Developmental potential and ultrastructural injuries of metaphase II (MII) mouse oocytes after slow freezing or vitrification

Purpose: The purpose of this study was to determine the developmental ability and ultrastructure of MII mouse oocytes after cryopreservation by slow freezing or vitrification.Methods: Ovulated MII mouse oocytes were allocated to slow frozen, vitrified and control groups. Oocytes in the slow frozen and vitrified groups were cryopreserved using 1,2 propandiol (PROH) and ethylene glycol (EG) respectively as cryoprotectants. After thawing, the surviving MII oocytes in both cryopreserved groups and the control group were inseminated and their developmental ability was compared. The ultrastructure of MII oocytes in both cryopreserved groups was assessed immediately after thawing and 10 h post insemination at the pronuclear stage, and compared with that of the control group.Results: The survival rates were nearly identical in both cryopreserved groups. The fertilization rates were also identical and comparable to that of the control group. The further development of vitrified oocytes was similar to that of the control oocytes, whereas it was severely limited in the slow-frozen oocytes. In the slow-frozen MII oocytes, the intermediate filaments were destroyed and the oolemma and microvilli were also modified. At the pronuclear stage deterioration of mitochondria and the presence of numerous vacuoles were also observed within the ooplasm. In the vitrified MII oocytes, the intermediate filaments were the only structures affected and these cytoskeletal elements were reorganized at the pronuclear stage.Conclusions: Vitrification results in less ultrastructural damage and better post fertilization development of MII mouse oocytes than slow freezing.