Effects of frozen timing on the spindle density, the angle between the polar body and spindle, and embryo development of intracytoplasmic sperm injection in mouse mature oocytes

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Better pregnancy outcomes can be obtained by human mature oocyte vitrification, but many problems remain to be resolved in human mature oocyte vitrification. Since mature oocyte development possesses its own maturity cycle, there should be the optimal timing for mature oocyte vitrification. The purpose of this study was to observe the effects of frozen timing on the spindle density, the angle between the polar body and spindle, and embryo development of intracytoplasmic sperm injection (ICSI) in vitrified mouse mature oocytes and explore its possible mechanism. Mouse oocytes were randomly divided into three groups according to different frozen timing including Groups A, B, and C in which oocytes were vitrified within 2 h after ovum pick-up, and 3-4 and 5-6 h after ovum pick-up, respectively. Spindle-related parameters were measured, ICSI was performed. The spindle occurrence rate of vitrified-thawed oocytes was 98.4% in Group A, 82.3% in Group B, and 75.8% in Group C, without statistical differences between pre-vitrification and post-thawing and among the three groups (P > 0.05). The angles between the polar body and spindle were larger after thawing than before vitrification (P < 0.01). The spindle retardance values were lower after thawing than before vitrification in Groups B and C (P < 0.05), but higher in Group A (P < 0.05). The spindle retardance values before vitrification were higher in Group B than in Groups A and C (P < 0.05), but the spindle retardance value, oocyte survival and two-cell rate after thawing were higher in Group A than in Groups B and C (P < 0.05). There were no statistical differences in ICSI fertility rate between the three groups (P > 0.05). The damage on the spindle is the slightest and embryo quality is the highest in the mouse oocytes vitrified within 2 h after ovum pick-up. The spindle retardance value is more valuable than the spindle occurrence rate in the evaluation of vitrified-thawed oocyte quality, and is positively correlated with embryo quality.