Fertilization
Embryo transfer
Embryo culture
The evolving physiological needs of the growing embryo in in-vitro culture conditions vary, necessitating different culture media compositions based on the developmental stage of the embryo. Mature oocytes (MII) and 2-cell stage zygotes have low oxygen consumption and primarily rely on carboxylic acids such as pyruvate as their energy source. Embryos before the compaction stage require pyruvate and non-essential amino acids in their culture medium. Post-compaction embryos exhibit a higher demand for glucose and essential amino acids.
Culturing embryos to the blastocyst stage offers several advantages, including improved viability assessment, enhanced synchronization, and the opportunity for PGD. Transferring embryos at the blastocyst stage results in higher implantation rates and lowers the risk of multiple pregnancies. The significantly higher implantation rates of blastocysts compared to cleavage-stage embryos is considered the most compelling evidence supporting the effectiveness of culturing embryos to an advanced developmental stage in in-vitro conditions.
The increased implantation rate of blastocysts indicates that embryos with the highest viability are being selected. Embryos of poorer quality, which might have implanted if transferred on day three, may not reach the blastocyst stage in culture. Therefore, culturing to the blastocyst stage is associated with a reduction in the number of transferable embryos. Although the higher implantation rate of blastocysts allows for the transfer of fewer embryos, the multiple pregnancy rate after transferring two blastocysts is equal to or even higher than when a larger number of cleavage-stage embryos are transferred.
Patients undergoing blastocyst-stage embryo transfer will have fewer surplus embryos for cryopreservation compared to those undergoing cleavage-stage embryo transfer. A major drawback of culturing embryos to the blastocyst stage and transferring them is the increased risk of cycle cancellation. Some studies on the outcomes of culturing to the blastocyst stage and blastocyst transfer have shown that the dizygotic twinning rate after transferring two blastocysts can reach 50%. In patients with a good prognosis, selective transfer of a single blastocyst can reduce the incidence of twinning without decreasing the overall pregnancy rate.
There are concerns that prolonged embryo culture may increase the risk of epigenetic mutations (imprinting). Evidence from animal studies, showing that the developmental programming of the embryo before implantation can be affected by laboratory manipulations, supports the need for well-defined and standardized culture media. Furthermore, long-term, detailed studies of children born after blastocyst transfer are necessary.
Source: Clinical Approaches to Infertility in Reproductive Biology
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