Integrity rate of pronuclei after cryopreservation of pronuclear-zygotes as a criteria for subsequent embryo development and pregnancy.

BACKGROUND The aim of this study was to examine whether the integrity rate of pronuclei, after cryopreservation of pronuclear zygotes, could be a predictor of future embryo development and implantation. METHODS Two-pronuclei stage zygotes (n = 862) were cryopreserved by aseptic rapid freezing in 15% ethylene glycol + 15% DMSO + 0.2 M sucrose with a 4-step exposure in 12% (v:v), 25, 50 and 100% rapid freezing solution for 2, 1, 1 min and 30-50 s, respectively, at room temperature, and then plunged into liquid nitrogen. Zygotes were rapidly warmed at a speed of 30,000 degrees C/min and subsequently expelled into a graded series of sucrose solutions (1.0, 0.5, 0.25, 0.12 and 0.06 M) at 2.5 min intervals and in vitro cultured for 5 days. If embryos had developed to blastocysts on the 5th day, they were transferred to the recipients. The rest of the embryos were fixed for evaluation of chromatin. RESULTS Zygote development, up to the expanded blastocyst stage, after in vitro culture, was 40%, if the integrity rate of pronuclei was high, and 4% if the integrity rate was low (P < 0.05). The pregnancy rate after transfer of the 5-day blastocysts depended on the pronuclear integrity rate: 43% (125 pregnancies aftertransfer of 291 blastocysts) for those with a high integrity, and only 1 in 5 (20%) for the few blastocysts which had shown a pronuclear low integrity rate (P < 0.05). CONCLUSIONS Integrity rate of pronuclei after cryopreservation of pronuclear zygotes was a predictor of future embryo development and implantation: high integrity rate resulted in high pregnancy rate, while zygotes with low integrity rate of pronuclei after cryopreservation had low developmental potential.