Lysis of oocytes by bovine sperm and seminal plasma

This study examined the lytic activity of bovine seminal plasma on zona-free oocytes commonly used to assess bovine capacitation. Exposure of hamster oocytes (intact or zona-free) to undiluted seminal plasma resulted in lysis within 5 min. With intact oocytes, 1:5 to 1:20 seminal plasma resulted in swelling of the oocytes. With zona-free oocytes, seminal plasma (1:0 to 1:1000) resulted in lysis within 1 min to 3 h depending on dilution. Heating seminal plasma to inactivate complement did not reduce lytic activity, but boiling destroyed it. Lytic activity was present in seminal plasma from vasectomized bulls and in seminal vesicle fluid. After elution of seminal plasma through Sephadex G-200, lytic activity was only associated with the fraction containing proteins of 200,000 to 45,000 dal. Lytic activity remained with washed capacitated bull sperm only when 10(7) sperm/ml or more were coincubated with zona-free oocytes at 37 degrees C for 3 h. In conclusion, both bovine seminal plasma and capacitated sperm were lytic in the zona-free hamster oocyte assay. The results may explain why so few sperm are normally found at the site of fertilization.