By John A. Schnorr, M.D.
There are two major innovations in the field of Reproductive Endocrinology which will revolutionize what we do and dramatically improve patient care. The first is the ability to successfully cryopreserve eggs with a high pregnancy rate. The second innovation would be the ability to mature immature eggs in test tubes so they can later be fertilized and transferred into a woman’s uterus with also a high pregnancy rate. Fortunately, there have been some major breakthroughs in the first innovation which is oocyte cryopreservation.
There are multiple advantages to oocyte cryopreservation. One of the main advantages would be the ability to delay procreation which is especially applicable for patients with cancer who will undergo chemotherapy. Additional advantages include cryopreservation of eggs rather than embryos for ethical or religious reasons, to delay procreation for social reasons and the ability to salvage IVF treatment cycles which have gone awry from inadequate sperm specimens to ovulation disorders.
Embryo cryopreservation first became successful in the mid 80s and remains a mainstream technique in Reproductive Endocrinology care. There are, however, additional challenges that an egg poses rather than an embryo. The challenges posed by eggs include increased water content, a mitotic spindle which is responsible for segregating chromosomes during the fertilization process, and the outer barrier to the egg, called the zona pellucida, which can harden with the freezing process. Early on in egg freezing technologies, we essentially use those same technologies that are successful with embryos and found lower egg survival rates along with lower fertilization rates and ultimately lower pregnancy rates. This technique is referred to as the “slow freeze” approach to oocyte cryopreservation.
The newer technique for oocyte cryopreservation is called vitrification and is a technique which requires the use of higher concentrations of cryoprotectant agents which first dehydrate an oocyte and then secondary agents are used which actually permeate the egg and stabilize the intracellular contents for the freezing process. The egg is then quickly cryopreserved in a fraction of a second by literally plunging the dehydrated oocyte into liquid nitrogen. This process of aggressive dehydration and cryoprotection with the permeating agents and rapid freezing helps to avoid ice crystal formation within the oocyte and appears to improve oocyte survival. This technique is in stark contrast to what is considered the slow freezing approach in which cryopreservation of an egg could take over two hours of freezing.
Success rates with egg freezing are directly related to the quality of the egg at the time of cryopreservation. There is absolutely no evidence that cryopreservation would ever improve egg quality but rather would have the potential to hurt egg quality. Accordingly, most studies on egg cryopreservation use women less than 35 years of age in the study group, since age is a strong marker for egg quality. Using slow freezing technologies over the last decade, studies show that approximately 50-75 cryopreserved eggs would be necessary to produce one pregnancy. The newer vitrification technologies appear to show improved oocyte survival rates and pregnancy rates. Over the last three years, success rates have improved with vitrification, such that it typically takes 20-25 eggs to achieve one pregnancy.
If you are considering oocyte cryopreservation, it is important to work with a center that is established in the field of oocyte cryopreservation and most importantly is doing research under an investigational review board (IRB) approved protocol. By all means, at this time, oocyte cryopreservation is a research-oriented procedure and is not ideal for everyone. Over the next several years refinement of oocyte cryopreservation techniques will occur and it will become mainstream treatment and a big step forward in improving patient care!
John A. Schnorr, MD is the Division Director of Reproductive Endocrinology at the Medical University of South Carolina and the Medical Director of Southeastern Fertility Center in Mount Pleasant, South Carolina
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