THE PROTECTIVE EFFECTS OF POLYMERS IN THE CRYOPRESERVATION OF HUMAN AND MOUSE ZONAE-PELLUCIDAE AND EMBRYOS

Objectives: To evaluate the occurrence of injury due to physical facto rs in embryo cryopreservation and the effect of the polymers dextran, polyvinylpyrrolidone (PVP), and Ficoll on this mechanical damage. Desi gn: Damage to the zona pellucida (ZP) observed after cryopreservation was taken as indication of cryoinjury caused exclusively by physical f actors. Human and mouse ZPs from oocytes remaining unfertilized after previous IVF attempts and mouse two-cell embryos were frozen in the pr esence of different polymers. After thawing, they were checked careful ly for signs of physical damage (cracks). A possible toxicity of the u se of the polymers in cryoprotection was evaluated by development to t he blastocyst stage of mouse two-cell embryos that survived the freezi ng and thawing process. Results: Incidences of damaged ZPs in groups o f human and mouse ZPs and two-cell embryos frozen without polymers wer e found to vary between 20% and 29%. The use of any of the tested poly mers 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 embr yos that survived the cryopreservation process, blastocyst formation w as not significantly different in groups frozen without polymer (80%) or in the presence of either dextran (90%) or Ficoll (82%); however, e mbryos 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. Polyviny lpyrrolidone is toxic to mouse two-cell embryos when present during fr eezing and thawing.