An article in the latest issue of the Journal of Assisted Reproduction and Genetics evaluates two different formulations of slow-freezing solutions for cleavage stage embryo cryopreservation. The authors, Dr L Fang et al, from the Zhejiang University School of Medicine in China investigated whether improved survival rates could be achieved using a new formulation of media. The aim was to investigate if survival rates similar to those obtained with vitrification could be obtained in their clinic with this new composition of solutions from Vitrolife AB from Sweden for freezing and thawing of cleavage stage embryos.

The evaluation was in two parts - the first being a retrospective analysis of results obtained after freezing and thawing day 3 embryos from 400 women using an older formulation of cryopreservation solutions. These results were compared with those from 108 women where cryopreservation had been performed using the new composition solution from Vitrolife. In total, 2274 embryos from 897 patients were thawed using the old formulation of solutions while 1273 embryos from 542 patients were frozen and thawed using the new solutions.

With the new media - FreezeKit Cleave™ and ThawKit Cleave™ Vitrolife  - the survival rate increased from 82.1 to 94.4 % and the complete embryo survival rate increased from 54.9 to 81.3 %. The implantation rate, clinical pregnancy rate per embryo transfer, and per cycle were 28.2, 45.2, and 43.7 %, respectively, using the old formulations of cryo solutions. With the new solutions, the results reached 33.7, 54.1, and 54.1 %, respectively - the differences being statistically significant. The new solutions were specially formulated to add new base medium composition, a modification of a G-Series culture medium containing amino acids for support of embryo viability and with the addition of MOPS buffer for physiological pH maintenance plus hyaluronan, which has been shown to increase embryo survival after cryopreservation.

The team, from the Department of Reproductive Endocrinology, Women's Hospital, Hangzhou, use a Planer freezer for slow freezing, in preference to vitrification methods. Traditionally with vitrification, higher survival rates were obtained compared to standard slow-freezing procedures and it was initially introduced as a quick and easy cryopreservation method. But many clinics find vitrification time consuming and prefer to cryopreserve pronuclear-stage and cleavage stage embryos using the conventional slow-freezing method. The authors conclude: “In our clinic, we consider slow freezing more practical but wanted to improve the results, especially the survival rate after thawing of the embryos. With the new formulation of the solutions for freezing and thawing of cleavage stage embryos and the modified protocol, we have found that we can achieve the same high survival rates with the traditional slow-freezing method as with vitrification. We can continue using our freezing machine and can spend time on other things in the lab while the machine is running".

You can read the abstract at

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