Closed versus open vitrification systems for human oocytes and embryos : a mini-review

Vitrification is an established and successful technique for preserving human oocytes and embryos. It can be achieved either by direct (open systems) or indirect (closed systems) contact with liquid nitrogen and there is not a consensus on the optimum vitrification protocol. Scientific societies agree that there are no particular concerns regarding vitrification other than direct contact with a non sterile product. Moreover, European directives pose the need for aseptic procedures as a critical point. Therefore, several strategies have been developed in order to avoid the risk of contamination, including closed devices and liquid nitrogen sterilization. There have been concerns with closed vitrification devices that a reduction in cooling rate compared to open vitrification systems due to thermal insulation of samples would cause ice crystal formation resulting in impaired results. It has been proposed that a correct exposure to cryoprotective agents before closed vitrification and a high warming rate can adequately compensate the reduction in cooling rates. This reduction in cooling rate can also be prevented with direct plunging of samples in sterile liquid nitrogen followed by hermetical cryostorage (semi-closed system). Studies comparing different protocols suggest that aseptic vitrification is an effective strategy both for embryos and oocytes.