Evaluation of DNA fragmentation of freeze-dried mouse sperm using a modified sperm chromatin structure assay.

Freeze-dried sperm is applicable to the storage and transport of genetic material. We recently reported that freeze-dried mouse sperm required temperatures lower than -80 degrees C for long-term preservation and concluded that it was necessary to explore freeze-drying conditions before long-term preservation of sperm becomes viable. In the current study, we determined the percentage of sperm with elevated levels of DNA fragmentation using a sperm chromatin structure assay (SCSA), a technique not previously reported for the evaluation of freeze-dried mouse sperm. We applied SCSA to mouse sperm freeze-dried under four conditions (various combinations of primary drying pressure of 0.04 and 0.37 hPa and storage temperatures of 4 and -80 degrees C) and compared the results with the embryonic developmental rates of freeze-dried sperm after intracytoplasmic sperm injection (ICSI) and with comet assay results. The DNA fragmentation index values under the four conditions determined by SCSA had good correlation with the developmental rate to the blastocyst stage of embryos from ICSI with freeze-dried mouse sperm. We concluded that the SCSA method applied to freeze-dried mouse sperm after storage will lead to not only clarification of the developmental rate derived from ICSI using freeze-dried sperm but also to improvements in the freeze-drying and storage processes.