Set up of a serum-free culture system for bovine embryos: Embryo development and quality before and after transient transfer

It is well known that serum in culture medium negatively affects blastocyst quality. The objective of this work was to develop and test a serum-free culture medium which could improve embryo quality, measured by the resistance to freezing, lipid and glutathione content of the resulting blastocysts, as well as the ability of the blastocysts to elongate after transient transfer to recipient cows. In a first experiment we showed that adding a mixture of insulin, transferrin and selenium to serum-free Synthetic Oviduct Fluid medium (SOF–ITS) improved embryo development and quality. In the second experiment, the addition of BSA to SOF–ITS further improved blastocyst development. Moreover, a reduction in lipid content of morulae was observed in SOF–ITS–BSA by comparison with morulae cultured with serum (SOF–FCS). The resistance to freezing measured by hatching rates 24 h post-thawing was also improved for blastocysts with a diameter between 160 and 180 μm cultured in SOF–ITS–BSA by comparison to those produced with serum. In order to evaluate the redox potential of the embryos, reduced glutathione content (GSH) was evaluated both before and after cryopreservation. A significant decrease in glutathione was observed after freezing, whatever the culture medium, but no difference was observed between culture conditions. Transient transfers were performed and elongated D-13 embryos were recovered. Elongation was more pronounced and the embryonic disk more often visible in embryos cultured in SOF–ITS–BSA than in embryos cultured with FCS. In conclusion, the serum-free system we developed to produce in vitro bovine embryos meets the developmental and qualitative requirements for a large-scale use.