The viability or developmental ability of porcine embryos after slow-freezing and thawing differs depending on the embryonic stage or the batch, which is defined as a group of embryos obtained from one donor at one time. We froze porcine blastocysts in batches and assessed their cryotolerance by using two expanded blastocysts (EBs) as samples to predict the developmental potential of other blastocysts from the same batch at different stages. Two EBs from the same batch that had been separately frozen were thawed and cultured in vitro for 48 h to examine their in vitro ability to develop to the hatched blastocyst stage. Thereafter, each batch was assigned to Grade A, B, or C according to the viability of the two EBs, i.e., 100% viability (2/2: number of hatched blastocystsumber of cultured EBs) was Grade A; 50% (1/2) was Grade B; and 0% (0/2) was Grade C. The viability of EBs after freeze-thawing and in vitro culture varied depending on the batch and was lower (31.0+/-10.2%, mean+/-S.E.M.; P<0.01) than that of unfrozen controls (96.8+/-2.3%). The viability of frozen-thawed hatched blastocysts (HBs) did not differ among the graded batches, but the blastocyst diameter decreased (from 409 to 326 microm) as the batch grade decreased (from A to C). When both EBs and HBs from batches of the same grade were transferred to recipients (average 11.7 EBs and 16.0 HBs per recipient), the rate of pregnancy and farrowing in recipients decreased (from 77.8% to 0%) and the number of piglets obtained decreased (from 15.3 to 0) as the batch grade decreased. However, when not only frozen-thawed EBs from Grade B or C batches, but also four helper embryos at the morula to early blastocyst stage (which were expected to support the pregnancy) were transferred, the number of piglets generated was higher from EBs from Grade B batches (16.0) than from EBs from Grade C batches (0.0). When frozen-thawed HBs and helper embryos were transferred, the number of piglets generated was higher from HBs from Grade B batches (12.7) than that from HBs from Grade C batches (1.9). After slow-freezing of porcine blastocysts, their rate of survival to the piglet stage differs batchwise, and in vitro viability assessment of sample EBs after freezing and thawing may help in assessing the post-freezing and post-thawing developmental potential of other blastocysts at different stages from the same batch.
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