In vivo development of cryopreserved murine demi-embryos split before or after freezing

The in vivo development of demi-embryos produced prior to or after freezing and thawing and the effect of cryopreserving demi-embryos within or withou t a zona pellucida were studied. Late morulae and blastocysts collected fro m superovulated NMRI mice were bisected and frozen either with (Group 1) or without (Group 2) zonae pellucidae. Group 3 consisted of demi-embryos deri ved by splitting embryos after freezing and thawing. Demiembryos of Groups 1, 2 and 3 as well as frozen-thawed intact embryos (Group 4) were transferr ed to 10, 10, 14 and 10 recipient mice, respectively, that had been mated w ith vasectomized males 24h later than the donors. Upon killing of the recip ients on D 13 after transfer, it was found that the initial pregnancy rate achieved with demi-embryos was lower than that of the controls (60, 50 and 64% for Groups 1-3, respectively, vs 90% for Group 4). Live fetuses were on ly recovered from Group 1 (9 % embryo survival) and Group 4 (44% embryo sur vival). We conclude that, when murine demi-embryos are to be cryopreserved, the embryos should be split before freezing and should be frozen within a zona pellucida.