Spindle-to-oocyte light retardance ratio as a noninvasive biomarker for oocyte quality assessment

Background – Polarized light (PL) imaging provides a noninvasive strategy to assess oocyte quality and maturity. This prospective study aims to identify PL parameters in metaphase II (MII) oocytes that serve as adjunctive indicators for usable blastocyst formation. Methods – From August 2024 to August 2025, we studied 581 mature oocytes from women aged ≤ 38 years undergoing autologous in vitro fertilization cycles at Lee Women’s Hospital. Before intracytoplasmic sperm injection, we used PL microscopy to measure birefringence signals in the spindle and zona pellucida (ZP). Fertilized embryos were cultured in a time-lapse (TL) system. We then categorized the oocytes into groups based on their birefringence signals to compare their developmental potential. Results – Through PL imaging, we identified the oocytes with superior developmental competence. Oocytes with a visible MII spindle achieved significantly higher developmental rates—notably for usable blastocyst development—than did those without an MII spindle (46.1% vs. 15.6%). MII oocytes with high spindle and ZP birefringence demonstrated the highest usable blastocyst rate (53.7%) and spindle-to-oocyte light retardance ratio (SOLRR; 0.011 ± 0.003). In multivariate analysis, SOLRR and anti-Müllerian hormone were significant positive and negative predictors of usable blastocyst formation (adjusted odds ratios = 1.148 and 0.939, respectively). SOLRR alone demonstrated moderate predictive power (area under the receiver operating characteristic curve = 0.63). Oocytes with high SOLRR (≥0.01127) consistently outperformed those with low SOLRR (<0.01127), achieving a higher usable blastocyst rate (61.8% vs. 38.0%). In TL analysis, the high SOLRR group demonstrated faster embryonic progression than the low SOLRR group did, indicated by faster blastocyst formation (108.2 vs. 111.2 h) and blastocoel expansion (8.7 vs. 10.2 h); this group also achieved a higher nonreverse cleavage rate (95.4% vs. 86.8%) and superior trophectoderm morphology (58.0% vs. 40.7%). Conclusions – PL imaging can be used to identify oocytes with superior developmental potential. SOLRR serves as an exploratory but quantifiably objective indicator of oocyte competence, independently predicting oocytes with relatively high usable blastocyst formation rates, rapid embryonic progression, and improved trophectoderm morphology.