Abstract
OBJECTIVE: Early-cleaving zygotes are more likely to develop to the blastocyst stage than their late-cleaving counterparts. The mechanism causing variation in the timing of the first zygotic cleavage has not been identified. This study aimed to investigate the relationship between meiotic spindle characteristics in human oocytes and the timing of the first zygotic cleavage after ICSI. DESIGN: Retrospective clinical study. MATERIALS AND METHODS: We evaluated 150 patients undergoing ICSI cycles, resulting in 509 metaphase II (MII) oocytes. Meiotic spindles and zona pellucida characteristics were imaged using the PolScope before ICSI in all MII oocytes. After ICSI, the oocytes were cultured and assessed for early cleavage (EC). The first mitotic division was checked 25–27 hours after ICSI. Embryos that had reached the two-cell stage at this point were classified as EC embryos. Embryos with intact nuclei or nuclear membrane breakdown were classified as late cleavage (LC) embryos. We compared the meiotic spindle and zona pellucida characteristics of EC with LC embryos. RESULTS: The meiotic spindle was visualized in 468 oocytes (92.0%), but could not be visualized in the remaining 41 (8.0%). After ICSI, the fertilization rate in oocytes with a visible spindle was 384/468 (82.1%). From 384 embryos, 163 (42.4%) were classed as EC and 221 (57.6%) were LC. The zona pellucida characteristics and spindle angle to the first polar body were similar in EC and LC embryos. EC embryos had significantly greater spindle areas (108.0 ± 17.8 versus 89.8 ± 19.2 μm2) and lengths (14.7 ± 2.0 versus 13.4 ± 2.2 μm) compared with LC embryos (P < 0.01). Spindle retardance in EC embryos was significantly higher than that in the LC embryos (P < 0.01). CONCLUSION: Meiotic spindle characteristics were significantly associated with EC score. Quantitative measurement of the size and retardance of the meiotic spindle could be valuable for identifying human embryo potential in the IVF laboratory.
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