Cryopreservation is a process of freezing the gametes or embryos and keeping them in liquid nitrogen at -196°C for future use. The survival rate (80%) of thawed embryos depends on their quality before freezing. Most of the time, the survival rates of embryos will decrease after thawing.
The duration of storage in liquid nitrogen will not affect the post-thawed embryos' survival rates. Cryopreservation does not cause damage to the embryos during the storage phase but it occurs during the freezing and thawing phase.
During the freezing process, the embryos are exposed to rapidly falling temperatures and two adverse events which affect the embryo's survival rates. Intracellular crystal formation and dehydration may occur and this will damage the embryos. To decrease the damage to the embryos, cryoprotectants are used during the freezing and thawing process.
Embryo freezing is done whenever there are more than three good embryos obtained from IVF or ICSI cycle. The best three embryos are chosen for embryo transfer while the remainder will be kept for freezing. Embryo freezing is also performed when the female partner cannot undergo embryo transfer in the same cycle because of conditions such as OHSS (ovarian hyperstimulation syndrome). Embryos freezing also helps in the embryo donation program.
The two commonest embryo freezing protocols practiced are slow freezing and vitrification. Slow freezing requires an expensive programmable freezing planner and involves a series of washing in freezing solutions which contain cryoprotectants. This is a time-consuming procedure because it takes three to four hours to complete the whole process.
Vitrification is a rapid freezing procedure and it does not require a programmable freezing planner and can be done within minutes. The embryos are plunged into liquid nitrogen directly after short exposure to the high-concentrated cryoprotectants. After freezing, either by slow freezing or vitrification, the embryos are stored in liquid nitrogen for as long as required.
The frozen embryos are thawed when the female partner is ready to receive the embryos. The female partner’s endometrium will be prepared and made ready to receive the embryos for implantation and development. This is done using drug stimulation (estrogen and progesterone). The pregnancy rates with frozen-thawed embryos are as high as those in IVF cycles using fresh embryos ( 50%).
To date, there is no indication that babies born following the transfer of frozen embryos have an increased risk of abnormality.
Programmable freezing planer,
used for slow freezing procedure.
The liquid nitrogen tanks
used for embryos storage.
Sperm freezing is done when a male partner is not available or may have difficulties producing semen on the day of IUI or IVF. Sperm freezing is also indicated for those who are going to undergo radiotherapy or chemotherapy.
The sperm is frozen and kept in liquid nitrogen. The frozen sperm is thawed when it is required for ART. As a routine, sperm freezing is not encouraged as the survival rate is much lower compared to fresh sperm. Fresh sperm is preferred for ART.
Oocyte freezing had been proven to be more difficult to achieve compared to embryo and sperm freezing due to their low post-thaw survival rates and fertilization rates.
The post-thaw survival rates of frozen eggs vary between 18% to 52% while their fertilization rates are usually below 50%. Thus, it is not a common practice for most fertility centers. Embryo freezing is preferred. It is indicated to those ladies who are going to undergo radiotherapy or chemotherapy.