Fertilization Time and Oocyte Spindle Integrity: Implication for Embryo Ploidy

The aim of this retrospective study in assisted reproductive technology was to provide new evidence that: (1) visibility of the meiotic spindle (MS) in human oocytes with an extruded first polar body predicts not only nuclear maturation but also embryo ploidy; and (2) the interval between ovulation triggering and ICSI-mediated fertilization affects the chromosomal status of embryos. MS visibility was assessed using polarized light microscopy, and the trigger-to-fertilization interval was adjusted accordingly to increase the likelihood of complete oocyte nuclear maturation. Extending the trigger-fertilization interval based on MS status significantly increased the proportion of mature oocytes (displaying both MS and polar body) from 79% to 92% (p < 0.05), indicating ongoing spindle assembly. Of 1,879 oocytes examined under polarized light, 462 (24.5%) developed into embryos undergoing PGT-A. Embryos derived from immature oocytes (without visible MS or with a spindle bridge to the polar body) had a euploidy rate of 14%, compared with 42% in embryos from mature (visible MS) oocytes (p < 0.05). Differences in embryo euploidy were also associated with the interval between ovulation trigger and ICSI fertilization. Cycles with a trigger-ICSI interval 40-44 hours had a significantly higher likelihood of euploidy (47%) compared with cycles in which the trigger-ICSI interval was 36-39 hours (33%) (p < 0.05). Namely in women of advanced maternal age, who exhibit an increased risk of embryonic aneuploidy, spindle-guided assessment of oocyte maturation and quality may improve the prediction of subsequent embryo ploidy.