Successful ultrarapid freezing of unfertilized oocytes

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Abstract
Successful application of ultrarapid freezing techniques to unfertilized murine oocytes has not been reported. In an effort to improve results, preovulatory murine oocytes were exposed to three ultrarapid freezing protocols involving varying sucrose concentrations (0.25, 0.5, and 1.0M) and 3.5M dimethyl sulfoxide (DMSO) as cryoprotectants prior to direct immersion in liquid nitrogen. Postthaw morphology and rates of in vitro fertilization and embryo development were compared with those obtained after freezing oocytes employing two established programmed cooling techniques. The rates of fertilization and development to the blastocyst stage in vitro of oocytes undergoing ultrarapid freezing after exposure to 3.5M DMSO and 0.5M sucrose were similar or superior to those obtained with programmed cooling techniques. Of oocytes which appeared morphologically normal postthaw, only those which underwent ultrarapid freezing with 0.25 or 0.5M sucrose and 3.5M DMSO reached the blastocyst stage at rates similar to those of controls. Ultrarapid freezing may represent a viable option for successful murine oocyte cryopreservation.