Physicians Vitrification
Cryopreservation is a method of storing tissues and cells for a long period of time in liquid nitrogen at a temperature of -196°С.
Cryopreservation of spermatozoids is performed using a special medium through the so-called “fast method”. They can be stored for dozens of years in a frozen state without any risk of injury. If necessary, spermatozoids are defrosted using a special method, after that are activated with specific mediums and are injected in the uterus after the so-called controlled intra-uterus insemination (IUI) method. The freezing of sperm is applied for the purpose of storing spermatozoids for the donor ship program, for facilitation of the spouses in their infertility treatment, or for men with pending radiotherapy or chemical therapy in connection with malign diseases.
Unlike the spermatozoids, embryos are rather vulnerable during the freezing and defrosting procedures – usually about 10-20% of them do not survive. For this reason cryopreservation is made only with respect to good quality embryos (I and II degree) and it is applied when a sufficient number of good quality embryos have remained after embryo transfer. Thus, in case of unsuccessful outcome of the IVF procedure, there is a possibility for embryo transfer of frozen/defrosted embryos in a consecutive non-stimulated cycle. This method is applied also in cases of severe forms of ovarian hyperstimulation syndrome, when an eventual pregnancy would deteriorate the critical state of the woman. For that very reason embryo transfer is not performed and all embryos are frozen.
There are two main methods for embryo freezing – slow freezing and rapid freezing (vitrification).
Cryopreservation performed using the so-called slow method with computer controlled freezer at an average speed of 1°С/3 min. Specific mediums with cryoprotectants are used, inducing extraction of the liquids from the embryos.
Recently the rapid cryopreservation has become a promising technology in the assisted reproduction. Vitrification is an ultrarapid cooling technique that produces a glasslike solidification of living cells that completely avoids ice crystal formation during cooling and, equally important, during warming to recover cells.
A major advantage of vitrification is elimination of mechanical damage caused by intra- or extracellular ice crystals and potential reduction of chilling damage by shortening the exposure to suboptimal temperature. Vitrification is a result of high cooling rates associated with high concentrations of cryoprotectants. Because of the precise nature of vitrification, it does require practice and experience. In Sofia Center of Reproductive Medicine and IVF the vitrification of different stages embryo is a routine practice. During the thawing of vitrified embryos the recovery rate is 95% and the pregnancy rates are equal to the rates of fresh embryo transfers.
