Objectives: To evaluate the occurrence of injury due to physical factors in embryo cryopreservation and the effect of the polymers dextran, polyvinylpyrrolidone (PVP), and Ficoll on this mechanical damage.
Design: Damage to the zona pellucida (ZP) observed after cryopreservation was taken as indication of cryoinjury caused exclusively by physical factors. Human and mouse ZPs from oocytes remaining unfertilized after previous IVF attempts and mouse two-cell embryos were frozen in the presence of different polymers. After thawing, they were checked carefully for signs of physical damage (cracks). A possible toxicity of the use of the polymers in cryoprotection was evaluated by development to the blastocyst stage of mouse two-cell embryos that survived the freezing and thawing process.
Results: Incidences of damaged ZPs in groups of human and mouse ZPs and two-cell embryos frozen without polymers were found to vary between 20% and 29%. The use of any of the tested polymers resulted in significantly lower incidences of damaged ZPs (0% to 15%). Damage to the ZP after freezing and thawing in mouse embryos was accompanied by low survival rates of the embryo itself. Of mouse embryos that survived the cryopreservation process, blastocyst formation was not significantly different in groups frozen without polymer (80%) or in the presence of either dextran (90%) or Ficoll (82%); however, embryos frozen in the presence of PVP showed low blastocyst formation (12%).
Conclusions: Polymers can protect embryos against cryoinjury by avoiding mechanical strain occurring during cryopreservation. Polyvinylpyrrolidine is toxic to mouse two-cell embryos when present during freezing and thawing.