media were compared by performing conditional logistic regression analysis that controlled for egg-specific factors.

MAIN OUTCOME MEASURE(S)

Proportion of mature oocytes and appearance of normal spindle-chromosome cytoarchitecture.

RESULT(S)

At 24 hours, more P1 oocytes than TC199 oocytes reached metaphase II (59.7% vs. 44.9%). At 48 hours, 71.7% of P1 oocytes and 61.0% of TC199 oocytes reached metaphase II, but this difference was not significant. Metaphase II oocytes in P1 were 34.3% more likely than those in TC199 to have a bipolar spindle with aligned chromosomes. Compared with oocytes at the germinal vesicle stage at 0 hour, those at metaphase I at 0 hour were more likely to progress to metaphase II (72.6% vs. 46.1% at 24 hours; 84.1% vs. 60.6% at 48 hours).

CONCLUSION(S)

P1 is superior to TC199 for in vitro maturation of granulosa-free human oocytes.

Human oocyte cryopreservation

Despite early success with sperm and embryo cryopreservation, human oocyte cryopreservation has met with limited success both in terms of survival and subsequent fertilization. However, compared with embryo cryopreservation, oocyte freezing has great potential for eliminating the ethical and legal problems of embryo storage.

Freezing immature oocytes

The establishment of a long-term preservation system for mammalian oocytes is important for the development of both biological and medical sciences. A number of efforts have been made to develop this system. In human reproductive medicine, the development of an oocyte cryopreservation system can improve the efficacy of the current assisted reproductive technology (ART) for infertile patients with severe reproductive disorders. In this article, the technical development of cryopreservation programs for human oocytes and its biological background were reviewed. Clinical outcome after the use of this technology was further introduced.

Cryopreservation of human germinal vesicle stage and in vitro matured M II oocytes: influence of cryopreservation media on the survival, fertilization, and early cleavage divisions

OBJECTIVE

To study the influence of low-sodium cryopreservation media (CPM) on the survival and development of frozen-thawed germinal vesicle (GV) stage and in vitro matured human oocytes.

DESIGN

Prospective experimental study.

SETTING

Academic hospital-based fertility center.

PATIENT(S)

Experimental groups: Oocytes cryopreserved at the GV (group A, n = 63 and group B, n = 64) or M II stage (group C, n = 62) with use of conventional (group A) or low-sodium CPM (groups B and C). Control groups: Sibling GV stage oocytes subjected to in vitro maturation (IVM; control group A, n = 64; control group B, n = 64).

INTERVENTION(S)

IVM, intracytoplasmic sperm injection and subsequent culture.

MAIN OUTCOME MEASURE(S)

Rates of survival, maturation, fertilization, and cleavage.

RESULT(S)

The postthaw survival was significantly lower in groups A (57.1%) and B (48.4%) compared to C (84.4%). In group A, maturation and cleavage rates were significantly lower, and fertilization rate was similar to controls (GVBD: 72.2% vs. 90.6%; progression to M II: 33.3% vs. 76.6%; cleavage: 42.9% vs. 88.2%; and fertilization: 58.3% vs. 69.4% in group A vs. control group A, respectively). There was no such difference in group B. In group C, despite a slight but significant lowering of the rate of 2 PN and an increase in that of 3 PN (2 PN: 47.4% vs. 70.2% and 3 PN: 15.8% vs. 3.2% in group C vs. total controls, respectively), embryonic cleavage per GV oocyte was significantly higher (25.8%) compared to group A (4.8%) but not to group B (15.6%). The rate of maturation and cleavage per surviving GV oocyte was significantly higher in group B than group A.

CONCLUSION(S)

Low-sodium-based CPM is beneficial for in vitro matured M II stage oocytes and is significantly better than the conventional sodium-based media for the GV stage oocytes.

Fundamental cryobiology of mammalian oocytes and ovarian tissue

Embryo cryopreservation is a widely used and relatively well-established procedure. By contrast, ovarian tissue and unfertilized oocytes are only rarely cryopreserved, even though for germ line storage these often would be preferable to embryo cryopreservation. There are many reasons for this discrepancy. Unfertilized mature (MII) stage oocytes are more difficult to cryopreserve than cleavage stage embryos of the same species. Many factors contribute to this including the oocyte's surface to volume ratio, single membrane, temperature-sensitive metaphase spindle and zona, and its susceptibility to parthenogenetic activation and chill-injury. A completely different set of problems applies to primordial follicles. Oocytes in primordial follicles are very small and tolerate cryopreservation by slow cooling very well. The problem lies in the difficulty in producing mature oocytes from these primordial follicles. Better and/or more convenient cryopreservation procedures for both oocytes and ovarian tissue are being developed. This paper describes some of the advances in this area and outlines the relative merits and limitations of several currently available egg and ovarian tissue cryopreservation procedures.

Improved human oocyte development after vitrification: a comparison of thawing methods

OBJECTIVE

To evaluate the developmental competence of vitrified human oocytes thawed using two different methods to establish an effective cryopreservation protocol.

DESIGN

In vitro model study.

SETTING

University-affiliated hospital.

PATIENT(S)

Patients who underwent a long protocol of ovarian stimulation with GnRH and gonadotropins.

INTERVENTION(S)

Vitrified oocytes from the patients were thawed using either a four-step method with 2.5-minute intervals or a four-step method with 5-minute intervals.

MAIN OUTCOME MEASURE(S)

Morphologic normality, maturation, fertilization, and development of the oocytes to the blastocyst stage.

RESULT(S)

The two thawing methods did not significantly affect the morphologic normality (84%-100%), maturation (75%-100%), fertilization (38%-71%), polyspermy (more than three pronuclei; 0%-20%), or parthenogenetic activation (only female pronucleus; 0%-8%) of the vitrified oocytes. However, more of the vitrified oocytes developed to the two-cell (71%-100% versus 50%-67%), four-cell (71%-93% versus 0%-50%), eight-cell (46%-71% versus 0%), and blastocyst (23%-36% versus 0%) stages after thawing using the four-step method with 2.5-minute intervals than using the four-step method with 5-minute intervals.

CONCLUSION(S)

Vitrified human oocytes developed to the blastocyst stage with IVF. A four-step thawing method with 2.5-minute intervals was more effective in supporting preimplantation embryo development than a four-step thawing method with 5-minute intervals.

Vitrification of mouse germinal vesicle oocytes: effect of treatment temperature and egg yolk on chromatin and spindle normality and cumulus integrity

The success rates for cryopreservation of immature oocytes from several species including human remain low, in contrast to major improvements with mature oocytes. In this study, a new approach has been developed using a short exposure ultra-rapid freezing protocol, examining the effect of temperature and egg yolk (two factors which may be expected to influence membrane flexibility) on the cryostability of immature mouse oocytes and cumulus complexes. These two factors were tested in various patterns for their cryoprotective effect using ethylene glycol as the principal cryoprotectant. The results showed that 37 degrees C pre- and post-freeze exposure significantly improved both survival and normal spindle configuration after in-vitro maturation. Egg yolk was found to produce further beneficial effects on both the oocyte and cumulus cell integrity, with the best effects being obtained at 37 degrees C with inclusion of egg yolk both before and after the freezing. This protocol produced > 80% normal survival post-thaw with intact and attached cumulus complex, 84% maturation rate and 45% normal metaphase configuration. In summary, a unique combination of high survival and meiotic normality together with good preservation of the attached cumulus cell mass has been achieved using a simple new vitrification procedure.

Preservation of fertility in women undergoing chemotherapy: current approach and future prospects

PURPOSE

Anticancer treatment causes ovarian failure.

METHODS

Some hormones may have a protective effect on the ovary. Cryopreservation (freezing) of oocytes has had very limited success, and therefore, currently its use before chemotherapy is not a feasible option. However, cryopreservation of embryos is possible. Another solution is oocyte donation followed by in vitro fertilization (IVF).

RESULTS

Ovarian cortical slices containing primordial follicles have been cryopreserved successfully. To restore fertility, cryopreserved-thawed tissue taken from cancer patients before therapy could be replanted after recovery. The possible risk of malignancy restoration could be eliminated by obtaining unilaminar follicles from cryopreserved-thawed tissue and growing them in vitro, followed by routine IVF.

CONCLUSIONS

Although women who undergo chemotherapy face limited options for fertility preservation, intensive studies in cryopreservation and in vitro maturation of follicles harbor hope for brighter prospects in the future.

Chromosome and spindle configurations of human oocytes matured in vitro after cryopreservation at the germinal vesicle stage

OBJECTIVE

To investigate effects of cryoprotectant and cryopreservation on the chromosome and microtubule configuration of human immature oocytes.

DESIGN

Intact cumulus-enclosed immature oocytes were collected from unstimulated ovaries and divided into three groups: group 1, no treatment (control); group 2, only 1,2-propanediol treatment, and group 3, cryopreserved oocytes. Oocytes in groups 1 and 2, and oocytes that survived after cryopreservation in group 3 were cultured for 48 hours.

SETTING

Infertility Medical Center at the CHA General Hospital, Seoul, Korea.

PATIENT(S)

Oocytes were obtained from patients undergoing gynecologic surgery.

MAIN OUTCOME MEASURE(S)

Maturation rate and abnormality in chromosomes by fluorescence in situ hybridization and in the spindle by immunostaining for tubulin.

RESULT(S)

There was no effect of propanediol-only treatment on the chromosomal (41.4%) and spindle abnormalities (35.3%) in group 2 compared with control oocytes (31.8% and 22.2%, respectively), whereas a statistically significant increase in abnormalities in chromosomes (77.8%) and spindles (70%) was found in group 3.

CONCLUSION(S)

Human oocytes matured in vitro after cryopreservation at the germinal vesicle stage showed increased incidence of chromosomal and spindle abnormalities. These abnormalities may impair the capacity for further development of the embryos derived from frozen-thawed oocytes.