Fertilization time and oocyte spindle integrity: implication for embryo ploidy

ABSTRACT: The aim of this retrospective study in assisted reproductive technology was to provide new evidence that i) the 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 ii) 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 of 40-44 h had a significantly higher likelihood of euploidy (47%) compared with cycles in which the trigger-ICSI interval was 36-39 h (33%) (P < 0.05). For example, 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. LAY SUMMARY: This IVF study investigated whether giving human eggs more time to mature before fertilization can improve embryo quality. Eggs showing clear signs of full maturity under the polarization microscope were more likely to develop into embryos with the correct number of chromosomes. Having the correct chromosome number is important for healthy embryo development and a successful pregnancy. Eggs that were given a few extra hours to mature before fertilization were more likely to produce healthy embryos with the correct number of chromosomes. The best results were seen when fertilization took place 40-44 h after the hormone injection, rather than 36-39 h later. Fertilization was carried out using a procedure in which a single sperm is injected directly into an egg. These findings suggest that careful assessment of egg maturity and optimization of fertilization timing may help improve IVF outcomes.