Advances in oocyte cryopreservation--part I: slow cool rapid thaw technique

The need for freezing oocytes has been established for females undergoing potential therapy that could damage their ovarian egg reserve, ethical or religious reasons (not having excess embryos frozen) or women nearing the age of lower fecundity but not married and not ready to use donor sperm. Applying the cryopreservation techniques for oocytes as used for embryos resulted in very poor pregnancy results.

Changes in methodology including fertilization by intracytoplasmic sperm injection because of zona hardness and using a sodium-deplete choline substitute freezing media are some of the changes made for the slow cool rapid thaw method.

These modifications have led to significant improved survival rates of frozen oocytes not to mention fertilization rates and subsequent pregnancy rates.

Since some in vitro fertilization (IVF) centers do poorly with frozen embryo transfer pregnancy rates despite good pregnancy rates following fresh embryo transfer, there is suspicion that the culpability may lie in the programmable freezer used in the slow cool technique. A simplified slow cool freezing technique using a bio-cool freezer instead of a programmable freezer has been described which has resulted in consistently good results with embryos. It would be interesting to see if this technique would work well with oocytes with the new changes to the freezing method.