Zona pellucida birefringence in in vivo and in vitro matured oocytes

Objective: To evaluate zona pellucida birefringence (ZPB) in immature and mature oocytes collected after controlled ovarian stimulation and to assess the influence of ZPB on oocyte development.

Design: Prospective study.

Setting(s): Private assisted reproduction centre.

Patient(s): Thirty patients undergoing intracytoplasmic sperm injection.

Intervention(s): The ZPB of mature and immature oocytes was evaluated using a polarization imaging software module, and the oocytes were classified as high birefringence (HB) or low birefringence.

Main outcome measure(s): The ZPB of in vivo and in vitro matured oocytes and its influence on spontaneous nuclear maturation in vitro, fertilization, and embryo quality.

Result(s): The percentage of HB oocytes was higher in immature than in mature oocytes (40.1 vs. 23.6%). Among immature oocytes, an increased percentage of HB in prophase-I stage oocytes compared to metaphase I stage oocytes was also observed (50.7 vs. 25.0%). However, the percentage of HB oocytes did not change when comparing oocytes before and after in vitro maturation for both prophase I and metaphase I oocytes. No influence of ZPB was observed on the spontaneous in vitro maturation potential. Exclusively for metaphase II retrieved oocytes, a positive influence of ZPB on fertilization (odds ratio [OR], 1.78; 95% confidence interval [CI], 1.27-2.49) and embryo quality (OR, 2.28; 95% CI, 1.04-4.99) was noted.

Conclusion(s): ZPB may be a useful tool to predict embryo quality for metaphase-II oocytes. Moreover, the completion of nuclear changes in the production of metaphase-II oocytes in vitro may not reflect their molecular maturity.