Embryo cryopreservation is particularly useful for maintaining the genotype of homozygous strains.
For most strains 500 8-cell embryos are collected and cryopreserved.
However, some strain backgrounds may not freeze well and some mutations may interfere with the cryopreservation process.
For these it is necessary to bank between 750 and 1,000 embryos. Quality control is performed on all strains once they have been cryopreserved.
Quality control consists of thawing one or two random vials of cryopreserved embryos, transferring to pseudopregnant females and recovering liveborn pups.
Sperm cryopreservation is a cost-effective method of cryopreserving mouse strains if maintaining homozygosity is not a concern.
Caudae epididymides are removed from 5-8 males (3-8 months old) and sperm samples are collected and cryopreserved in multiple vials.
Quality control consists of performing in vitro fertilization (IVF) with a randomly selected thawed sperm sample and hybrid oocytes.
Following transfer of the IVF embryos to pseudopregnant females, liveborn pups are recovered.
Ovary cryopreservation is a cost-effective method of cryopreserving strains. Ovaries are removed from 4-5 females and cryopreserved.
This method is recommended for strains in which sperm or embryo cryopreservation is not efficient.
The frozen and thawed half-ovaries are implanted in ovariectomized female recipients, which after recovery from surgery will be mated with selected males,
obtaining pregnancies derived from the implanted ovary.
In vitro fertilization may be effectively used to rapidly expand a colony either for experimental or breeding purposes.
It is particularly effective for strains that are maintained by forced heterozygosity in a continuous backcross to an inbred strain.
In this case it is possible for a single male to produce 500 or more offspring in a single IVF.