Freezing Unfertilized Eggs
Cryopreservation of oocytes (unfertilized eggs) is available to women who have not chosen the father of their future child at the time that their future fertility is threatened by cancer or another disease (Cancer & Fertility). It may also be of interest to young women who have to defer childbearing and are concerned about the decline in fertility due to advancing age (What is Age Factor?).
Egg freezing has presented much greater challenges in comparison to freezing of embryos (fertilized eggs) which are hardier. Freshly ovulated mature eggs are at a very sensitive stage of the meiotic cell division that prepares the egg for fertilization. The chromosomes are precisely aligned within a fragile structure known as the spindle apparatus which is shown below. Cryopreservation can disrupt the meiotic spindle and thus cause chromosomal abnormalities which then lead to arrested or abnormal development.
Mature unfertilized egg with a spindle apparatus
In addition, freezing can trigger premature hardening of the zona pellucida which surrounds the egg. Normally zona hardening occurs only after the first sperm penetrates the egg thus protecting it from being penetrated by other sperm.
These inherent difficulties have limited the success of oocyte cryopreservation in the past. However, several recent studies have reported improved results with adjustments in the freezing protocols. In particular, extremely rapid cooling rates with a technique known as vitrification appear to minimize chromosomal damage. The addition of intra-cytoplamic sperm injection (What is ICSI?) to overcome the effects of premature zona hardening have also contributed to improved success rates. Nonetheless, SART still considers oocyte cryopreservation to be an experimental procedure unlike the freezing of embryos which is a clinically proven and safe technique.