Media temperature control: a potentially important quality control parameter in human oocyte vitrification

Abstract: The purpose of this study was to evaluate the role of controlling media temperature during human oocyte vitrification. This retrospective study compared fertilisation rates, cleavage-stage embryo development, and blastulation between two separate periods defined by uncontrolled versus controlled media temperature during oocyte vitrification. A retrospective observational study was conducted at Ventus IVF Center, Cyprus, evaluating the effect of vitrification solution temperature during vitrification on oocyte survival and embryological outcomes. Oocytes were vitrified either under uncontrolled ambient conditions (n = 87 oocytes) or with temperature control (n = 83 oocytes), with the experimental groups assessed asynchronously. Vitrification and equilibration solution temperatures were recorded using a calibrated UNI-T UT322 thermometer. There were no significant differences in baseline characteristics of oocyte donors between the controlled and uncontrolled temperature groups (P > 0.05). During vitrification, vitrification media temperature was significantly higher in the controlled group (23.85 ± 0.43°C) compared to the uncontrolled group (19.65 ± 0.43°C; P < 0.001). No significant differences were observed in oocyte survival post-warming (82/83 vs 80/87; P = 0.08) or fertilisation rates (69/82 vs 70/87; P = 0.67) between groups. Compared to the controlled group, the uncontrolled group showed markedly higher developmental arrest at the pronuclear (10/70 vs 0/69; P < 0.001), post-pronuclear (20/70 vs 0/69; P < 0.001), and cleavage (35/70 vs 0/69; P < 0.001) stages, and significantly reduced blastocyst formation (5/70 vs 56/69; P < 0.001). Maintaining vitrification solution temperature rather than relying on ambient conditions significantly improves oocyte vitrification efficiency and pre-implantation embryo development parameters. Lay summary: Freezing human eggs is widely used in fertility treatment and fertility preservation, but small laboratory factors can strongly influence success. This study investigated whether controlling the temperature of the freezing solutions during egg freezing into a glass-like state affects how well eggs survive and develop into embryos. Eggs from donor cycles were frozen either under room conditions or by controlling the freezing solution temperature at a stable temperature. Although egg survival and fertilisation rates were similar in both groups, eggs frozen without temperature control showed very high rates of early embryo development failure and produced very few embryos following a five-day culture. In contrast, eggs frozen with controlled solution temperatures developed into embryos following the five-day culture far more often. These findings show that maintaining the temperature of freezing solutions is a critical but often overlooked laboratory quality control factor and that simple temperature control measures could significantly improve the success of egg freezing in fertility clinics.