The first pregnancy after replacement of a frozen-thawed blastocyst occurred in 1985. Since then there have been wide-ranging results reported from frozen blastocyst transfers (22% with co-cultured embryos to lower rates with non-co-cultured embryos). Cryopreservation protocol, the culture system used for blastocyst development and the quality of the embryos frozen may all determine the outcome.
Blastocyst freezing has advantages over pronuclear and multicellular stage freezing. Loss or damage of some cells, particularly trophectoderm, may be tolerated better at this later stage of development. The process of embryo selection has taken place before cryopreservation, therefore there will be fewer embryos to freeze reducing cryopreservation and transfer costs. However, as with fresh blastocyst transfers, there is the risk of losing embryos, that if frozen and transferred at earlier stages may implant, but due to suboptimal culture conditions fail to form viable blastocysts.
At this time, blastocyst freezing at Reproductive Sciences Center is generally limited to embryos left over after a cleavage stage (day 2 or 3) transfer, that after further culture develop into satisfactory quality blastocysts by day 5 or 6.
What is Embryo Culture?
The culture of fertilized eggs in the laboratory to allow them to develop into early cleavage stage embryos and progress thorough to the pre-implantation blastocyst stage.
Fertilized eggs (zygotes) may be cultured up to day 2 (-48 h post insemination) or day 3 (-72h post insemination) with embryo transfers performed at the 2-4 cell and 6-8 cell stages respectively. Prolonging culture to day 3 enables the embryologist to monitor embryo development through the first 3 cleavage divisions (during embryonic gene activation), thus potentially improving selection of the most viable embryos. However, Day 2 transfers may have advantages for some patients (for example, when there are small numbers of embryos in culture and /or the embryo quality is poor). Extending culture to day 5 to allow embryos to reach the blastocyst stage may also offer potential benefits for some patients (e.g. patients that have had repeated IVF failures or are at high risk for a multiple gestation).
An optimal embryo culture system is required for cleavage stage embryos to obtain high pregnancy and implantation rates and to provide a basis for further culture to the blastocyst stage. Successful embryo culture depends on optimizing pH and temperature control, eliminating adverse environmental conditions and using high quality media and tissue culture-ware. Strict daily quality control is performed in the embryo laboratory to maintain optimal culture conditions.
It is the general policy of Reproductive Sciences Center to provide flexibility on a case-by-case basis with respect to length of time embryos are cultured. Embryos are cultured in microdroplets under oil using specifically defined culture media. The first media is designed to support the early cleavage stage embryo (up to day 3). The embryos are then transferred to a different media specifically designed to promote the later stages of pre-implantation development (8-cell up to the blastocyst stage.)
Embryos may be co-cultured with follicular cells collected from the follicles during the egg aspiration to provide extra nourishment and remove any embryo-toxic factors that may be present.