Changes in freezing tolerance of pig blastocysts in peri-hatching stage.

Freezing tolerance of pig embryos at various peri-hatching stages was investi-gated. Survival of expanded blastocysts, hatching blastocysts and hatched blastocysts, as well as earlier staged embryos, i.e. morulae and blastocysts as control, was evaluated by culturing after they were frozen at -20°C with 1.5 M DMSO. Although none of the early staged embryos survived after freezing, 16 (48.5%) of 33 expanded blastocysts, 13 (56.5%) of 23 hatching blastocysts and 26 (47.3%) of 55 hatched blastocysts showed development after freezing and thawing. The highest survival rate (25/30, 83.3%) was obtained by the freezing of the hatched blastocysts which had developed in vitro from the expanded blastocysts. These results indicate that the expanded blastocysts of pig is at a critical stage where the freezing tolerance increases, and that the hatched blastocyst is tolerant of freezing.