blastocyst cultivation, extended embryo culture
Prolonged embryo cultivation is understood as cultivation to the blastocyst stage (Pic. 1), i.e. 5 or 6 days after fertilization. The logic behind extending embryo culture until the day 5 is not to improve its quality, but to increase the probability of choosing the embryo with better developmental and implantation potential and to increase the pregnancy rate with minimizing the risk of multiple gestation.
When embryos reach the blastocyst stage it means it survive the 4-to-8-cell stage (Pic. 2), in which development fails most commonly. In addition, standard cultivation takes 3 days (cleavage stage embryos), but blastocyst transfer is more consistent with the natural conception when the embryo reaches the uterus at days 5-6 after fertilization.
When culturing to the blastocyst stage (Pic. 3), sequential or single system are used. In sequential system, the embryo is sequentially placed in different media. One medium may be used for culture to day 3, and a second medium is used for culture thereafter. Artificial embryo culture media basically contain glucose, pyruvate, and energy-providing components, but the addition of amino acids, nucleotides, vitamins, and cholesterol improve the performance of embryonic growth and development. Methods to permit dynamic embryo culture with fluid flow and embryo movement are also available. In single medium system, composition of this medium is constant and contains all the components needed during its development and the composition of this single medium does not change during embryo culture.
A new method in development uses the uterus as an incubator and the naturally occurring intrauterine fluids as culture medium by encapsulating the embryos in a permeable intrauterine vessel. A review in 2013 of commercially available IVF culture media was unable to identify a specific media that was superior in terms of pregnancy outcome.
Extending the duration of embryo culture to the blastocyst stage for assisted reproductive techniques (ART) offers several theoretical advantages over the transfer of cleavage-stage embryos.
These include:
However, blastocysts have certain drawbacks. With current techniques, some cleavage embryos do not develop into blastocysts in vitro and some blastocysts cannot be well cryopreserved. Therefore, it is still questionable whether the transfer of blastocyst-stage embryos is beneficial to all infertile patients. Technique is not recommended for patients with less embryos since embryos are longer exposure to artificial environment with its influence compare to shorter cultivation.
After prolonged culture, decreased number of embryos available for transfer and embryo cryopreservation, so the cumulative clinical pregnancy rates are increased with cleavage stage transfer. On average only 50% of cultured embryos reach the blastocyst stage.
Some studies indicated that embryo transfer after prolonged culture to the blastocyst stage was associated with higher odds of preterm birth compared to cleavage stage transfer. In order to explain the above they suggested that longer in vitro culture might have a deleterious effect on subsequent placentation.
Blastocyst transfer might reduce the multiple pregnancy rate due to use of fewer embryos, but still maintaining pregnancy success. If only single blastocyst is transferred, it could still split and result in twin pregnancy.
Epigenetic abnormalities in embryos having undergone embryo culture were also detected, especially in prolonged culture.
Often, an additional incubator is required due to additional 2-3 days during which the embryos remain in culture. Blastocyst culture is much labour intensive, and laboratory staff may be required to perform more weekend work, particularly if the embryos from two different stages of development are required to be cryopreserved. For patients, a higher chance of cancellation of the treatment cycle prior to embryo transfer may result in a lower treatment cost.
The blastocyst culture may increase the likelihood of selectively transferring viable and genetically normal embryos, i.e. embryos with higher implantation potential. Although the development to the blastocyst stage is not a guarantee of chromosomal normality, most embryos that fail to continue to develop in extended culture show multiple aneuploidies (abnormal number of chromosomes). Embryo culture until the blastocyst stage confers a significant increase in live birth rate per embryo transfer.
Furthermore, to perform embryo transfer at the blastocyst stage has additional advantages such as better synchronization between embryo developmental stage and uterine environment, and decreased uterine contractility, resulting in a higher implantation rate without affecting pregnancy rate. The high implantation rates could allow fewer-transferring but higher-quality embryos at the blastocyst stage, thus avoiding the risk of multiple pregnancies.
Usage of low oxygen concentrations of 5% rather than about 20% in the atmosphere has been shown to increase live birth rate to a relative probability of 1.24, without any evidence of increased risk for multiple pregnancies, miscarriages or congenital abnormalities.
