Timing of the first cleavage post-insemination affects cryosurvival of in vitro-produced bovine blastocysts.

Abstract

The time of the first cleavage of bovine zygotes during in vitro culture can affect the rate of development and cell number of the blastocysts. The aim of this work was to study the effect of the timing of first cleavage on the cryosurvival of the resulting blastocysts. Following standard IVM and IVF, zygotes were cultured in modified synthetic oviduct fluid (SOF), with 10% fetal calf serum (FCS) added 48 hr post insemination, in a humidified atmosphere of 5% CO2, 5% O2 and 90% N2. Embryos which cleaved by 24, 27, 30, 33, or 36 hr after insemination (IVF) were harvested and further cultured to the blastocyst stage (day 7 or day 8 post IVF). All developing blastocysts on days 7 and 8 were classified into three groups and were cryopreserved by vitrification. Group A consisted of blastocysts (<150 microm, small blastocysts); group B consisted of expanded or hatching blastocysts (>150 microm, large blastocysts); and group C consisted of morphologically poor quality blastocysts. The vitrification solution consisted of 6.5 M glycerol and 6% bovine serum albumin in PBS (VS3a). Thawed embryos were cultured further and survival was defined as the re-expansion and maintenance of the blastocoel over 24, 48, and 72 hr, respectively. Overall survival and hatching at 72 hr post-thawing was higher in blastocysts formed by day 7 than those formed by day 8 (60% vs. 40% survival; 63% vs. 45% hatching). Large blastocysts from day-7 and day-8 groups survived significantly better than small or poor quality blastocysts (76% vs. 63% and 31%; 72% vs. 30% and 26%, respectively; P < 0.05). Day-7 blastocysts from the 27- and 30-hr cleavage groups survived significantly better than those from the 36-hr group (63% and 66% vs. 25%, P < 0.05). Day-8 blastocysts from later cleaved (30 hr) zygotes had a higher survival than the 27-hr cleavage groups (52% vs. 26%, P < 0.05). These results indicate that the day of blastocyst appearance, developmental stage, and timing of the first cleavage post-insemination can influence the cryosurvival of bovine blastocysts following vitrification.