Vitrification of human oocytes and embryos

Oocytes
The clinical application of human oocytes cryopreservation is important (Oktay et al, 2006). (1) to preserve eggs for patients prior to chemotherapy and (2) to establish oocyte banks. Because human metaphase II oocytes are chilling sensitive and are among the largest cells in the human, slow controlled-rate freezing procedures show suboptimal results (Oktay et al, 2006). Oktay et al (2006) and Kuwayama et al (2005) clearly demonstrated that there was a trend for better outcome when vitrification techniques are applied. However,these observations are not validated yet in large controlled trials.
Embryos
Cryopreservation of embryos has always been an integral part in IVF and infertility treatment. Embryos can be frozen as zygotes on day 1, as cleavage stage embryos on day 2 en 3 and as blastocysts on day 5 and 6. To avoid the iatrogenic epidemic situation of twin and triplet pregnancies only one embryo is replaced since 2003 in Belgium. Furthermore one should consider to avoid the occurrence of twins and triplets when frozen and thawed embryos are replaced. It is clear that for such strategies to be implemented successfully, efficiënt cryopreservation programmes are extremely important. Recent publications (Liebermann et al, 2005; Kuwayama, 2006) indicated that vitrification might be a promising "innovative" technique for the cryopreservation of human embryos. However, neither these observations are not validated yet in large controlled trials.
Vitrification is the complete solidification of a solution without ice crystal formation. The solution is said to become a glass. The solidification is brought about by using high concentrations of cryoprotectant in the vitrification solutions. Penetrating cryoprotectants (DMSO, propylene glycol and ethyleneglycol) protect the cells intra and extra cellularly, non penetrating cryoprotectants (sugars) protect the cell through osmotic dehydration. For succesfull vitrification an exact interplay between three important parameters is essential needs to occur: (1) the concentration and type of cryoprotectants, and the time and temperature of exposure; (2) cooling and warming rates; (3) carrier systems used (= the system that is used to carry the embryo) (Pegg,2005; Vajta and Nagy, 2006; Vajta and Kuwayama, 2006; Yavin and Arav, 2007). The implementation of the EU directives for the storage of cells and tissues will imply that closed carrier systems are used for the vitrification of human oocytes and embryos in order to avoid the risk of cross-contamination from pathogens in liquid nitrogen. Paynter in 2005, described that cryopreservation protocols should be developed which are extremely robust, to avoid variability in results between and within centres. Therefore it is better to develop "Equilibrium" vitrification procedures as suggested by Pegg (2005). For all the above we think it is justified to initiate research on the vitrification of human oocytes and embryos. The aim of the following project will be to develop and evaluate an efficient, consistent and safe vitrification protocol for vitrification of human oocytes and embryos.