FACTORS AFFECTING SURVIVAL RATES OF IN-VITRO PRODUCED BOVINE EMBRYOS AFTER VITRIFICATION AND DIRECT IN-STRAW REHYDRATION

The aim of this work was to investigate the possibilities of simplific ation, and to outline the limits of application, of a vitrification me thod for cow embryos. Morulae and blastocysts were produced by in vitr o fertilization of slaughterhouse-derived, in vitro matured oocytes wi th frozen-thawed bull semen, and subsequent culture on a granulosa cel l monolayer. Vitrification was performed by equilibration of embryos w ith 12.5% ethylene glycol and 12.5% dimethylsulphoxide at 20-22 degree s C for 60 s, then with 25% ethylene glycol and 25% dimethylsulphoxide at 4 degrees C for another 60 s. Embryos were then loaded in straws, placed in liquid nitrogen vapour for 2 min, and then plunged. Straws w ere thawed in a 22 degrees C water-bath, the embryos were directly reh ydrated and further incubated in straw, and were then expelled and cul tured in vitro for 72 h. In the first experiment, embryos of different age and developmental stage (Day 5 compacted morulae, Day 6 early bla stocysts, Days 6 and 7 blastocysts, Day 7 expanded blastocysts and Day 8 hatched blastocysts) as well as Days 7 and 5 blastocysts previously subjected to partial zona dissection were vitrified. After thawing, t he re-expansion rates of blastocysts and zona-dissected embryos did no t differ (67 and 87%, respectively), and hatching was more frequent fo r blastocysts frozen in advanced developmental stages (34, 47 and 63% for early blastocysts, blastocysts and expanded blastocysts, respectiv ely). The re-expansion rate of morulae was lower (10%) and no hatching of these embryos was observed. In the second experiment, Day 7 expand ed blastocysts were vitrified using PBS, PBS + albumin, TCM199 and TCM 199 + calf serum as holding media. No differences in re-expansion and hatching rates were seen. However, when incubation with the concentrat ed cryoprotectant solution was performed at 20-22 degrees C, the embry o survival rate decreased (PBS + albumin) or no embryo survived (TCM19 9 + calf serum) the vitrification procedure. In the third experiment, Day 7 expanded blastocysts were vitrified, thawed, cultured for 1 day, and then re-expanded embryos were again vitrified and thawed. Out of the 87% that survived the first cycle, 73% re-expanded and 47% hatched following the second vitrification and thawing. These observations pr ove that the vitrification procedure described is relatively harmless, that it can he used for blastocysts of different developmental stages and that an intact zona is not required to obtain high survival rates .