Possibility of long-term preservation of freeze-dried mouse spermatozoa

Freeze-dried mouse spermatozoa are capable of participating in normal embryonic development after injection into oocytes. When the freeze-dried spermatozoa are used as a method for storage of genetic materials, however, it is essential to assure the relevance of long-term preservation over several decades or centuries. Thus, we applied the theory of accelerated degradation kinetics to freeze-dried mouse spermatozoa. Thermal denaturation kinetics were determined based on Arrhenius plots derived from transition-state theory analysis at three elevated temperatures: 30, 40, and 50 degrees C. Accelerated degradation kinetics were calculated by extrapolation of Arrhenius plots. This theory also is being applied to the long-term stability of drugs. The estimated rate of development to the blastocyst stage at 3 and 6 mo and at 1, 10, and 100 yr of sperm storage at 4 degrees C were 21.60%, 7.91%, 1.00%, 0%, and 0%, respectively. At -80 degrees C, estimated development rates to the blastocyst stage that would be expected after 100 yr of storage did not decline significantly. In addition, after 3 or 6 mo of storage at 4 or -80 degrees C, preimplantation development of the embryos derived from intracytoplasmic sperm injection (ICSI) was examined. The actual developmental rates to the blastocyst stage from ICSI by freeze-dried sperm stored for 3 mo at 4 and -80 degrees C were 21% and 62%, respectively, and the rates for such sperm stored for 6 mo were 13% and 59%, respectively. These results indicate that the determination of accelerated degradation kinetics can be applied to the preservation of freeze-dried mouse spermatozoa. Furthermore, for long-term preservation, freeze-dried mouse spermatozoa appear to require being kept at lower than -80 degrees C.