In vitro fertilization and development of immature and mature bovine oocytes cryopreserved by ethylene glycol with sucrose.

Immature and mature bovine oocytes were frozen slowly in 1.8 M ethylene glycol (EG) with different concentrations of sucrose (0, 0.1, and 0.2 M). After thawing and dilution of the cryoprotectant by a one-step procedure, the survival of the oocytes was assessed morphologically and also by in vitro fertilization and culture. Sucrose had no effect on the survival and fertilization rates of frozen-thawed oocytes. However, higher cleavage rates of frozen-thawed immature oocytes were obtained with 0.2 M sucrose used in combination with EG. Moreover, none of the immature oocytes frozen in EG without sucrose developed into blastocysts, but about 1% of immature oocytes frozen with sucrose developed into blastocysts. The cleavage rates of mature oocytes frozen in EG were significantly lower with sucrose than without sucrose. However, no significant difference was observed in the development to blastocysts. Transfer of four blastocysts derived from frozen-thawed immature oocytes into two recipient heifers resulted in one pregnancy.