A modified two-step method for cryopreservation of mouse embryos for purposes of embryo banking.

So far, more than 17,000 embryos have been frozen in the repository of the cryobank in the Institute of Molecular Genetics. The freezing of embryos from more than 20 inbred, congenic and mutant strains has been completed. For purposes of embryo banking, a modified two-step method was used. The first step includes slow freezing, after seeding, to -25 degrees C, rate 0.3 degrees C/min, and is followed by a second step involving direct transfer of the embryos into liquid nitrogen. A relatively high percentage of survival is found (85-95%) after thawing, depending on the strain used. If thawed embryos are cultivated briefly and transferred into recipients, 20-35% living offspring are obtained. Mice born after embryo transfer are subjected to genetic control in order to check the correct procedure of embryo banking.