Oocyte Cryopreservation: Slow Cooling Versus Vitrification Techniques on Oocyte Survival

Overview

Oocyte cryopreservation has been studied for many years without much success in refining a method that has consistent, reliable results in producing viable embryos and clinical pregnancies. In 1986 the first baby was born from an embryo created from a frozen oocyte; however, since then there have been less than 150 births from frozen eggs. To date, there are no reportable adverse outcomes in the children born from frozen oocytes. The research continues to look at different methods of oocyte cryopreservation. Many smaller studies have been conducted with some success but larger clinical trials are needed to replicate these findings. The conventional cryopreservation technique has been slow cooling with differing methods of freezing; however, vitrification is now being researched as the potential cryopreserving method that holds some promise for the future. Our hypothesis is the use of vitrification (quick freezing) to cryopreserve oocytes in patients undergoing in-vitro fertilization will be more successful than slow freezing in oocyte survival, fertilization rate with ICSI and subsequent embryo development, implantation rate and pregnancy rate.

Cryopreservation of oocytes is desirable because it: 1) would allow infertility patients to store excess oocytes instead of embryos, eliminating some of the ethical and religious concerns that accompany embryo storage; 2) permit storage of donor oocytes to avoid donor-recipient synchronization difficulties; and 3) can help women who may face sterilization due to chemotherapy or radiation. Oocyte cryopreservation is therefore gaining in popularity as an option for infertility treatment as well as fertility preservation. Oocyte cryopreservation using conventional slow-cooling methods has not had much success; however more recent results have provided more optimism (Boldt et al., 2003; Porcu et al., 1997; 2000; 2002; Yang et al., 1998; 1999; 2002; Winslow et al., 2001). Vitrification has also been employed (Hong et al., 1999; Kuleshova et al., 1999; Yoon et al., 2000, 2003; Chung et al 2000; Wu et al., 2001: Kuwayama et al., 2005) with increased oocyte survival rate and live births. Vitrification is performed by suspending the oocytes in a solution containing a high concentration of cryoprotectants and then plunging them directly into liquid nitrogen (Rall and Fahy, 1985). The advantage of this technique is to prevent the formation of ice crystals within the oocyte. However the toxic effect of the high concentration of the cryoprotectant media has been a concern. New vitrification techniques which attempt to accelerate the cooling rate by decreasing the cryosolution volume and concentration, may reduce the potential toxicity. In addition, a more rapid cooling rate results in reduced chilling injury (Vajta et al., 1998).