Objective: Cryopreservation of embryos and oocytes has become an essential service for infertility treatment. The clinical application of this technology should ensure optimal survival of the embryos and oocytes that are stored and subsequently thawed for transfer. The aim of this review is to compare the widely employed slow cooling procedures with vitrification to evaluate and recommend the more effective and safer procedure.
Design: The review is mainly based on a comparison of the principles, procedures, and results reported in the literature. A historical description of vitrification and personal experiences with this technology are also included.
Setting: University-based hospitals and private clinics that treat infertility and have published information on cryopreservation.
Patient(s): Women being treated for infertility and reproductive technology clinics.
Intervention(s): The application of slow cooling involving a range of cooling rates is compared with vitrification using rapid and ultrarapid cooling in simple containers. The purpose of both techniques is the induction of a glasslike state in cells to protect them from damage by ice crystals. The early development of vitrification involved the use of long pre-equilibration procedures. Improved methods resulted from the use of mixtures of penetrating and nonpenetrating solutes that are not toxic and a range of cooling rates.
Main outcome measure(s): Reported number of pregnancies established after transfer of embryos that were cryopreserved by vitrification, or transfer of embryos derived from vitrified oocytes.
Result(s): Both slow cooling and vitrification procedures have resulted in the successful cryopreservation of human embryos and oocytes. Both procedures have resulted in healthy births, although the slow cooling of oocytes gives very low success rates. Vitrification is a promising novel technique in assisted reproductive technology, but comparative success rates are yet to be established.
Conclusion(s): Vitrification is a simple procedure that requires less time and is likely to become safer and more cost effective than slow cooling.