O-20: Slow-Freezing-Induced Changes of Birefringent Structures in Human Oocytes Are Related to Responsiveness to Ovulation Induction

Background: The slow-freezing method is one of the most popular techniques to freeze human oocytes and it is also used in routine in vitro-fertilization (IVF) programs, particularly in those countries where embryo freezing is not allowed. Slow-freezing damages some of the structures of the egg, among which the meiotic spindle (MS) and the zona pellucida (ZP). Polarized Light Microscopy (PLM) permits detection of both the MS and the ZP without damaging the oocyte; they appear birefringent when enlighted by polarized light. Materials and Methods: Herein we used PLM to study the variations induced by slow-freezing procedure on the MS and the ZP of mature human oocytes, and we analyzed the relationship between the effects of slow-freezing on the egg and some clinical characteristics of the patient (age, body mass index, ovarian responsiveness to hormonal stimulation). Ovarian responsiveness was calculated as the FSH dose needed to retrieve an oocyte at ovarian puncture (FSH dose / oocyte). Results: We observed that both the MS and the ZP (particularly its inner part) underwent significant changes during the slow freezing procedure. In fact, the MS became thinner and structurally less ordered, whereas the ZP became thicker and its inner part lost structural organization. These morphological changes were not related to the patient’s age or to BMI, but ZP variations were significantly related to ovarian responsiveness to the hormones used for ovulation induction. Conclusion: These results suggest that patients with a higher responsiveness to FSH (that need less FSH to get an oocyte) produce oocytes able to better preserve their characteristics after the freezing/thawing procedure.