Cryopreservation for bovine embryos in serum-free freezing medium containing silk protein sericin

Because the use of serum in the embryo cryopreservation increases the probability of animal health problems such as bovine spongiform encephalopathy (BSE) and viral infections, this study was conducted to examine the effects of sericin supplementation for serum-free freezing medium on the survival and development of bovine embryos after freezing-thawing and direct transfer to recipients. When in vitro-produced bovine embryos were frozen conventionally in the freezing medium supplemented with various concentrations (0.1%, 0.5%, and 1.0%) of sericin, the percentages of damaged zona pellucida, survival, and development of frozen-thawed embryos were similar to those of embryos frozen in freezing medium supplemented with 0.4% bovine serum albumin (BSA) and 20% fetal bovine serum (FBS) (0.4BSA/20F; control). When in vivo-derived embryos were frozen with 0.4BSA/20F (control), 0.5% sericin +20% FBS (0.5S/20F) or 0.5% sericin (0.5S) and were subsequently transferred directly to recipients, the percentages of recipients with pregnancy and normal calving in the 0.5S/20F group were higher than those in the control group (47.3% vs. 40.1% and 94.6% vs. 87.3%, respectively). Moreover, the percentages of recipients with pregnancy and normal calving (42.2% and 92.4%, respectively) in the 0.5S group were similar with those of other groups. In conclusion, these results indicate that serum-free freezing medium supplemented with sericin is available for the cryopreservation of bovine embryos and that it is beneficial for the elimination of a potential source of biological contamination by serum or BSA.