Cryopreservation of in vitro-produced bovine embryos by vitrification: In pursuit of a simplified, standardized procedure that improves pregnancy rates to promote cattle industry use

Abstract

The goal of cryopreservation is to retain the original stage of gametes and embryos after they have endured cooling and warming. Slow freezing is a standard method for in vivo-derived bovine embryo cryopreservation, threefifths of such embryos being frozen by this method globally. However, it is evident that slow freezing is not efficient for cryopreserving in vitro-produced bovine embryos. Hence, only one-third of in vitro-produced bovine embryos are cryopreserved. Vitrification is a preferred method for storage of human embryos; consequently, it has been explored as a novel means to store in vitro-produced bovine embryos, for which it shows considerable promise as an alternative to slow freezing. This is due to several reasons: vitrification is often less time-consuming than slow freezing; it does not need expensive slow rate freezing machines; and it has been proven to have comparatively higher survival rates. Yet, in the cattle industry vitrification continues to present shortcomings, such as possible toxicity of vitrification solutions and failure to standardize methods, which pose a challenge for its application to in vitro-produced bovine embryos. Therefore, determining the most suitable procedure is crucial to make vitrification more practical in commercial settings.

Specifics of vitrification of in vitro-produced cattle embyos at various development stages

“…Vitrification of embryos at various stages of the development is being debated (Juanpanich et al, 2018;Do, & Taylor-Robinson, 2020). Using cryoconservation of blastocysts, embryos can be transferred right after freezing, avoiding the re-cultivation of embryos in in vitro conditions (Aono et al, 2013).…”

“…However, Sommerfeld & Niemann (1999) indicate that expanded blastocysts survive after vitrification better than nonexpanded -42% and 12%, respectively. On the other hand, cryoconservation of embryos at early stage slows the in vitro production of embryos, since it is important that those embryos continue to be in vtro cultivated after thawing, developing to the blastocyst phase, when they can be transplanted to recipients (Do & Taylor-Robinson, 2020).…”