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 postthawing
was also improved for blastocysts with a diameter between 160 and 180 mm 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.