Differing requirements for capacitation in vitro of mouse spermatozoa from two strains

Concentrated suspensions of epididymal spermatozoa obtained from two strains of mice, TO and C57BL/10, were preincubated for 20 min, 1 h or 2 h before dilution and addition of (C57BL/10 X CBA)F1 eggs. While all 3 groups of TO spermatozoa demonstrated high fertility (greater than 90% of eggs subsequently cleaved), C57BL/10 spermatozoa preincubated for 20 min and 1 h gave significantly reduced fertilization rates compared with those in the 2 h group. Furthermore, the penetration rate of the C57BL/10 spermatozoa preincubated for 20 min was significantly slower than that for similarly treated TO spermatozoa, as demonstrated by a delay in the first cleavage. A long preincubation of C57BL/10 spermatozoa in a diluted rather than a concentrated suspension did not improve fertility, suggesting that optimal capacitation may be sperm concentration-dependent in some, if not all, strains of mice.