STUDY ON THE RELATIONSHIP BETWEEN THE ANGLE OF MII SPINDLE AND THE DIFFERENCE IN THE SIZE OF 2PN IN HUMAN ZYGOTE USING TIME-LAPSE SYSTEM

Abstract

Previous studies have shown that the angle of spindle in MII oocyte is associated with embryo development. The incidence of zygotes with significant pronucleus size asynchrony is considered to be an indication of the poor embryo development. However, the correlation between angle the of spindle and difference in the size of 2PN have not been studied. In the present study, we investigated the correlation between the angle of spindle and the difference in the size of 2PN, and the effect of its correlation on clinical outcomes. Data were retrospectively collected from polarized light microscopy and time-lapse system performed from March 2018 to February 2020 in the Fertility center of CHA Gangnam Medical Center. We analyzed 235 zygotes from 66 patients. We measured the angle of spindle prior to ICSI using a LC-PolscopeTM (CRI). Oocytes were divided into two groups according to the angle of MII spindle (<30° group =189, ≥30° group =46) and were cultured in EmbryoscopeTM (Vitrolife). The size of 2PN were analyzed by measuring the horizontal diameter at the same focal plane before PN fade. Also, an abnormal division (direct division and not available cell number due to poor quality)in 2-cell embryo was checked. The average of difference in the size of 2PN was significantly increased in ≥30° group (<30° group; 1.4 um vs. ≥30° group; 3.5 um, p< 0.05). The rate of abnormal division (8.5% vs.23.9%) was highly increased but the rate of good quality (63.5% vs. 47.8%) was decreased in the ≥30° group compared with <30° group (p <0.05). When only embryos from the <30° group were selected for transfer, the pregnancy and implantation rates were 67.7% and 57.1%. But when only embryos from the ≥30° group were transferred, those rates were 25.0% and 20.0%, respectively (p <0.05). This is the first study that has analyzed the correlation between spindle angle and difference in the size of 2PN. Further studies with larger sample size are needed. This study could provide a new insight for the embryo selection by enabling noninvasive judgement of embryos that are more likely to become pregnant at the earliest time point. Also, it could be especially useful to assess zygote quality in countries with strict policies.