VITRIFICATION OF MATURE MOUSE OOCYTES - IMPROVED RESULTS FOLLOWING ADDITION OF POLYETHYLENE-GLYCOL TO A DIMETHYL-SULFOXIDE SOLUTION

Oocytes have been successfully cryopreserved using rapid and slow free zing procedures; however, variability in the success of replicates has limited its practical application. We have evaluated the potentially beneficial effects of adding 1 mg/ml of the polymer polyethylene glyco l (PEG) (M-r 8000) to a 6 M dimethyl sulfoxide (Me2SO) vitrification s olution. Stepwise addition of cryoprotectant, either with or without P EG, was performed at room temperature (19-21 degrees C). Oocytes were then loaded in plastic insemination straws and held in liquid nitrogen vapour at -140 degrees C for 3 min prior to storage in liquid nitroge n. Oocytes were warmed rapidly to room temperature and removal of cryo protective agent was effected in the presence of 1 M sucrose solution. Viability was assessed by in vitro fertilization. Oocytes cryopreserv ed after exposure to 6 M Me2SO in the absence of PEG showed 60% normal ity, 80% fertilization, and 55% development to blastocyst, median of 1 1 replicate experiments (191 oocytes). Individual replicates yielded h ighly variable survival which ranged from 0 to 100%. The addition of P EG significantly improved oocyte normality to 95% (range, 76-100%; med ian of 9 replicate experiments, 301 oocytes). Rates of fertilization ( 91%; 60-100%) and development to blastocyst (73%; 67-92%) were also im proved. The addition of 1 mg/ml PEG to a 6 M Me2SO solution resulted i n greatly improved viability of oocytes following cryopreservation and vastly reduced the variability seen with the Me2SO solution alone. (C ) 1997 Academic Press.