Evaluation of meiotic spindles in thawed oocytes after vitrification using polarized light microscopy

Use of confocal microscopy and intracytoplasmic sperm injection (ICSI) to assess viability of equine oocytes from young and old mares after vitrification

BACKGROUND

The impact of vitrification on oocyte developmental competence as a function of donor age remains an important issue in assisted reproductive technologies (ARTs).

METHODS

Equine germinal vesicle (GV) or metaphase II (M(II) oocytes were vitrified using the Cryotop® method. Spindle organization and chromosome alignment were evaluated from confocal imaging data sets of in vivo (IVO) or in vitro (IVM) matured oocytes subjected to vitrification or not. Intracytoplasmic sperm injection (ICSI) from the same groups was used to assess developmental potential.

RESULTS

An increase in chromosome misalignment was observed in spindles from older mares when compared to those of younger mares (P < 0.05). When MII oocytes subjected to vitrification were examined following warming, there was no difference in the percentage of oocytes displaying chromosome misalignment. Next, GV oocytes, collected from the ovaries of younger and older mares, were compared between fresh IVM and IVM following vitrification and warming. For nonvitrified samples, an age difference was again noted for spindle organization and chromosome alignment, with a higher (P < 0.05) percentage of normal bipolar meiotic spindles with aligned chromosomes observed in nonvitrified oocytes from young versus older mares. Vitrification led to a reduction of spindle length (P < 0.05) for oocytes from old mares, whether vitrified at GV or MII stages, whereas this effect was not observed in oocytes from young mares except those vitrified at GV and subjected to IVM. Oocyte developmental potential after vitrification was evaluated after ICSI of vitrified and warmed MII or GV oocytes from young mares. From 25 MII oocytes, 18 oocytes were injected with sperm, and six blastocysts were produced, which, upon transfer to mares' uteri, resulted in four pregnancies. Immature (GV) oocytes collected from live mares were also vitrified, warmed, and matured in vitro before ICSI. In this group, nonvitrified, control, and vitrified oocytes did not differ (P > 0.05) with respect to the incidence of maturation to MII, cleavage after ICSI, or blastocyst development.

CONCLUSION

These findings demonstrate an effect of maternal age in an equine model at the level of meiotic spindle integrity and chromosome positioning that is influenced by both the meiotic stage at which oocytes are vitrified and whether meiotic maturation occurred in vivo or in vitro.

The effects of vitrification on oocyte quality

Vitrification, is an ultra-rapid, manual cooling process that produces glass-like (ice crystal free) solidification. Water is prevented from forming intercellular and intracellular ice crystals during cooling as a result of oocyte dehydration and the use of highly concentrated cryoprotectant. Though oocytes can be cryopreserved without ice crystal formation through vitrification, it is still not clear whether the process of vitrification causes any negative impact (temperature change/chilling effect, osmotic stress, cryoprotectant toxicity, and/or phase transitions) on oocyte quality that translate to diminished embryo developmental potential or subsequent clinical outcomes. In this review, we attempt to assess the technique's potential effects and the consequence of these effects on outcomes.

Vitrification negatively affects the Ca2+-releasing and activation potential of mouse oocytes, but vitrified oocytes are potentially useful for diagnostic purposes

RESEARCH QUESTION

To what extent does vitrification affect the Ca²⁺-releasing and activation potential of mouse oocytes, which are commonly used to determine the oocyte activation potential of human spermatozoa?

DESIGN

The effect of mouse oocyte vitrification on Ca²⁺ dynamics and developmental competence after oocyte activation was assessed and compared with fresh mouse oocytes. Moreover, the Ca²⁺ store content of the endoplasmic reticulum was determined at different time points during the vitrification-warming procedure. Finally, the Ca²⁺ pattern induced by cryoprotectant exposure was determined.

RESULTS

After human sperm injection into mouse oocytes, Ca²⁺ dynamics but not fertilization rates were significantly altered by vitrification warming (P < 0.05). Ca²⁺ dynamics in response to SrCl₂ or ionomycin were also altered by oocyte vitrification. In contrast, activation and blastocyst rates after SrCl₂ exposure were not affected (P > 0.05), whereas activation rates after ionomycin exposure were significantly lower in vitrified-warmed oocytes (P < 0.05); blastocyst rates were not affected (P > 0.05). Cryoprotectant exposure was associated with a strong drop in endoplasmic reticulum Ca²⁺ store content. Oocytes rapidly recovered during warming and recovery in Ca²⁺-containing media; a threshold area under the curve of Ca²⁺ dynamics to obtain activation rates above 90% was determined.

CONCLUSIONS

Vitrified-warmed mouse oocytes display reduced Ca²⁺-releasing potential upon oocyte activation, caused by cryoprotectant exposure. With adapted classification criteria, these oocytes could be used for diagnosing oocyte activation deficiencies in patients. Evaluating the Ca²⁺-signalling machinery in vitrified-warmed human oocytes is required.

Vitrification by Cryotop and the Maturation, Fertilization, and Developmental Rates of Mouse Oocytes

Background:

Oocyte cryopreservation is an important part of modern fertility treatment. The effect of vitrification on the fertilization and developmental rates of embryo is still a matter of debate.

Objectives:

This study aimed to investigate the effect of vitrification on the success of mouse oocyte maturation, fertilization, and preimplantation development in vitro.

Materials and Methods:

In this experimental study, a total of 200 germinal vesicle (GV) and 200 metaphase II (MII) oocytes were obtained from ovaries and fallopian tubes of NMRI mice, respectively and divided into two control and experimental (vitrified) groups. Oocytes in the experimental group were vitrified by Cryotop using vitrification medium (Origio, Denmark) and kept in liquid nitrogen for one month. Then, they were cultured in maturation medium for 24 hours. In vitro maturated metaphase 2 (IVM-MII) and ovulated metaphase 2 (OV-MII) oocytes were inseminated and the fertilized embryos assessed until the hatching blastocyst stage. Outcomes were assessed for statistical significance by Chi-square test using SPSS software.

Results:

Vitrification caused a significant reduction in the maturation rate of oocytes. Of those that matured, the fertilization rate of vitrified IVM-MII (44.1%) and OV-MII oocytes (50%) was not significantly different from each other but both were significantly lower than the control group (P < 0.05). There was no significant difference in developmental rates of both vitrified groups and the control group.

Conclusions:

The present study showed that vitrification using Cryotop and freezing medium can damage oocytes by reducing the maturation and fertilization rates in both developmental stages.

Label-free characterization of vitrification-induced morphology changes in single-cell embryos with full-field optical coherence tomography

Vitrification is an increasingly popular method of embryo cryopreservation that is used in assisted reproductive technology. Although vitrification has high post-thaw survival rates compared to other freezing techniques, its long-term effects on embryo development are still poorly understood. We demonstrate an application of full-field optical coherence tomography (FF-OCT) to visualize the effects of vitrification on live single-cell (2 pronuclear) mouse embryos without harmful labels. Using FF-OCT, we observed that vitrification causes a significant increase in the aggregation of structures within the embryo cytoplasm, consistent with reports in literature based on fluorescence techniques. We quantify the degree of aggregation with an objective metric, the cytoplasmic aggregation (CA) score, and observe a high degree of correlation between the CA scores of FF-OCT images of embryos and of fluorescence images of their mitochondria. Our results indicate that FF-OCT shows promise as a label-free assessment of the effects of vitrification on embryo mitochondria distribution. The CA score provides a quantitative metric to describe the degree to which embryos have been affected by vitrification and could aid clinicians in selecting embryos for transfer.

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

PURPOSE

To evaluate the effect of different exposure temperatures during the dilution process on the survival rate of vitrified oocytes and following development.

METHODS

Patients were divided at random into two groups for different dilution temperature (20-22 °C, RT group; 37 °C,37 °C group) according to computer-generated random numbers on the day of oocyte warming. The survival and fertilization rates of vitrified oocytes as well as the implantation and clinical pregnancy rates of the resulting embryos were recorded.

RESULTS

A total of 662 and 676 oocytes were warmed in the 37 °C group and RT group, respectively, and significant difference was observed in the survival rate between 37 °C group (88.37%) and RT group (79.88%) (P = 0.0000). There was significant difference between the survival rate of 37 °C group (87.27%) and RT group (75.64%) in nondonor patients (P = 0.0001). Multiple linear regression analysis showed that dilution temperature (β = 0.079, P = 0.017) and clinical outcomes of fresh cycles (β = 0.063, P = 0.001) were significantly and independently associated with survival rate. No significant difference was found between the 37 °C group and RT group in: fertilization rate (66.67 versus 65.37%), implantation rate (20.0 versus 19.46%), clinical pregnancy rate (37.5 versus 35.0%).

CONCLUSIONS

In conclusion, the results of this study give supportive evidence of the application of 37 °C in the dilution process, especially for oocytes of poor quality. Further studies with well-controlled experimental groups are needed to optimize protocols for human oocyte vitrification.

Impact of vitrification on the meiotic spindle and components of the microtubule-organizing center in mouse mature oocytes

Cryopreservation of oocytes is becoming a valuable method for fertility preservation in women. However, various unphysiological alterations occur in the oocyte during the course of cryopreservation, one of which is the disappearance of the meiotic spindle. Fortunately, the meiotic spindle does regenerate after thawing the frozen oocytes, which enables completion of meiosis and further development after fertilization. Nonetheless, the mechanistic understanding of the meiotic spindle regeneration after cryopreservation is still scarce. Here, to gain insight into the mechanisms of the spindle disappearance and regeneration, we examined the status of spindle microtubules as well as the key components of the microtubule-organizing center (MTOC), specifically gamma-Tubulin, NEDD1, and Pericentrin, in mature (metaphase II) mouse oocytes at different steps of vitrification, a major cryopreservation technique. We found that the configuration of the spindle microtubules dynamically changed during the process of vitrification and that spindle regeneration was preceded by excessive microtubule polymerization, followed by reduction into the normal size and shape. Also, all three MTOC components exhibited disappearance and reappearance during the vitrification process, although Pericentrin appeared to regenerate in earlier steps compared to the other components. Furthermore, we found that the localization of the MTOC components to the spindle poles persisted even after depolymerization of spindle microtubules, suggesting that the MTOC components are impacted by vitrification independently from the integrity of the microtubules. The present study would set the stage for future investigations on the molecular mechanisms of the meiotic spindle regeneration, which may contribute to further improving protocols for oocyte cryopreservation.

The formin protein mDia2 serves as a marker of spindle pole dynamics in vitrified-warmed mouse oocytes

The mouse diaphanous 2 (mDia2) protein belongs to the formin family and has been shown to nucleate actin filaments and stabilize microtubules, thus indicating a role in cytoskeleton organization. Our previous study, which showed that mDia2 specifically localizes to spindle poles of metaphase I mouse oocytes and NIH3T3 cells, provided the first evidence of its spindle pole-associated cellular function. In the present study, we aim to determine whether spindle pole proteins, such as mDia2 and pericentrin, can be used to monitor the status of spindle poles in cryopreserved mouse oocytes. We show herein that mDia2 exhibits an overlapping distribution with pericentrin, which is a crucial component of centrosomes and microtubule organizing centers (MTOCs). In vitrified-warmed oocytes, the overlapping distribution of mDia2 and pericentrin was immediately detected after thawing, thereby suggesting that mDia2 maintains a tight association with the spindle pole machinery. Interestingly, we observed that microtubules extend from mDia2 clusters in cytoplasmic MTOCs after thawing. This result suggests that mDia2 is a major MTOC component that is closely associated with pericentrin and that it plays a role in microtubule growth from MTOCs. Collectively, our results provide evidence that mDia2 is a novel marker of spindle pole dynamics before and after cryopreservation.

Effects of frozen timing on the spindle density, the angle between the polar body and spindle, and embryo development of intracytoplasmic sperm injection in mouse mature oocytes

Better pregnancy outcomes can be obtained by human mature oocyte vitrification, but many problems remain to be resolved in human mature oocyte vitrification. Since mature oocyte development possesses its own maturity cycle, there should be the optimal timing for mature oocyte vitrification. The purpose of this study was to observe the effects of frozen timing on the spindle density, the angle between the polar body and spindle, and embryo development of intracytoplasmic sperm injection (ICSI) in vitrified mouse mature oocytes and explore its possible mechanism. Mouse oocytes were randomly divided into three groups according to different frozen timing including Groups A, B, and C in which oocytes were vitrified within 2 h after ovum pick-up, and 3-4 and 5-6 h after ovum pick-up, respectively. Spindle-related parameters were measured, ICSI was performed. The spindle occurrence rate of vitrified-thawed oocytes was 98.4% in Group A, 82.3% in Group B, and 75.8% in Group C, without statistical differences between pre-vitrification and post-thawing and among the three groups (P > 0.05). The angles between the polar body and spindle were larger after thawing than before vitrification (P < 0.01). The spindle retardance values were lower after thawing than before vitrification in Groups B and C (P < 0.05), but higher in Group A (P < 0.05). The spindle retardance values before vitrification were higher in Group B than in Groups A and C (P < 0.05), but the spindle retardance value, oocyte survival and two-cell rate after thawing were higher in Group A than in Groups B and C (P < 0.05). There were no statistical differences in ICSI fertility rate between the three groups (P > 0.05). The damage on the spindle is the slightest and embryo quality is the highest in the mouse oocytes vitrified within 2 h after ovum pick-up. The spindle retardance value is more valuable than the spindle occurrence rate in the evaluation of vitrified-thawed oocyte quality, and is positively correlated with embryo quality.

Ultrastructure of human mature oocytes after vitrification

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

Cryopreservation of oocytes in experimental models

Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models has provided the opportunity for research that may not have been possible with human material. Today, results of these studies still continue to form the basis of oocyte cryobiology. This review discusses these studies, especially the physiological impacts of cryopreservation on oocyte biology. It will also focus on the role that animal models have played in improvement strategies, validation before translating new techniques into the human model and the advances made in the human in IVF because of these animal models. Finally, existing investigations and their potential impact in other areas of research will be discussed. Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models provided the opportunity for research that may not have been possible with human material. Today, animal models still continuously provide imperative data that facilitate further advancements in oocyte cryobiology. This review will focus on the physiological impacts, current improvement strategies and future applications of oocyte cryopreservation using animal models as they benefit not only human oocyte cryopreservation procedures, but also the human species through their usefulness in agriculture, medicine and conservation.

Impact of phase transition on the mouse oocyte spindle during vitrification

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

A PolScope evaluation of meiotic spindle dynamics in frozen-thawed oocytes

In mature human oocytes, the metaphase II (MII) spindle presence and birefringence signal detected through the PolScope may vary before and after freezing. In particular, spindle dynamics during the first few hours after thawing is still under study. In this study, oocytes from stimulated ovaries were cryopreserved in 1.5 mol/l 1,2-propanediol with 0.3 mol/l sucrose using a slow freezing-rapid thawing method. Oocytes were examined with the PolScope for the presence, intensity of signal birefringence and size of the meiotic spindle before freezing and at 0, 1 and 2 h post-thaw (where 0 h = the time of the end of the thawing procedure). Of the 173 surviving oocytes exhibiting a spindle before freezing, 82.7% (143/173) showed spindle birefringence within 1 h of thawing. However, at the end of the thawing procedure the intensity of spindle birefringence (retardance) and the spindle length were smaller in comparison to the pre-freezing condition. These parameters increased after 1 h, although were not restored to the value observed before freezing. No significant changes were observed by extending the culture to 2 h.

Embryo development of fresh 'versus' vitrified metaphase II oocytes after ICSI: a prospective randomized sibling-oocyte study

BACKGROUND

A successful oocyte cryopreservation programme is of utmost importance where a limited number of oocytes can be inseminated per cycle, to overcome legal and ethical issues related to embryo storage, for oocyte donation programmes and for fertility preservation (especially for cancer patients). Vitrification has been recently proposed as an effective procedure for this purpose.

METHODS

In order to validate the effectiveness of oocyte vitrification a non-inferiority trial was started on sibling metaphase II (MII) oocytes. To demonstrate the non-inferiority based on an absolute difference of 17% in the fertilization rate per sibling oocyte, a minimum of 222 oocytes were required. After oocyte denudation, MII oocytes with normal morphology were randomly allocated to fresh ICSI insemination or to vitrification procedure. If pregnancy was not obtained a subsequent ICSI cycle was performed with warmed oocytes of the same cohort. In both groups, three oocytes were inseminated per cycle by ICSI procedure. Primary end-points were fertilization rates calculated per warmed and per injected oocytes. Secondary end-points were zygote and embryo morphology.

RESULTS

A total of 244 oocytes were involved in this study. Of the 120 fresh sibling oocytes inseminated, 100 were fertilized (83.3%). Survival rate of sibling vitrified oocytes was 96.8% (120/124 oocytes). Fertilization rate after ICSI was 76.6% (95/124) per warmed oocyte and 79.2% (95/120) per survived/inseminated oocyte. No statistical difference in fertilization rates was observed between the two groups when calculated per sibling oocytes (absolute difference -6.73%; OR: 0.65; 95% CI = 0.33-1.29; P = 0.20) and per inseminated oocyte (absolute difference -4.17%; OR: 0.76; 95% CI = 0.37-1.53; P = 0.50). Embryo development was also similar in both treatment groups up till Day 2. The percentage of excellent quality embryos was 52.0% (52/100) in the fresh group and 51.6% (49/95) in the vitrification group (absolute difference -0.43%; OR: 0.98; 95% CI = 0.53-1.79; P = 0.9). The mean age of the 40 patients included in this study was 35.5 +/- 4.8 years (range 26-42). Fifteen clinical pregnancies were obtained in the vitrification cycles of 39 embryo transfers performed (37.5% per cycle, 38.5% per embryo transfer), with an implantation rate of 20.2% (19/94). Three spontaneous miscarriages occurred (20%). Twelve pregnancies are ongoing (30.0% per cycle, 30.8% per embryo transfer) beyond 12 weeks of gestation.

CONCLUSIONS

Our results indicate that oocyte vitrification procedure followed by ICSI is not inferior to fresh insemination procedure, with regard to fertilization and embryo developmental rates. Moreover, ongoing clinical pregnancy is compatible with this procedure, even with a restricted number of oocytes available for insemination. The promising clinical results obtained, in a population of infertile patients, need to be confirmed on a larger scale.

CLINICAL TRIALS REGISTRATION NUMBER

iSRCTN60158641.

A modified cryoloop vitrification protocol in the cryopreservation of mature mouse oocytes

In this study, we examined a modified cryoloop vitrification protocol in the cryopreservation of mature mouse oocytes. The mature mouse oocytes were first vitrified and then warmed up in a modified cryoloop vitrification medium [15% ethylene glycol (EG) + 15% dimethyl sulphoxide (ME2SO) + 5.8 mg/ml Ficoll 400 (F) + 0.58 mol/l sucrose (S)]. These oocytes were later studied along with fresh oocytes, which served as the control group.

Based on the post-warm-up incubation time, the oocytes in the study group were divided into three subgroups: 0 h, 1 h and 2 h. We then examined the configurations of spindles and chromosomes, the fragmentation of DNA, and the oocyte's ability to be fertilized and developed into blastocysts. By evaluating the vitrified oocytes' morphology, we confirmed that 601 out of 612 (98.2%) oocytes survived this protocol. The percentage of oocytes with normal spindle and chromosome configurations in the study groups 0 h, 1 h and 2 h were all quite similar to each other and not statistically different from that of the control group. Similar results were also observed in the percentage of oocytes containing fragmented DNA. The fertilization rate and blastocyst formation rate of the thawed oocytes were not statistically different from that of the control group either. However, if not handled properly (too much remnant medium on oocytes in the process of freezing or too long a time of oocytes in the vitrification medium before freezing), the cryopreserved oocytes could show dramatic difference from the control group in terms of the morphologically survival rate, the configuration of the spindles and chromosomes, and the DNA fragmentation. In conclusion, when followed correctly, this modified cryoloop vitrification protocol had little effect on the survival rate and development potential of mature mouse oocytes.

Nuclear transfer and oocyte cryopreservation

Somatic cells can be reprogrammed to a totipotent state through nuclear transfer or cloning, because it has been demonstrated that the oocyte has the ability to reprogramme an adult nucleus into an embryonic state that can initiate the development of a new organism. Therapeutic cloning, whereby nuclear transfer is used to derive patient-specific embryonic stem cells, embraces an entire new opportunity for regenerative medicine. However, a key obstacle for human therapeutic cloning is that the source of fresh human oocytes is extremely limited. In the present review, we propose prospective sources of human oocytes by using oocyte cryopreservation, such as an oocyte bank and immature oocytes. We also address some potential issues associated with nuclear transfer when using cryopreserved oocytes. In the future, if the efficacy and efficiency of cryopreserved oocytes are comparable to those of fresh oocytes in human therapeutic cloning, the use of cryopreserved oocytes would be invaluable and generate a great impact to regenerative medicine.

Initial experience with a donor egg bank

OBJECTIVE

To report on the establishment of a commercial donor egg bank (CryoEggs International, LP) and to present our initial experience from the first four patients to receive eggs.

DESIGN

Case report.

SETTING

Private fertility clinic.

PATIENT(S)

The four recipient women were aged 43, 43, 40, and 33 years. All had cycle day FSH levels greater than 25 mIU/mL. All were given the option of fresh donor egg IVF but opted to use frozen donor oocytes.

INTERVENTION(S)

Purchased and quarantined frozen donor eggs were thawed and inseminated using intracytoplasmic sperm injection (ICSI). Subsequent embryos were transferred on day 3.

MAIN OUTCOME MEASURE(S)

Clinical pregnancy as defined by presence of cardiac activity.

RESULT(S)

There was a thawed egg survival rate of 76%, a fertilization rate of 74%, a pregnancy rate (PR) of 50%, with an average of 2.75 embryos per transfer and an implantation rate of 27%.

CONCLUSION(S)

Although very preliminary, these results indicate that more widespread use of frozen donor eggs obtained from a commercial egg bank may be feasible in the future, changing the landscape of donor egg IVF.

Human oocyte cryopreservation: 5-year experience with a sodium-depleted slow freezing method

A slow freezing/rapid thawing method for the cryopreservation of human oocytes has been employed using a sodium-depleted culture media. In 53 frozen egg-embryo transfer (FEET) cycles, a 60.4% survival rate post-thaw was obtained and a 62.0% fertilization rate following intracytoplasmic sperm injection. Overall pregnancy rates were 26.4% per thaw attempt, 30.4% per patient, and 32.6% per embryo transfer. Pregnancy rates using sodium-depleted phosphate-buffered saline (PBS) as the base medium were 20.0% per thaw, 21.7% per patient, and 26.3% per transfer. With sodium-depleted modified human tubal fluid (mHTF) as the base for the cryopreservation medium, rates were 32.1% per thaw attempt, 39.1% per patient, 37.5% per transfer. The overall implantation rates were 4.2% per thawed oocyte and 13.6% per embryo, (PBS: 3.0% per egg, 10.6% per embryo; mHTF:5.3% per oocyte; 15.9% per embryo). These data indicate that the use of a sodium-depleted media with slow freezing and rapid thawing can yield acceptable pregnancy rates after FEET.