ABSTRACT
INTRODUCTION e announcement in 1972 of the survival of mouse embryos aer cryopreservation at −196°C and thawing was groundbreaking.1 Since then, the impact of cryopreservation on the growth and improved eciency of assisted reproduction in humans has become increasingly appreciated. e percentage of frozen human embryos has risen steadily over the years, with approximately one and a quarter million babies born following cryopreservation. Moreover, cryopreservation has also been shown to increase overall pregnancy rates, while allowing for further selection of embryos. Indeed, it is possible to achieve implantation and pregnancy rates with frozen-thawed embryos as high as those achieved with fresh embryos. Routine in vitro blastocyst culture and cryopreservation have been shown to increase pregnancy rates, while allowing for better selection of embryos. In the early days of human in vitro fertilization (IVF) history, before reliable cryopreservation protocols, there was an emphasis on transferring as many embryos as possible into patients. With more reliable cryopreservation techniques such as “vitrication,” lower numbers of embryos are now being transferred, resulting in less high-order multiple pregnancies, as well as increased healthy implantations. In addition, decreased numbers of embryos are transferred, so increasing the potential for more embryos to be placed into frozen storage, thus reducing the number of fresh cycles. e fundamental objectives for successful cryostorage of cells in liquid nitrogen (LN2) at −196°C can be summarized as follows: (1) arresting the metabolism reversibly; (2) maintaining structural and genetic integrity; (3) achieving acceptable survival rates aer thawing; (4) maintenance of developmental competency post-thaw; and (5) the technique has to be reliable and repeatable.
