In vivo fertilization improved the cryotolerance and developmental ability of vitrified-warmed rat fertilized oocytes.

The cryopreservation of rat embryos is useful for efficiently archiving rat resources in bioresource repositories. The cryopreserved fertilized oocytes can be quickly reanimated to rats with homozygous mutations using embryo transfer. In addition, cryopreserved rat fertilized oocytes are easier to transport than live animals. Before cryopreservation, fertilized oocytes are obtained by mating or in vitro fertilization. However, it is not clear which fertilized oocytes are most suited to cryopreservation. In this study, we performed a systematic comparison of the fertilizing ability, cryotolerance, and developmental ability of cryopreserved fertilized oocytes at the pronuclear stage produced either by mating (in vivo) or in vitro fertilization (in vitro) in SD and F344 rats. In vivo-fertilized oocytes had higher cryotolerance and developmental ability than in vitro-fertilized oocytes in SD and F344 rats. Furthermore, the fertilization ability, cryotolerance, and developmental ability of vitrified-warmed fertilized oocytes differed between SD and F344 rats. In conclusion, our study suggests that in vivo-fertilized rat oocytes were more suitable for cryopreservation. Our protocol provides an optimized system for the management of rat colonies using fertilized oocytes cryopreservation and contributes to the 3Rs principle by reducing the number of animals used for research.