Cryopreservation and Vitrification
How are sperm, eggs and embryos stored?
For many years, cells have been frozen in liquid nitrogen (-196°C). This is particularly easy with very small cells, such as sperm cells. This led to the creation of sperm banks several decades ago. With larger cells or tissue fragments, freezing in liquid nitrogen is much more difficult, since crystals form in the cytoplasm (liquid part of the cell) that may damage the organelles and genetic material (cell nucleus). This is why freezing the largest cell in the human body, the egg cell, was an extremely difficult process until very recently, using the slow freeze technique. Even the freezing of embryos, which are made up of smaller cells, gave poor to moderate results.
Vitrification, however, is a new technique by means of which eggs and embryos can be frozen without loss of quality. Vitrification is much simpler and is bound to become cheaper than the other techniques in the future. Expensive materials and procedures, such as a computer-controlled freezing programme, are not required. In comparison with the slow freeze technique, a larger amount of cryoprotectant (freezing medium) is used, which cools the eggs considerably more quickly. The freezing time with vitrification (fast freezing) has been reduced from three hours to ten minutes.
This reduction in the freezing time is very significant, because it means that there is no time for crystals to form. After all, an egg is a giant cell - in comparison, the egg is the largest cell in the human (female) body - which means that sharp ice crystals that damage the egg can easily form during freezing. Thus vitrification offers a solution: after all, the chance of vitrified eggs surviving after thawing is over 90%.
What is more, vitrification allows the freezing of more complex structures, such as more highly developed embryos that are already at the blastocyst stage on day five after oocyte retrieval. This gives us the opportunity to make a better, stronger selection of the embryos to be frozen. Clearly that significantly increases the chance of a pregnancy by embryo transfer.