Slow-freezing-induced changes of birefringent structures in human oocytes are related to responsiveness to ovulation induction

The slow-freezing method is widely used to freeze human oocytes, both for fertility preservation and in routine IVF programmes. Slow freezing damages some of the cell’s structures, including the meiotic spindle (MS) and the zona pellucida (ZP). Polarized light microscopy was used to study the variations induced by slow freezing on the MS and the ZP of human oocytes and to analyse the relationship between slow-freezing effects on the gamete and some clinical characteristics, such as age, body mass index and ovarian responsiveness to ovulation induction (expressed as total follicle-stimulating hormone dose/retrieved oocyte). Both the MS and the ZP (particularly its inner layer) underwent significant changes during slow-freezing procedure. The MS became thinner and structurally less organized (lower retardance) (P < 0.001 and P < 0.05, respectively), whereas the ZP became thicker and its inner layer lost structural organization (both P < 0.05). These morphological changes were unrelated to the patient’s age or body mass index, but ZP variations in thickness and retardance were significantly related to ovarian responsiveness (P = 0.033 and P = 0.026, respectively), suggesting that patients with a higher response to gonadotrophins produce oocytes better able to preserve their characteristics after freezing–thawing.