Effects of cryopreservation on glucose metabolism and survival of bovine morulae and blastocysts derived in vitro or in vivo.

Bovine morulae and blastocysts were either produced in vitro through maturation, fertilization and culture of immature oocytes recovered from slaughterhouse-derived ovaries, collected in vivo or obtained after 24 h in vitro culture of in vivo collected embryos. The morulae and blastocysts were classified into four categories of embryo quality and two stages of embryonic development. Embryos were frozen by a controlled freezing method using 10% glycerol as a cryoprotectant. The ability of individual embryos to withstand freeze/thawing was measured immediately before and after cryopreservation by changes in CO2 production from (U-14C)glucose during a 2 h incubation period in a non-invasive closed system immediately before and after cryopreservation. Post-thaw survival was assessed by development in vitro during a 48 h culture period. Survival rates and oxidative metabolism after freeze/thawing were similar in embryos of the two developmental stages. However, after freeze/thawing, the rate of CO2 production of in vitro produced embryos was reduced to one half of their pre-freeze levels and was associated with poor survival rates. In vivo collected embryos had a significantly better tolerance to freezing and higher survival rates. However, when in vivo embryos were exposed to in vitro culture conditions, the rates of CO2 production and survival were significantly reduced. Pre-freeze embryo quality affected post-thaw in vitro development and metabolic activity markedly in embryos produced in vitro or pre-exposed to in vitro culture conditions. While there was no relationship between pre-freeze levels of CO2 production and post-thaw in vitro embryo development, all embryos which developed in vitro after freezing/thawing retained at least 58% of the pre-freeze levels of CO2 production regardless of their origin. Results of the present study indicate that embryos produced in vitro or pre-exposed to in vitro culture conditions are more sensitive to cryo-injury. This sensitivity is affected by embryo quality and is similarly reflected at the biochemical level. Determination of oxidative metabolism offers a feasibility for selection of viable morulae/blastocysts after freezing/thawing.