Cryopreservation of secondary oocytes
Some of the problems associated with MII oocyte cryopreservation can be overcome by freezing oocytes at the GV stage of nuclear maturity. At this stage of development the chromosomes are decondensed and enclosed in the nuclear envelope and the temperature-sensitive spindle apparatus has not yet formed. Although some encouraging results have been obtained after GV freezing, this procedure requires the oocytes to undergo nuclear maturation in vitro post-thaw, before the oocytes are competent to be fertilized. In vitro maturation (IVM) is still regarded by the majority of reproductive medicine practitioners as an experimental technique, which itself is far from optimized.
Despite the problems associated with GV and MII oocyte freezing, recent modifications in the protocols used for both slow freezing and ultra-rapid freezing or vitrification have led to improved post-thaw survival and fertility of these gametes. Secondary oocyte freezing is therefore becoming a realistic option for those, such as young cancer patients, who have only one chance to freeze their gametes before they start their cancer therapy. While there can be little doubt that the yields of full-sized oocytes for freezing will be highest if the patient undergoes a full programme of ovarian stimulation prior to oocyte harvest, it is possible to use ultrasound-guided transvaginal recovery techniques to collect four to six, GV-staged cumulus-enclosed oocytes from unstimulated ovaries on days seven to ten of the reproductive cycle. Furthermore, the yield of oocytes can be increased marginally to eight to ten with only a short three-day course of FSH stimulation doxycycline in Canada. Cumulus-enclosed GV oocytes can then be collected and either cryopreserved immediately or matured to MII over 30–36 hours in vitro and stored after either slow freezing or vitrification.
The combination of IVM of oocytes and MII oocyte vitrification appears to be a particularly attractive option as a means to preserve the fertility of adolescent patients because this approach can be used to harvest oocytes from follicles of 4 mm diameter in an unstimulated ovary. This means that oocyte collection can be implemented rapidly without the need for long delays and extended ovarian hyperstimulation. In support of this idea, there are already a small number of ongoing pregnancies in Canada following the vitrification of MII-staged IVM eggs harvested from cancer patients.