Cryopreservation of gametes and cleavage stage embryos.

The initial idea by the pioneers of in-vitro fertilization of human eggs on cryopreserving those fertilized oocytes or early embryonic stages was to avoid their replacement into cycles of ovarian hyperstimulation. In this article, a historical review and an assessment of freezing methods and means of cryopreservation is presented. Advantages and disadvantages of freezing early cleavage stages and blastocysts are discussed. Subsequently, ethical arguments are taken into consideration, which under certain circumstances such as legislation in the Federal Republic of Germany, support the procedure of cryopreserving pronuclear oocytes rather than unfertilized eggs or embryonic stages. Finally, there are some most actual calculations reported which reflect attempts to improve the overall success rate of in-vitro fertilization by means of various therapeutic strategies in which cryopreserved oocytes and embryos are essentially involved.