Nowadays, cryopreservation of embryos is considered a usual and effective application in the Assisted Reproduction Technology, thanks to the method of vitrification. This technique is usually preferred in ART cycles that there is a surplus of good quality embryos or in cases when for some reason, embryo transfer is not recommended.
Vitrification is the most modern and promising technique in the field of embryo cryopreservation. The method is based on the use of special cryoprotectant substances in consecutive higher concentrations, along with the ultra-high freezing rates. The cryoprotectants are of a double importance, as they remove the water from the inner of the cell and at the same time they protect the cell membranes from the extremely low temperatures. Both these two factors result in avoiding the appearance of ice crystals inside the cell and as a consequence the survival of the embryos. After this process, the vitrified embryos are stored in liquid nitrogen tanks in the temperature of -196o. The embryos can remain stored and vitrified, without any effect on their implantation ability. Based on the Greek law 3305/2005, concerning the assisted reproduction, vitrified embryos can remain stored for 5 years long, with a possible extension of this period for 5 more years. In particular, the higher survival rate (97%) of the embryos after the vitrification comparing with the older technique of slow freezing must be noted. In addition, the implantation and clinical pregnancy rates after thawing the vitrified embryos are also noteworthy, being even comparable with those of the fresh embryos.
The time of vitrification depends on factors, such as the embryo quality, the surplus number of the embryos, as well as the kind of the ART cycle that the couple will follow. Usually, embryos are vitrified on the 3rd or the 5th day, when they are at the cleavage (6-8 cells) or the blastocyst stage (>100 cells), respectively. Otherwise, depending on each single case, the vitrification can be done even during the second day of the culture, as with the cases that preimplantation diagnosis (PGS/ PGD) is programmed.