Effects of cryopreservation on survival and development of interphase- and mitotic-stage 1-cell mouse embryos

The effects of cryopreservation with 1,2-propanediol on two groups of 1-cell mouse embryos were studied in terms of survival after thawing, growth in vitro until the blastocyst stage and development in vivo assessed by the number of implantations and living fetuses. The two groups were divided according to different stages in the cell cycle: cells in (i) interphase with two distinct pronuclei or (ii) mitosis just prior to the first cleavage division. Zygotes in the interphase stage proved to be more resistant to freezing and thawing procedures, showing a significantly higher survival rate after thawing than zygotes in mitosis (78.5 versus 61.3%, P < 0.05). Blastocyst formation was similar in the two experimental groups: 72.7% for interphase and 60.8% for mitosis (P = 0.06), but for both groups fewer blastocysts formed when compared with the control group (86.7%) (P < or = 0.01). The implantation rates were not statistically different: 54.2% for the interphase cells and 47.4% for the control group and 44.0% for the mitotic cells and 49.4% for the control group. The formation of living fetuses was similar between the experimental and control groups: 36.5% for the interphase group (40.0% for its control group) and 22.6% for the mitotic group (38.8% for its control group). We conclude that freezing embryos during nuclear division is detrimental for their survival after thawing.