The physiological condition of the female reproductive tract reflects the requirements of the early mammalian embryo. In the human oviduct
and uterus, concentrations of carbohydrates differ between ovulation and the time of implantation. Amino acids are involved throughout preimplantation development. Hyaluronan is also present in the reproductive tract and is involved in different processes. It has been shown that culture of embryos in media containing hyaluronan improves cryosurvival, /.e. results in an increased viability after thawing. Overall, embryo requirements differ for the pre- and post-compaction embryo. As a consequence, to meet the needs of human embryos, culture conditions have to be adapted to the developmental stage. Optimized human embryo culture in vitro requires physiological conditions from the moment of oocyte collection until the replacement of the embryo. A new media system, the Gill Series™, has been developed taken into account the changes in embryo physiology and requirements which occur as the embryo develops from the oocyte to the blastocyst stage.
Results form different clinics have shown that high pregnancy and implantation rates can be obtained using the Gill Series™. As compared to the previous media (G1.2/G2.2), Gardner et al. (Fertile. Steril., 78 (Suppl. 1), S8, 2001) reported an increased clinical pregnancy rate (from 69.8 % to 78 %) and an increased implantation rate (from 46 % to 53 %), for day 5 embryo transfers with the Gill Series™. Comparing the same culture systems, Balaban et al. (RRM online, 10, 485-491, 2005) reported an increase from 37.8 % to 50.3 % and from 14.5 % to 25.7 % for clinical pregnancy and implantation rate respectively for day 3 embryo transfers. For day 5 transfers the respective rates increased from 51.4 % to 69.4 % and from 29.0 % to 45.1 %. For embryos cultured in the presence of hyaluronan before ryopreservation Balaban et al. also reported increased development in vitro between thawing and replacement (from 26.2 % in G1.2-G2.2 to 58.3 % in Gill) as well as increased clinical implantation (from 7.8 % in G1.2-G2.2 to 15.1 % in Gill). The improvement of the in vitro environment allows performing single embryo transfer with good success rates. A clinical pregnancy rate of 36.9 % and an implantation rate of 32.1 % have been reported in a programme where an average of 1.2 embryos (75 % single embryo transfer) was replaced on day 2. Based on the latest research, the Gill series has been developed to minimize stress to oocytes and embryos and to culture them under physiological conditions. Clinical results obtained by different clinics confirm that large numbers of embryos can develop in vitro and further develop to term after replacement on different days of development.