Room temperature storage of mouse epididymal spermatozoa: exploration of factors affecting sperm survival

To explore optimal conditions for in vitro sperm survival, we examined the effects of several media used for murine egg culture and in vitro fertilization (IVF; including M16, M2, PB1, TYH, and CZB) on motility of murine spermatozoa stored at 22 degrees C under paraffin oil. Of media tested, M2 medium, that had been adjusted to pH 7.2 by adding N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), was found to be the best. Addition of various concentrations of HEPES to TYH did not improve sperm survival, suggesting that HEPES (and probably neutral pH) do not enhance survival of murine sperm. Since M16 has higher amounts of bicarbonate than M2 (25 mM versus 4.15 mM), four variations of M16 media containing 4.15, 8.30, 16.60, or 33.20 mM bicarbonate were prepared and tested. The modified M16 media with 4.15-16.60 mM bicarbonate yielded good sperm survival (comparable to M2 medium), while relatively high concentrations of bicarbonate (ranging from 16.60 to 33.20 mM) were deleterious to isolated sperm, suggesting the need for a minimum level of residual bicarbonate. However, the mechanism by which the lifespan of spermatozoa is extended remains unknown. The in vitro fertilizing abilities of spermatozoa left in M2 medium for 1, 3, and 5 days at 22 degrees C were 52.5, 21.8, and 7.0%, respectively, when the cleavage rate to the two-cell stage was examined. Transfer of two-cell embryos produced in vitro with spermatozoa stored for 1, 3, and 5 days at 22 degrees C resulted in production of fetuses with efficiencies of 42.5, 23.4, and 12.5%, respectively, which were lower than that of embryos derived from in vitro fertilization with fresh spermatozoa (68.1%). In conclusion, spermatozoa kept in M2 medium for up to 5 days at 22 degrees C can fertilize oocytes.