New aspects of cryopreservation of oocytes and embryos in assisted reproduction and future perspectives.

Cryopreservation of human gametes and tissue has been introduced into assisted reproduction in recent years. The current status as well as new developments of cryopreservation, especially in conjunction with new techniques, are reviewed. Cryopreservation of pronuclear stage oocytes or embryos at the cleavage stage are routine procedures worldwide after conventional in-vitro fertilization (IVF). The data on children born show no increase in malformation rates, or any impaired psychomotor development. The combination of cryopreservation with intracytoplasmic sperm injection (ICSI) has given the same results. Cryopreservation of unfertilized mature and immature oocytes has lead only to a few pregnancies until now. It is still unknown whether ICSI may help to improve these results. Experience using cryopreservation of blastocysts is still limited, with a wide range of pregnancy rates (0-53 %) being reported. Further studies are required before exact numbers for these procedures can be given. The combination of biopsy and cryopreservation appears to have a different impact for subsequent embryo development in animals and humans. Possibly, there is a reduction in embryo developmental potential after these procedures. Currently, the most promising technique is the cryopreservation of ovarian tissue, which has already led to the birth of lambs, mice and rats. The future development of cryopreservation is discussed.