Cryostorage for fertility preservation
Fertility preservation is a vital branch of reproductive medicine and involves the preservation of gametes (sperm and oocytes), embryos and reproductive tissues (ovarian and testicular tissues) for use in artificial reproduction.
Cryopreservation
The most common fertility preservation technique is cryopreservation, which involves freezing cells and tissues at cryogenic temperatures. Cryopreserved cells and tissues can endure storage for decades with almost no change in functionality or genetic information, making this storage method highly attractive.
The development of safe and reliable cryopreservation protocols has facilitated the widespread use of gametes and embryos that have been frozen and subsequently thawed in assisted reproduction technologies (ART).
Cryopreservation improves the success of assisted reproduction
Inclusion of embryo cryopreservation into IVF programmes helps to reduce the risk of multiple gestation and offers couples greater flexibility in their family planning. It may also be recommended if certain hormone levels are too high during the IVF cycle or to avoid the risk of a condition called ovarian hyperstimulation syndrome.
Cryopreservation is also a valuable option for couples who anticipate an imminent decline in gamete viability (fertility) as a consequence of medical causes or in cases where advanced maternal age is a growing fertility concern.
Cryopreservation has facilitated the development of gamete banking (for medial or social reasons) as well as the formation of gamete and embryo donation programmes.
Methods of cryopreservation
Major developments in cryopreservation technology over the last 2 decades means that it is now possible to cryopreserve sperm, embryos and more recently oocyte at their various stages of development.
There are two major techniques for cryopreservation: freeze-thaw processes and vitrification.
The major difference between them is the total avoidance of ice formation in vitrification.
1.Freeze- thaw process
This is a traditional cryopreservation method which allows ice to form and solute concentrations to rise during the preservation process. However, both ice and high solute concentrations can cause cell damage during thawing and as such this process has been superseded by the vitrification technique (below).
2.Vitrification
Vitrification is an ultra-rapid method of cryopreservation whereby a cell is transitioned from 37 to −196 °C in <1 second, resulting in extremely fast rates of cooling. High concentrations of cryoprotectants together with rapid cooling rates are essential to cryopreserve embryos in a vitrified, glass-like state. It enables the cells to be cooled to cryogenic temperatures in the absence of ice.
Vitrification has proved to be a game-changer in assisted reproduction and has received widespread recognition. It has now become the method of choice over traditional slow freezing methods. Its efficiency and reliability have encouraged single embryo transfers (and thus reduction in the risk of multiple pregnancies), the safe storage of remaining ‘extra’ embryos and a reduction in the risk of ovarian hyperstimulation syndrome in pregnancy. This method does not require expensive equipment and is not time-consuming.
