Cryopreservation of mature human oocytes by vitrification with ethylene glycol in straws

Objective: To examine the effect of vitrification with ethylene glycol (EG) for mature human oocytes in straws. Design: Prospective, randomized, in vitro experiments. Setting: Reproductive unit of a university hospital. Patient(s): Immature oocytes from 110 patients undergoing intracytoplasmic sperm injection (ICSI). Intervention(s): The immature oocytes were incubated to reach metaphase II (MII). The MII oocytes were treated with EG-based cryoprotectants and vitrified in straws. They were diluted in sucrose solutions, inseminated by ICSI, and cultured in vitro. Main Outcome Measure(s): Survival, fertilization, and embryo cleavage. Result(s): The survival rates were greater for oocytes pretreated with 1.5 M of EG (65% for 0 minute, 93% for 5 minutes, and 96% for 10 minutes). The oocytes vitrified in 60 and 90 seconds had a greater rate of fertilization than those vitrified in 120 seconds. There were no differences in survival and fertilization for vitrified oocytes diluted by three or four steps. The cleavage rates to the six- to eight-cell stage were comparable with controls. However, no blastocyst formation was observed in vitrified oocytes. Conclusion(s): Vitrification of human oocytes with EG in straws achieves a high rate of survival, fertilization, and early cleavage of embryos. Further studies should be conducted for the improvement of blastocyst formation. (Fertil Steril (R) 2000,74:804-8. (C) 2000 by American Society for Reproductive Medicine.).