[Cryopreservation of unfertilized rat oocytes by ultrarapid freezing]

Unfertilized rat oocytes were placed in a highly concentrated solution of cryoprotectant (DAP 224:2M dimethylsulphoxide, 2M acetamide, 4M propylene glycol in PB1) in 0.5 ml sampling tubes and then immediately immersed into liquid nitrogen; thawing was conducted in a 37 degrees C waterbath. After thawing, 630 out of 968 oocytes (65.1%) were morphologically normal. After insemination in vitro of cryopreserved oocytes, the proportion of pronuclear oocytes with spermatail (s), male (s) and female pronuclei (8-10 h post insemination), and 2-cell embryos with two identical blastomeres (28-30 h post insemination) was 60.8% (152/250) and 29.8% (39/131), respectively. One hundred and fifty oocytes that were judged as pronuclear oocytes under the inverted microscope 8-10 h after insemination were transferred to the oviducts of pseudopregnant recipients; 18.7% (28/150) of the oocytes developed to normal young.