Development of serum-free media for the culture and transfer of human blastocysts

The culture and transfer of the blastocyst stage embryo has several advantages for assisted reproduction in the human. However, due to inadequacies of present culture conditions in human in-vitro fertilization (IVF), embryos are routinely transferred to the uterus on either day 2 or day 3 of development around the 4- to 8-cell stage, with resultant implantation rates of only 10-25%. In other mammalian species the transfer of cleavage stage embryos, which normally reside in the oviduct, results in a significantly lower implantation rate compared with the transfer of blastocysts. Extended culture of human embryos in vitro will help to identify those embryos with little, if any, developmental potential. It is therefore plausible that the blastocyst has an intrinsically higher viability than the cleavage stage embryo. It has now been shown in human IVF that sequential serum-free media can support > 50% blastocyst development, with an implantation rate per blastocysts of 50%, double that obtained for cleavage stage embryos. As the implantation rate of the blastocyst is higher than the cleavage stage embryo, fewer blastocysts are required for transfer. The development of completely defined embryo culture media may prove feasible by the replacement of protein with the glycosaminoglycan hyaluronate. Hyaluronate, which is protein-free, is more suitable than albumin in supporting implantation in the mouse, and can eliminate the biological variation inherent when using protein and the potential for contamination when using blood products such as albumin.