[Initial results of the cryopreservation of human oocytes]

Embryology outcomes after oocyte vitrification with super-cooled slush nitrogen are similar to outcomes with conventional liquid nitrogen: a randomized controlled trial

OBJECTIVE

To determine whether the use of slush nitrogen (SN), a super-cooled form of nitrogen with a temperature from -207 to -210 °C, can improve oocyte survival after vitrification and warming compared with conventional liquid nitrogen (LN).

DESIGN

Randomized controlled trial.

SETTING

Academic-affiliated private practice.

PATIENT(S)

A total of 556 metaphase II oocytes from 32 oocyte donor cycles were included.

INTERVENTION(S)

Donor oocytes were block randomized to undergo vitrification with either SN or LN. Vitrification was followed by warming, fertilization with donor sperm, embryo culture to the blastocyst stage, and preimplantation genetic testing for aneuploidy via trophectoderm biopsy with targeted next-generation sequencing.

MAIN OUTCOME MEASURE(S)

The primary outcome was oocyte survival after vitrification and warming. Secondary outcomes included rates of fertilization, usable blastocyst formation, and whole chromosome aneuploidy.

RESULT(S)

Half of the metaphase II oocytes (n = 278) were randomized to undergo vitrification with SN, whereas the other half (n = 278) were randomized to undergo vitrification with LN. There were no statistically significant differences noted in oocyte survival rate (85.3% vs. 86.3%), fertilization rate (84.0% vs. 80.0%), rate of usable blastocyst formation (54.3% vs. 55.7%), or rate of whole chromosome aneuploidy (9.4% vs. 11.7%) between the SN and LN arms, respectively.

CONCLUSION(S)

The implementation of an SN oocyte vitrification protocol resulted in similar embryology outcomes compared with LN. The use of SN did not lead to any demonstrable improvement in oocyte survival after vitrification and warming.

CLINICAL TRIAL REGISTRATION NUMBER

NCT04342364.

Chapter 1 Historical Background on Gamete and Embryo Cryopreservation

This chapter describes the development of the science of cryopreservation of gametes and embryos of various species including human. It attempts to record in brief the main contributions of workers in their attempts to cryopreserve gametes and embryos. The initial difficulties faced and subsequent developments and triumphs leading to present-day state of the art are given in a concise manner. The main players and their contributions are mentioned and the authors' aim is to do justice to them. This work also attempts to ensure that credit is correctly attributed for significant advances in gamete and embryo cryopreservation. In general this chapter has tried to describe the historical development of the science of cryopreservation of gametes and embryos as accurately as possible without bias or partiality.

Improvement of the vitrification method suppressing the disturbance of meiotic spindle and chromosome systems in mature oocytes

Vitrification method is widely used in oocyte cryopreservation for IVF but the birth rates are lower than that of the fresh oocyte. One of the known main reasons is structural instability of meiotic spindle and chromosome systems of mature oocyte. To get the best way for keeping competence of matured oocytes, we studied the best conditions for vitrification focused on equilibration times. The mature oocytes were underwent vitrification with current popular method and analyzed the survival rates, microtubule stability and DNA integrity. The survival rates of recovered oocyte are almost same between groups and are more than 93%. The structural configuration of meiotic spindle was well kept in 10 min equilibration group and the stability rate was almost same with that of control. The chromosomal breakdown was observed in all experimental groups, but the chromosomal stability was higher in 10 min equilibration group than the other groups. The 10 min equilibration group showed best condition compared with the other groups. Based on these results, the equilibration time is one of the key factors in successful keeping for competence of mature oocyte. Although, more fine analysis about the effects of physical stress on oocyte during vitrification is needed to define the optimal condition, it is suggested that the optimal equilibration time to get competent oocyte in mouse is 10 min. Information acquired this study may provide insight into intracellular structural events occurring in human oocytes after vitrification and application for cryopreservation of human oocyte.

Efficiency of oocyte cryopreservation: a meta-analysis

OBJECTIVE

To determine the efficiency of oocyte cryopreservation relative to IVF with unfrozen oocytes.

DESIGN

Meta-analysis.

SETTING

Academic assisted reproduction center.

PATIENT(S)

Results of all reports from January 1997 to June 2005 with the patients undergoing IVF-intracytoplasmic sperm injection (ICSI) with cryopreserved cycles between 1996 and 2004 were compared with those of patients who underwent IVF-ICSI with unfrozen oocytes in 2002 and 2003 in our program.

INTERVENTION(S)

Mean age and number of ET cycles originating from unfrozen oocytes was matched with those for thaw cycles originating from oocytes cryopreserved with a slow-freezing (SF) protocol. Vitrification (VF) reports were not included in the comparative analysis because of a small number of pregnancies (10) before June 2005.

MAIN OUTCOME MEASURE(S)

The comparison of fertilization rate, clinical pregnancy, and live-birth rates per injected oocyte, clinical pregnancy and live-birth rates per transfer, and implantation rate between IVF-ICSI cycles with frozen and unfrozen oocytes.

RESULT(S)

Live-birth rates per oocyte thawed were 1.9% and 2.0% for SF and VF, respectively, before June 2005. Live-birth rates per injected oocyte and ET, respectively, were 3.4% and 21.6% for SF and were 6.6% and 60.4% for IVF with unfrozen oocytes. Compared to women who underwent IVF after SF, IVF with unfrozen oocytes resulted in significantly better rates of fertilization (odds ratio [95% confidence interval]); 2.22 (1.80, 2.74), of live birth per injected oocyte; 1.5 (1.26, 1.79), and of implantation; 4.66 (3.93, 5.52). These odds ratios were lower when oocyte cryopreservation success rates from 2002-2004 were compared with those for IVF with unfrozen oocytes. When the reports after June 2005 were considered, this trend did not appear to continue. With the consideration of VF reports after June 2005, however, higher pregnancy rates were achieved.

CONCLUSION(S)

In vitro fertilization success rates with slow-frozen oocytes are significantly lower when compared with the case of IVF with unfrozen oocytes. Although oocyte cryopreservation with the SF method appears to be justified for preserving fertility when a medical indication exists, its value for elective applications remains to be determined. Pregnancy rates with VF appear to have improved, but further studies will be needed to determine the efficiency and safety of this technique.

Live births after vitrification of oocytes in a stimulated in vitro fertilization-embryo transfer program

OBJECTIVE

To assess the usefulness of the vitrification method in clinical practice.

DESIGN

Clinical study of vitrification of human oocytes.

SETTING

A university-affiliated hospital.

PATIENT(S)

Thirty-four patients who agreed to undergo additional IVF-ET after the failed fresh cycle using the previously vitrified/thawed oocytes.

INTERVENTION(S)

Surplus oocytes from the IVF-ET patients were vitrified for the next cycle.

MAIN OUTCOME MEASURES

Morphologic normality of post-thawed oocytes, fertilization, cleavage, pregnancy, and implantation rate.

RESULT(S)

Overall morphological survival and fertilization rates of vitrified/liquefied oocytes were 68.6% (325/474) and 71.7% (142/198), respectively. Pregnancy rate and implantation rate per embryo transfer were 21.4% (6/28) and 6.4% (8/125), respectively. All pregnancies resulted in the delivery of healthy babies (1 twin and 5 singletons/6 pregnancies).

CONCLUSION(S)

The feasibility of the vitrification method for human oocytes was confirmed by our clinical results. Subsequent studies on vitrification and thawing procedures should be undertaken for further optimizing the vitrification method.

Effects of chilling to 0 degrees C on the morphology of meiotic spindles in human metaphase II oocytes

OBJECTIVE

To determine the effects of chilling to 0 degrees C on the meiotic spindle of human metaphase II oocytes, as observed by optical sectioning microscopy.

DESIGN

Laboratory study.

SETTING

Academic research laboratory in a medical school.

PATIENT(S)

Seventy-two women undergoing infertility treatment donated a total of 108 oocytes.

INTERVENTION(S)

Metaphase II oocytes were stripped of their cumulus cells, cooled directly to 0 degrees C, and held for periods of 1 to 10 minutes. They were then fixed at 37 degrees C, stained for immunofluorescence, and examined microscopically.

MAIN OUTCOME MEASURE(S)

Morphology of the meiotic spindle in chilled and control oocytes.

RESULT(S)

Microscopic evaluations of 46 chilled oocytes revealed various time-dependent changes in microtubules compared to 9 control oocytes. After 1 minute at 0 degrees C, spindle damage was negligible, but in oocytes cooled for 2 or 3 minutes, there was obvious shortening of the spindle and loss of polarity. Cooling to 0 degrees C for 4 to 9 minutes resulted in increasingly more drastic changes; by 10 minutes the spindles had totally disappeared. Despite depolymerization of microtubular tubulin at 0 degrees C, the chromosomes did not become dispersed, but remained anchored even in the absence of spindles.

CONCLUSION(S)

Even brief exposure of human oocytes to temperatures near 0 degrees C causes profound alterations of the meiotic spindle.

Use of fluorescence in situ hybridization to assess the chromosomal status of embryos obtained from cryopreserved oocytes

OBJECTIVE

To analyze the chromosomal status of human embryos obtained from frozen-thawed oocytes.

DESIGN

Fluorescence in situ hybridization analysis of embryos obtained after oocyte cryopreservation.

SETTING

Department of Obstetrics and Gynecology at the University of Perugia, Italy, and the Instituto Valenciano de Infertilidad, Spain.

PATIENT(S)

Oocyte donors (n = 43). Fertilization, development, and chromosomal status of the embryos were compared with a control group (n = 18) of patients undergoing preimplantation genetic diagnosis for sex chromosome-linked diseases.

INTERVENTION(S)

Collection of oocytes after conventional ovarian stimulation and cryopreservation using propanediol as the cryoprotectant and a slow freezing procedure. Microinjection of surviving metaphase II oocytes and evaluation of fertilization and embryo development up to blastocyst stage. Chromosomal analysis after embryo biopsy.

MAIN OUTCOME MEASURE(S)

Survival, fertilization, and blastocyst rates. Embryo chromosomal analysis employing specific probes for chromosomes 13,18,21, X and Y.

RESULT(S)

The overall survival rate was 59.4%. There was no difference between cryopreservation and control groups in fertilization rates (76.5% vs. 90.5%) or blastocyst development (29.6% vs. 35%). The percentage of blastocysts from the original number of cryopreserved oocytes was only 5.6%, comparable to the 5.9% obtained in the control group. The percentage of embryos with abnormal number of chromosomes in the cryopreservation group (28.6%) was comparable to the 26% observed in the controls.

CONCLUSION(S)

Fertilization and cleavage rates after oocyte freezing are acceptable. Survival is, however, still poor, leading to overall results that make the technique clinically inefficient. There is no increase in the rate of chromosomal abnormalities, indicating that the technique is, nevertheless, safe enough to be further explored and improved.

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.