Considerations in Immature Oocyte Cryopreservation

Safe and effective oocyte cryopreservation will have a considerable impact on the clinical practice in assisted reproduction. Current freezing technology, both of slow-freezing and vitrification method, has markedly improved oocyte survival rate after warming, and the pregnancy rates are comparable to those achieved with fresh oocytes. Although immature oocytes can be cryopreserved successfully, clinical outcomes do not reach those of the matured oocytes cryopreserved. It seems that oocytes should be cryopreserved at mature metaphase II (MII) stage following IVM rather than at the immature germinal vesicle (GV) stage, because the structure of cumulus-oocyte complex (COC) is damaged with the procedure of cryopreservation with immature oocytes, in which the structure of COC is essential during oocyte maturation, and potential for oocyte maturation is reduced when cryopreservation is performed at the immature stage.