[Slow freezing and vitrification of human mature and immature oocytes]

Performance and security questions in human oocyte cryopreservation have been taking researchers for about two decades. Oocytes are usually frozen at metaphase II. Immature oocytes cryopreservation is still a research alternative. Two techniques are currently available for oocyte cryopreservation: slow freezing and vitrification. Experimental data suggest that vitrification has less impact on oocyte physiology than classical slow freezing. After slow freezing of mature oocytes, survival and fertilization rates reach 70 to 80% whereas cleavage rates are around 90%, leading to five implantations and 1.2 births per 100 thawed oocytes. After vitrification of mature oocytes, survival and cleavage rates reach 90% leading to 11 implantations and 1.8 births per 100 thawed oocytes. The obstetrical and neonatal prognosis of these pregnancies is reassuring. No increased risk of congenital anomalies has been observed. However, further evaluation is needed to guarantee the safety of cryopreservation procedures. Immature oocyte cryopreservation is not currently perfected but some indications appear of great interest.