Effects of sperm concentration on the capacitation of rat spermatozoa

Rat epididymal spermatozoa were preincubated for various times in concentrated (9.6–14.9 × 106 sperm/ml) or diluted form (0.7–1.2 × 106 sperm/ml) and mixed at similar concentrations with eggs recovered from superovulated immature rats. When the eggs were examined at various times after insemination, the penetration rates, the time of sperm penetration, and the fertilizing capacity of sperm were greatly reduced following the preincubation of spermatozoa in a concentrated form for 5–7.5 hours. By preincubation of sperm in diluted form, capacitation of spermatozoa was achieved in 3–5.5 hours, the penetration rates were increased, and the interval between insemination and sperm penetration was progressively reduced as the duration of preincubation increased up to 7.5 hours. It appears that the sperm concentration plays an important role for the capacitation of rat spermatozoa in vitro and the maintenance of their fertilizing capacity.