Stability of cryopreserved samples of mutant mice.

Genetically modified animals are unique models with enormous scientific potential. Cryopreservation of pre-implantation embryos or of spermatozoa is a common approach to save those lines. The breeding of a line can be discontinued if a sufficient number of samples have been cryopreserved. To maintain the opportunity to recover a line, it is mandatory to assess the quality of the cryopreserved samples and to assure safe long-term storage conditions. Here, we investigated the revitalization rate of cryopreserved pre-implantation embryos stored in-house up to 158 months, of imported (and shipped) embryos, and of embryos received after in vitro fertilization. The storage period did not affect the revitalization rate, whereas the recovery of imported embryos was significantly reduced, possibly due to shipment conditions. The genotypes of genetically modified pups received following embryo-transfer were slightly smaller than expected by Mendelian laws. Intensive investigations of the hygienic state of the cryopreserved samples and the equipment used never showed microbiological contamination of a sample within a cryo-tube. However, environmental organisms were found frequently in the permanent freezers and dry shippers used. Since such contamination cannot be completely excluded and an embryo-transfer might not lead in all cases to a secure rederivation, foster mothers and revitalized pups should be housed in an intermediate facility and their health assessed before introducing them into the target facility.