Evidence-based ET: What is the best protocol for cryopreserving IVF-derived bovine embryos?

Evidence-based ET: What is the best protocol for cryopreserving IVF-derived bovine embryos?

John F. Hasler

Note: The title of this column was suggested by our AETA board of directors

There has been concern regarding the best way to cryopreserve bovine IVF-derived embryos ever since commercial in vitro embryo production (IVP) started in the early 1990s. Donors in the early days were primarily infertile, problem donors, and annual IVP embryo production in the United States was limited to a few thousand embryos, at most. Production of IVP embryos has increased substantially in recent years, and in 2013, it was reported that 48,112 embryos were produced from OPU collections compared with 301,671 in vivo embryos collected from superovulated cattle. Thus, 13.8% of the total embryos produced were from IVF procedures and 55% of them were reported to have been frozen. It is anticipated that reported IVP production will be substantially higher in 2014. Understandably, the companies providing IVF services are reluctant to share details of their cryopreservation services. However, because fresh IVP embryos are often shipped overnight to ET practitioners/donor owners for transfer on-farm, embryo numbers sometimes exceed the number of available recipients. Consequently, not infrequently, practitioners are faced with cryopreserving leftover IVP embryos. Even today, however, there does not seem to be any widely agreed upon, best protocol for cryopreserving IVP embryos. In the last few years it has been publically reported that IVP embryos have been commercially cryopreserved by vitrification and slow freezing with both ethylene glycol (EG) and glycerol used as the cryoprotectants.


Evidence-based ET: Does the inclusion of sucrose in EG freezing medium improve embryo survival and pregnancy rates?

Categories: Evidence-Based ET
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Published on: June 17, 2014

Evidence-based ET

John F. Hasler

Does the inclusion of sucrose in EG freezing medium improve embryo survival and pregnancy rates?

As pointed out previously in this column, efficacious cryopreservation of bovine embryos is critical to the commercial ET industry because, as has been the case for some years now, more than 70% of embryos collected in the US are frozen, while fewer than 30% are transferred fresh. Following the published report of Voelkel and Hu in 1992 on cryopreservation with EG, the commercial bovine ET industry rather quickly switched from glycerol to EG as the major cryoprotectant in freezing media. The overall percentage of embryos frozen in EG rose rapidly starting in 1992 and reached 97% in 2008, the last year that the AETA collected data on this specific statistic.

Several companies provide 1.5 M EG freezing media with or without the inclusion of 0.1 M sucrose. The question posed here is whether including sucrose in the freezing medium makes any difference. In the normal range of ambient temperatures under which most embryos are placed in freezing media and loaded into straws, sucrose does not enter the cells.  Although sucrose readily diffuses through the zona, it remains outside the cells and, at the relatively low concentration of 0.1 M, exerts a mild osmotic imbalance, thereby pulling some water out of the cells. Whether this improves embryo survival is in question and may, in fact, be somewhat related to embryo stage of development. (more…)

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