Cryopreservation of porcine embryos would greatly facilitate the maintenance of genetic resources and the practical application of embryo transfer programmes. In this study, the effect of the stage of development of porcine embryos (prehatch vs post hatch) on the post thaw viability of blastocysts was evaluated. The blastocysts had been recovered from superovulated donor gilts and frozen in 1.5M glycerol according to a standard slow cooling protocol. From 444 frozenthawed embryos, 302 (68 per cent) were judged to be viable and were used for a 24-hour culture experiment in modified Whitten's medium (n=89) or transferred to three synchronous (n=72) or seven asynchronous (-24 hours) recipients (n=141). The proportion of embryos surviving in culture was significantly (P=0.05) greater for those frozen as post hatch (17/34) than as pre-hatch blastocysts (8/36). Although transfer of frozen-thawed embryos to synchronous recipients did not result in a pregnancy, two of the asynchronous recipients, which received 15 and 22 embryos, became pregnant and farrowed five and three piglets, respectively, at normal term. In spite of the overall inefficiency of the cryopreservation procedure used in this study, the birth of piglets from two successful transfers of frozen-thawed blastocysts suggests that further study of the cryopreservation protocols would be worthwhile.
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