Freezing Cleavage-stage Embryos by Vitrification Improves Outcome July 9, 2009 Embryo cryopreservation is known to offer several advantages during ART cycles, including enhancing cumulative pregnancy rates, preventing ovarian hyperstimulation syndrome, reducing multiple pregnancy rates, and lowering treatment costs. After the vitrification technology for cryopreservation was developed, several studies have compared the slow freezing technique and vitrification method in relation to post-thaw survival, implantation, and live-birth rates. Now, a new retrospective study published in the Journal of Assisted Reproduction and Genetics highlights the efficacy of cleavage-stage embryo vitrification in improving the survival rate, post-thaw embryo morphology, and pregnancy outcomes, compared to the slow-freezing technique. Mojtaba Rezazadeh Valojerdi and colleagues, from the Embryology Department, Royan Institute, Iran, compared the effect of vitrification against slow-freezing of cleavage-stage embryos with regard to post-thaw survival rate, embryo morphology, and clinical outcomes. Cleavage-stage embryos of 305 patients were either subjected to vitrification (n=153) or slow-freezing (n=152) procedures. The following results observed during the study demonstrated that vitrification is a better cryopreservation technique compared to the slow-freezing method. Variables Vitrification (%) Slow-freezing (%) Odds Ratio Survival rate 96.9 82.8 6.607 Morphology with intact blastomeres 91.8 56.2 8.769 Clinical pregnancy rate 40.5 21.4 2.427 Implantation rate 16.6 6.8 2.726 Previously, Loutradi et al (Fertility and Sterility, 2008) conducted a systemic review and meta-analysis to compare post-thaw survival rates following vitrification and slow-freezing of human embryos. The investigators analyzed four studies, including three randomized controlled trials, comprising of 7,482 vitrified and 1,342 slow-frozen human blastocysts/cleavage stage embryos. A substantially higher cleavage stage embryo survival rate was observed in the vitrification group as compared to the slow-freezing group (OR=15.57; random effects model). Post-thaw survival rate of blastocysts was also found to be considerably greater in the vitrification group than the slow-freezing group (OR=2.20; fixed effects model). The conventional cryopreservation, by means of the slow-rate freezing protocol is associated with disadvantages such as osmotic shock, cryoprotectant toxicity, and mainly intracellular ice formation that can damage the cell wall and structure. In contrast, vitrification, the ultra-rapid cryopreservation method, eliminates the formation of ice crystals, thereby reducing the chances of cellular damage. The superiority of vitrification over slow-freezing for embryo preservation has been documented by several authors. Balaban et al (Human Reproduction, 2008) demonstrated that vitrification has a lower effect on embryo metabolic rate, compared to slow-freezing; as evident by the higher survival rate and subsequent in vitro development. Apart from the potential advantages of embryo vitrification, the ultra-rapid technique of cryopreservation has also shown its superiority in oocyte and sperm cryopreservation, and is hence becoming a more favorable procedure in comparison to the slow-freezing technique. In a more recent review study, Kolibianakis and colleagues (Current Opinion in Obstetrics and Gynecology, 2009) noted that vitrification was significantly better than slow-freezing with regard to post-thaw survival rates and embryo development of cleavage-stage embryos and blastocysts. However, the clinical pregnancy rates per transfer were comparable between the two groups. Although there seems to be ample evidence from retrospective studies and meta-analyses on the potential benefits of vitrification compared to the conventional freezing techniques, further prospective, randomized controlled trials are mandated for validating these findings and also to assuage the concerns of embryo toxicity due to the cryoprotectants used for vitrification.
1. Rezazadeh Valojerdi M, Eftekhari-Yazdi P, Karimian L, Hassani F, Movaghar B. Vitrification versus slow freezing gives excellent survival, post warming embryo morphology and pregnancy outcomes for human cleaved embryos. J Assist Reprod Genet. 2009 Jun 10. [Epub ahead of print]
2. Loutradi KE, Kolibianakis EM, Venetis CA, et al. Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertil Steril. 2008 Jul;90(1):186-93.
3. Balaban B, Urman B, Ata B, et al. A randomized controlled study of human Day 3 embryo cryopreservation by slow freezing or vitrification: vitrification is associated with higher survival, metabolism and blastocyst formation. Hum Reprod. 2008 Sep;23(9):1976-82.
4. Kolibianakis EM, Venetis CA, Tarlatzis BC. Cryopreservation of human embryos by vitrification or slow freezing: which one is better? Curr Opin Obstet Gynecol. 2009 Jun;21(3):270-4.